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Mid-Infrared Few-Cycle Pulse Generation and Amplification

Mid-Infrared Few-Cycle Pulse Generation and Amplification hv photonics Review Mid-Infrared Few-Cycle Pulse Generation and Amplification Kan Tian, Linzhen He, Xuemei Yang and Houkun Liang * College of Electronics and Information Engineering, Sichuan University, Chengdu 610064, China; tiankan@stu.scu.edu.cn (K.T.); helinzhen@stu.scu.edu.cn (L.H.); yangxuemei@stu.scu.edu.cn (X.Y.) * Correspondence: hkliang@scu.edu.cn Abstract: In the past decade, mid-infrared (MIR) few-cycle lasers have attracted remarkable research efforts for their applications in strong-field physics, MIR spectroscopy, and bio-medical research. Here we present a review of MIR few-cycle pulse generation and amplification in the wavelength range spanning from 2 to ~20 m. In the first section, a brief introduction on the importance of MIR ultrafast lasers and the corresponding methods of MIR few-cycle pulse generation is provided. In the second section, different nonlinear crystals including emerging non-oxide crystals, such as CdSiP , ZnGeP , GaSe, LiGaS , and BaGa Se , as well as new periodically poled crystals such as 2 2 2 4 7 OP-GaAs and OP-GaP are reviewed. Subsequently, in the third section, the various techniques for MIR few-cycle pulse generation and amplification including optical parametric amplification, optical parametric chirped-pulse amplification, and intra-pulse difference-frequency generation with all sorts of designs, pumped by miscellaneous lasers, and with various MIR output specifications in terms of pulse energy, average power, and pulse width are reviewed. In addition, high-energy MIR single-cycle pulses are ideal tools for isolated attosecond pulse generation, electron dynamic investigation, and tunneling ionization harness. Thus, in the fourth section, examples of state-of-the- art work in the field of MIR single-cycle pulse generation are reviewed and discussed. In the last section, prospects for MIR few-cycle lasers in strong-field physics, high-fidelity molecule detection, and cold tissue ablation applications are provided. Keywords: mid-infrared; few-cycle pulse; optical parametric amplification; optical parametric chirped-pulse amplification; intra-pulse difference-frequency generation Citation: Tian, K.; He, L.; Yang, X.; Liang, H. Mid-Infrared Few-Cycle Pulse Generation and Amplification. Photonics 2021, 8, 290. https:// doi.org/10.3390/photonics8080290 1. Introduction The mid-infrared (MIR) wavelength is usually defined in the range of 2–20 m Received: 31 May 2021 (500–5000 cm ). With its unique properties and wide application prospects, lasers in this Accepted: 14 July 2021 band have attracted a great deal of attention from researchers all over the world. The main Published: 21 July 2021 characteristics of an MIR laser can be summarized as the following two aspects. First, as the pondermotive energy is quadratically proportional to the driving laser wavelength, MIR Publisher’s Note: MDPI stays neutral lasers with high peak power have been routinely pursued as the driving sources for novel with regard to jurisdictional claims in strong-field phenomenon [1–9], such as extreme ultra-violet and X-ray generation [10–13], published maps and institutional affil- attosecond pulse generation [14], and terahertz generation [15–17]. Second, most of the iations. vibrational peaks of different molecules fall in the MIR band, which is also called the “molecular fingerprint” regime. Therefore, MIR coherent spectroscopy is a unique method for high-fidelity and high-sensitivity molecule detection and identification [18–24]. As for MIR solid-state lasers, there are two main technique streams to generate MIR Copyright: © 2021 by the authors. pulses, namely the direct emission of doped ions and optical parametric down conversion. Licensee MDPI, Basel, Switzerland. The former is based on a process wherein the gain medium is stimulated after energy This article is an open access article storage, and the output wavelength depends on the energy level structures of the gain distributed under the terms and media. The biggest challenge of this process is that the relaxation energy in the MIR conditions of the Creative Commons wavelength coincides with the phonon vibration energy, which reduces the gain and Attribution (CC BY) license (https:// hinders the lasing process at the MIR wavelength. The second technique is based on the creativecommons.org/licenses/by/ parametric frequency conversion that is mainly assisted by nonlinear crystals that create 4.0/). Photonics 2021, 8, 290. https://doi.org/10.3390/photonics8080290 https://www.mdpi.com/journal/photonics Photonics 2021, 8, 290 2 of 24 the phase-matching conditions. In this process, there is no thermal accumulation, and broadband laser amplification can be realized through broadband phase matching, which supports the generation of few-cycle MIR pulses. At present, parametric down conversion has become an indispensable means to expand the new laser spectrum, generating pulses covering deep ultraviolet, visible, near-infrared, MIR, and THz wavelength regimes. In this paper, we review typical MIR nonlinear crystals and then summarize the techniques for the generation and amplification of ultrafast MIR lasers, including optical parametric amplification (OPA), optical parametric chirped-pulse amplification (OPCPA), and intra- pulse difference-frequency generation (IPDFG) with various kinds of designs. Subsequently, examples of state-of-the-art work in the field of MIR single-cycle pulse generation are reviewed and discussed. In the last part, new prospects for MIR few-cycle lasers in strong-field physics, high-fidelity molecule detection, and cold tissue ablation applications are provided. 2. MIR Nonlinear Crystals Nonlinear crystals that are commonly used in MIR pulse generation and amplification mainly include KTiOAsO (KTA), KTiOPO (KTP), and LiNbO (LNO), which belong a 4 4 3 group known as oxide crystals, and ZnGeP (ZGP), CdSiP (CSP), AgGaS (AGS), AgGaSe 2 2 2 2 (AGSe), GaSe, BaGa S (BGS), BaGa Se (BGSe), LiGaS (LGS), and LiGaSe (LGSe), which 4 7 4 7 2 2 are classified as non-oxide crystals. Generally, the damage threshold and mechanical hardness of oxide crystals are excellent, but their transparent range is limited to less than 5 m, which is not conducive for the generation of long-wavelength MIR pulses. The effective nonlinear coefficient of non-oxide crystals is higher, and the transparency range can reach more than 10 m, which is commonly used in the generation of long-wavelength infrared pulses. However, the bandgap energy of such crystals is generally around 2 eV, which makes the two-photon absorption non-negligible, and significantly reduces the damage threshold when pumped at an ~1 m wavelength with high peak power. LGS and LGSe are relatively new MIR nonlinear crystals with large bandgap energy, which enables high peak power pump at an ~1 m wavelength. However, the transparent range of LGS and LGSe are limited to 10 m. High quality new MIR nonlinear crystals with large bandgap energy and a broader transparent range are desired. BGSe is one of the candidates, however, the crystal growth quality still needs substantial improvement. In addition to the above MIR nonlinear crystals, periodically poled crystals such as periodically poled LiNbO (PPLN), orientation-patterned GaAs (OP-GaAs), and orientation-patterned GaP (OP-GaP) have become an emerging stream in the MIR parametric down conversion with their excellent quasi-phase matching bandwidth and large nonlinear coefficients, although their aperture size is the current bottleneck for pulse energy upscaling. The optical specifications such as the transparent range, effective nonlinear coefficient, and bandgap energy of the commonly used MIR nonlinear crystals are compared and summarized in Table 1. Table 1. Comparison of different MIR nonlinear crystals. Nonlinear Nonlinear Crystal Transparency (m) Bandgap (eV) Reference Coefficient (pm/V) AGS 0.5–13 13.4 2.76 [25] AGSe 0.75–15 26.8 1.83 [25] BGSe 0.47–18 24.3 2.64 [26] CSP 0.5–9 84.5 2.45 [27] ZGP 2–12 72 2.2 [28] GaSe 0.65–18 57 2.1 [29] LGS 0.32–11.6 5.9 3.76 [30] OP-GaAs 0.9–17 94 2.1 [31] OP-GaP 0.57–12 70 2.26 [32] Photonics 2021, 8, 290 3 of 24 3. MIR Generation Among all kinds of nonlinear polarizations, the most commonly used in down con- version is probably the second-order nonlinear effect due to the good efficiency. The three-wave mixing process introduced by the second-order nonlinear effect is the basis of second harmonics generation (SHG), difference-frequency generation (DFG), and OPA. DFG and OPA are the main methods of broadband MIR pulse generation and amplifica- tion. In a further step, combined with the theory of OPA and chirped-pulse amplification (CPA) technologies, OPCPA have emerged to scale up the pulse energy and peak power of MIR few-cycle lasers. In this section, we will review the state-of-the-art works using MIR few-cycle pulse generation and amplification via OPA, OPCPA, and IPDFG. 3.1. OPA OPA is an old parametric technique with emission in the visible and near-infrared wavelength regimes. It has the merits of broadband emission and simple dispersion control. Recently, the OPA emission bad has been extended to MIR wavelengths pumped at ~1 and ~2 m wavelengths, as summarized in Table 2. Typical work using MIR OPA are selected and reviewed in the following paragraphs. Table 2. Parameters of selected OPA system pumped by 1 and 2 m. Repetition Pump Wavelength Pulse Energy Power Pulse Width Optical Rate Reference (m) (m) (J) (mW) (fs) Cycle (kHz) 1 7.6–11.5 0.59 100 59 126 3.8 [33] 1 5–11 0.22 50 11 32 1.2 [34] 2 2.5–9 33 1 33 12.4 0.88 [35] 2 4.2–16 3.4 1 3.4 19 0.64 [36] 2.4 3–10 130 1 130 318 15.1 [37] In 2017, H. K. Liang demonstrated a sub-single-cycle MIR pulse synthesizer based on a MIR OPA, pumped by an OPCPA at 2 m and at a 1 kHz repetition rate, as shown in Figure 4a [35]. A ZGP crystal was employed as the nonlinear crystal for its wide trans- parent range, large nonlinear coefficient, and broad phase-matching bandwidth. A CaF wedge divided the pump beam into two paths. One (~10 J) was used as the pump to gen- erate a signal via supercontinuum generation in a BaF plate, while the other path entered the pump line of the MIR OPA. A broadband MIR emission covering 2.5–9 m was demon- strated with 33 J pulse energy. A pulse width of 12.4 fs was measured, corresponding to 0.88 optical cycles at a 4.2 m centre wavelength, as shown in Figure 4b–d. Subsequently, in 2019, H. K. Liang’s group extended the centre wavelength of the single-cycle OPA to 8.8 m using a GaSe nonlinear crystal that had a broader transmission range on the long-wavelength side compared to that of ZGP [36]. As shown in Figure 1a, the 2.15 m pump with a duration of 51 fs and a repetition rate of 1 kHz was derived using a Ti:sapphire laser pumped OPA (TOPAS). A 1-mm-thick GaSe crystal was employed as the OPA crystal. An ultra-broadband idler pulse with its spectrum spanning from 4.2 to 16 m supporting a Fourier-transform-limited pulse width of 19 fs and centered at 8.8 m was generated, as presented in Figure 1b. Considering all of the losses, the idler pulse energy of 3.4 J through a Si plate and a long pass filter was obtained. Photonics 2021, 8, x FOR PEER REVIEW 4 of 24 Figure 1. (a) The schematic of the high-energy phase-stable sub-cycle MIR OPA. Polarizations of the beams are marked by double-headed arrows and concentric circles. 300-μm thick Si wafers at Brew- ster angle were used as polarization beam splitter and beam combiner to transmit the 2.1-μm pump pulse and reflected the signal and idler pulses. The synthesized pulses and a branch of 2.1-μm ref- erence pulses were sent into XFROG with 30-μm thick GaSe nonlinear crystal. The synthesis of a sub-cycle MIR pulse by coherently combining the sub-2-cycle signal and idler pulses is shown con- ceptually at the top of the figure. (b) The signal spectrum from WLG at BaF2, measured after a 2400 nm long-pass filter. (c) The measured output spectrum of the MIR OPA. The dotted line separates Photonics 2021, 8, 290 the signal and idler spectra. The retrieved temporal (d) intensity profiles of the synthesized pulse. 4 of 24 Reprinted with permission from [35]. Figure 1. (a) The schematic of the GaSe-based MIR OPA. The pump, the generated supercontinuum, and the amplified MIR Figure 2. (a) The schematic of the GaSe-based MIR OPA. The pump, the generated supercontinuum, pulses are shown and the amplif in maroon, ied M purple, IR pu andlse pink, s are shown in respectively mar . (HWP: oon, pu halfrple, an wave plate, d pinDL: k, re delay spectiline vely , .LPF: (HWP: h long a pass lf filter, Photonics 2021, 8, x FOR PEER REVIEW 5 of 24 wave plate, DL: delay line, LPF: long pass filter, BD: beam dump, CS: characterization setup, and BD: beam dump, CS: characterization setup, and L1–L6 are CaF lenses). The measured (solid blue, 20 nm resolution) and L1–L6 are CaF2 lenses). The measured (solid blue, 20 nm resolution) and simulated (dashed red) simulated (dashed red) spectra of the amplified signal (b) and the idler pulses (c). Reprinted with permission from [36]. spectra of the amplified signal (b) and the idler pulses (c). Reprinted with permission from [36]. S. S. Nam et. al. presented an Nam et al. presented an octave-spanning octave-spanning 3–10 3–10 μ m m MIR OPA MIR OPA system system bas based edon on a a ZGP crystal, pumped by a 1 kHz, 2.4 m, 250 fs Cr:ZnSe CPA [37]. The OPA system was ZGP crystal, pumped by a 1 kHz, 2.4 μm, 250 fs Cr:ZnSe CPA [37]. The OPA system was seeded either by white light generation from a YAG plate or optical parametric gener- seeded either by white light generation from a YAG plate or optical parametric generation ation (OPG) in a ZGP crystal, as shown in Figure 2. By combining the signal seed and (OPG) in a ZGP crystal, as shown in Figure 3. By combining the signal seed and pump pump with orthogonal polarization through a Si wafer placed at the Brewster angle, the with orthogonal polarization through a Si wafer placed at the Brewster angle, the signal signal beam was amplified in a 5 mm-long ZGP crystal, and the long-wavelength idler beam was amplified in a 5 mm-long ZGP crystal, and the long-wavelength idler pulses pulses were generated. With 0.55 mJ pump pulse energy, 130 J or 55 J overall OPA were generated. With 0.55 mJ pump pulse energy, 130 μJ or 55 μJ overall OPA output output (signal + idler) were obtained from the MIR OPA seeded by OPG or white light (signal + idler) were obtained from the MIR OPA seeded by OPG or white light generation, generation, which corresponds to a pump to signal + idler conversion efficiency of 23% which corresponds to a pump to signal + idler conversion efficiency of 23% or 10%, re- or 10%, respectively. spectively. Figure 3. Experimental setup of MIR ZGP OPA. Red lines, 2.4 μm pump beam; green line, signal Figure 2. Experimental setup of MIR ZGP OPA. Red lines, 2.4 m pump beam; green line, signal (seed) beam; blue dotted line, idler beam. M1–M4: Al mirrors; L1–L4: CaF2 lenses; F1: 3 μm LPF; F2: (seed) beam; blue dotted line, idler beam. M1–M4: Al mirrors; L1–L4: CaF lenses; F1: 3 m 4.5 or 7 μm LPF; W: ZnSe wedge; D: delay stage; NL: nonlinear crystal (YAG or ZGP); B: 300-μm- LPF; F2: 4.5 or 7 m LPF; W: ZnSe wedge; D: delay stage; NL: nonlinear crystal (YAG or ZGP); thick Brewster Si plate; WP: λ/2 waveplate. Reprinted with permission from [37]. B: 300-m-thick Brewster Si plate; WP: /2 waveplate. Reprinted with permission from [37]. In 2018, M. Seidel et. al. employed PPLN crystal and wide-bandgap nonlinear crystal In 2018, M. Seidel et al. employed PPLN crystal and wide-bandgap nonlinear crystal BGS in a MIR OPA pumped by a thin-disc laser at ~1 μm. As shown in Figure 4, MIR BGS in a MIR OPA pumped by a thin-disc laser at ~1 m. As shown in Figure 3, MIR emission covering a spectral range of 2 to 11 μm was generated, with an output power of emission covering a spectral range of 2 to 11 m was generated, with an output power of 5 W at 4.1 μm and 1.3 W at 8.5 μm [38]. Subsequently, in 2019, B. Chen reported a MIR 5 W at 4.1 m and 1.3 W at 8.5 m [38]. Subsequently, in 2019, B. Chen reported a MIR laser source with a spectrum covering the 5–11 μm range and a pulse width of 32 fs [34]. The generated MIR pulse had a pulse energy of 220 nJ at a 50 kHz repetition rate. Figure 4. Tuning curves. (a) Generated MIR power for maximal pump power and tuning periods from 28 to 25.5 μm of the PPLN. The spectrum centered at 4.2 μm is shaped through CO2 absorption. The power was measured 25 cm behind the nonlinear crystal. (b) Tuning curve measured with a type I phase-matched LGS crystal. The OPA operated the most powerfully around 8.2 μm (slightly blue-shifted from type II). Upon detuning from this central point, the phase-matched wavelengths split, allowing the generation of very broadband spectra (vine red line). The spectra below 6 μm (black line) Photonics 2021, 8, x FOR PEER REVIEW 5 of 24 S. Nam et. al. presented an octave-spanning 3–10 μm MIR OPA system based on a ZGP crystal, pumped by a 1 kHz, 2.4 μm, 250 fs Cr:ZnSe CPA [37]. The OPA system was seeded either by white light generation from a YAG plate or optical parametric generation (OPG) in a ZGP crystal, as shown in Figure 3. By combining the signal seed and pump with orthogonal polarization through a Si wafer placed at the Brewster angle, the signal beam was amplified in a 5 mm-long ZGP crystal, and the long-wavelength idler pulses were generated. With 0.55 mJ pump pulse energy, 130 μJ or 55 μJ overall OPA output (signal + idler) were obtained from the MIR OPA seeded by OPG or white light generation, which corresponds to a pump to signal + idler conversion efficiency of 23% or 10%, re- spectively. Figure 3. Experimental setup of MIR ZGP OPA. Red lines, 2.4 μm pump beam; green line, signal (seed) beam; blue dotted line, idler beam. M1–M4: Al mirrors; L1–L4: CaF2 lenses; F1: 3 μm LPF; F2: 4.5 or 7 μm LPF; W: ZnSe wedge; D: delay stage; NL: nonlinear crystal (YAG or ZGP); B: 300-μm- thick Brewster Si plate; WP: λ/2 waveplate. Reprinted with permission from [37]. In 2018, M. Seidel et. al. employed PPLN crystal and wide-bandgap nonlinear crystal BGS in a MIR OPA pumped by a thin-disc laser at ~1 μm. As shown in Figure 4, MIR Photonics 2021, 8, 290 5 of 24 emission covering a spectral range of 2 to 11 μm was generated, with an output power of 5 W at 4.1 μm and 1.3 W at 8.5 μm [38]. Subsequently, in 2019, B. Chen reported a MIR laser source with a spectrum covering the 5–11 μm range and a pulse width of 32 fs [34]. laser source with a spectrum covering the 5–11 m range and a pulse width of 32 fs [34]. The generated MIR pulse had a pulse energy of 220 nJ at a 50 kHz repetition rate. The generated MIR pulse had a pulse energy of 220 nJ at a 50 kHz repetition rate. Figure 4. Tuning curves. (a) Generated MIR power for maximal pump power and tuning periods from 28 to 25.5 μm of Figure 3. Tuning curves. (a) Generated MIR power for maximal pump power and tuning periods the PPLN. The spectrum centered at 4.2 μm is shaped through CO2 absorption. The power was measured 25 cm behind from 28 to 25.5 m of the PPLN. The spectrum centered at 4.2 m is shaped through CO absorption. the nonlinear crystal. (b) Tuning curve measured with a type I phase-matched LGS crystal. The OPA operated the most The power was measured 25 cm behind the nonlinear crystal. (b) Tuning curve measured with a powerfully around 8.2 μm (slightly blue-shifted from type II). Upon detuning from this central point, the phase-matched type I phase-matched LGS crystal. The OPA operated the most powerfully around 8.2 m (slightly wavelengths split, allowing the generation of very broadband spectra (vine red line). The spectra below 6 μm (black line) blue-shifted from type II). Upon detuning from this central point, the phase-matched wavelengths split, allowing the generation of very broadband spectra (vine red line). The spectra below 6 m (black line) and above 10 m (light blue line) needed different delays to be generated because of the uncompressed seed pulse. With type I phase matching, a maximal MIR power of 1.0 W could be generated. Type I, however, allowed the generation of slightly more broadband spectra than type II. Power spectral density is provided in units of mW/cm and W/f , where f = 37.5 MHz is the rep rep oscillator repetition rate. Reprinted with permission from [38]. In 2020, Heiner et al. presented an OPA system pumped by a Yb:KGd(WO ) laser 4 2 system with a repetition rate of 100 kHz and pulse width of 180 fs at 1028 nm [33]. The pump was divided to produce white light as the seed of the amplification stage, where the crystal could be BGS or LGS with large bandgap energy. After the Ge lens, average pulse power of 59 mW at 10 m and 81 mW at 8.1 m through the BGS and LGS crystals of the same length respectively were obtained, and their pulse widths were measured as 126 fs (3.8 cycle) and 121 fs (4.5 cycle), as shown in Figure 5. This comparison indicates that BGS is a promising candidate for 1-m-pumped OPAs generating MIR emission beyond 5 m because of its larger size availability and longer transmission cutoff up to 13.7 m. 3.2. MIR OPCPA 3.2.1. 2–4 m OPCPA OPCPA systems have superior energy and power upscaling capability. With careful dispersion management, amplified pulses with a pulse width close to the transform limit could be generated. High-energy, few-cycle light sources with a central wavelength of 2–4 m, and multi-millijoule pulse energy have been realized in many research groups via OPCPA techniques. Table 3 summarizes the specifications of few-cycle 2–4 m OPCPA. Subsequently, some a variety of previously published works are selected to elaborate in detail. Photonics 2021, 8, x FOR PEER REVIEW 4 of 24 Photonics 2021, 8, 290 6 of 24 Photonics 2021, 8, x FOR PEER REVIEW 6 of 24 and above 10 μm (light blue line) needed different delays to be generated because of the uncompressed seed pulse. With type I phase matching, a maximal MIR power of 1.0 W could be generated. Type I, however, allowed the generation of slightly more broadband spectra than type II. More information is provided in section S5. Power spectral density is pro- −1 vided in units of mW/cm and μW/frep, where frep = 37.5 MHz is the oscillator repetition rate. Reprinted with permission from [38]. Figure 1. (a) The schematic of the high-energy phase-stable sub-cycle MIR OPA. Polarizations of the Figure 4. (a) The schematic of the high-energy phase-stable sub-cycle MIR OPA. Polarizations of In 2020, Heiner et. al. presented an OPA system pumped by a Yb:KGd(WO4)2 laser beams are marked by double-headed arrows and concentric circles. 300-μm thick Si wafers at Brew- the beams are marked by double-headed arrows and concentric circles. 300-m thick Si wafers at system with a repetition rate of 100 kHz and pulse width of 180 fs at 1028 nm [33]. The ster angle were used as polarization beam splitter and beam combiner to transmit the 2.1-μm pump Brewster angle were used as polarization beam splitter and beam combiner to transmit the 2.1-m pump was divided to produce white light as the seed of the amplification stage, where pulse and reflected the signal and idler pulses. The synthesized pulses and a branch of 2.1-μm ref- pump pulse and reflected the signal and idler pulses. The synthesized pulses and a branch of 2.1-m the crystal could be BGS or LGS with large bandgap energy. After the Ge lens, average erence pulses were sent into XFROG with 30-μm thick GaSe nonlinear crystal. The synthesis of a reference pulses were sent into XFROG with 30-m thick GaSe nonlinear crystal. The synthesis of pulse power of 59 mW at 10 μm and 81 mW at 8.1 μm through the BGS and LGS crystals sub-cycle MIR pulse by coherently combining the sub-2-cycle signal and idler pulses is shown con- a sub-cycle MIR pulse by coherently combining the sub-2-cycle signal and idler pulses is shown of the same length respectively were obtained, and their pulse widths were measured as ceptually at the top of the figure. (b) The signal spectrum from WLG at BaF2, measured after a 2400 conceptually at the top of the figure. (b) The signal spectrum from WLG at BaF , measured after 126 fs (3.8 cycle) and 121 fs (4.5 cycle), as shown in Figure 5. This compa 2 rison indicates nm long-pass filter. (c) The measured output spectrum of the MIR OPA. The dotted line separates a 2400 nm long-pass filter. (c) The measured output spectrum of the MIR OPA. The dotted line tthe signal and hat BGS is a promisin idler spectra g c.a T ndid he retrieved te ate for 1-μ mporal ( m-pumped d) inte OPAs nsity profile geners o ating MI f the synthesiz R emission ed pu be- lse. separates Reprinted with the signal permission fro and idler spectra. m [35].The retrieved temporal (d) intensity profiles of the synthesized yond 5 μm because of its larger size availability and longer transmission cutoff up to 13.7 pulse. Reprinted with permission from [35]. μm. Figure 5. Illustration of BGS OPA laser system. BS, beam sampler; PR, partial reflector; WLC, white Figure 5. Illustration of BGS OPA laser system. BS, beam sampler; PR, partial reflector; WLC, white light continuum generation unit; L, lens; BD, beam dump. DM1, dichroic mirror, high reflection light continuum generation unit; L, lens; BD, beam dump. DM1, dichroic mirror, high reflection (HR) at 1.03 μm and high transmission (HT) at >1.1 μm; DM2, dichroic mirror, HR at 1.0–1.2 μm (HR) at 1.03 m and high transmission (HT) at >1.1 m; DM2, dichroic mirror, HR at 1.0–1.2 m and and HT at 6–12 μm; OPA, LGS or BGS crystal; Ge, germanium-based temporal chirp compensation HT at 6–12 m; OPA, LGS or BGS crystal; Ge, germanium-based temporal chirp compensation unit. unit. Reprinted with permission from [33]. Reprinted with permission from [33]. 3.2. MIR OPCPA In a pioneer work, F. Krausz’s research group employed the broadband Ti:sapphire 3.2.1. 2–4 μm OPCPA laser (oscillator and amplifier) as both the signal and pump at the same time [39]. As shown in Figure 6, the 1030 nm component of the broadband spectrum output from the OPCPA systems have superior energy and power upscaling capability. With careful Figure 2. (a) The schematic of the GaSe-based MIR OPA. The pump, the generated supercontinuum, Ti:sapphire oscillator was extracted and injected into a Yb:YAG thin disk amplifier to obtain dispersion management, amplified pulses with a pulse width close to the transform limit and the amplified MIR pulses are shown in maroon, purple, and pink, respectively. (HWP: half the pump light synchronized with the signal beam. Non-collinear OPAs (NOPAs) were could wave plate, D be gene Lrated. : delay H lin igh e, L -e P nergy, F: long pass fil few-cycle ter, BD: b light sour eam du ces w mpi, C th S a ce : chntral wave aracterization se lengtup, and th of 2– carried out in two stages using PPLN crystals and during the last power amplifier stage L1–L6 are CaF2 lenses). The measured (solid blue, 20 nm resolution) and simulated (dashed red) 4 μm, and multi-millijoule pulse energy have been realized in many research groups via with spectr aa of t LNO he crystal. amplified si Finally gnal ,(a b)MIR and the output idler at pua lse central s (c). Rewavelength printed with p of ermi 2.1ssio m n fro and ma [36 repetition ]. OPCPA techniques. Table 3 summarizes the specifications of few-cycle 2–4 μm OPCPA. Subsequently, some a variety of previously published works are selected to elaborate in detail. Table 3. Parameters of selected 2–4 μm OPCPA system. Wavelength Pulse Energy Repetition Rate Power Pulse Width Optical Cycle Reference (µm) (mJ) (kHz) (W) (fs) 2.1 1.2 3 3.6 10.5 1.5 [39] 2.1 2.7 10 27 30 4.3 [40] 2.1 2.6 1 2.6 39 5.6 [41] 2.2 0.25 100 25 16.5 2.2 [42] 2.5 0.126 100 12.6 14.4 1.7 [43] 3 0.3 10 3 21 2.1 [44] 3 2.4 10 24 50 5 [45] 3.07 0.01 125 1.25 72 7 [46] 3.1 0.125 100 12.5 73 7 [47] 3.2 0.152 100 15.2 38 3.6 [48] 3.25 0.06 160 9.6 14.5 1.4 [49] 3.4 0.012 50 0.6 41.6 3.7 [50] Photonics 2021, 8, 290 7 of 24 frequency of 3 kHz with a pulse energy of 1.2 mJ and a pulse width of 10.5 fs (1.5 cycles) was obtained. Table 3. Parameters of selected 2–4 m OPCPA system. Photonics 2021, 8, x FOR PEER REVIEW 7 of 24 Wavelength Pulse Energy Repetition Rate Power Pulse Width Optical Cycle Reference (m) (mJ) (kHz) (W) (fs) 2.1 1.2 3 3.6 10.5 1.5 [39] 3.425 13.3 0.01 0.133 111 9.7 [51] 2.1 2.7 10 27 30 4.3 [40] 3.9 8 0.02 0.16 83 6.4 [52] 2.1 2.6 1 2.6 39 5.6 [41] 4 2.6 0.1 0.26 21.5 1.6 [53] 2.2 0.25 100 25 16.5 2.2 [42] 2.5 0.126 100 12.6 14.4 1.7 [43] 3 0.3 10 3 21 2.1 [44] In a pioneer work, F. Krausz’s research group employed the broadband Ti:sapphire 3 2.4 10 24 50 5 [45] laser (oscillator and amplifier) as both the signal and pump at the same time [39]. As 3.07 0.01 125 1.25 72 7 [46] shown in Figure 6, the 1030 nm component of the broadband spectrum output from the 3.1 0.125 100 12.5 73 7 [47] Ti:sapphire oscillator was extracted and injected into a Yb:YAG thin disk amplifier to ob- 3.2 0.152 100 15.2 38 3.6 [48] tain the pump light synchronized with the signal beam. Non-collinear OPAs (NOPAs) 3.25 0.06 160 9.6 14.5 1.4 [49] were carried out in two stages using PPLN crystals and during the last power amplifier 3.4 0.012 50 0.6 41.6 3.7 [50] 3.425 13.3 stage with a 0.01 LNO crystal. Fina 0.133 lly, a MIR out111 put at a central w 9.7 avelength of 2.1 [51μm a ] nd a 3.9 8 0.02 0.16 83 6.4 [52] repetition frequency of 3 kHz with a pulse energy of 1.2 mJ and a pulse width of 10.5 fs 4 2.6 0.1 0.26 21.5 1.6 [53] (1.5 cycles) was obtained. Figure 6. Schematic of 2.1 μm few-cycle OPCPA system. Reprinted with permission from [39]. Figure 6. Schematic of 2.1 m few-cycle OPCPA system. Reprinted with permission from [39]. In 2020, U. Keller ’s research group presented an OPCPA system centered at a wave- In 2020, U. Keller’s research group presented an OPCPA system centered at a wave- length of 2.2 m, generating 16.5 fs pulses (2.2 cycles) with 25 W of average power at length of 2.2 μm, generating 16.5 fs pulses (2.2 cycles) with 25 W of average power at 100 100 kHz [42]. As shown in Figure 7, the seed from a Ti:sapphire oscillator was amplified kHz [42]. As shown in Figure 7, the seed from a Ti:sapphire oscillator was amplified in a in a BBO crystal. The idler was then generated from the NOPA in another BBO. Through BBO crystal. The idler was then generated from the NOPA in another BBO. Through three three NOPA stages, idler pulses were amplified to 300 J. Finally, after the compressor, NOPA stages, idler pulses were amplified to 300 μJ. Finally, after the compressor, pulses pulses of 250 J and 16.5 fs were obtained. Based on the MIR OPCPA, soft-X ray emission of 250 μJ and 16.5 fs were obtained. Based on the MIR OPCPA, soft-X ray emission with with the spectrum extending to 0.6 keV was demonstrated. the spectrum extending to 0.6 keV was demonstrated. The most famous MIR OPCPA in the 3–4 m band is probably from the research team led by A. Baltuska. The MIR laser output with a central wavelength of 3.9 m, pulse energy of 8–20 mJ, pulse width of ~90 fs, and a repetition rate of 20 Hz is demonstrated [52]. As shown in Figure 8, a Yb:KGW Kerr-lens mode-locked oscillator was used as the seed source. A signal light with an energy of 65 J and a central wavelength of 1460 nm generated from white light continuum was then obtained using successive three-stage parametric amplification based on KTP crystals. On the other hand, the 1064 nm component of the output spectrum of the oscillator was extracted and amplified into a pulse energy of 250 mJ by Nd:YAG CPAs, which served as the pumping source of the subsequent OPCPA system. Two-stage OPCPA was then constructed to obtain a 1.46 m signal light of 22 mJ and a 3.9 m idler light of 13 mJ. The output pulse with 8 mJ, ~80 fs was obtained by compressing the idle light. Further energy upscaling of the system to ~20 mJ has been demonstrated in subsequent work. Photonics 2021, 8, x FOR PEER REVIEW 7 of 24 3.425 13.3 0.01 0.133 111 9.7 [51] 3.9 8 0.02 0.16 83 6.4 [52] 4 2.6 0.1 0.26 21.5 1.6 [53] In a pioneer work, F. Krausz’s research group employed the broadband Ti:sapphire laser (oscillator and amplifier) as both the signal and pump at the same time [39]. As shown in Figure 6, the 1030 nm component of the broadband spectrum output from the Ti:sapphire oscillator was extracted and injected into a Yb:YAG thin disk amplifier to ob- tain the pump light synchronized with the signal beam. Non-collinear OPAs (NOPAs) were carried out in two stages using PPLN crystals and during the last power amplifier stage with a LNO crystal. Finally, a MIR output at a central wavelength of 2.1 μm and a repetition frequency of 3 kHz with a pulse energy of 1.2 mJ and a pulse width of 10.5 fs (1.5 cycles) was obtained. Figure 6. Schematic of 2.1 μm few-cycle OPCPA system. Reprinted with permission from [39]. In 2020, U. Keller’s research group presented an OPCPA system centered at a wave- length of 2.2 μm, generating 16.5 fs pulses (2.2 cycles) with 25 W of average power at 100 kHz [42]. As shown in Figure 7, the seed from a Ti:sapphire oscillator was amplified in a BBO crystal. The idler was then generated from the NOPA in another BBO. Through three NOPA stages, idler pulses were amplified to 300 μJ. Finally, after the compressor, pulses Photonics 2021, 8, 290 of 250 μJ and 16.5 fs were obtained. Based on the MIR OPCPA, soft-X ray emission with 8 of 24 the spectrum extending to 0.6 keV was demonstrated. Photonics 2021, 8, x FOR PEER REVIEW 8 of 24 Figure 7. (a) 2.2 μm OPCPA layout. The inset on the top right shows the long-term output stability of the system and beam profile after cylindrical reshaping telescopes. (b) The retrieved pulse shape of the amplifier output. (c) Blue line, measured spectrum; blue-dashed line, retrieved spec- trum; orange line, retrieved phase. Reprinted with permission from [42]. The most famous MIR OPCPA in the 3–4 μm band is probably from the research team led by A. Baltuska. The MIR laser output with a central wavelength of 3.9 μm, pulse en- ergy of 8–20 mJ, pulse width of ~90 fs, and a repetition rate of 20 Hz is demonstrated [52]. As shown in Figure 8, a Yb:KGW Kerr-lens mode-locked oscillator was used as the seed source. A signal light with an energy of 65 μJ and a central wavelength of 1460 nm gener- ated from white light continuum was then obtained using successive three-stage paramet- ric amplification based on KTP crystals. On the other hand, the 1064 nm component of the output spectrum of the oscillator was extracted and amplified into a pulse energy of 250 mJ by Nd:YAG CPAs, which served as the pumping source of the subsequent OPCPA system. Two-stage OPCPA was then constructed to obtain a 1.46 μm signal light of 22 mJ and a 3.9 μm idler light of 13 mJ. The output pulse with 8 mJ, ~80 fs was obtained by Figure 7. (a) 2.2 m OPCPA layout. The inset on the top right shows the long-term output stability of the system and beam compressing the idle light. Further energy upscaling of the system to ~20 mJ has been profile after cylindrical reshaping telescopes. (b) The retrieved pulse shape of the amplifier output. (c) Blue line, measured demonstrated in subsequent work. spectrum; blue-dashed line, retrieved spectrum; orange line, retrieved phase. Reprinted with permission from [42]. Figure 8. Layout of the 3.9 m OPCPA system. Reprinted with permission from [52]. Figure 8. Layout of the 3.9 μm OPCPA system. Reprinted with permission from [52]. In the 3–4 m band, J. Biegert’s team demonstrated a high-average-power MIR OPCPA In the 3–4 μm band, J. Biegert’s team demonstrated a high-average-power MIR with 21 W output power at a central wavelength of 3.25 m and a repetition rate of OPCPA with 21 W output power at a central wavelength of 3.25 μm and a repetition rate 160 kHz [49]. As shown in Figure 9, the MIR seed at 3.25 m was generated by a two-color of 160 kHz [49]. As shown in Figure 9, the MIR seed at 3.25 μm was generated by a two- fiber front-end in combination with a DFG stage. Afterwards, the MIR pulses were stretched color fiber front-end in combination with a DFG stage. Afterwards, the MIR pulses were and consecutively amplified in a preamplifier and two booster amplifiers. As it was being stretched and consecutively amplified in a preamplifier and two booster amplifiers. As it pumped, a Nd:YVO -based master oscillator power amplifier (MOPA) was employed, was being pumped, a Nd:YVO4-based master oscillator power amplifier (MOPA) was em- providing 1.1 mJ, 9 ps pulses at a 1064 nm wavelength at a 160 kHz repetition rate. After ployed, providing 1.1 mJ, 9 ps pulses at a 1064 nm wavelength at a 160 kHz repetition rate. three pre-amplifiers and four booster amplifiers, a MIR pulse with a 131 J pulse energy After three pre-amplifiers and four booster amplifiers, a MIR pulse with a 131 μJ pulse and a 21 W average power was obtained. The MIR laser output was then compressed down energy and a 21 W average power was obtained. The MIR laser output was then com- to 1.35 cycles via soliton self-compression in a noble gas filled anti-resonance photonic pressed down to 1.35 cycles via soliton self-compression in a noble gas filled anti-reso- crystal fiber, yielding 14.5 fs pulses at 3.3 m with a 9.6 W average power. nance photonic crystal fiber, yielding 14.5 fs pulses at 3.3 μm with a 9.6 W average power. Photonics 2021, 8, x FOR PEER REVIEW 9 of 24 Photonics 2021, 8, 290 9 of 24 Figure 9. Setup of the high-power, MIR OPCPA system. The seed was generated by a two-color fiber front-end in combi- Figure 9. Setup of the high-power, MIR OPCPA system. The seed was generated by a two-color fiber front-end in nation with a DFG stage. Afterward, the MIR pulses were stretched and consecutively amplified in a pre-amplifier and combination with a DFG stage. Afterward, the MIR pulses were stretched and consecutively amplified in a pre-amplifier two booster amplifiers. Maximum conversion efficiencies were achieved by multiple uses of the pump beam and by indi- and two booster amplifiers. Maximum conversion efficiencies were achieved by multiple uses of the pump beam and by vidually tailored seed-to-pump pulse durations. Reprinted with permission from [49]. individually tailored seed-to-pump pulse durations. Reprinted with permission from [49]. Y. Leng’s research team reported a 4 μm OPCPA with a 2.6 mJ pulse energy and a Y Y. Leng’s research te . Leng’s research team am r reported eported a 4 µ a 4 m m OPCPA OPCPA with with a a 2.6 2.6 m mJJpulse pulse ener energy gy and and a a 1.6 cycle pulse width [53]. As shown in Figure 10, a CEP-sTable 4 μm seed with ~120 μJ 1.6 cycle pulse width [53]. As shown in Figure 10, a CEP-sTable 4 m seed with ~120 J 1.6 cycle pulse width [53]. As shown in Figure 10, a CEP-sTable 4 µm seed with ~120 µJ energy was generated from a home-built OPA pumped by a commercial Ti:sapphire energy was generated from a home-built OPA pumped by a commercial Ti:sapphire energy was generated from a home-built OPA pumped by a commercial Ti:sapphire femtosecond laser. The pump laser for the MIR OPCPA was from a picosecond Nd:YAG femtosecond laser. The pump laser for the MIR OPCPA was from a picosecond Nd:YAG femtosecond laser. The pump laser for the MIR OPCPA was from a picosecond Nd:YAG laser that could deliver a 1064 nm pulses with up to 300 mJ energy and 50 ps pulse dura- laser that could deliver a 1064 nm pulses with up to 300 mJ energy and 50 ps pulse duration laser that could deliver a 1064 nm pulses with up to 300 mJ energy and 50 ps pulse dura- tion running at 100 Hz repetition rate. After two amplifier stages, the amplified 4 μm running at 100 Hz repetition rate. After two amplifier stages, the amplified 4 m chirped tion running at 100 Hz repetition rate. After two amplifier stages, the amplified 4 µm chirped pulse with 11.8 mJ energy was compressed to 105 fs by employing a grating com- pulse with 11.8 mJ energy was compressed to 105 fs by employing a grating compressor. In chirped pulse with 11.8 mJ energy was compressed to 105 fs by employing a grating com- pressor. In order to obtain a near-single-cycle pulse at 4 μm, a noble gas filled hollow-core order to obtain a near-single-cycle pulse at 4 m, a noble gas filled hollow-core fiber with pressor. In order to obtain a near-single-cycle pulse at 4 µm, a noble gas filled hollow-core fiber with a 1-mm inner core diameter and 3-m length was employed. Combined with a a 1-mm inner core diameter and 3-m length was employed. Combined with a CaF bulk fiber with a 1-mm inner core diameter and 3-m length was employed. Combined with a CaF2 bulk material, the MIR pulse with a pulse energy of 2.6 mJ was further compressed material, the MIR pulse with a pulse energy of 2.6 mJ was further compressed to 21.5 fs, CaF2 bulk material, the MIR pulse with a pulse energy of 2.6 mJ was further compressed to 21.5 fs, corresponding to a 1.6 optical cycle at a 4 μm centre wavelength. corresponding to a 1.6 optical cycle at a 4 m centre wavelength. to 21.5 fs, corresponding to a 1.6 optical cycle at a 4 µm centre wavelength. Figure 10. Schematic of the 4 μm OPCPA and post-compression system. Reprinted with permis- Figure 10. Schematic of the 4 µm OPCPA and post-compression system. Reprinted with permis- Figure 10. Schematic of the 4 m OPCPA and post-compression system. Reprinted with permission sion from [53]. sion from [53]. from [53]. In 2021, H. K. Liang’s team demonstrated a high-energy and high-power 3 μm Table 4. Parameters of the long-wavelength MIR OPCPA systems. OPCPA pumped by a shaped flat-top beam [45]. With the combination of a commercial diffractive phase plate and a focus lens, a 1 μm Gaussian pump was transformed into a Wavelength Pulse Energy Repetition Rate Power Pulse Width Optical Cycle Reference flat-top profile with a high beam shaping ratio of more than 95%, as shown in Figure 11a. (m) (mJ) (kHz) (W) (fs) An MIR OPCPA stage with a 2-fold efficiency enhancement of up to 13.5% was achieved 5 3.4 1 3.4 89.4 5.4 [54] with the shaped flat-top pump beam. 2.7 mJ, 27 W, 125 fs, 3 μm pulses with a 10 kHz 7 0.7 0.1 0.07 188 8 [55] repetition rate were generated, as measured in Figure 11b–d. The amplified flat-top-like 9 0.014 10 0.14 142 4.7 [56] MIR pulse was subsequently compressed to 50 fs through nonlinear compression in a thin YAG crystal, corresponding to 5 optical cycles, with ~90% compression efficiency. Equip- In 2021, H. K. Liang’s team demonstrated a high-energy and high-power 3 m OPCPA ping an OPCPA with a diffractive phase plate as the beam shaper is a simple, robust and pumped by a shaped flat-top beam [45]. With the combination of a commercial diffractive cost-effective method. It could also, in principle, be applied to other parametric conver- phase plate and a focus lens, a 1 m Gaussian pump was transformed into a flat-top sions at other wavelength ranges. profile with a high beam shaping ratio of more than 95%, as shown in Figure 11a. An MIR Photonics 2021, 8, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/photonics Photonics 2021, 8, 290 10 of 24 OPCPA stage with a 2-fold efficiency enhancement of up to 13.5% was achieved with the shaped flat-top pump beam. 2.7 mJ, 27 W, 125 fs, 3 m pulses with a 10 kHz repetition rate were generated, as measured in Figure 11b–d. The amplified flat-top-like MIR pulse was subsequently compressed to 50 fs through nonlinear compression in a thin YAG crystal, corresponding to 5 optical cycles, with ~90% compression efficiency. Equipping an OPCPA with a diffractive phase plate as the beam shaper is a simple, robust and cost-effective Photonics 2021, 8, x FOR PEER REVIEW 10 of 24 method. It could also, in principle, be applied to other parametric conversions at other wavelength ranges. Figure 11. (a) The schematic of flat-top beam shaping of the high-energy and high-average-power 3 μm OPCPA. The MIR Figure 11. (a) The schematic of flat-top beam shaping of the high-energy and high-average-power 3 m OPCPA. The MIR pulses centered at 3 μm were generated and amplified to 300 μJ from 3-stage OPCPA preamplifiers via periodically poled pulses centered at 3 m were generated and amplified to 300 J from 3-stage OPCPA preamplifiers via periodically poled lithium niobate (PPLN) and KTA crystals. The 4th OPCPA stage was designed to boost up the MIR output and enhance lithium niobate (PPLN) and KTA crystals. The 4th OPCPA stage was designed to boost up the MIR output and enhance the the parametric efficiency through the flat-top beam shaping. The Gaussian pump beam of the 4th-stage OPCPA was sent parametric efficiency through the flat-top beam shaping. The Gaussian pump beam of the 4th-stage OPCPA was sent to a to a flat-top beam shaper consisting of a phase plate and a focus lens, and the flat-top pump beam is formed at the imaging flat-top beam shaper consisting of a phase plate and a focus lens, and the flat-top pump beam is formed at the imaging plane of the lens. The Gaussian idler beam generated from the first 3 OPCPA stages was amplified with a flat-top pump, plane of the lens. The Gaussian idler beam generated from the first 3 OPCPA stages was amplified with a flat-top pump, producing a high-energy and high-average-power flat-top-like 3 μm output. The measured pump beam profiles (b) with pr and ( oducing c) wiathout the high-ener flat-t gy and op b high-average-power eam shaper on the K flat-top-like TA crystal.3 Th m e c output. ross-seThe ction b measur eam pro ed pump files obeam n the x a profiles nd y a (bx ) es with are included too. (d) The pulse energy measurements of the 3 μm idler pulse from the OPCPA with flat-top (red) and Gaussian and (c) without the flat-top beam shaper on the KTA crystal. The cross-section beam profiles on the x and y axes are (black) pump beam profiles. 2.7 mJ and 1.45 mJ MIR pulse energy were obtained from the flat-top and Gaussian pump, included too. (d) The pulse energy measurements of the 3 m idler pulse from the OPCPA with flat-top (red) and Gaussian corresponding to 7% and 13.5% pump-to-idler efficiency for the 4th-OPCPA stage, respectively. Reprinted with permis- (black) pump beam profiles. 2.7 mJ and 1.45 mJ MIR pulse energy were obtained from the flat-top and Gaussian pump, sion from [45]. corresponding to 7% and 13.5% pump-to-idler efficiency for the 4th-OPCPA stage, respectively. Reprinted with permission from [45]. 3.2.2. 5–10 μm OPCPA 3.2.2. In recent 5–10 m OPCP years, there h A ave been emerging reports of long-wavelength MIR OPCPA systems with lasing wavelengths exceeding 5 μm. Table 4 summarizes long-wavelength In recent years, there have been emerging reports of long-wavelength MIR OPCPA MIR OPCPA systems with the centre wavelengths of 5 μm, 7 μm, and 9 μm. systems with lasing wavelengths exceeding 5 m. Table 4 summarizes long-wavelength MIR OPCPA systems with the centre wavelengths of 5 m, 7 m, and 9 m. Table 4. Parameters of the long-wavelength MIR OPCPA systems. In 2020, a 5 m OPCPA system delivering multi-10 GW peak power femtosecond pulses at a 1 kHz repetition rate were reported [54]. The signal and pump seed pulses Wavelength Pulse Energy Repetition Rate Power Pulse Width Optical Cycle Reference were provided by a fiber-based 40 MHz multi-color laser system, as shown in Figure 12. (µm) (mJ) (kHz) (W) (fs) It contained three separate erbium (Er)-doped fiber amplifiers and generated pulses at 5 3.4 1 3.4 89.4 5.4 [54] 1.55 m as well as two supercontinua. The latter pulses were then generated separately in 7 0.7 0.1 0.07 188 8 [55] two nonlinear fibers, optimized for supercontinuum with center wavelengths at 1.0 and 9 0.014 10 0.14 142 4.7 [56] 2.0 m, respectively. The signal at 3.5 m was provided by DFG using a 1.5 m oscillator and 1 m supercontinuum pulses. The pulses with a center of gravity at 2 m served In 2020, a 5 μm OPCPA system delivering multi-10 GW peak power femtosecond as seed for the pump 36 mJ pump. Subsequently, through four-stage OPAs, a 5 m MIR pulses at a 1 kHz repetition rate were reported [54]. The signal and pump seed pulses emission with 3.4 mJ pulse energy and 89.4 fs pulse width was obtained. were provided by a fiber-based 40 MHz multi-color laser system, as shown in Figure 12. It contained three separate erbium (Er)-doped fiber amplifiers and generated pulses at 1.55 μm as well as two supercontinua. The latter pulses were then generated separately in two nonlinear fibers, optimized for supercontinuum with center wavelengths at 1.0 and 2.0 μm, respectively. The signal at 3.5 μm was provided by DFG using a 1.5 μm oscillator and 1 μm supercontinuum pulses. The pulses with a center of gravity at 2 μm served as seed for the pump 36 mJ pump. Subsequently, through four-stage OPAs, a 5 μm MIR emission with 3.4 mJ pulse energy and 89.4 fs pulse width was obtained. Photonics 2021, 8, x FOR PEER REVIEW 11 of 24 Photonics 2021, 8, x FOR PEER REVIEW 11 of 24 Photonics 2021, 8, 290 11 of 24 Figure 12. Setup of the 2 μm pumped MIR OPCPA. Reprinted with permission from [54]. Figure 12. Setup of the 2 μm pumped MIR OPCPA. Reprinted with permission from [54]. Figure 12. Setup of the 2 m pumped MIR OPCPA. Reprinted with permission from [54]. In 2016, J. Biegert’s team demonstrated a high-energy, few-cycle 7 μm OPCPA [55]. In 2016, J. Biegert’s team demonstrated a high-energy, few-cycle 7 μm OPCPA [55]. In 2016, J. Biegert’s team demonstrated a high-energy, few-cycle 7 m OPCPA [55]. As shown in Figure 13, the system started with an Er:Tm:Ho:fiber laser which generated As shown in Figure 13, the system started with an Er:Tm:Ho:fiber laser which generated As shown in Figure 13, the system started with an Er:Tm:Ho:fiber laser which generated a a 7 μm seed via DFG in a CSP crystal. The 7 μm seed pulse was then amplified in a ZGP- a 7 μm seed via DFG in a CSP crystal. The 7 μm seed pulse was then amplified in a ZGP- 7 m seed via DFG in a CSP crystal. The 7 m seed pulse was then amplified in a ZGP- based OPCPA chain pumped by a cryogenic-cooled Ho:YLF CPA system with a 260 mJ based OPCPA chain pumped by a cryogenic-cooled Ho:YLF CPA system with a 260 mJ based OPCPA chain pumped by a cryogenic-cooled Ho:YLF CPA system with a 260 mJ pump energy at 2052 nm pump energy and a at 2052 nm 16 ps pulse w and a idth. Th 16 ps pulse w e pulses wer idth. Th e fir e pulses wer st put through both e first put through both pump energy at 2052 nm and a 16 ps pulse width. The pulses were first put through both a pre-amplifier a p and b re-am oost pleir am fier and b plifieroost stage, er am and plifin fier a st lly p age, ut t and hroug finh a ally p com ut t ph ress roug or, h a aft com er pressor, after a pre-amplifier and booster amplifier stage, and finally put through a compressor, after which a 7 μm pulses with a 0.75 mJ pulse energy and a 188 fs pulse width were obtained. which which a a 77 μ m m pulses pulses with with a a 0.75 0.75 m mJ J pulse pulse ener energy an gy andd a 188 fs pulse a 188 fs pulse width width were o were obtained. btained. Figure 13. Layout Figure 13. of the 7 μm OPCPA. The Layout of the 7 μ MIR se m OPCPA. The ed was generated using the two broa MIR seed was generated using the two broa dband dband Figure 13. Layout of the 7 m OPCPA. The MIR seed was generated using the two broadband femtosecond outputs from femtosecond outputs from a three-color fiber frontend via DFG. Afterwards, the MIR pulses were femtosecond outputs from a three-color fiber frontend via DFG. Afterwards, the MIR pulses were a three-color fiber frontend via DFG. Afterwards, the MIR pulses were stretched in a dielectric bulk and consecutively stretched in a dielec stretched tric bu in a lk an d d conse ielectric cu bu tive lk a ly n amplified d consecu in tive a pre-amplif ly amplified ier in and a a pre-amplif booster ier ampli- and a booster ampli- amplified in a pre-amplifier and a booster amplifier separated with a chirp inversion stage. Maximum efficiency of the fier separated with a chirp inversion stage. Maximum efficiency of the OPCPA was achieved by fier separated with a chirp inversion stage. Maximum efficiency of the OPCPA was achieved by OPCPA was achieved by tailoring the seed-to-pump pulse durations in the pre-amplifier and booster amplifier. The tailoring the seed-to-pump pulse durations in the pre-amplifier and booster amplifier. The broad- tailoring the seed-to-pump pulse durations in the pre-amplifier and booster amplifier. The broad- broadband high-energy MIR pulses were recompressed using a dielectric bulk rod of BaF . Reprinted with permission band high-energy MIR pulses were recompressed using a dielectric bulk rod of BaF2. Reprinted band high-energy MIR pulses were recompressed using a dielectric bulk rod of BaF2. Reprinted from [55]. with permission from [55]. with permission from [55]. Recently, H. K. Liang’s research group reported a 9 m, few cycle MIR OPCPA based Recently, H. K. Li Recent ang’s ly res , H e.arch K. Li group ang’s res repe o arch rted a group 9 μm, re fp ew cy orted a cle 9 Mμ Im, R OP few cy CPA c l ba e M seI d R OPCPA based on LiGaS crystals pumped by a 1 m Yb:YAG laser at a 10 kHz repetition rate [56]. This on LiGaS2 crystals pumped by a 1 μm Yb:YAG laser at a 10 kHz repetition rate [56]. This on LiGaS2 crystals pumped by a 1 μm Yb:YAG laser at a 10 kHz repetition rate [56]. This is the first long-wavelength MIR OPCPA pumped at the 1 m wavelength. As shown in is the first long-wavelength MIR OPCPA pumped at the 1 μm wavelength. As shown in is the first long-wavelength MIR OPCPA pumped at the 1 μm wavelength. As shown in Figure 14, a small fraction separated from the Yb:YAG pump was injected into the YAG Figure 14, a small fraction separated from the Yb:YAG pump was injected into the YAG Figure 14, a small fraction separated from the Yb:YAG pump was injected into the YAG crystal to produce a white-light continuum with a central wavelength of 1.16 m. The crystal to produce cryst a wh al toit pro e-light duce cont a wh inu itu em wit -light cont h a ce innt uu ral w m wit ave h le a ngt cent h of ral w 1.1 a6 ve μ le m. Th ngth of e 1.16 μm. The stretched signal pulses were amplified in two consecutive amplification stages. Finally, stretched signalst pu retlse ched s wsig ere nal ampl puif lse ied s w in e t re wampl o con ifsied ecut in ive tw ampl o con ifs icat ecut ion ive stampl ages. F ificat inaion lly, st ages. Finally, long-wavelength MIR idler pulses centered at 9 m with a 14 J pulse energy and a 142 fs long-wavelength MIR idler pulses centered at 9 μm with a 14 μJ pulse energy and a 142 long-wavelength MIR idler pulses centered at 9 μm with a 14 μJ pulse energy and a 142 (4.7 optical cycles) duration at a 10 kHz repetition rate were achieved. The 9 m pulses were fs (4.7 optical cycles) duration at a 10 kHz repetition rate were achieved. The 9 μm pulses fs (4.7 optical cycles) duration at a 10 kHz repetition rate were achieved. The 9 μm pulses further compressed to 45 fs corresponding to 1.5 optical cycles by nonlinear compression were further compressed to 45 fs corresponding to 1.5 optical cycles by nonlinear com- were further compressed to 45 fs corresponding to 1.5 optical cycles by nonlinear com- using a KrS-5 bulk material [57]. pression using a KrS-5 bulk material [57]. pression using a KrS-5 bulk material [57]. Photonics 2021, 8, x FOR PEER REVIEW 12 of 24 Photonics 2021, 8, 290 12 of 24 Figure 14. (a) The schematic of the 9 μm OPCPA. YAG, Yttrium aluminum garnet; ZnSe, zinc sele- Figure 14. (a) The schematic of the 9 m OPCPA. YAG, Yttrium aluminum garnet; ZnSe, zinc selenide nide window; HR, high reflective mirror; TFP, thin film polarizer; BS, beam splitter; LGS, LiGaS2 window; HR, high reflective mirror; TFP, thin film polarizer; BS, beam splitter; LGS, LiGaS crystal; crystal; Ge, germanium window. For TFP, the reflectance of the S-polarized pump and the trans- Ge, germanium window. For TFP, the reflectance of the S-polarized pump and the transmittance of mittance of the P-polarized signal were measured as > 99% and 91% respectively. (b) The spectra the P-polarized signal were measured as >99% and 91% respectively. (b) The spectra of signal pulses of signal pulses after SC generation (blue dotted), the pre-amplification stage (red) and the main- after SC generation (blue dotted), the pre-amplification stage (red) and the main-amplification stage amplification stage (black dashed); (c) the measured (black) and simulated (red dashed) spectra of the ou (blacktpu dashed); t idler pu (c) lse. the Reprinted with measured (black) permission and simulated from [56] (red .dashed) spectra of the output idler pulse. Reprinted with permission from [56]. 3.3. MIR Intra-Pulse DFG 3.3. MIR Intra-Pulse DFG DFG has remarkable advantages such as a single pass structure without complex DFG has remarkable advantages such as a single pass structure without complex cavity adjustment and a broad tuning range of the output spectrum. Intra-pulse DFG cavity adjustment and a broad tuning range of the output spectrum. Intra-pulse DFG (IPDFG) is a special DFG process that uses the low and high frequency components of an (IPDFG) is a special DFG process that uses the low and high frequency components of an ultra-broadband pump pulse to realize MIR femtosecond emission. In this method, only ultra-broadband pump pulse to realize MIR femtosecond emission. In this method, only an ultra-broadband pump laser with a few-cycle pulse width is needed, which further an ultra-broadband pump laser with a few-cycle pulse width is needed, which further simplifies the DFG process. Table 5 lists the state-of-the-art work that have generated few- simplifies the DFG process. Table 5 lists the state-of-the-art work that have generated cycle MIR laser pulses. few-cycle MIR laser pulses. Table 5. Parameters of selected IPDFG source. Table 5. Parameters of selected IPDFG source. Pump Wavelength IPDFG Spectral Span Conversion Efficiency Nonlinear Crystal Reference Pump Wavelength IPDFG Spectral Span Conversion Efficiency (µm) Nonlinear Crystal (µm) (%) Reference (m) (m) (%) 1.03 LGS 8–11 0.037 [58] 1.03 LGS 8–11 0.037 [58] 1.03 LGS 6.8–16.4 0.11 [59] 1.03 LGS 6.8–16.4 0.11 [59] 1.57 OP-GaP 4–12 0.071 [24] 1.57 OP-GaP 4–12 0.071 [24] 1.9 Ga 1.9 S GaSee 3.7 3.7–18–18 1.41.4 [60[6]0] 2 ZnSe 2.7–20 0.51 [61] 2 ZnSe 2.7–20 0.51 [61] 2 GaSe 4.5–20 0.13 [62] 2 GaSe 4.5–20 0.13 [62] 2.1 AGSe 7–11 0.8 [63] 2.1 AGSe 7–11 0.8 [63] 2.5 GaSe 4.3–17.6 0.22 [64] 2.5 GaSe 4.3–17.6 0.22 [64] 2.5 ZGP 5.8–12.5 3.3 [64] 3 GaSe 6–13.2 5.3 [65] 2.5 ZGP 5.8–12.5 3.3 [64] 3 GaSe 6–13.2 5.3 [65] In 2015, I. Pupeza et al. presented a pioneer work on MIR IPDFG. As shown In 2015, I. Pupeza et. al. presented a pioneer work on MIR IPDFG. As shown in Figure in Figure 15, a Yb:YAG high-average power laser was employed to pump a LGS crys- 15, a Yb:YAG high-average power laser was employed to pump a LGS crystal that had tal that had large bandgap energy. The compact apparatus generated MIR pulses with a large bandgap energy. The compact apparatus generated MIR pulses with a 0.1 W output 0.1 W output power and a spectral coverage of 6.8–16.4 m. A 66 fs pulse width corre- psponding ower and a to sp sub-two ectral cove cycles rage center of 6.8 ed –16 at .4 11.5 μm. A m 66 wavel fs pulse width ength wascorrespondin measured. Thr g to sub- ough a two cycles centered at 11.5 μm wavelength was measured. Through a proof-of-concept Photonics 2021, 8, x FOR PEER REVIEW 13 of 24 demonstration, the MIR IPDFG provided a simple and robust method for generating MIR pulses with high average power and high dynamic range that could for molecular spec- troscopy and hyperspectral imaging applications as well as time-domain coherent control of vibrational dynamics [59]. Based on MIR nonlinear crystals with a larger nonlinear coefficient, such as ZGP, GaSe, and AGSe, a number of research groups have performed MIR IPDFG pumped at an ~2 μm wavelength. C. Gaida et. al, demonstrated a source of coherent MIR radiation with the combination of 150 mW average power and 3.7–18 μm spectral coverage in the molec- ular fingerprint region pumped at 1.9 μm wavelength, as shown in Figure 16 [60]. J. Zhang et. al. demonstrated few-cycle pulse generation by means of the soliton self- compression of the pump pulse in a silica-core photonic-crystal fiber, and subsequently, LWIR generation using IPDFG, resulting in a two-octave-spanning spectrum (−30 dB) from 5 to 20 μm at an average power of 24 mW, as shown in Figure 17 [62]. O. Novak et. al. implemented a MIR IPDFG pumped by a 2 μm OPCPA in a AGSe crystal in 2018 [63], where carrier-envelope phase-stable idler pulses covering the wave- length range of 7–11 μm were achieved, as shown in Figure 18. Photonics 2021, 8, 290 13 of 24 S. Vasilyev et. al. demonstrated efficient generation of coherent long-wave MIR tran- sients using, a compact 2.5 μm Cr:ZnS MOPA laser system as a pump source that could directly produce < 20 fs pulses without additional pulse compression, as shown in Figure proof-of-concept demonstration, the MIR IPDFG provided a simple and robust method for 19 [64]. A ZGP crystal was suited for the generation of an octave-wide spectrum (5.8–12.5 generating MIR pulses with high average power and high dynamic range that could for μm) with an output power of 0.15 W and an optical conversion efficiency of 3% while a molecular spectroscopy and hyperspectral imaging applications as well as time-domain GaSe crystal allowed, with types I and II phase matching, the coverage of a 2 octave spec- coherent control of vibrational dynamics [59]. tral span (4.3–17.6 μm), although with lower output power (13 mW). Figure Figure 15 15.. (( aa )) MIR MIR generatio generationnand and dete detect ctio ionn setup. ( setup. (b b )) Normalized Normalized PSD PSD of ofthe theFourier Fourier transforms transforms for for the EOS tim the EOS time-domain e-domain trace trace of the retrieved fiel of the retrieved field d and and of the NIR probe pulse. of the NIR probe pulse. The dynamic meas- The dynamic urement range, determined as the peak of the signal PSD divided by the average detector noise floor measurement range, determined as the peak of the signal PSD divided by the average detector noise (blue, continuous line), was 2.7 × 10 . The absolute power per comb li 4 ne of the generated MIR radi- floor (blue, continuous line), was 2.7  10 . The absolute power per comb line of the generated MIR ation, obtained by calibrating the normalized PSD of the retrieved MIR power spectrum by the in- radiation, obtained by calibrating the normalized PSD of the retrieved MIR power spectrum by the dependently measured total power and considering the pulse repetition frequency, is shown on the independently measured total power and considering the pulse repetition frequency, is shown on the right axis. Reprinted with permission from [59]. right axis. Reprinted with permission from [59]. Based on MIR nonlinear crystals with a larger nonlinear coefficient, such as ZGP, GaSe, and AGSe, a number of research groups have performed MIR IPDFG pumped at an ~2 m wavelength. C. Gaida et. al, demonstrated a source of coherent MIR radiation with the combination of 150 mW average power and 3.7–18 m spectral coverage in the molecular fingerprint region pumped at 1.9 m wavelength, as shown in Figure 20 [60]. J. Zhang et al. demonstrated few-cycle pulse generation by means of the soliton self-compression of the pump pulse in a silica-core photonic-crystal fiber, and subsequently, LWIR generation using IPDFG, resulting in a two-octave-spanning spectrum (30 dB) from 5 to 20 m at an average power of 24 mW, as shown in Figure 16 [62]. O. Novak et al. implemented a MIR IPDFG pumped by a 2 m OPCPA in a AGSe crys- tal in 2018 [63], where carrier-envelope phase-stable idler pulses covering the wavelength range of 7–11 m were achieved, as shown in Figure 17. S. Vasilyev et al. demonstrated efficient generation of coherent long-wave MIR tran- sients using, a compact 2.5 m Cr:ZnS MOPA laser system as a pump source that could directly produce < 20 fs pulses without additional pulse compression, as shown in Fig- ure 18 [64]. A ZGP crystal was suited for the generation of an octave-wide spectrum (5.8–12.5 m) with an output power of 0.15 W and an optical conversion efficiency of 3% while a GaSe crystal allowed, with types I and II phase matching, the coverage of a 2 octave spectral span (4.3–17.6 m), although with lower output power (13 mW). Photonics 2021, 8, x FOR PEER REVIEW 14 of 24 Figure 16. (a) Schematic experimental setup for IPDFG. (b) Brightness comparison of IPDPG and synchrotron radiation. High-power MIR spectra generated by IPDFG pumped at 2 μm wavelength (this work, red). The brightness of the high-power table-top MIR source in this work exceeds that of large-scale facility synchrotrons, e.g., the Diamond B22 IR beamline [66], by 4 orders of magnitude in the 7.5–15 μm wavelength range. Reprinted with permission from [60]. The pump-to-MIR conversion efficiency of IPDFG could be improved by using a long driving wavelength to reduce the quantum defect. In 2019, H. K. Liang’s group investi- gated a MIR IPDFG pumped at a 3 μm pump wavelength in a GaSe crystal. As shown in Figure 20, the MIR output had a 5 μJ pulse energy and an average power of 50 mW. It spanned Photonics 2021, 8, 290 14 of 24 over a spectral range from 6–13.2 μm, with a record-high conversion efficiency of up to 5.3% [65]. Photonics 2021, 8, x FOR PEER REVIEW 15 of 24 Photonics 2021, 8, x FOR PEER REVIEW Figure 17. (a) Self-compression and MIR generation setup. (b) MIR spectrum and beam profile. The 15 of 24 Figure 16. (a) Self-compression and MIR generation setup. (b) MIR spectrum and beam profile. The MIR spectrum (red line) together with the noise floor (gray line) measured using a monochromator. MIR spectrum (red line) together with the noise floor (gray line) measured using a monochromator. −1 −1 The MIR spectrum extends from 500 cm to 2250 cm (−30 dB), corresponding to the wavelength 1 1 The MIR spectrum extends from 500 cm to 2250 cm (30 dB), corresponding to the wavelength range from 4.5 to 20 μm. Inset: the beam profile measured using a Pyrocam beam profiler. Reprinted range from 4.5 to 20 range from 4.5 to 20μ m. m.Inset Inset: : the beam profi the beam profile le measured us measured using ing a Pyrocam a Pyrocambeam profiler. beam profiler. Reprinted Reprinted with permission from [62]. with permission from [62]. with permission from [62]. Figure 18. Figure 17. Schematic Schematic of of the theexperimental experimental se setup tup fo for IPDFG. r IPDFG NDF . NDF, , neutral neudensity tral density f filter; ilter /2,; half-wave λ/2, half- Figure 18. Schematic of the experimental setup for IPDFG. NDF, neutral density filter; λ/2, half- wave plate at 2020 nm in a rotation mount; L, lens; AGSe, 2 mm thick AGSe crystal; LPF, long-pass wave plate at 2020 nm in a rotation mount; L, lens; AGSe, 2 mm thick AGSe crystal; LPF, long-pass plate at 2020 nm in a rotation mount; L, lens; AGSe, 2 mm thick AGSe crystal; LPF, long-pass filter; filter; P, power meter; OAP, off-axis parabola; FB, MIR fiber; M, MIR monochromator with MCT filter; P, power meter; OAP, off-axis parabola; FB, MIR fiber; M, MIR monochromator with MCT P, power meter; OAP, off-axis parabola; FB, MIR fiber; M, MIR monochromator with MCT detector. detector. Reprinted with permission from [63]. detector. Reprinted with permission from [63]. Reprinted with permission from [63]. Figure 19. (a) Schematic of the IPDFG setup. Normalized IPDFG spectra: (b) Obtained from ZGP at 4.5 W (black line) and Figure 19. (a) Schematic of the IPDFG setup. Normalized IPDFG spectra: (b) Obtained from ZGP at 4.5 W (black line) and Figure 18. (a) Schematic of the IPDFG setup. Normalized IPDFG spectra: (b) Obtained from ZGP at 4.5 W (black line) and 5.9 W (red line) pump. The spectral power density at the peak was 76 μW/nm and 71 μW/nm, respectively. (c) Obtained 5.9 W (red line) pump. The spectral power density at the peak was 76 μW/nm and 71 μW/nm, respectively. (c) Obtained 5.9 W (red line) pump. The spectral power density at the peak was 76 W/nm and 71 W/nm, respectively. (c) Obtained from GaSe for type I and type II phase-matching at 5.9 W pump, with the peak IPDFG spectral density of 3.1 μW/nm and from GaSe for type I and type II phase-matching at 5.9 W pump, with the peak IPDFG spectral density of 3.1 μW/nm and from GaSe for type I and type II phase-matching at 5.9 W pump, with the peak IPDFG spectral density of 3.1 W/nm and 5.2 μW/nm, respectively. Scattered dots show the noise floor. Gray background shows transmission of 1 m standard air. 5.2 μW/nm, respectively. Scattered dots show the noise floor. Gray background shows transmission of 1 m standard air. 5.2 W/nm, respectively. Scattered dots show the noise floor. Gray background shows transmission of 1 m standard air. Reprinted with permission from [64]. Reprinted with permission from [64]. Reprinted with permission from [64]. Figure 20. (a) The schematic of experimental setups. (b) The measured IPDFG spectrum with p- Figure 20. (a) The schematic of experimental setups. (b) The measured IPDFG spectrum with p- polarization at 5 μJ output energy. Reprinted with permission from [65]. polarization at 5 μJ output energy. Reprinted with permission from [65]. Photonics 2021, 8, x FOR PEER REVIEW 15 of 24 range from 4.5 to 20 μm. Inset: the beam profile measured using a Pyrocam beam profiler. Reprinted with permission from [62]. Figure 18. Schematic of the experimental setup for IPDFG. NDF, neutral density filter; λ/2, half- wave plate at 2020 nm in a rotation mount; L, lens; AGSe, 2 mm thick AGSe crystal; LPF, long-pass filter; P, power meter; OAP, off-axis parabola; FB, MIR fiber; M, MIR monochromator with MCT detector. Reprinted with permission from [63]. Photonics 2021, 8, 290 15 of 24 The pump-to-MIR conversion efficiency of IPDFG could be improved by using a long driving wavelength to reduce the quantum defect. In 2019, H. K. Liang’s group Figure 19. (a) Schematic of the IPDFG setup. Normalized IPDFG spectra: (b) Obtained from ZGP at 4.5 W (black line) and investigated a MIR IPDFG pumped at a 3 m pump wavelength in a GaSe crystal. As 5.9 W (red line) pump. The spectral power density at the peak was 76 μW/nm and 71 μW/nm, respectively. (c) Obtained shown in Figure 19, the MIR output had a 5 J pulse energy and an average power of from GaSe for type I and type II phase-matching at 5.9 W pump, with the peak IPDFG spectral density of 3.1 μW/nm and 50 mW. It spanned over a spectral range from 6–13.2 m, with a record-high conversion 5.2 μW/nm, respectively. Scattered dots show the noise floor. Gray background shows transmission of 1 m standard air. efficiency of up to 5.3% [65]. Reprinted with permission from [64]. Photonics 2021, 8, x FOR PEER REVIEW 14 of 24 Figure Figure 20. 19. ((a a) T ) The he schematic of schematic of experimental setups. ( experimental setups. b) The (b) The measured IPD measured F IPDFG G spectrum with p- spectrum with polarization at 5 μJ output energy. Reprinted with permission from [65]. p-polarization at 5 J output energy. Reprinted with permission from [65]. Figure 16. (a) Schematic experimental setup for IPDFG. (b) Brightness comparison of IPDPG and Figure 20. (a) Schematic experimental setup for IPDFG. (b) Brightness comparison of IPDPG and synchrotron radiation. High-power MIR spectra generated by IPDFG pumped at 2 μm wavelength synchrotron radiation. High-power MIR spectra generated by IPDFG pumped at 2 m wavelength (this work, red). The brightness of the high-power table-top MIR source in this work exceeds that of (this work, red). The brightness of the high-power table-top MIR source in this work exceeds that of large-scale facility synchrotrons, e.g., the Diamond B22 IR beamline [66], by 4 orders of magnitude large-scale facility synchrotrons, e.g., the Diamond B22 IR beamline [66], by 4 orders of magnitude in in the 7.5–15 μm wavelength range. Reprinted with permission from [60]. the 7.5–15 m wavelength range. Reprinted with permission from [60]. The pump-to-MIR conversion efficiency of IPDFG could be improved by using a long 4. Single-Cycle MIR Generation driving wavelength to reduce the quantum defect. In 2019, H. K. Liang’s group investi- High energy single- or sub-cycle MIR pulses can provide unique opportunities to gated a MIR IPDFG pumped at a 3 μm pump wavelength in a GaSe crystal. As shown in explore phase-sensitive strong-field light-matter interactions in atoms, molecules, and Figure 20, the MIR output had a 5 μJ pulse energy and an average power of 50 mW. It spanned over a spectral range from 6–13.2 μm, with a record-high conversion efficiency of up to 5.3% [65]. Figure 17. (a) Self-compression and MIR generation setup. (b) MIR spectrum and beam profile. The MIR spectrum (red line) together with the noise floor (gray line) measured using a monochromator. −1 −1 The MIR spectrum extends from 500 cm to 2250 cm (−30 dB), corresponding to the wavelength Photonics 2021, 8, 290 16 of 24 solids. Tremendous efforts have been made to reduce the duration of a laser pulse down to a few-cycle or to the single-cycle regime. Obtaining such an ultrashort laser pulse requires spectral broadening and phase control over the ultrabroad spectrum that supports a single-cycle pulse. At present, four methods have been used to generate single-cycle MIR pulses, namely DFG, four-wave mixing (FWM), OPA, and IPDFG. Table 6 shows several works that have generated single- or sub-cycle MIR pulses. Table 6. Parameters of single- or sub-cycle MIR. Wavelength Repetition Rate Pulse Energy Pulse Width Method Optical Cycle Reference (m) (kHz) (J) (fs) 5–300 1 0.4 46 1 [67] DFG 3–18 1 2 45 1.2 [68] 1.8–4.4 1 1.5 11 1.2 [69] FWM 2–20 1 0.25 7.4 0.57 [70] OPA 2.5–9 1 33 12.4 0.88 [35] 5 6 4–12 1  10 2.5  10 [24] IPDFG 6–18 5  10 0.01 43 1.16 [71] 4.1. MIR Single-Cycle Pulse Generation via DFG In 2010, F. Junginger et al. employed a cascaded OPA and DFG system, which produced a phase-stable single-cycle transients with frequency components of 1–60 THz and a peak field intensity of 12 MV/cm, pumped by a high pulse energy of a regenerative Ti:sapphire amplifier [67]. This amplifier delivered a 5 mJ, 1 kHz pulse, pumping two OPAs both seeded by a shared white-light continuum. The output wavelengths of the two OPAs were 1.28 m and 1.18 m with a pulse energy of 150 J and 360 J, respectively, serving as the input of the MIR DFG. The two broadband outputs from two OPAs were mixed in a type-I GaSe crystal. The DFG output was a single-cycle MIR pulse with a 46 fs pulse width, with its spectrum centered at the 22 THz. In 2015, A. A. Lanin et al. presented a MIR transient centered at a wavelength of 7.9 m with the pulse width of 45 fs (~1.2 cycle) and the spectrum ranged from 3–18 m at 1 kHz repetition rate [68]. As shown in Figure 21, first, a seed produced by supercontinuum in a sapphire plate, which was driven by 810 nm, 0.8 mJ, 65 fs, 1 kHz pulses delivered by a Ti:sappire laser. This seed was sent to an OPA with a BBO crystal, generating tunable signal and idler pulses that ranged from 1150–1580 nm and 1620–2300 nm, respectively. In the second step, the signal and idler from the OPA were used for DFG in an AGS crystal, producing a MIR pulse with a pulse duration of 150 fs and pulse energy of ~2 J at the central wavelength of 7.9 m. After that, the MIR radiation underwent spectral broadening and self-compression in a 5 mm GaAs plate with high nonlinearity, leading to a spectrum covering 3–18 m, and the pulse width compressed into 45 fs. In 2016, A. A. Lanin and his group repeated the experiment and changed the centre wavelength of the DFG output to 6.8 m, the spectrum of which hit the point of zero group-velocity dispersion (GVD). Using this method, even more efficient spectral broadening and self-compression could take place, which generated a source of sub-cycle pulse with 1 J pulse energy [72]. In 2017, P. Krogen et al. used the adiabatic DFG (ADFG) to transfer the near-infrared frequencies to the MIR spectrum and produce a single-cycle MIR pulse [69]. The ADFG system consisted of an octave spanning Ti:sapphire oscillator seed source, which was used to seed a 1 kHz Nd:YLF CPA, and a 2-stage OPCPA system pumped by the same Nd:YLF amplifier. The OPCPA system used 2 grism pairs and an acousto-optic programmable dispersive filters to chirp the near-IR pulses to an approximately 3 ps duration for efficient amplification by the second harmonic of the Nd:YLF laser. The resulting chirped near- infrared pulses were down-converted to the MIR using chirped pulse difference frequency generation with the narrowband 1047 nm output of the Nd:YLF amplifier using the ADFG crystal to generate chirped pulses in the MIR. Finally, these were compressed in a 21-mm thick silicon block back to their transform-limited duration. The MIR output spanning Photonics 2021, 8, x FOR PEER REVIEW 17 of 24 Photonics 2021, 8, x FOR PEER REVIEW 17 of 24 amplification by the second harmonic of the Nd:YLF laser. The resulting chirped near- amplification by the second harmonic of the Nd:YLF laser. The resulting chirped near- infrared pulses were down-converted to the MIR using chirped pulse difference fre- infrared pulses were down-converted to the MIR using chirped pulse difference fre- quency generation with the narrowband 1047 nm output of the Nd:YLF amplifier using Photonics 2021, 8, 290 17 of 24 quency generation with the narrowband 1047 nm output of the Nd:YLF amplifier using the ADFG crystal to generate chirped pulses in the MIR. Finally, these were compressed the ADFG crystal to generate chirped pulses in the MIR. Finally, these were compressed in a 21-mm thick silicon block back to their transform-limited duration. The MIR output in a 21-mm thick silicon block back to their transform-limited duration. The MIR output spanning 1.8–4.4 μm at −10 dB from the peak, with a pulse duration of 10.7 fs (1.2 optical 1.8–4.4 m at 10 dB from the peak, with a pulse duration of 10.7 fs (1.2 optical cycles), spanning 1.8–4.4 μm at −10 dB from the peak, with a pulse duration of 10.7 fs (1.2 optical cycles), and a pulse energy of 1 μJ at 1 kHz repetition rate are shown in Figure 22. and a pulse energy of 1 J at 1 kHz repetition rate are shown in Figure 22. cycles), and a pulse energy of 1 μJ at 1 kHz repetition rate are shown in Figure 22. Figure 21. Experimental setup. Ti:S, mode-locked Ti:sapphire master oscillator; MPA, multipass am- Figure 21. Experimental setup. Ti:S, mode-locked Ti:sapphire master oscillator; MPA, multipass Figure 21. Experimental setup. Ti:S, mode-locked Ti:sapphire master oscillator; MPA, multipass am- plifier; OPA, optical parametric amplifier; AGS, AgGaS2 crystal; LPF, longpass filter; PM, parabolic amplifier; OPA, optical parametric amplifier; AGS, AgGaS crystal; LPF, longpass filter; PM, parabolic plifier; OPA, optical parametric amplifier; AGS, AgGaS2 crystal; LPF, longpass filter; PM, parabolic mirror; L, BK7 glass lens; OD, optical delay line; PMH, parabolic mirror with a hole; FWM, four- mirror; L, BK7 glass lens; OD, optical delay line; PMH, parabolic mirror with a hole; FWM, four- mirror; L, BK7 glass lens; OD, optical delay line; PMH, parabolic mirror with a hole; FWM, four- wave mixing in a gas medium; SPF, shortpass filter; Spec, spectrometer. Reprinted with permission wave mixing in a gas medium; SPF, shortpass filter; Spec, spectrometer. Reprinted with permission wave mixing in a gas medium; SPF, shortpass filter; Spec, spectrometer. Reprinted with permission from [68]. from [68]. from [68]. Figure 22. (a) A chirped adiabatic frequency conversion scheme employed the uncompressed broad- Figure Figure 22. 22.(a() aA ) chirped A chirped adiaba adiabatic tic frequ frequency ency conversion conversion schem scheme e employed t employed he uncompressed broad- the uncompressed band output of a near-IR OPCPA mixed with a longer-wavelength picosecond pump pulse in a band output of a near-IR OPCPA mixed with a longer-wavelength picosecond pump pulse in a periodica broadband lly po output led qu ofasi-phas a near-IR e-matching grating, OPCPA mixed with a bu a longer lk sili -wavelength con post-com picosecond pressor and pump a frequ pulse ency in-a periodically poled quasi-phase-matching grating, a bulk silicon post-compressor and a frequency- resolved optical gating (FROG) characterization device. Measurement of a single-cycle MIR pulse. periodically poled quasi-phase-matching grating, a bulk silicon post-compressor and a frequency- resolved optical gating (FROG) characterization device. Measurement of a single-cycle MIR pulse. (b) Retrieved pulse as a function of time (blue, temporal intensity; gray, temporal phase) showing a resolved optical gating (FROG) characterization device. Measurement of a single-cycle MIR pulse. (b) Retrieved pulse as a function of time (blue, temporal intensity; gray, temporal phase) showing a 10.7-fs full-width at half-maximum (FWHM) pulse duration (1.2 optical cycles at the central wave- (b) Retrieved pulse as a function of time (blue, temporal intensity; gray, temporal phase) showing 10.7-fs full-width at half-maximum (FWHM) pulse duration (1.2 optical cycles at the central wave- length of 2.8 μm), which is 1.15 times the transform-limited duration. (c) Spectral intensity (red, a 10.7-fs full-width at half-maximum (FWHM) pulse duration (1.2 optical cycles at the central length of 2.8 μm), which is 1.15 times the transform-limited duration. (c) Spectral intensity (red, measured; blue, retrieved) and retrieved spectral phase (gray). Reprinted with permission from [69]. wavelength of 2.8 m), which is 1.15 times the transform-limited duration. (c) Spectral intensity (red, measured; blue, retrieved) and retrieved spectral phase (gray). Reprinted with permission from [69]. measured; blue, retrieved) and retrieved spectral phase (gray). Reprinted with permission from [69]. 4.2. MIR Single-Cycle Pulse Generation via FWM 4.2. MIR Single-Cycle Pulse Generation via FWM 4.2. MIR Single-Cycle Pulse Generation via FWM In 2012, Y. Nomura et. al. investigated MIR sub-cycle pulse generation via FWM. The In 2012, Y. Nomura et. al. investigated MIR sub-cycle pulse generation via FWM. The fundamental mode and second-harmonic pulses of a 25 fs Ti:sappire amplifier output In 2012, Y. Nomura et al. investigated MIR sub-cycle pulse generation via FWM. The fundamental mode and second-harmonic pulses of a 25 fs Ti:sappire amplifier output with the wavelength of 800 nm and energy of 0.9 mJ at 1 kHz were focused into argon gas, fundamental mode and second-harmonic pulses of a 25 fs Ti:sappire amplifier output with with the wavelength of 800 nm and energy of 0.9 mJ at 1 kHz were focused into argon gas, producing a phase-stable sub-cycle MIR pulse through FWM assisted by filament [70]. As the wavelength of 800 nm and energy of 0.9 mJ at 1 kHz were focused into argon gas, producing a phase-stable sub-cycle MIR pulse through FWM assisted by filament [70]. As producing a phase-stable sub-cycle MIR pulse through FWM assisted by filament [70]. As shown in Figure 23, a phase-stable 250 nJ, 7.4 fs (0.57 cycles) MIR conical emission centered at 3.9 m with its spectrum coverage of 2–20 m was created. Photonics 2021, 8, x FOR PEER REVIEW 18 of 24 Photonics 2021, 8, x FOR PEER REVIEW 18 of 24 Photonics 2021, 8, 290 18 of 24 shown in Figure 23, a phase-sTable 250 nJ, 7.4 fs (0.57 cycles) MIR conical emission cen- shown in Figure 23, a phase-sTable 250 nJ, 7.4 fs (0.57 cycles) MIR conical emission cen- tered at 3.9 μm with its spectrum coverage of 2–20 μm was created. tered at 3.9 μm with its spectrum coverage of 2–20 μm was created. Figure 23. (a) Experimental and (b) retrieved XFROG traces. The retrieved pulse in (c) time and Figure 23. (a) Experimental and (b) retrieved XFROG traces. The retrieved pulse in (c) time and (d) (d) frequency domain. The spectrum measured with Fourier transform spectrometer (brown solid frequency domain. The Figure 23. (a) Experimental and ( spectrum measured with b) retrieved XFROG Fourier transform spectr traces. The retrieved ometer (brown sol pulse in (c) time id and (d) cu curve) frequency domain. The rve) is is al also so sshown. hown. Reprinted with Reprinted spectrum measured with with permis permission sion from [70] from Fourier transform spectr [70.]. ometer (brown solid curve) is also shown. Reprinted with permission from [70]. 4.3. MIR Single-Cycle Pulse Generation via OPA 4.3. MIR Single-Cycle Pulse Generation via OPA In 2017, H. K. Liang et al. generated a MIR sub-cycle pulse from an OPA with the 4.3. MIR Single-Cycle Pulse Generation via OPA In 2017, H. K. Liang et. al. generated a MIR sub-cycle pulse from an OPA with the signal and idler pulses at 3.2 m and 6.4 m [35]. It was demonstrated that with the stable In 2017, H. K. Liang et. al. generated a MIR sub-cycle pulse from an OPA with the signal and idler pulses at 3.2 μm and 6.4 μm [35]. It was demonstrated that with the stable carrier-envelope phase for both the signal and idler pulses and the careful control of the signal and idler pulses at 3.2 μm and 6.4 μm [35]. It was demonstrated that with the stable carrier-envelope phase for both the signal and idler pulses and the careful control of the relative delay, the signal and idler pulses were synthesized without extra coherent control. carrier-envelope phase for both the signal and idler pulses and the careful control of the relative delay, the signal and idler pulses were synthesized without extra coherent control. As shown in Figure 24, the synthesized pulse had a spectral coverage from 2.5 to 9.0 m, relative delay, the signal and idler pulses were synthesized without extra coherent control. As shown in Figure 24, the synthesized pulse had a spectral coverage from 2.5 to 9.0 μm, and and a pulse width of 12.4 fs which corresponded to 0.88 cycles for a central wavelength As shown in Figure 24, the synthesized pulse had a spectral coverage from 2.5 to 9.0 μm, and a pulse width of 12.4 fs which corresponded to 0.88 cycles for a central wavelength of 4.2 μm. of 4.2 m. a pulse width of 12.4 fs which corresponded to 0.88 cycles for a central wavelength of 4.2 μm. Figure 24. Temporal characterisation of the synthesised MIR pulse. The retrieved spectral (a) and temporal ( Figure 24. b) in Temporal characterisation of the tensity profiles of the synthesised pulse. synthesised M The dotted curves IR pulse. The re are the retrie trieved spectra ved phase l (a) and . Figure 24. Temporal characterisation of the synthesised MIR pulse. The retrieved spectral (a) and Pulse width (12.4 fs) at full width at half maximum is measured with a centre wavelength at 4.2 μm. temporal (b) intensity profiles of the synthesised pulse. The dotted curves are the retrieved phase. temporal (b) intensity profiles of the synthesised pulse. The dotted curves are the retrieved phase. It c Pulse orresponds width (12 to 0.88 .4 fs opti ) at full width at ha cal cycle. Reprinted with permission from [35]. lf maximum is measured with a centre wavelength at 4.2 μm. Pulse width (12.4 fs) at full width at half maximum is measured with a centre wavelength at 4.2 m. It corresponds to 0.88 optical cycle. Reprinted with permission from [35]. It corresponds to 0.88 optical cycle. Reprinted with permission from [35]. 4.4. MIR Single-Cycle Pulse Generation via IPDFG 4.4. MIR Single-Cycle Pulse Generation via IPDFG 4.4. MIR Single-Cycle Pulse Generation via IPDFG In 2018, H. Timmers et. al. presented a scheme for generating super-octave spanning In 2018, H. Timmers et. al. presented a scheme for generating super-octave spanning MIR frequenc In 2018, y combs with a band H. Timmers et al. presented width sp aannin scheme g fro for mgenerating 4 to 12 μm super through IPDFG in a -octave spanning n MIR frequency combs with a bandwidth spanning from 4 to 12 μm through IPDFG in an OP-Ga MIR frP equency crystal driven combs by with a a few-c bandwidth ycle Er-pump spanning infr fr aom struct 4 to ure 12 [28 m ]. As through shown IPDFG in Figure in an OP-GaP OP-GaP crystal crystadriven l driven by by a few-cycle a few-cycle Er-pump Er-pump infrastr infructur astruct e [u 28 re ]. [As 28]. As shown shown in Figu inr Fi e 25 gure a, 25a, the output of an Er mode-locked laser was amplified using an Er-doped fiber ampli- the 25a, the o outputu of tput of an Eran Er m mode-locked ode-loc laser ked laser was amplified was ampliusing fied us an ing an Er-doped fibe Er-doped fiber amplifier r ampl.i- fier. The amplified pulses then undergo nonlinear broadening in a nonlinear fiber and were compre The fieramplified . The amp ssed to pulses lified p a few then ulses -cycle pulse under then undergo go width se nonlinear nornline v br inoadening ga ar br s thoaden e pu in ming in a p o nonlinear f IPD a nonl FG. M fiber ine IRa rr f and adib iaer and wer tione compr were compre essed to ssed to a few-cycle a few-cycle pulse pulse width width se serving rvas ingthe as tpump he pum of p o IPDFG. f IPDFG MIR . MIR radiation radiation spanning from 4 to 12 μm was generated from an OP-GaP crystal. Subsequently, in 2019, spanning spanning fr fro om m 4 4 to to 12 12 μ m m was was generated generated from from an an OP-GaP OP-GaP crystal. crystal.Subsequently Subsequently , in , in 2019, 2019, from the same group, A. S. Kowligy et. al. measured the temporal profile of the MIR pulse from the same group, A. S. Kowligy et al. measured the temporal profile of the MIR pulse from the same group, A. S. Kowligy et. al. measured the temporal profile of the MIR pulse via electro-optical sampling using an ultra-short near-infrared reference pulse. A 1.2-cycle MIR pulse oscillating at a 7.6 m centre wavelength was obtained [73]. Photonics 2021, 8, x FOR PEER REVIEW 19 of 24 Photonics 2021, 8, 290 19 of 24 via electro-optical sampling using an ultra-short near-infrared reference pulse. A 1.2-cycle Photonics 2021, 8, x FOR PEER REVIEW 19 of 24 MIR pulse oscillating at a 7.6 μm centre wavelength was obtained [73]. via electro-optical sampling using an ultra-short near-infrared reference pulse. A 1.2-cycle MIR pulse oscillating at a 7.6 μm centre wavelength was obtained [73]. Figure 25. (a) Experimental layout for IPDFG comb generation. (b) The spectrum of this few-cycle Figure 25. (a) Experimental layout for IPDFG comb generation. (b) The spectrum of this few-cycle driver. The inset of (b) displays the measured intensity profile of the pump pulse, corresponding driver. The inset of (b) displays the measured intensity profile of the pump pulse, corresponding Figure 25. (a) Experimental layout for IPDFG comb generation. (b) The spectrum of this few-cycle to a pulse duration of 10.6 fs. (c) Super-octave longwave infrared (LWIR) spectra containing up to to a pulse duration of 10.6 fs. (c) Super-octave longwave infrared (LWIR) spectra containing up to driver. The inset of (b) displays the measured intensity profile of the pump pulse, corresponding 0.25 mW of power. Reprinted with permission from [24]. 0.25 mW of power. Reprinted with permission from [24]. to a pulse duration of 10.6 fs. (c) Super-octave longwave infrared (LWIR) spectra containing up to 0.25 mW of power. Reprinted with permission from [24]. In 2019, T. P. Butler et. al. reported a phase-stable source with watt-scale average In 2019, T. P. Butler et al. reported a phase-stable source with watt-scale average power and broad bandwidth (6–18 μm) via the IPDFG in GaSe crystal pumped by thu- power In 2 and 019 br , T. oad P. But bandwidth ler et. al(6–18 . reported m)a p viahthe ase-IPDFG stable so in urce w GaSeitcrystal h watt-sc pumped ale averby age thulium- lium-doped fiber-laser system at 2 μm [71]. The 2 μm pump pulses from the fiber chirped- power and broad bandwidth (6–18 μm) via the IPDFG in GaSe crystal pumped by thu- doped fiber-laser system at 2 m [71]. The 2 m pump pulses from the fiber chirped-pulse p lium ulse -doped amplif fiber-lase ier had r syst an ave em at 2 rage pμm o [ wer 71]. The 2 of 100 μW an m pump pulse d were com s from t preh ssed b e fibery chirped- grating pairs to amplifier had an average power of 100 W and were compressed by grating pairs to the pulse amplifier had an average power of 100 W and were compressed by grating pairs to the output of 40 W. The compressed 2 μm output was then divided into two parts and output of 40 W. The compressed 2 m output was then divided into two parts and further the output of 40 W. The compressed 2 μm output was then divided into two parts and further compressed nonlinearly through photonic crystal fibers to 13 fs with 4.5 W power compressed nonlinearly through photonic crystal fibers to 13 fs with 4.5 W power and 32 fs further compressed nonlinearly through photonic crystal fibers to 13 fs with 4.5 W power and 32 fs with a 30 W power, respectively. The 32 fs pulse was focused into a 1 mm GaSe with a 30 W power, respectively. The 32 fs pulse was focused into a 1 mm GaSe crystal for and 32 fs with a 30 W power, respectively. The 32 fs pulse was focused into a 1 mm GaSe crystal for MIR IPDFG. As shown in Figure 26, after a 6 mm of bulk germanium for com- MIR IPDFG. As shown in Figure 26, after a 6 mm of bulk germanium for compression, a crystal for MIR IPDFG. As shown in Figure 26, after a 6 mm of bulk germanium for com- pression, a MIR transient with a broad spectral range (6–18 μm) and ultra-short pulse MIR transient with a broad spectral range (6–18 m) and ultra-short pulse duration of 43 fs pression, a MIR transient with a broad spectral range (6–18 μm) and ultra-short pulse duration of 43 fs (~1 cycle) at 50 MHz was characterized through electro-optical sampling (~1 cycle) at 50 MHz was characterized through electro-optical sampling with the help of duration of 43 fs (~1 cycle) at 50 MHz was characterized through electro-optical sampling the with the hel 13 fs near p -infrar of the ed 13r fs efer ne ence. ar-infrared reference. with the help of the 13 fs near-infrared reference. Figure 26. (a) Measured (left)and retrieved (right) FROG spectrograms of the longer pulse PCF com- Figure 26. (a) Measured (left)and retrieved (right) FROG spectrograms of the longer pulse PCF pression channel. (b) Retrieved FROG temporal intensity and phase. (c) Retrieved FROG spectral compression channel. (b) Retrieved FROG temporal intensity and phase. (c) Retrieved FROG spectral Figure 26. (a) Measured (left)and retrieved (right) FROG spectrograms of the longer pulse PCF com- intensity and phase compared to independently measured spectrum obtained using a NIR grating pression channel. (b) Retrieved FROG temporal intensity and phase. (c) Retrieved FROG spectral spectrometer. (d–f) shows the same information as (a–c), this time measured for the shorter pulse PCF channel. Reprinted with permission from [71]. Photonics 2021, 8, 290 20 of 24 5. Prospects of High-Power Broad-Band Few-Cycle MIR Lasers The development of high-power broadband few-cycle MIR lasers has been driven by a number of applications in the field of strong-field physics, high-fidelity molecule detection, and cold tissue ablation applications. In strong-field physics, high-order harmonic genera- tion (HHG) with excellent spatial coherence is probably one of the biggest driving forces of strong MIR OPCPA. Extreme ultra-violet harmonics with the photon energy exceeding the water absorption window have been generated via the MIR OPCPA pump [17]. The famous 3.9 m OPCPA has enabled the generation of soft X-ray HHG with the photon energy extended to 1500 eV. Besides HHG, femtosecond hard X-ray covering tens of keV photon energy has been excited through plasma generation in a metallic target, pumped by the MIR OPCPA [13]. Attosecond pulse generation is another main application of MIR OPCPA in the field of strong-field physics. A total of 40–50 as isolated attosecond pulses have been generated pumped by 1.8 m OPCPA/OPA [1]. Moreover, terahertz generation with a high conversion efficiency of 2.36% has been achieved via MIR two-colour filamentation in air pumped by MIR OPCPA centred at 3.9 m. Besides the peak power of the MIR OPCPA system, which sets the threshold and cut-off of the aforementioned strong-field applications, the average power is another important parameter to pursue, which accounts for photon flux. Therefore, it is suggested high-average power MIR OPCPAs with decent pulse energy be the next research phase focus serving as the enablers to reveal a more uncharted continent in the field of strong-field physics. Molecule detection is another important application of MIR OPCPA. Laser filamen- tation in air pumped by energetic MIR OPCPAs has been realized at the 3.9 and 2 m wavelengths [74,75]. Stand-off detection of ambient air molecules such as CO have been demonstrated via air filamentation pumped using MIR OPCPA [22]. By using the atmo- spheric transparent windows in the MIR wavelength region, namely the 2–5.5 m and 8–14 m bands, more molecules in the air could be detected with MIR OPCPAs, especially at longer wavelengths such as 5 and 9 m. In addition, dual-frequency combs (DFCs) based on broadband MIR lasers have been developed for sensitive and precision molecule sensing. With DFCs at the 3–5 m wavelength range, the detection of molecular species in 13 18 17 15 34 a gas mixture, including isotopologues containing isotopes such as C, O, O, N, S, S, and deuterium, with part-per-billion sensitivity and sub-Doppler resolution has been demonstrated [23]. At longer wavelengths covering 4 to 12 m, DFCs have enabled the high-precision vapor detection of methanol and ethanol. With the high-average power of the MIR broadband laser sources at a 100 MHz repetition rate, a good signal-to-noise ratio 1/2 (67 Hz ) has been achieved with a sub-ms acquisition time. Besides strong-field physics and spectroscopic applications, high-power, broadband, few-cycle lasers have also been used in minimally invasive surgery. However, limited by the available femtosecond laser wavelengths, its current applications in bio-medical micro processing/surgery are only limited to cataract surgery [76,77] and myopia correction surgery [78–80]. An MIR wavelength at 3–10 m coincides with strong molecular resonant peaks, which results in strong and sharp absorption peaks for various molecules. The strong absorption resonances of water, protein, and lipids have been investigated using MIR femtosecond laser exposure from a free-electron laser facility in ocular, brain, and dermis tissues. A new mechanism for tissue ablation was proposed. It was found that when a MIR femtosecond laser at the 6.2–6.7 m wavelength is chosen, the laser output power is absorbed by both water and proteins. Reaching ~60 C, collagen undergoes structural transitions from highly ordered arrays to amorphous gelatin with less resilience, which enables better tissue ablation efficiency and less lateral damages. With the emerging and development of high-power MIR femtosecond lasers at more flexible wavelengths, we foresee promising prospects for watt-level MIR femtosecond lasers in the soft and hard tissue cold ablation applications. Photonics 2021, 8, 290 21 of 24 6. Conclusions In this paper, we have shown that the field of high-energy, high-power, few-cycle MIR lasers has experienced rapid development over the last 10 years that has been driven by the demands of strong-field physics (such as in HHG, attosecond generation, and terahertz generation experiments), high-fidelity molecule detection (such as in MIR dual- comb spectroscopy), and tissue cold ablation. Possible trends for the next phase would include increasing the high average power (high repetition rate), for example, by a few to a few tens watts of MIR few-cycle pulses with mJ pulse energy output, would be highly desired to boost the photon flux, enhance the signal-to-noise ratio, and reduce detection time. This puts requirements and challenges in place to improve on both the high-power pump source and techniques to raise the parametric conversion efficiency, for example, by using a quasi-parametric amplifier [81,82], cascaded extraction from OPA [83], or a flat-top pumped parametric process. Author Contributions: Writing—original draft preparation, K.T., L.H. and X.Y.; writing—review and editing, H.L. All authors have read and agreed to the published version of the manuscript. Funding: National Natural Science Foundation of China (62075144) and the Engineering Featured Team Fund of Sichuan University (2020SCUNG105). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: No new data were created or analyzed in this study. Data sharing is not applicable to this article. Conflicts of Interest: The authors declare no conflict of interest. References 1. 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Mid-Infrared Few-Cycle Pulse Generation and Amplification

Photonics , Volume 8 (8) – Jul 21, 2021

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hv photonics Review Mid-Infrared Few-Cycle Pulse Generation and Amplification Kan Tian, Linzhen He, Xuemei Yang and Houkun Liang * College of Electronics and Information Engineering, Sichuan University, Chengdu 610064, China; tiankan@stu.scu.edu.cn (K.T.); helinzhen@stu.scu.edu.cn (L.H.); yangxuemei@stu.scu.edu.cn (X.Y.) * Correspondence: hkliang@scu.edu.cn Abstract: In the past decade, mid-infrared (MIR) few-cycle lasers have attracted remarkable research efforts for their applications in strong-field physics, MIR spectroscopy, and bio-medical research. Here we present a review of MIR few-cycle pulse generation and amplification in the wavelength range spanning from 2 to ~20 m. In the first section, a brief introduction on the importance of MIR ultrafast lasers and the corresponding methods of MIR few-cycle pulse generation is provided. In the second section, different nonlinear crystals including emerging non-oxide crystals, such as CdSiP , ZnGeP , GaSe, LiGaS , and BaGa Se , as well as new periodically poled crystals such as 2 2 2 4 7 OP-GaAs and OP-GaP are reviewed. Subsequently, in the third section, the various techniques for MIR few-cycle pulse generation and amplification including optical parametric amplification, optical parametric chirped-pulse amplification, and intra-pulse difference-frequency generation with all sorts of designs, pumped by miscellaneous lasers, and with various MIR output specifications in terms of pulse energy, average power, and pulse width are reviewed. In addition, high-energy MIR single-cycle pulses are ideal tools for isolated attosecond pulse generation, electron dynamic investigation, and tunneling ionization harness. Thus, in the fourth section, examples of state-of-the- art work in the field of MIR single-cycle pulse generation are reviewed and discussed. In the last section, prospects for MIR few-cycle lasers in strong-field physics, high-fidelity molecule detection, and cold tissue ablation applications are provided. Keywords: mid-infrared; few-cycle pulse; optical parametric amplification; optical parametric chirped-pulse amplification; intra-pulse difference-frequency generation Citation: Tian, K.; He, L.; Yang, X.; Liang, H. Mid-Infrared Few-Cycle Pulse Generation and Amplification. Photonics 2021, 8, 290. https:// doi.org/10.3390/photonics8080290 1. Introduction The mid-infrared (MIR) wavelength is usually defined in the range of 2–20 m Received: 31 May 2021 (500–5000 cm ). With its unique properties and wide application prospects, lasers in this Accepted: 14 July 2021 band have attracted a great deal of attention from researchers all over the world. The main Published: 21 July 2021 characteristics of an MIR laser can be summarized as the following two aspects. First, as the pondermotive energy is quadratically proportional to the driving laser wavelength, MIR Publisher’s Note: MDPI stays neutral lasers with high peak power have been routinely pursued as the driving sources for novel with regard to jurisdictional claims in strong-field phenomenon [1–9], such as extreme ultra-violet and X-ray generation [10–13], published maps and institutional affil- attosecond pulse generation [14], and terahertz generation [15–17]. Second, most of the iations. vibrational peaks of different molecules fall in the MIR band, which is also called the “molecular fingerprint” regime. Therefore, MIR coherent spectroscopy is a unique method for high-fidelity and high-sensitivity molecule detection and identification [18–24]. As for MIR solid-state lasers, there are two main technique streams to generate MIR Copyright: © 2021 by the authors. pulses, namely the direct emission of doped ions and optical parametric down conversion. Licensee MDPI, Basel, Switzerland. The former is based on a process wherein the gain medium is stimulated after energy This article is an open access article storage, and the output wavelength depends on the energy level structures of the gain distributed under the terms and media. The biggest challenge of this process is that the relaxation energy in the MIR conditions of the Creative Commons wavelength coincides with the phonon vibration energy, which reduces the gain and Attribution (CC BY) license (https:// hinders the lasing process at the MIR wavelength. The second technique is based on the creativecommons.org/licenses/by/ parametric frequency conversion that is mainly assisted by nonlinear crystals that create 4.0/). Photonics 2021, 8, 290. https://doi.org/10.3390/photonics8080290 https://www.mdpi.com/journal/photonics Photonics 2021, 8, 290 2 of 24 the phase-matching conditions. In this process, there is no thermal accumulation, and broadband laser amplification can be realized through broadband phase matching, which supports the generation of few-cycle MIR pulses. At present, parametric down conversion has become an indispensable means to expand the new laser spectrum, generating pulses covering deep ultraviolet, visible, near-infrared, MIR, and THz wavelength regimes. In this paper, we review typical MIR nonlinear crystals and then summarize the techniques for the generation and amplification of ultrafast MIR lasers, including optical parametric amplification (OPA), optical parametric chirped-pulse amplification (OPCPA), and intra- pulse difference-frequency generation (IPDFG) with various kinds of designs. Subsequently, examples of state-of-the-art work in the field of MIR single-cycle pulse generation are reviewed and discussed. In the last part, new prospects for MIR few-cycle lasers in strong-field physics, high-fidelity molecule detection, and cold tissue ablation applications are provided. 2. MIR Nonlinear Crystals Nonlinear crystals that are commonly used in MIR pulse generation and amplification mainly include KTiOAsO (KTA), KTiOPO (KTP), and LiNbO (LNO), which belong a 4 4 3 group known as oxide crystals, and ZnGeP (ZGP), CdSiP (CSP), AgGaS (AGS), AgGaSe 2 2 2 2 (AGSe), GaSe, BaGa S (BGS), BaGa Se (BGSe), LiGaS (LGS), and LiGaSe (LGSe), which 4 7 4 7 2 2 are classified as non-oxide crystals. Generally, the damage threshold and mechanical hardness of oxide crystals are excellent, but their transparent range is limited to less than 5 m, which is not conducive for the generation of long-wavelength MIR pulses. The effective nonlinear coefficient of non-oxide crystals is higher, and the transparency range can reach more than 10 m, which is commonly used in the generation of long-wavelength infrared pulses. However, the bandgap energy of such crystals is generally around 2 eV, which makes the two-photon absorption non-negligible, and significantly reduces the damage threshold when pumped at an ~1 m wavelength with high peak power. LGS and LGSe are relatively new MIR nonlinear crystals with large bandgap energy, which enables high peak power pump at an ~1 m wavelength. However, the transparent range of LGS and LGSe are limited to 10 m. High quality new MIR nonlinear crystals with large bandgap energy and a broader transparent range are desired. BGSe is one of the candidates, however, the crystal growth quality still needs substantial improvement. In addition to the above MIR nonlinear crystals, periodically poled crystals such as periodically poled LiNbO (PPLN), orientation-patterned GaAs (OP-GaAs), and orientation-patterned GaP (OP-GaP) have become an emerging stream in the MIR parametric down conversion with their excellent quasi-phase matching bandwidth and large nonlinear coefficients, although their aperture size is the current bottleneck for pulse energy upscaling. The optical specifications such as the transparent range, effective nonlinear coefficient, and bandgap energy of the commonly used MIR nonlinear crystals are compared and summarized in Table 1. Table 1. Comparison of different MIR nonlinear crystals. Nonlinear Nonlinear Crystal Transparency (m) Bandgap (eV) Reference Coefficient (pm/V) AGS 0.5–13 13.4 2.76 [25] AGSe 0.75–15 26.8 1.83 [25] BGSe 0.47–18 24.3 2.64 [26] CSP 0.5–9 84.5 2.45 [27] ZGP 2–12 72 2.2 [28] GaSe 0.65–18 57 2.1 [29] LGS 0.32–11.6 5.9 3.76 [30] OP-GaAs 0.9–17 94 2.1 [31] OP-GaP 0.57–12 70 2.26 [32] Photonics 2021, 8, 290 3 of 24 3. MIR Generation Among all kinds of nonlinear polarizations, the most commonly used in down con- version is probably the second-order nonlinear effect due to the good efficiency. The three-wave mixing process introduced by the second-order nonlinear effect is the basis of second harmonics generation (SHG), difference-frequency generation (DFG), and OPA. DFG and OPA are the main methods of broadband MIR pulse generation and amplifica- tion. In a further step, combined with the theory of OPA and chirped-pulse amplification (CPA) technologies, OPCPA have emerged to scale up the pulse energy and peak power of MIR few-cycle lasers. In this section, we will review the state-of-the-art works using MIR few-cycle pulse generation and amplification via OPA, OPCPA, and IPDFG. 3.1. OPA OPA is an old parametric technique with emission in the visible and near-infrared wavelength regimes. It has the merits of broadband emission and simple dispersion control. Recently, the OPA emission bad has been extended to MIR wavelengths pumped at ~1 and ~2 m wavelengths, as summarized in Table 2. Typical work using MIR OPA are selected and reviewed in the following paragraphs. Table 2. Parameters of selected OPA system pumped by 1 and 2 m. Repetition Pump Wavelength Pulse Energy Power Pulse Width Optical Rate Reference (m) (m) (J) (mW) (fs) Cycle (kHz) 1 7.6–11.5 0.59 100 59 126 3.8 [33] 1 5–11 0.22 50 11 32 1.2 [34] 2 2.5–9 33 1 33 12.4 0.88 [35] 2 4.2–16 3.4 1 3.4 19 0.64 [36] 2.4 3–10 130 1 130 318 15.1 [37] In 2017, H. K. Liang demonstrated a sub-single-cycle MIR pulse synthesizer based on a MIR OPA, pumped by an OPCPA at 2 m and at a 1 kHz repetition rate, as shown in Figure 4a [35]. A ZGP crystal was employed as the nonlinear crystal for its wide trans- parent range, large nonlinear coefficient, and broad phase-matching bandwidth. A CaF wedge divided the pump beam into two paths. One (~10 J) was used as the pump to gen- erate a signal via supercontinuum generation in a BaF plate, while the other path entered the pump line of the MIR OPA. A broadband MIR emission covering 2.5–9 m was demon- strated with 33 J pulse energy. A pulse width of 12.4 fs was measured, corresponding to 0.88 optical cycles at a 4.2 m centre wavelength, as shown in Figure 4b–d. Subsequently, in 2019, H. K. Liang’s group extended the centre wavelength of the single-cycle OPA to 8.8 m using a GaSe nonlinear crystal that had a broader transmission range on the long-wavelength side compared to that of ZGP [36]. As shown in Figure 1a, the 2.15 m pump with a duration of 51 fs and a repetition rate of 1 kHz was derived using a Ti:sapphire laser pumped OPA (TOPAS). A 1-mm-thick GaSe crystal was employed as the OPA crystal. An ultra-broadband idler pulse with its spectrum spanning from 4.2 to 16 m supporting a Fourier-transform-limited pulse width of 19 fs and centered at 8.8 m was generated, as presented in Figure 1b. Considering all of the losses, the idler pulse energy of 3.4 J through a Si plate and a long pass filter was obtained. Photonics 2021, 8, x FOR PEER REVIEW 4 of 24 Figure 1. (a) The schematic of the high-energy phase-stable sub-cycle MIR OPA. Polarizations of the beams are marked by double-headed arrows and concentric circles. 300-μm thick Si wafers at Brew- ster angle were used as polarization beam splitter and beam combiner to transmit the 2.1-μm pump pulse and reflected the signal and idler pulses. The synthesized pulses and a branch of 2.1-μm ref- erence pulses were sent into XFROG with 30-μm thick GaSe nonlinear crystal. The synthesis of a sub-cycle MIR pulse by coherently combining the sub-2-cycle signal and idler pulses is shown con- ceptually at the top of the figure. (b) The signal spectrum from WLG at BaF2, measured after a 2400 nm long-pass filter. (c) The measured output spectrum of the MIR OPA. The dotted line separates Photonics 2021, 8, 290 the signal and idler spectra. The retrieved temporal (d) intensity profiles of the synthesized pulse. 4 of 24 Reprinted with permission from [35]. Figure 1. (a) The schematic of the GaSe-based MIR OPA. The pump, the generated supercontinuum, and the amplified MIR Figure 2. (a) The schematic of the GaSe-based MIR OPA. The pump, the generated supercontinuum, pulses are shown and the amplif in maroon, ied M purple, IR pu andlse pink, s are shown in respectively mar . (HWP: oon, pu halfrple, an wave plate, d pinDL: k, re delay spectiline vely , .LPF: (HWP: h long a pass lf filter, Photonics 2021, 8, x FOR PEER REVIEW 5 of 24 wave plate, DL: delay line, LPF: long pass filter, BD: beam dump, CS: characterization setup, and BD: beam dump, CS: characterization setup, and L1–L6 are CaF lenses). The measured (solid blue, 20 nm resolution) and L1–L6 are CaF2 lenses). The measured (solid blue, 20 nm resolution) and simulated (dashed red) simulated (dashed red) spectra of the amplified signal (b) and the idler pulses (c). Reprinted with permission from [36]. spectra of the amplified signal (b) and the idler pulses (c). Reprinted with permission from [36]. S. S. Nam et. al. presented an Nam et al. presented an octave-spanning octave-spanning 3–10 3–10 μ m m MIR OPA MIR OPA system system bas based edon on a a ZGP crystal, pumped by a 1 kHz, 2.4 m, 250 fs Cr:ZnSe CPA [37]. The OPA system was ZGP crystal, pumped by a 1 kHz, 2.4 μm, 250 fs Cr:ZnSe CPA [37]. The OPA system was seeded either by white light generation from a YAG plate or optical parametric gener- seeded either by white light generation from a YAG plate or optical parametric generation ation (OPG) in a ZGP crystal, as shown in Figure 2. By combining the signal seed and (OPG) in a ZGP crystal, as shown in Figure 3. By combining the signal seed and pump pump with orthogonal polarization through a Si wafer placed at the Brewster angle, the with orthogonal polarization through a Si wafer placed at the Brewster angle, the signal signal beam was amplified in a 5 mm-long ZGP crystal, and the long-wavelength idler beam was amplified in a 5 mm-long ZGP crystal, and the long-wavelength idler pulses pulses were generated. With 0.55 mJ pump pulse energy, 130 J or 55 J overall OPA were generated. With 0.55 mJ pump pulse energy, 130 μJ or 55 μJ overall OPA output output (signal + idler) were obtained from the MIR OPA seeded by OPG or white light (signal + idler) were obtained from the MIR OPA seeded by OPG or white light generation, generation, which corresponds to a pump to signal + idler conversion efficiency of 23% which corresponds to a pump to signal + idler conversion efficiency of 23% or 10%, re- or 10%, respectively. spectively. Figure 3. Experimental setup of MIR ZGP OPA. Red lines, 2.4 μm pump beam; green line, signal Figure 2. Experimental setup of MIR ZGP OPA. Red lines, 2.4 m pump beam; green line, signal (seed) beam; blue dotted line, idler beam. M1–M4: Al mirrors; L1–L4: CaF2 lenses; F1: 3 μm LPF; F2: (seed) beam; blue dotted line, idler beam. M1–M4: Al mirrors; L1–L4: CaF lenses; F1: 3 m 4.5 or 7 μm LPF; W: ZnSe wedge; D: delay stage; NL: nonlinear crystal (YAG or ZGP); B: 300-μm- LPF; F2: 4.5 or 7 m LPF; W: ZnSe wedge; D: delay stage; NL: nonlinear crystal (YAG or ZGP); thick Brewster Si plate; WP: λ/2 waveplate. Reprinted with permission from [37]. B: 300-m-thick Brewster Si plate; WP: /2 waveplate. Reprinted with permission from [37]. In 2018, M. Seidel et. al. employed PPLN crystal and wide-bandgap nonlinear crystal In 2018, M. Seidel et al. employed PPLN crystal and wide-bandgap nonlinear crystal BGS in a MIR OPA pumped by a thin-disc laser at ~1 μm. As shown in Figure 4, MIR BGS in a MIR OPA pumped by a thin-disc laser at ~1 m. As shown in Figure 3, MIR emission covering a spectral range of 2 to 11 μm was generated, with an output power of emission covering a spectral range of 2 to 11 m was generated, with an output power of 5 W at 4.1 μm and 1.3 W at 8.5 μm [38]. Subsequently, in 2019, B. Chen reported a MIR 5 W at 4.1 m and 1.3 W at 8.5 m [38]. Subsequently, in 2019, B. Chen reported a MIR laser source with a spectrum covering the 5–11 μm range and a pulse width of 32 fs [34]. The generated MIR pulse had a pulse energy of 220 nJ at a 50 kHz repetition rate. Figure 4. Tuning curves. (a) Generated MIR power for maximal pump power and tuning periods from 28 to 25.5 μm of the PPLN. The spectrum centered at 4.2 μm is shaped through CO2 absorption. The power was measured 25 cm behind the nonlinear crystal. (b) Tuning curve measured with a type I phase-matched LGS crystal. The OPA operated the most powerfully around 8.2 μm (slightly blue-shifted from type II). Upon detuning from this central point, the phase-matched wavelengths split, allowing the generation of very broadband spectra (vine red line). The spectra below 6 μm (black line) Photonics 2021, 8, x FOR PEER REVIEW 5 of 24 S. Nam et. al. presented an octave-spanning 3–10 μm MIR OPA system based on a ZGP crystal, pumped by a 1 kHz, 2.4 μm, 250 fs Cr:ZnSe CPA [37]. The OPA system was seeded either by white light generation from a YAG plate or optical parametric generation (OPG) in a ZGP crystal, as shown in Figure 3. By combining the signal seed and pump with orthogonal polarization through a Si wafer placed at the Brewster angle, the signal beam was amplified in a 5 mm-long ZGP crystal, and the long-wavelength idler pulses were generated. With 0.55 mJ pump pulse energy, 130 μJ or 55 μJ overall OPA output (signal + idler) were obtained from the MIR OPA seeded by OPG or white light generation, which corresponds to a pump to signal + idler conversion efficiency of 23% or 10%, re- spectively. Figure 3. Experimental setup of MIR ZGP OPA. Red lines, 2.4 μm pump beam; green line, signal (seed) beam; blue dotted line, idler beam. M1–M4: Al mirrors; L1–L4: CaF2 lenses; F1: 3 μm LPF; F2: 4.5 or 7 μm LPF; W: ZnSe wedge; D: delay stage; NL: nonlinear crystal (YAG or ZGP); B: 300-μm- thick Brewster Si plate; WP: λ/2 waveplate. Reprinted with permission from [37]. In 2018, M. Seidel et. al. employed PPLN crystal and wide-bandgap nonlinear crystal BGS in a MIR OPA pumped by a thin-disc laser at ~1 μm. As shown in Figure 4, MIR Photonics 2021, 8, 290 5 of 24 emission covering a spectral range of 2 to 11 μm was generated, with an output power of 5 W at 4.1 μm and 1.3 W at 8.5 μm [38]. Subsequently, in 2019, B. Chen reported a MIR laser source with a spectrum covering the 5–11 μm range and a pulse width of 32 fs [34]. laser source with a spectrum covering the 5–11 m range and a pulse width of 32 fs [34]. The generated MIR pulse had a pulse energy of 220 nJ at a 50 kHz repetition rate. The generated MIR pulse had a pulse energy of 220 nJ at a 50 kHz repetition rate. Figure 4. Tuning curves. (a) Generated MIR power for maximal pump power and tuning periods from 28 to 25.5 μm of Figure 3. Tuning curves. (a) Generated MIR power for maximal pump power and tuning periods the PPLN. The spectrum centered at 4.2 μm is shaped through CO2 absorption. The power was measured 25 cm behind from 28 to 25.5 m of the PPLN. The spectrum centered at 4.2 m is shaped through CO absorption. the nonlinear crystal. (b) Tuning curve measured with a type I phase-matched LGS crystal. The OPA operated the most The power was measured 25 cm behind the nonlinear crystal. (b) Tuning curve measured with a powerfully around 8.2 μm (slightly blue-shifted from type II). Upon detuning from this central point, the phase-matched type I phase-matched LGS crystal. The OPA operated the most powerfully around 8.2 m (slightly wavelengths split, allowing the generation of very broadband spectra (vine red line). The spectra below 6 μm (black line) blue-shifted from type II). Upon detuning from this central point, the phase-matched wavelengths split, allowing the generation of very broadband spectra (vine red line). The spectra below 6 m (black line) and above 10 m (light blue line) needed different delays to be generated because of the uncompressed seed pulse. With type I phase matching, a maximal MIR power of 1.0 W could be generated. Type I, however, allowed the generation of slightly more broadband spectra than type II. Power spectral density is provided in units of mW/cm and W/f , where f = 37.5 MHz is the rep rep oscillator repetition rate. Reprinted with permission from [38]. In 2020, Heiner et al. presented an OPA system pumped by a Yb:KGd(WO ) laser 4 2 system with a repetition rate of 100 kHz and pulse width of 180 fs at 1028 nm [33]. The pump was divided to produce white light as the seed of the amplification stage, where the crystal could be BGS or LGS with large bandgap energy. After the Ge lens, average pulse power of 59 mW at 10 m and 81 mW at 8.1 m through the BGS and LGS crystals of the same length respectively were obtained, and their pulse widths were measured as 126 fs (3.8 cycle) and 121 fs (4.5 cycle), as shown in Figure 5. This comparison indicates that BGS is a promising candidate for 1-m-pumped OPAs generating MIR emission beyond 5 m because of its larger size availability and longer transmission cutoff up to 13.7 m. 3.2. MIR OPCPA 3.2.1. 2–4 m OPCPA OPCPA systems have superior energy and power upscaling capability. With careful dispersion management, amplified pulses with a pulse width close to the transform limit could be generated. High-energy, few-cycle light sources with a central wavelength of 2–4 m, and multi-millijoule pulse energy have been realized in many research groups via OPCPA techniques. Table 3 summarizes the specifications of few-cycle 2–4 m OPCPA. Subsequently, some a variety of previously published works are selected to elaborate in detail. Photonics 2021, 8, x FOR PEER REVIEW 4 of 24 Photonics 2021, 8, 290 6 of 24 Photonics 2021, 8, x FOR PEER REVIEW 6 of 24 and above 10 μm (light blue line) needed different delays to be generated because of the uncompressed seed pulse. With type I phase matching, a maximal MIR power of 1.0 W could be generated. Type I, however, allowed the generation of slightly more broadband spectra than type II. More information is provided in section S5. Power spectral density is pro- −1 vided in units of mW/cm and μW/frep, where frep = 37.5 MHz is the oscillator repetition rate. Reprinted with permission from [38]. Figure 1. (a) The schematic of the high-energy phase-stable sub-cycle MIR OPA. Polarizations of the Figure 4. (a) The schematic of the high-energy phase-stable sub-cycle MIR OPA. Polarizations of In 2020, Heiner et. al. presented an OPA system pumped by a Yb:KGd(WO4)2 laser beams are marked by double-headed arrows and concentric circles. 300-μm thick Si wafers at Brew- the beams are marked by double-headed arrows and concentric circles. 300-m thick Si wafers at system with a repetition rate of 100 kHz and pulse width of 180 fs at 1028 nm [33]. The ster angle were used as polarization beam splitter and beam combiner to transmit the 2.1-μm pump Brewster angle were used as polarization beam splitter and beam combiner to transmit the 2.1-m pump was divided to produce white light as the seed of the amplification stage, where pulse and reflected the signal and idler pulses. The synthesized pulses and a branch of 2.1-μm ref- pump pulse and reflected the signal and idler pulses. The synthesized pulses and a branch of 2.1-m the crystal could be BGS or LGS with large bandgap energy. After the Ge lens, average erence pulses were sent into XFROG with 30-μm thick GaSe nonlinear crystal. The synthesis of a reference pulses were sent into XFROG with 30-m thick GaSe nonlinear crystal. The synthesis of pulse power of 59 mW at 10 μm and 81 mW at 8.1 μm through the BGS and LGS crystals sub-cycle MIR pulse by coherently combining the sub-2-cycle signal and idler pulses is shown con- a sub-cycle MIR pulse by coherently combining the sub-2-cycle signal and idler pulses is shown of the same length respectively were obtained, and their pulse widths were measured as ceptually at the top of the figure. (b) The signal spectrum from WLG at BaF2, measured after a 2400 conceptually at the top of the figure. (b) The signal spectrum from WLG at BaF , measured after 126 fs (3.8 cycle) and 121 fs (4.5 cycle), as shown in Figure 5. This compa 2 rison indicates nm long-pass filter. (c) The measured output spectrum of the MIR OPA. The dotted line separates a 2400 nm long-pass filter. (c) The measured output spectrum of the MIR OPA. The dotted line tthe signal and hat BGS is a promisin idler spectra g c.a T ndid he retrieved te ate for 1-μ mporal ( m-pumped d) inte OPAs nsity profile geners o ating MI f the synthesiz R emission ed pu be- lse. separates Reprinted with the signal permission fro and idler spectra. m [35].The retrieved temporal (d) intensity profiles of the synthesized yond 5 μm because of its larger size availability and longer transmission cutoff up to 13.7 pulse. Reprinted with permission from [35]. μm. Figure 5. Illustration of BGS OPA laser system. BS, beam sampler; PR, partial reflector; WLC, white Figure 5. Illustration of BGS OPA laser system. BS, beam sampler; PR, partial reflector; WLC, white light continuum generation unit; L, lens; BD, beam dump. DM1, dichroic mirror, high reflection light continuum generation unit; L, lens; BD, beam dump. DM1, dichroic mirror, high reflection (HR) at 1.03 μm and high transmission (HT) at >1.1 μm; DM2, dichroic mirror, HR at 1.0–1.2 μm (HR) at 1.03 m and high transmission (HT) at >1.1 m; DM2, dichroic mirror, HR at 1.0–1.2 m and and HT at 6–12 μm; OPA, LGS or BGS crystal; Ge, germanium-based temporal chirp compensation HT at 6–12 m; OPA, LGS or BGS crystal; Ge, germanium-based temporal chirp compensation unit. unit. Reprinted with permission from [33]. Reprinted with permission from [33]. 3.2. MIR OPCPA In a pioneer work, F. Krausz’s research group employed the broadband Ti:sapphire 3.2.1. 2–4 μm OPCPA laser (oscillator and amplifier) as both the signal and pump at the same time [39]. As shown in Figure 6, the 1030 nm component of the broadband spectrum output from the OPCPA systems have superior energy and power upscaling capability. With careful Figure 2. (a) The schematic of the GaSe-based MIR OPA. The pump, the generated supercontinuum, Ti:sapphire oscillator was extracted and injected into a Yb:YAG thin disk amplifier to obtain dispersion management, amplified pulses with a pulse width close to the transform limit and the amplified MIR pulses are shown in maroon, purple, and pink, respectively. (HWP: half the pump light synchronized with the signal beam. Non-collinear OPAs (NOPAs) were could wave plate, D be gene Lrated. : delay H lin igh e, L -e P nergy, F: long pass fil few-cycle ter, BD: b light sour eam du ces w mpi, C th S a ce : chntral wave aracterization se lengtup, and th of 2– carried out in two stages using PPLN crystals and during the last power amplifier stage L1–L6 are CaF2 lenses). The measured (solid blue, 20 nm resolution) and simulated (dashed red) 4 μm, and multi-millijoule pulse energy have been realized in many research groups via with spectr aa of t LNO he crystal. amplified si Finally gnal ,(a b)MIR and the output idler at pua lse central s (c). Rewavelength printed with p of ermi 2.1ssio m n fro and ma [36 repetition ]. OPCPA techniques. Table 3 summarizes the specifications of few-cycle 2–4 μm OPCPA. Subsequently, some a variety of previously published works are selected to elaborate in detail. Table 3. Parameters of selected 2–4 μm OPCPA system. Wavelength Pulse Energy Repetition Rate Power Pulse Width Optical Cycle Reference (µm) (mJ) (kHz) (W) (fs) 2.1 1.2 3 3.6 10.5 1.5 [39] 2.1 2.7 10 27 30 4.3 [40] 2.1 2.6 1 2.6 39 5.6 [41] 2.2 0.25 100 25 16.5 2.2 [42] 2.5 0.126 100 12.6 14.4 1.7 [43] 3 0.3 10 3 21 2.1 [44] 3 2.4 10 24 50 5 [45] 3.07 0.01 125 1.25 72 7 [46] 3.1 0.125 100 12.5 73 7 [47] 3.2 0.152 100 15.2 38 3.6 [48] 3.25 0.06 160 9.6 14.5 1.4 [49] 3.4 0.012 50 0.6 41.6 3.7 [50] Photonics 2021, 8, 290 7 of 24 frequency of 3 kHz with a pulse energy of 1.2 mJ and a pulse width of 10.5 fs (1.5 cycles) was obtained. Table 3. Parameters of selected 2–4 m OPCPA system. Photonics 2021, 8, x FOR PEER REVIEW 7 of 24 Wavelength Pulse Energy Repetition Rate Power Pulse Width Optical Cycle Reference (m) (mJ) (kHz) (W) (fs) 2.1 1.2 3 3.6 10.5 1.5 [39] 3.425 13.3 0.01 0.133 111 9.7 [51] 2.1 2.7 10 27 30 4.3 [40] 3.9 8 0.02 0.16 83 6.4 [52] 2.1 2.6 1 2.6 39 5.6 [41] 4 2.6 0.1 0.26 21.5 1.6 [53] 2.2 0.25 100 25 16.5 2.2 [42] 2.5 0.126 100 12.6 14.4 1.7 [43] 3 0.3 10 3 21 2.1 [44] In a pioneer work, F. Krausz’s research group employed the broadband Ti:sapphire 3 2.4 10 24 50 5 [45] laser (oscillator and amplifier) as both the signal and pump at the same time [39]. As 3.07 0.01 125 1.25 72 7 [46] shown in Figure 6, the 1030 nm component of the broadband spectrum output from the 3.1 0.125 100 12.5 73 7 [47] Ti:sapphire oscillator was extracted and injected into a Yb:YAG thin disk amplifier to ob- 3.2 0.152 100 15.2 38 3.6 [48] tain the pump light synchronized with the signal beam. Non-collinear OPAs (NOPAs) 3.25 0.06 160 9.6 14.5 1.4 [49] were carried out in two stages using PPLN crystals and during the last power amplifier 3.4 0.012 50 0.6 41.6 3.7 [50] 3.425 13.3 stage with a 0.01 LNO crystal. Fina 0.133 lly, a MIR out111 put at a central w 9.7 avelength of 2.1 [51μm a ] nd a 3.9 8 0.02 0.16 83 6.4 [52] repetition frequency of 3 kHz with a pulse energy of 1.2 mJ and a pulse width of 10.5 fs 4 2.6 0.1 0.26 21.5 1.6 [53] (1.5 cycles) was obtained. Figure 6. Schematic of 2.1 μm few-cycle OPCPA system. Reprinted with permission from [39]. Figure 6. Schematic of 2.1 m few-cycle OPCPA system. Reprinted with permission from [39]. In 2020, U. Keller ’s research group presented an OPCPA system centered at a wave- In 2020, U. Keller’s research group presented an OPCPA system centered at a wave- length of 2.2 m, generating 16.5 fs pulses (2.2 cycles) with 25 W of average power at length of 2.2 μm, generating 16.5 fs pulses (2.2 cycles) with 25 W of average power at 100 100 kHz [42]. As shown in Figure 7, the seed from a Ti:sapphire oscillator was amplified kHz [42]. As shown in Figure 7, the seed from a Ti:sapphire oscillator was amplified in a in a BBO crystal. The idler was then generated from the NOPA in another BBO. Through BBO crystal. The idler was then generated from the NOPA in another BBO. Through three three NOPA stages, idler pulses were amplified to 300 J. Finally, after the compressor, NOPA stages, idler pulses were amplified to 300 μJ. Finally, after the compressor, pulses pulses of 250 J and 16.5 fs were obtained. Based on the MIR OPCPA, soft-X ray emission of 250 μJ and 16.5 fs were obtained. Based on the MIR OPCPA, soft-X ray emission with with the spectrum extending to 0.6 keV was demonstrated. the spectrum extending to 0.6 keV was demonstrated. The most famous MIR OPCPA in the 3–4 m band is probably from the research team led by A. Baltuska. The MIR laser output with a central wavelength of 3.9 m, pulse energy of 8–20 mJ, pulse width of ~90 fs, and a repetition rate of 20 Hz is demonstrated [52]. As shown in Figure 8, a Yb:KGW Kerr-lens mode-locked oscillator was used as the seed source. A signal light with an energy of 65 J and a central wavelength of 1460 nm generated from white light continuum was then obtained using successive three-stage parametric amplification based on KTP crystals. On the other hand, the 1064 nm component of the output spectrum of the oscillator was extracted and amplified into a pulse energy of 250 mJ by Nd:YAG CPAs, which served as the pumping source of the subsequent OPCPA system. Two-stage OPCPA was then constructed to obtain a 1.46 m signal light of 22 mJ and a 3.9 m idler light of 13 mJ. The output pulse with 8 mJ, ~80 fs was obtained by compressing the idle light. Further energy upscaling of the system to ~20 mJ has been demonstrated in subsequent work. Photonics 2021, 8, x FOR PEER REVIEW 7 of 24 3.425 13.3 0.01 0.133 111 9.7 [51] 3.9 8 0.02 0.16 83 6.4 [52] 4 2.6 0.1 0.26 21.5 1.6 [53] In a pioneer work, F. Krausz’s research group employed the broadband Ti:sapphire laser (oscillator and amplifier) as both the signal and pump at the same time [39]. As shown in Figure 6, the 1030 nm component of the broadband spectrum output from the Ti:sapphire oscillator was extracted and injected into a Yb:YAG thin disk amplifier to ob- tain the pump light synchronized with the signal beam. Non-collinear OPAs (NOPAs) were carried out in two stages using PPLN crystals and during the last power amplifier stage with a LNO crystal. Finally, a MIR output at a central wavelength of 2.1 μm and a repetition frequency of 3 kHz with a pulse energy of 1.2 mJ and a pulse width of 10.5 fs (1.5 cycles) was obtained. Figure 6. Schematic of 2.1 μm few-cycle OPCPA system. Reprinted with permission from [39]. In 2020, U. Keller’s research group presented an OPCPA system centered at a wave- length of 2.2 μm, generating 16.5 fs pulses (2.2 cycles) with 25 W of average power at 100 kHz [42]. As shown in Figure 7, the seed from a Ti:sapphire oscillator was amplified in a BBO crystal. The idler was then generated from the NOPA in another BBO. Through three NOPA stages, idler pulses were amplified to 300 μJ. Finally, after the compressor, pulses Photonics 2021, 8, 290 of 250 μJ and 16.5 fs were obtained. Based on the MIR OPCPA, soft-X ray emission with 8 of 24 the spectrum extending to 0.6 keV was demonstrated. Photonics 2021, 8, x FOR PEER REVIEW 8 of 24 Figure 7. (a) 2.2 μm OPCPA layout. The inset on the top right shows the long-term output stability of the system and beam profile after cylindrical reshaping telescopes. (b) The retrieved pulse shape of the amplifier output. (c) Blue line, measured spectrum; blue-dashed line, retrieved spec- trum; orange line, retrieved phase. Reprinted with permission from [42]. The most famous MIR OPCPA in the 3–4 μm band is probably from the research team led by A. Baltuska. The MIR laser output with a central wavelength of 3.9 μm, pulse en- ergy of 8–20 mJ, pulse width of ~90 fs, and a repetition rate of 20 Hz is demonstrated [52]. As shown in Figure 8, a Yb:KGW Kerr-lens mode-locked oscillator was used as the seed source. A signal light with an energy of 65 μJ and a central wavelength of 1460 nm gener- ated from white light continuum was then obtained using successive three-stage paramet- ric amplification based on KTP crystals. On the other hand, the 1064 nm component of the output spectrum of the oscillator was extracted and amplified into a pulse energy of 250 mJ by Nd:YAG CPAs, which served as the pumping source of the subsequent OPCPA system. Two-stage OPCPA was then constructed to obtain a 1.46 μm signal light of 22 mJ and a 3.9 μm idler light of 13 mJ. The output pulse with 8 mJ, ~80 fs was obtained by Figure 7. (a) 2.2 m OPCPA layout. The inset on the top right shows the long-term output stability of the system and beam compressing the idle light. Further energy upscaling of the system to ~20 mJ has been profile after cylindrical reshaping telescopes. (b) The retrieved pulse shape of the amplifier output. (c) Blue line, measured demonstrated in subsequent work. spectrum; blue-dashed line, retrieved spectrum; orange line, retrieved phase. Reprinted with permission from [42]. Figure 8. Layout of the 3.9 m OPCPA system. Reprinted with permission from [52]. Figure 8. Layout of the 3.9 μm OPCPA system. Reprinted with permission from [52]. In the 3–4 m band, J. Biegert’s team demonstrated a high-average-power MIR OPCPA In the 3–4 μm band, J. Biegert’s team demonstrated a high-average-power MIR with 21 W output power at a central wavelength of 3.25 m and a repetition rate of OPCPA with 21 W output power at a central wavelength of 3.25 μm and a repetition rate 160 kHz [49]. As shown in Figure 9, the MIR seed at 3.25 m was generated by a two-color of 160 kHz [49]. As shown in Figure 9, the MIR seed at 3.25 μm was generated by a two- fiber front-end in combination with a DFG stage. Afterwards, the MIR pulses were stretched color fiber front-end in combination with a DFG stage. Afterwards, the MIR pulses were and consecutively amplified in a preamplifier and two booster amplifiers. As it was being stretched and consecutively amplified in a preamplifier and two booster amplifiers. As it pumped, a Nd:YVO -based master oscillator power amplifier (MOPA) was employed, was being pumped, a Nd:YVO4-based master oscillator power amplifier (MOPA) was em- providing 1.1 mJ, 9 ps pulses at a 1064 nm wavelength at a 160 kHz repetition rate. After ployed, providing 1.1 mJ, 9 ps pulses at a 1064 nm wavelength at a 160 kHz repetition rate. three pre-amplifiers and four booster amplifiers, a MIR pulse with a 131 J pulse energy After three pre-amplifiers and four booster amplifiers, a MIR pulse with a 131 μJ pulse and a 21 W average power was obtained. The MIR laser output was then compressed down energy and a 21 W average power was obtained. The MIR laser output was then com- to 1.35 cycles via soliton self-compression in a noble gas filled anti-resonance photonic pressed down to 1.35 cycles via soliton self-compression in a noble gas filled anti-reso- crystal fiber, yielding 14.5 fs pulses at 3.3 m with a 9.6 W average power. nance photonic crystal fiber, yielding 14.5 fs pulses at 3.3 μm with a 9.6 W average power. Photonics 2021, 8, x FOR PEER REVIEW 9 of 24 Photonics 2021, 8, 290 9 of 24 Figure 9. Setup of the high-power, MIR OPCPA system. The seed was generated by a two-color fiber front-end in combi- Figure 9. Setup of the high-power, MIR OPCPA system. The seed was generated by a two-color fiber front-end in nation with a DFG stage. Afterward, the MIR pulses were stretched and consecutively amplified in a pre-amplifier and combination with a DFG stage. Afterward, the MIR pulses were stretched and consecutively amplified in a pre-amplifier two booster amplifiers. Maximum conversion efficiencies were achieved by multiple uses of the pump beam and by indi- and two booster amplifiers. Maximum conversion efficiencies were achieved by multiple uses of the pump beam and by vidually tailored seed-to-pump pulse durations. Reprinted with permission from [49]. individually tailored seed-to-pump pulse durations. Reprinted with permission from [49]. Y. Leng’s research team reported a 4 μm OPCPA with a 2.6 mJ pulse energy and a Y Y. Leng’s research te . Leng’s research team am r reported eported a 4 µ a 4 m m OPCPA OPCPA with with a a 2.6 2.6 m mJJpulse pulse ener energy gy and and a a 1.6 cycle pulse width [53]. As shown in Figure 10, a CEP-sTable 4 μm seed with ~120 μJ 1.6 cycle pulse width [53]. As shown in Figure 10, a CEP-sTable 4 m seed with ~120 J 1.6 cycle pulse width [53]. As shown in Figure 10, a CEP-sTable 4 µm seed with ~120 µJ energy was generated from a home-built OPA pumped by a commercial Ti:sapphire energy was generated from a home-built OPA pumped by a commercial Ti:sapphire energy was generated from a home-built OPA pumped by a commercial Ti:sapphire femtosecond laser. The pump laser for the MIR OPCPA was from a picosecond Nd:YAG femtosecond laser. The pump laser for the MIR OPCPA was from a picosecond Nd:YAG femtosecond laser. The pump laser for the MIR OPCPA was from a picosecond Nd:YAG laser that could deliver a 1064 nm pulses with up to 300 mJ energy and 50 ps pulse dura- laser that could deliver a 1064 nm pulses with up to 300 mJ energy and 50 ps pulse duration laser that could deliver a 1064 nm pulses with up to 300 mJ energy and 50 ps pulse dura- tion running at 100 Hz repetition rate. After two amplifier stages, the amplified 4 μm running at 100 Hz repetition rate. After two amplifier stages, the amplified 4 m chirped tion running at 100 Hz repetition rate. After two amplifier stages, the amplified 4 µm chirped pulse with 11.8 mJ energy was compressed to 105 fs by employing a grating com- pulse with 11.8 mJ energy was compressed to 105 fs by employing a grating compressor. In chirped pulse with 11.8 mJ energy was compressed to 105 fs by employing a grating com- pressor. In order to obtain a near-single-cycle pulse at 4 μm, a noble gas filled hollow-core order to obtain a near-single-cycle pulse at 4 m, a noble gas filled hollow-core fiber with pressor. In order to obtain a near-single-cycle pulse at 4 µm, a noble gas filled hollow-core fiber with a 1-mm inner core diameter and 3-m length was employed. Combined with a a 1-mm inner core diameter and 3-m length was employed. Combined with a CaF bulk fiber with a 1-mm inner core diameter and 3-m length was employed. Combined with a CaF2 bulk material, the MIR pulse with a pulse energy of 2.6 mJ was further compressed material, the MIR pulse with a pulse energy of 2.6 mJ was further compressed to 21.5 fs, CaF2 bulk material, the MIR pulse with a pulse energy of 2.6 mJ was further compressed to 21.5 fs, corresponding to a 1.6 optical cycle at a 4 μm centre wavelength. corresponding to a 1.6 optical cycle at a 4 m centre wavelength. to 21.5 fs, corresponding to a 1.6 optical cycle at a 4 µm centre wavelength. Figure 10. Schematic of the 4 μm OPCPA and post-compression system. Reprinted with permis- Figure 10. Schematic of the 4 µm OPCPA and post-compression system. Reprinted with permis- Figure 10. Schematic of the 4 m OPCPA and post-compression system. Reprinted with permission sion from [53]. sion from [53]. from [53]. In 2021, H. K. Liang’s team demonstrated a high-energy and high-power 3 μm Table 4. Parameters of the long-wavelength MIR OPCPA systems. OPCPA pumped by a shaped flat-top beam [45]. With the combination of a commercial diffractive phase plate and a focus lens, a 1 μm Gaussian pump was transformed into a Wavelength Pulse Energy Repetition Rate Power Pulse Width Optical Cycle Reference flat-top profile with a high beam shaping ratio of more than 95%, as shown in Figure 11a. (m) (mJ) (kHz) (W) (fs) An MIR OPCPA stage with a 2-fold efficiency enhancement of up to 13.5% was achieved 5 3.4 1 3.4 89.4 5.4 [54] with the shaped flat-top pump beam. 2.7 mJ, 27 W, 125 fs, 3 μm pulses with a 10 kHz 7 0.7 0.1 0.07 188 8 [55] repetition rate were generated, as measured in Figure 11b–d. The amplified flat-top-like 9 0.014 10 0.14 142 4.7 [56] MIR pulse was subsequently compressed to 50 fs through nonlinear compression in a thin YAG crystal, corresponding to 5 optical cycles, with ~90% compression efficiency. Equip- In 2021, H. K. Liang’s team demonstrated a high-energy and high-power 3 m OPCPA ping an OPCPA with a diffractive phase plate as the beam shaper is a simple, robust and pumped by a shaped flat-top beam [45]. With the combination of a commercial diffractive cost-effective method. It could also, in principle, be applied to other parametric conver- phase plate and a focus lens, a 1 m Gaussian pump was transformed into a flat-top sions at other wavelength ranges. profile with a high beam shaping ratio of more than 95%, as shown in Figure 11a. An MIR Photonics 2021, 8, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/photonics Photonics 2021, 8, 290 10 of 24 OPCPA stage with a 2-fold efficiency enhancement of up to 13.5% was achieved with the shaped flat-top pump beam. 2.7 mJ, 27 W, 125 fs, 3 m pulses with a 10 kHz repetition rate were generated, as measured in Figure 11b–d. The amplified flat-top-like MIR pulse was subsequently compressed to 50 fs through nonlinear compression in a thin YAG crystal, corresponding to 5 optical cycles, with ~90% compression efficiency. Equipping an OPCPA with a diffractive phase plate as the beam shaper is a simple, robust and cost-effective Photonics 2021, 8, x FOR PEER REVIEW 10 of 24 method. It could also, in principle, be applied to other parametric conversions at other wavelength ranges. Figure 11. (a) The schematic of flat-top beam shaping of the high-energy and high-average-power 3 μm OPCPA. The MIR Figure 11. (a) The schematic of flat-top beam shaping of the high-energy and high-average-power 3 m OPCPA. The MIR pulses centered at 3 μm were generated and amplified to 300 μJ from 3-stage OPCPA preamplifiers via periodically poled pulses centered at 3 m were generated and amplified to 300 J from 3-stage OPCPA preamplifiers via periodically poled lithium niobate (PPLN) and KTA crystals. The 4th OPCPA stage was designed to boost up the MIR output and enhance lithium niobate (PPLN) and KTA crystals. The 4th OPCPA stage was designed to boost up the MIR output and enhance the the parametric efficiency through the flat-top beam shaping. The Gaussian pump beam of the 4th-stage OPCPA was sent parametric efficiency through the flat-top beam shaping. The Gaussian pump beam of the 4th-stage OPCPA was sent to a to a flat-top beam shaper consisting of a phase plate and a focus lens, and the flat-top pump beam is formed at the imaging flat-top beam shaper consisting of a phase plate and a focus lens, and the flat-top pump beam is formed at the imaging plane of the lens. The Gaussian idler beam generated from the first 3 OPCPA stages was amplified with a flat-top pump, plane of the lens. The Gaussian idler beam generated from the first 3 OPCPA stages was amplified with a flat-top pump, producing a high-energy and high-average-power flat-top-like 3 μm output. The measured pump beam profiles (b) with pr and ( oducing c) wiathout the high-ener flat-t gy and op b high-average-power eam shaper on the K flat-top-like TA crystal.3 Th m e c output. ross-seThe ction b measur eam pro ed pump files obeam n the x a profiles nd y a (bx ) es with are included too. (d) The pulse energy measurements of the 3 μm idler pulse from the OPCPA with flat-top (red) and Gaussian and (c) without the flat-top beam shaper on the KTA crystal. The cross-section beam profiles on the x and y axes are (black) pump beam profiles. 2.7 mJ and 1.45 mJ MIR pulse energy were obtained from the flat-top and Gaussian pump, included too. (d) The pulse energy measurements of the 3 m idler pulse from the OPCPA with flat-top (red) and Gaussian corresponding to 7% and 13.5% pump-to-idler efficiency for the 4th-OPCPA stage, respectively. Reprinted with permis- (black) pump beam profiles. 2.7 mJ and 1.45 mJ MIR pulse energy were obtained from the flat-top and Gaussian pump, sion from [45]. corresponding to 7% and 13.5% pump-to-idler efficiency for the 4th-OPCPA stage, respectively. Reprinted with permission from [45]. 3.2.2. 5–10 μm OPCPA 3.2.2. In recent 5–10 m OPCP years, there h A ave been emerging reports of long-wavelength MIR OPCPA systems with lasing wavelengths exceeding 5 μm. Table 4 summarizes long-wavelength In recent years, there have been emerging reports of long-wavelength MIR OPCPA MIR OPCPA systems with the centre wavelengths of 5 μm, 7 μm, and 9 μm. systems with lasing wavelengths exceeding 5 m. Table 4 summarizes long-wavelength MIR OPCPA systems with the centre wavelengths of 5 m, 7 m, and 9 m. Table 4. Parameters of the long-wavelength MIR OPCPA systems. In 2020, a 5 m OPCPA system delivering multi-10 GW peak power femtosecond pulses at a 1 kHz repetition rate were reported [54]. The signal and pump seed pulses Wavelength Pulse Energy Repetition Rate Power Pulse Width Optical Cycle Reference were provided by a fiber-based 40 MHz multi-color laser system, as shown in Figure 12. (µm) (mJ) (kHz) (W) (fs) It contained three separate erbium (Er)-doped fiber amplifiers and generated pulses at 5 3.4 1 3.4 89.4 5.4 [54] 1.55 m as well as two supercontinua. The latter pulses were then generated separately in 7 0.7 0.1 0.07 188 8 [55] two nonlinear fibers, optimized for supercontinuum with center wavelengths at 1.0 and 9 0.014 10 0.14 142 4.7 [56] 2.0 m, respectively. The signal at 3.5 m was provided by DFG using a 1.5 m oscillator and 1 m supercontinuum pulses. The pulses with a center of gravity at 2 m served In 2020, a 5 μm OPCPA system delivering multi-10 GW peak power femtosecond as seed for the pump 36 mJ pump. Subsequently, through four-stage OPAs, a 5 m MIR pulses at a 1 kHz repetition rate were reported [54]. The signal and pump seed pulses emission with 3.4 mJ pulse energy and 89.4 fs pulse width was obtained. were provided by a fiber-based 40 MHz multi-color laser system, as shown in Figure 12. It contained three separate erbium (Er)-doped fiber amplifiers and generated pulses at 1.55 μm as well as two supercontinua. The latter pulses were then generated separately in two nonlinear fibers, optimized for supercontinuum with center wavelengths at 1.0 and 2.0 μm, respectively. The signal at 3.5 μm was provided by DFG using a 1.5 μm oscillator and 1 μm supercontinuum pulses. The pulses with a center of gravity at 2 μm served as seed for the pump 36 mJ pump. Subsequently, through four-stage OPAs, a 5 μm MIR emission with 3.4 mJ pulse energy and 89.4 fs pulse width was obtained. Photonics 2021, 8, x FOR PEER REVIEW 11 of 24 Photonics 2021, 8, x FOR PEER REVIEW 11 of 24 Photonics 2021, 8, 290 11 of 24 Figure 12. Setup of the 2 μm pumped MIR OPCPA. Reprinted with permission from [54]. Figure 12. Setup of the 2 μm pumped MIR OPCPA. Reprinted with permission from [54]. Figure 12. Setup of the 2 m pumped MIR OPCPA. Reprinted with permission from [54]. In 2016, J. Biegert’s team demonstrated a high-energy, few-cycle 7 μm OPCPA [55]. In 2016, J. Biegert’s team demonstrated a high-energy, few-cycle 7 μm OPCPA [55]. In 2016, J. Biegert’s team demonstrated a high-energy, few-cycle 7 m OPCPA [55]. As shown in Figure 13, the system started with an Er:Tm:Ho:fiber laser which generated As shown in Figure 13, the system started with an Er:Tm:Ho:fiber laser which generated As shown in Figure 13, the system started with an Er:Tm:Ho:fiber laser which generated a a 7 μm seed via DFG in a CSP crystal. The 7 μm seed pulse was then amplified in a ZGP- a 7 μm seed via DFG in a CSP crystal. The 7 μm seed pulse was then amplified in a ZGP- 7 m seed via DFG in a CSP crystal. The 7 m seed pulse was then amplified in a ZGP- based OPCPA chain pumped by a cryogenic-cooled Ho:YLF CPA system with a 260 mJ based OPCPA chain pumped by a cryogenic-cooled Ho:YLF CPA system with a 260 mJ based OPCPA chain pumped by a cryogenic-cooled Ho:YLF CPA system with a 260 mJ pump energy at 2052 nm pump energy and a at 2052 nm 16 ps pulse w and a idth. Th 16 ps pulse w e pulses wer idth. Th e fir e pulses wer st put through both e first put through both pump energy at 2052 nm and a 16 ps pulse width. The pulses were first put through both a pre-amplifier a p and b re-am oost pleir am fier and b plifieroost stage, er am and plifin fier a st lly p age, ut t and hroug finh a ally p com ut t ph ress roug or, h a aft com er pressor, after a pre-amplifier and booster amplifier stage, and finally put through a compressor, after which a 7 μm pulses with a 0.75 mJ pulse energy and a 188 fs pulse width were obtained. which which a a 77 μ m m pulses pulses with with a a 0.75 0.75 m mJ J pulse pulse ener energy an gy andd a 188 fs pulse a 188 fs pulse width width were o were obtained. btained. Figure 13. Layout Figure 13. of the 7 μm OPCPA. The Layout of the 7 μ MIR se m OPCPA. The ed was generated using the two broa MIR seed was generated using the two broa dband dband Figure 13. Layout of the 7 m OPCPA. The MIR seed was generated using the two broadband femtosecond outputs from femtosecond outputs from a three-color fiber frontend via DFG. Afterwards, the MIR pulses were femtosecond outputs from a three-color fiber frontend via DFG. Afterwards, the MIR pulses were a three-color fiber frontend via DFG. Afterwards, the MIR pulses were stretched in a dielectric bulk and consecutively stretched in a dielec stretched tric bu in a lk an d d conse ielectric cu bu tive lk a ly n amplified d consecu in tive a pre-amplif ly amplified ier in and a a pre-amplif booster ier ampli- and a booster ampli- amplified in a pre-amplifier and a booster amplifier separated with a chirp inversion stage. Maximum efficiency of the fier separated with a chirp inversion stage. Maximum efficiency of the OPCPA was achieved by fier separated with a chirp inversion stage. Maximum efficiency of the OPCPA was achieved by OPCPA was achieved by tailoring the seed-to-pump pulse durations in the pre-amplifier and booster amplifier. The tailoring the seed-to-pump pulse durations in the pre-amplifier and booster amplifier. The broad- tailoring the seed-to-pump pulse durations in the pre-amplifier and booster amplifier. The broad- broadband high-energy MIR pulses were recompressed using a dielectric bulk rod of BaF . Reprinted with permission band high-energy MIR pulses were recompressed using a dielectric bulk rod of BaF2. Reprinted band high-energy MIR pulses were recompressed using a dielectric bulk rod of BaF2. Reprinted from [55]. with permission from [55]. with permission from [55]. Recently, H. K. Liang’s research group reported a 9 m, few cycle MIR OPCPA based Recently, H. K. Li Recent ang’s ly res , H e.arch K. Li group ang’s res repe o arch rted a group 9 μm, re fp ew cy orted a cle 9 Mμ Im, R OP few cy CPA c l ba e M seI d R OPCPA based on LiGaS crystals pumped by a 1 m Yb:YAG laser at a 10 kHz repetition rate [56]. This on LiGaS2 crystals pumped by a 1 μm Yb:YAG laser at a 10 kHz repetition rate [56]. This on LiGaS2 crystals pumped by a 1 μm Yb:YAG laser at a 10 kHz repetition rate [56]. This is the first long-wavelength MIR OPCPA pumped at the 1 m wavelength. As shown in is the first long-wavelength MIR OPCPA pumped at the 1 μm wavelength. As shown in is the first long-wavelength MIR OPCPA pumped at the 1 μm wavelength. As shown in Figure 14, a small fraction separated from the Yb:YAG pump was injected into the YAG Figure 14, a small fraction separated from the Yb:YAG pump was injected into the YAG Figure 14, a small fraction separated from the Yb:YAG pump was injected into the YAG crystal to produce a white-light continuum with a central wavelength of 1.16 m. The crystal to produce cryst a wh al toit pro e-light duce cont a wh inu itu em wit -light cont h a ce innt uu ral w m wit ave h le a ngt cent h of ral w 1.1 a6 ve μ le m. Th ngth of e 1.16 μm. The stretched signal pulses were amplified in two consecutive amplification stages. Finally, stretched signalst pu retlse ched s wsig ere nal ampl puif lse ied s w in e t re wampl o con ifsied ecut in ive tw ampl o con ifs icat ecut ion ive stampl ages. F ificat inaion lly, st ages. Finally, long-wavelength MIR idler pulses centered at 9 m with a 14 J pulse energy and a 142 fs long-wavelength MIR idler pulses centered at 9 μm with a 14 μJ pulse energy and a 142 long-wavelength MIR idler pulses centered at 9 μm with a 14 μJ pulse energy and a 142 (4.7 optical cycles) duration at a 10 kHz repetition rate were achieved. The 9 m pulses were fs (4.7 optical cycles) duration at a 10 kHz repetition rate were achieved. The 9 μm pulses fs (4.7 optical cycles) duration at a 10 kHz repetition rate were achieved. The 9 μm pulses further compressed to 45 fs corresponding to 1.5 optical cycles by nonlinear compression were further compressed to 45 fs corresponding to 1.5 optical cycles by nonlinear com- were further compressed to 45 fs corresponding to 1.5 optical cycles by nonlinear com- using a KrS-5 bulk material [57]. pression using a KrS-5 bulk material [57]. pression using a KrS-5 bulk material [57]. Photonics 2021, 8, x FOR PEER REVIEW 12 of 24 Photonics 2021, 8, 290 12 of 24 Figure 14. (a) The schematic of the 9 μm OPCPA. YAG, Yttrium aluminum garnet; ZnSe, zinc sele- Figure 14. (a) The schematic of the 9 m OPCPA. YAG, Yttrium aluminum garnet; ZnSe, zinc selenide nide window; HR, high reflective mirror; TFP, thin film polarizer; BS, beam splitter; LGS, LiGaS2 window; HR, high reflective mirror; TFP, thin film polarizer; BS, beam splitter; LGS, LiGaS crystal; crystal; Ge, germanium window. For TFP, the reflectance of the S-polarized pump and the trans- Ge, germanium window. For TFP, the reflectance of the S-polarized pump and the transmittance of mittance of the P-polarized signal were measured as > 99% and 91% respectively. (b) The spectra the P-polarized signal were measured as >99% and 91% respectively. (b) The spectra of signal pulses of signal pulses after SC generation (blue dotted), the pre-amplification stage (red) and the main- after SC generation (blue dotted), the pre-amplification stage (red) and the main-amplification stage amplification stage (black dashed); (c) the measured (black) and simulated (red dashed) spectra of the ou (blacktpu dashed); t idler pu (c) lse. the Reprinted with measured (black) permission and simulated from [56] (red .dashed) spectra of the output idler pulse. Reprinted with permission from [56]. 3.3. MIR Intra-Pulse DFG 3.3. MIR Intra-Pulse DFG DFG has remarkable advantages such as a single pass structure without complex DFG has remarkable advantages such as a single pass structure without complex cavity adjustment and a broad tuning range of the output spectrum. Intra-pulse DFG cavity adjustment and a broad tuning range of the output spectrum. Intra-pulse DFG (IPDFG) is a special DFG process that uses the low and high frequency components of an (IPDFG) is a special DFG process that uses the low and high frequency components of an ultra-broadband pump pulse to realize MIR femtosecond emission. In this method, only ultra-broadband pump pulse to realize MIR femtosecond emission. In this method, only an ultra-broadband pump laser with a few-cycle pulse width is needed, which further an ultra-broadband pump laser with a few-cycle pulse width is needed, which further simplifies the DFG process. Table 5 lists the state-of-the-art work that have generated few- simplifies the DFG process. Table 5 lists the state-of-the-art work that have generated cycle MIR laser pulses. few-cycle MIR laser pulses. Table 5. Parameters of selected IPDFG source. Table 5. Parameters of selected IPDFG source. Pump Wavelength IPDFG Spectral Span Conversion Efficiency Nonlinear Crystal Reference Pump Wavelength IPDFG Spectral Span Conversion Efficiency (µm) Nonlinear Crystal (µm) (%) Reference (m) (m) (%) 1.03 LGS 8–11 0.037 [58] 1.03 LGS 8–11 0.037 [58] 1.03 LGS 6.8–16.4 0.11 [59] 1.03 LGS 6.8–16.4 0.11 [59] 1.57 OP-GaP 4–12 0.071 [24] 1.57 OP-GaP 4–12 0.071 [24] 1.9 Ga 1.9 S GaSee 3.7 3.7–18–18 1.41.4 [60[6]0] 2 ZnSe 2.7–20 0.51 [61] 2 ZnSe 2.7–20 0.51 [61] 2 GaSe 4.5–20 0.13 [62] 2 GaSe 4.5–20 0.13 [62] 2.1 AGSe 7–11 0.8 [63] 2.1 AGSe 7–11 0.8 [63] 2.5 GaSe 4.3–17.6 0.22 [64] 2.5 GaSe 4.3–17.6 0.22 [64] 2.5 ZGP 5.8–12.5 3.3 [64] 3 GaSe 6–13.2 5.3 [65] 2.5 ZGP 5.8–12.5 3.3 [64] 3 GaSe 6–13.2 5.3 [65] In 2015, I. Pupeza et al. presented a pioneer work on MIR IPDFG. As shown In 2015, I. Pupeza et. al. presented a pioneer work on MIR IPDFG. As shown in Figure in Figure 15, a Yb:YAG high-average power laser was employed to pump a LGS crys- 15, a Yb:YAG high-average power laser was employed to pump a LGS crystal that had tal that had large bandgap energy. The compact apparatus generated MIR pulses with a large bandgap energy. The compact apparatus generated MIR pulses with a 0.1 W output 0.1 W output power and a spectral coverage of 6.8–16.4 m. A 66 fs pulse width corre- psponding ower and a to sp sub-two ectral cove cycles rage center of 6.8 ed –16 at .4 11.5 μm. A m 66 wavel fs pulse width ength wascorrespondin measured. Thr g to sub- ough a two cycles centered at 11.5 μm wavelength was measured. Through a proof-of-concept Photonics 2021, 8, x FOR PEER REVIEW 13 of 24 demonstration, the MIR IPDFG provided a simple and robust method for generating MIR pulses with high average power and high dynamic range that could for molecular spec- troscopy and hyperspectral imaging applications as well as time-domain coherent control of vibrational dynamics [59]. Based on MIR nonlinear crystals with a larger nonlinear coefficient, such as ZGP, GaSe, and AGSe, a number of research groups have performed MIR IPDFG pumped at an ~2 μm wavelength. C. Gaida et. al, demonstrated a source of coherent MIR radiation with the combination of 150 mW average power and 3.7–18 μm spectral coverage in the molec- ular fingerprint region pumped at 1.9 μm wavelength, as shown in Figure 16 [60]. J. Zhang et. al. demonstrated few-cycle pulse generation by means of the soliton self- compression of the pump pulse in a silica-core photonic-crystal fiber, and subsequently, LWIR generation using IPDFG, resulting in a two-octave-spanning spectrum (−30 dB) from 5 to 20 μm at an average power of 24 mW, as shown in Figure 17 [62]. O. Novak et. al. implemented a MIR IPDFG pumped by a 2 μm OPCPA in a AGSe crystal in 2018 [63], where carrier-envelope phase-stable idler pulses covering the wave- length range of 7–11 μm were achieved, as shown in Figure 18. Photonics 2021, 8, 290 13 of 24 S. Vasilyev et. al. demonstrated efficient generation of coherent long-wave MIR tran- sients using, a compact 2.5 μm Cr:ZnS MOPA laser system as a pump source that could directly produce < 20 fs pulses without additional pulse compression, as shown in Figure proof-of-concept demonstration, the MIR IPDFG provided a simple and robust method for 19 [64]. A ZGP crystal was suited for the generation of an octave-wide spectrum (5.8–12.5 generating MIR pulses with high average power and high dynamic range that could for μm) with an output power of 0.15 W and an optical conversion efficiency of 3% while a molecular spectroscopy and hyperspectral imaging applications as well as time-domain GaSe crystal allowed, with types I and II phase matching, the coverage of a 2 octave spec- coherent control of vibrational dynamics [59]. tral span (4.3–17.6 μm), although with lower output power (13 mW). Figure Figure 15 15.. (( aa )) MIR MIR generatio generationnand and dete detect ctio ionn setup. ( setup. (b b )) Normalized Normalized PSD PSD of ofthe theFourier Fourier transforms transforms for for the EOS tim the EOS time-domain e-domain trace trace of the retrieved fiel of the retrieved field d and and of the NIR probe pulse. of the NIR probe pulse. The dynamic meas- The dynamic urement range, determined as the peak of the signal PSD divided by the average detector noise floor measurement range, determined as the peak of the signal PSD divided by the average detector noise (blue, continuous line), was 2.7 × 10 . The absolute power per comb li 4 ne of the generated MIR radi- floor (blue, continuous line), was 2.7  10 . The absolute power per comb line of the generated MIR ation, obtained by calibrating the normalized PSD of the retrieved MIR power spectrum by the in- radiation, obtained by calibrating the normalized PSD of the retrieved MIR power spectrum by the dependently measured total power and considering the pulse repetition frequency, is shown on the independently measured total power and considering the pulse repetition frequency, is shown on the right axis. Reprinted with permission from [59]. right axis. Reprinted with permission from [59]. Based on MIR nonlinear crystals with a larger nonlinear coefficient, such as ZGP, GaSe, and AGSe, a number of research groups have performed MIR IPDFG pumped at an ~2 m wavelength. C. Gaida et. al, demonstrated a source of coherent MIR radiation with the combination of 150 mW average power and 3.7–18 m spectral coverage in the molecular fingerprint region pumped at 1.9 m wavelength, as shown in Figure 20 [60]. J. Zhang et al. demonstrated few-cycle pulse generation by means of the soliton self-compression of the pump pulse in a silica-core photonic-crystal fiber, and subsequently, LWIR generation using IPDFG, resulting in a two-octave-spanning spectrum (30 dB) from 5 to 20 m at an average power of 24 mW, as shown in Figure 16 [62]. O. Novak et al. implemented a MIR IPDFG pumped by a 2 m OPCPA in a AGSe crys- tal in 2018 [63], where carrier-envelope phase-stable idler pulses covering the wavelength range of 7–11 m were achieved, as shown in Figure 17. S. Vasilyev et al. demonstrated efficient generation of coherent long-wave MIR tran- sients using, a compact 2.5 m Cr:ZnS MOPA laser system as a pump source that could directly produce < 20 fs pulses without additional pulse compression, as shown in Fig- ure 18 [64]. A ZGP crystal was suited for the generation of an octave-wide spectrum (5.8–12.5 m) with an output power of 0.15 W and an optical conversion efficiency of 3% while a GaSe crystal allowed, with types I and II phase matching, the coverage of a 2 octave spectral span (4.3–17.6 m), although with lower output power (13 mW). Photonics 2021, 8, x FOR PEER REVIEW 14 of 24 Figure 16. (a) Schematic experimental setup for IPDFG. (b) Brightness comparison of IPDPG and synchrotron radiation. High-power MIR spectra generated by IPDFG pumped at 2 μm wavelength (this work, red). The brightness of the high-power table-top MIR source in this work exceeds that of large-scale facility synchrotrons, e.g., the Diamond B22 IR beamline [66], by 4 orders of magnitude in the 7.5–15 μm wavelength range. Reprinted with permission from [60]. The pump-to-MIR conversion efficiency of IPDFG could be improved by using a long driving wavelength to reduce the quantum defect. In 2019, H. K. Liang’s group investi- gated a MIR IPDFG pumped at a 3 μm pump wavelength in a GaSe crystal. As shown in Figure 20, the MIR output had a 5 μJ pulse energy and an average power of 50 mW. It spanned Photonics 2021, 8, 290 14 of 24 over a spectral range from 6–13.2 μm, with a record-high conversion efficiency of up to 5.3% [65]. Photonics 2021, 8, x FOR PEER REVIEW 15 of 24 Photonics 2021, 8, x FOR PEER REVIEW Figure 17. (a) Self-compression and MIR generation setup. (b) MIR spectrum and beam profile. The 15 of 24 Figure 16. (a) Self-compression and MIR generation setup. (b) MIR spectrum and beam profile. The MIR spectrum (red line) together with the noise floor (gray line) measured using a monochromator. MIR spectrum (red line) together with the noise floor (gray line) measured using a monochromator. −1 −1 The MIR spectrum extends from 500 cm to 2250 cm (−30 dB), corresponding to the wavelength 1 1 The MIR spectrum extends from 500 cm to 2250 cm (30 dB), corresponding to the wavelength range from 4.5 to 20 μm. Inset: the beam profile measured using a Pyrocam beam profiler. Reprinted range from 4.5 to 20 range from 4.5 to 20μ m. m.Inset Inset: : the beam profi the beam profile le measured us measured using ing a Pyrocam a Pyrocambeam profiler. beam profiler. Reprinted Reprinted with permission from [62]. with permission from [62]. with permission from [62]. Figure 18. Figure 17. Schematic Schematic of of the theexperimental experimental se setup tup fo for IPDFG. r IPDFG NDF . NDF, , neutral neudensity tral density f filter; ilter /2,; half-wave λ/2, half- Figure 18. Schematic of the experimental setup for IPDFG. NDF, neutral density filter; λ/2, half- wave plate at 2020 nm in a rotation mount; L, lens; AGSe, 2 mm thick AGSe crystal; LPF, long-pass wave plate at 2020 nm in a rotation mount; L, lens; AGSe, 2 mm thick AGSe crystal; LPF, long-pass plate at 2020 nm in a rotation mount; L, lens; AGSe, 2 mm thick AGSe crystal; LPF, long-pass filter; filter; P, power meter; OAP, off-axis parabola; FB, MIR fiber; M, MIR monochromator with MCT filter; P, power meter; OAP, off-axis parabola; FB, MIR fiber; M, MIR monochromator with MCT P, power meter; OAP, off-axis parabola; FB, MIR fiber; M, MIR monochromator with MCT detector. detector. Reprinted with permission from [63]. detector. Reprinted with permission from [63]. Reprinted with permission from [63]. Figure 19. (a) Schematic of the IPDFG setup. Normalized IPDFG spectra: (b) Obtained from ZGP at 4.5 W (black line) and Figure 19. (a) Schematic of the IPDFG setup. Normalized IPDFG spectra: (b) Obtained from ZGP at 4.5 W (black line) and Figure 18. (a) Schematic of the IPDFG setup. Normalized IPDFG spectra: (b) Obtained from ZGP at 4.5 W (black line) and 5.9 W (red line) pump. The spectral power density at the peak was 76 μW/nm and 71 μW/nm, respectively. (c) Obtained 5.9 W (red line) pump. The spectral power density at the peak was 76 μW/nm and 71 μW/nm, respectively. (c) Obtained 5.9 W (red line) pump. The spectral power density at the peak was 76 W/nm and 71 W/nm, respectively. (c) Obtained from GaSe for type I and type II phase-matching at 5.9 W pump, with the peak IPDFG spectral density of 3.1 μW/nm and from GaSe for type I and type II phase-matching at 5.9 W pump, with the peak IPDFG spectral density of 3.1 μW/nm and from GaSe for type I and type II phase-matching at 5.9 W pump, with the peak IPDFG spectral density of 3.1 W/nm and 5.2 μW/nm, respectively. Scattered dots show the noise floor. Gray background shows transmission of 1 m standard air. 5.2 μW/nm, respectively. Scattered dots show the noise floor. Gray background shows transmission of 1 m standard air. 5.2 W/nm, respectively. Scattered dots show the noise floor. Gray background shows transmission of 1 m standard air. Reprinted with permission from [64]. Reprinted with permission from [64]. Reprinted with permission from [64]. Figure 20. (a) The schematic of experimental setups. (b) The measured IPDFG spectrum with p- Figure 20. (a) The schematic of experimental setups. (b) The measured IPDFG spectrum with p- polarization at 5 μJ output energy. Reprinted with permission from [65]. polarization at 5 μJ output energy. Reprinted with permission from [65]. Photonics 2021, 8, x FOR PEER REVIEW 15 of 24 range from 4.5 to 20 μm. Inset: the beam profile measured using a Pyrocam beam profiler. Reprinted with permission from [62]. Figure 18. Schematic of the experimental setup for IPDFG. NDF, neutral density filter; λ/2, half- wave plate at 2020 nm in a rotation mount; L, lens; AGSe, 2 mm thick AGSe crystal; LPF, long-pass filter; P, power meter; OAP, off-axis parabola; FB, MIR fiber; M, MIR monochromator with MCT detector. Reprinted with permission from [63]. Photonics 2021, 8, 290 15 of 24 The pump-to-MIR conversion efficiency of IPDFG could be improved by using a long driving wavelength to reduce the quantum defect. In 2019, H. K. Liang’s group Figure 19. (a) Schematic of the IPDFG setup. Normalized IPDFG spectra: (b) Obtained from ZGP at 4.5 W (black line) and investigated a MIR IPDFG pumped at a 3 m pump wavelength in a GaSe crystal. As 5.9 W (red line) pump. The spectral power density at the peak was 76 μW/nm and 71 μW/nm, respectively. (c) Obtained shown in Figure 19, the MIR output had a 5 J pulse energy and an average power of from GaSe for type I and type II phase-matching at 5.9 W pump, with the peak IPDFG spectral density of 3.1 μW/nm and 50 mW. It spanned over a spectral range from 6–13.2 m, with a record-high conversion 5.2 μW/nm, respectively. Scattered dots show the noise floor. Gray background shows transmission of 1 m standard air. efficiency of up to 5.3% [65]. Reprinted with permission from [64]. Photonics 2021, 8, x FOR PEER REVIEW 14 of 24 Figure Figure 20. 19. ((a a) T ) The he schematic of schematic of experimental setups. ( experimental setups. b) The (b) The measured IPD measured F IPDFG G spectrum with p- spectrum with polarization at 5 μJ output energy. Reprinted with permission from [65]. p-polarization at 5 J output energy. Reprinted with permission from [65]. Figure 16. (a) Schematic experimental setup for IPDFG. (b) Brightness comparison of IPDPG and Figure 20. (a) Schematic experimental setup for IPDFG. (b) Brightness comparison of IPDPG and synchrotron radiation. High-power MIR spectra generated by IPDFG pumped at 2 μm wavelength synchrotron radiation. High-power MIR spectra generated by IPDFG pumped at 2 m wavelength (this work, red). The brightness of the high-power table-top MIR source in this work exceeds that of (this work, red). The brightness of the high-power table-top MIR source in this work exceeds that of large-scale facility synchrotrons, e.g., the Diamond B22 IR beamline [66], by 4 orders of magnitude large-scale facility synchrotrons, e.g., the Diamond B22 IR beamline [66], by 4 orders of magnitude in in the 7.5–15 μm wavelength range. Reprinted with permission from [60]. the 7.5–15 m wavelength range. Reprinted with permission from [60]. The pump-to-MIR conversion efficiency of IPDFG could be improved by using a long 4. Single-Cycle MIR Generation driving wavelength to reduce the quantum defect. In 2019, H. K. Liang’s group investi- High energy single- or sub-cycle MIR pulses can provide unique opportunities to gated a MIR IPDFG pumped at a 3 μm pump wavelength in a GaSe crystal. As shown in explore phase-sensitive strong-field light-matter interactions in atoms, molecules, and Figure 20, the MIR output had a 5 μJ pulse energy and an average power of 50 mW. It spanned over a spectral range from 6–13.2 μm, with a record-high conversion efficiency of up to 5.3% [65]. Figure 17. (a) Self-compression and MIR generation setup. (b) MIR spectrum and beam profile. The MIR spectrum (red line) together with the noise floor (gray line) measured using a monochromator. −1 −1 The MIR spectrum extends from 500 cm to 2250 cm (−30 dB), corresponding to the wavelength Photonics 2021, 8, 290 16 of 24 solids. Tremendous efforts have been made to reduce the duration of a laser pulse down to a few-cycle or to the single-cycle regime. Obtaining such an ultrashort laser pulse requires spectral broadening and phase control over the ultrabroad spectrum that supports a single-cycle pulse. At present, four methods have been used to generate single-cycle MIR pulses, namely DFG, four-wave mixing (FWM), OPA, and IPDFG. Table 6 shows several works that have generated single- or sub-cycle MIR pulses. Table 6. Parameters of single- or sub-cycle MIR. Wavelength Repetition Rate Pulse Energy Pulse Width Method Optical Cycle Reference (m) (kHz) (J) (fs) 5–300 1 0.4 46 1 [67] DFG 3–18 1 2 45 1.2 [68] 1.8–4.4 1 1.5 11 1.2 [69] FWM 2–20 1 0.25 7.4 0.57 [70] OPA 2.5–9 1 33 12.4 0.88 [35] 5 6 4–12 1  10 2.5  10 [24] IPDFG 6–18 5  10 0.01 43 1.16 [71] 4.1. MIR Single-Cycle Pulse Generation via DFG In 2010, F. Junginger et al. employed a cascaded OPA and DFG system, which produced a phase-stable single-cycle transients with frequency components of 1–60 THz and a peak field intensity of 12 MV/cm, pumped by a high pulse energy of a regenerative Ti:sapphire amplifier [67]. This amplifier delivered a 5 mJ, 1 kHz pulse, pumping two OPAs both seeded by a shared white-light continuum. The output wavelengths of the two OPAs were 1.28 m and 1.18 m with a pulse energy of 150 J and 360 J, respectively, serving as the input of the MIR DFG. The two broadband outputs from two OPAs were mixed in a type-I GaSe crystal. The DFG output was a single-cycle MIR pulse with a 46 fs pulse width, with its spectrum centered at the 22 THz. In 2015, A. A. Lanin et al. presented a MIR transient centered at a wavelength of 7.9 m with the pulse width of 45 fs (~1.2 cycle) and the spectrum ranged from 3–18 m at 1 kHz repetition rate [68]. As shown in Figure 21, first, a seed produced by supercontinuum in a sapphire plate, which was driven by 810 nm, 0.8 mJ, 65 fs, 1 kHz pulses delivered by a Ti:sappire laser. This seed was sent to an OPA with a BBO crystal, generating tunable signal and idler pulses that ranged from 1150–1580 nm and 1620–2300 nm, respectively. In the second step, the signal and idler from the OPA were used for DFG in an AGS crystal, producing a MIR pulse with a pulse duration of 150 fs and pulse energy of ~2 J at the central wavelength of 7.9 m. After that, the MIR radiation underwent spectral broadening and self-compression in a 5 mm GaAs plate with high nonlinearity, leading to a spectrum covering 3–18 m, and the pulse width compressed into 45 fs. In 2016, A. A. Lanin and his group repeated the experiment and changed the centre wavelength of the DFG output to 6.8 m, the spectrum of which hit the point of zero group-velocity dispersion (GVD). Using this method, even more efficient spectral broadening and self-compression could take place, which generated a source of sub-cycle pulse with 1 J pulse energy [72]. In 2017, P. Krogen et al. used the adiabatic DFG (ADFG) to transfer the near-infrared frequencies to the MIR spectrum and produce a single-cycle MIR pulse [69]. The ADFG system consisted of an octave spanning Ti:sapphire oscillator seed source, which was used to seed a 1 kHz Nd:YLF CPA, and a 2-stage OPCPA system pumped by the same Nd:YLF amplifier. The OPCPA system used 2 grism pairs and an acousto-optic programmable dispersive filters to chirp the near-IR pulses to an approximately 3 ps duration for efficient amplification by the second harmonic of the Nd:YLF laser. The resulting chirped near- infrared pulses were down-converted to the MIR using chirped pulse difference frequency generation with the narrowband 1047 nm output of the Nd:YLF amplifier using the ADFG crystal to generate chirped pulses in the MIR. Finally, these were compressed in a 21-mm thick silicon block back to their transform-limited duration. The MIR output spanning Photonics 2021, 8, x FOR PEER REVIEW 17 of 24 Photonics 2021, 8, x FOR PEER REVIEW 17 of 24 amplification by the second harmonic of the Nd:YLF laser. The resulting chirped near- amplification by the second harmonic of the Nd:YLF laser. The resulting chirped near- infrared pulses were down-converted to the MIR using chirped pulse difference fre- infrared pulses were down-converted to the MIR using chirped pulse difference fre- quency generation with the narrowband 1047 nm output of the Nd:YLF amplifier using Photonics 2021, 8, 290 17 of 24 quency generation with the narrowband 1047 nm output of the Nd:YLF amplifier using the ADFG crystal to generate chirped pulses in the MIR. Finally, these were compressed the ADFG crystal to generate chirped pulses in the MIR. Finally, these were compressed in a 21-mm thick silicon block back to their transform-limited duration. The MIR output in a 21-mm thick silicon block back to their transform-limited duration. The MIR output spanning 1.8–4.4 μm at −10 dB from the peak, with a pulse duration of 10.7 fs (1.2 optical 1.8–4.4 m at 10 dB from the peak, with a pulse duration of 10.7 fs (1.2 optical cycles), spanning 1.8–4.4 μm at −10 dB from the peak, with a pulse duration of 10.7 fs (1.2 optical cycles), and a pulse energy of 1 μJ at 1 kHz repetition rate are shown in Figure 22. and a pulse energy of 1 J at 1 kHz repetition rate are shown in Figure 22. cycles), and a pulse energy of 1 μJ at 1 kHz repetition rate are shown in Figure 22. Figure 21. Experimental setup. Ti:S, mode-locked Ti:sapphire master oscillator; MPA, multipass am- Figure 21. Experimental setup. Ti:S, mode-locked Ti:sapphire master oscillator; MPA, multipass Figure 21. Experimental setup. Ti:S, mode-locked Ti:sapphire master oscillator; MPA, multipass am- plifier; OPA, optical parametric amplifier; AGS, AgGaS2 crystal; LPF, longpass filter; PM, parabolic amplifier; OPA, optical parametric amplifier; AGS, AgGaS crystal; LPF, longpass filter; PM, parabolic plifier; OPA, optical parametric amplifier; AGS, AgGaS2 crystal; LPF, longpass filter; PM, parabolic mirror; L, BK7 glass lens; OD, optical delay line; PMH, parabolic mirror with a hole; FWM, four- mirror; L, BK7 glass lens; OD, optical delay line; PMH, parabolic mirror with a hole; FWM, four- mirror; L, BK7 glass lens; OD, optical delay line; PMH, parabolic mirror with a hole; FWM, four- wave mixing in a gas medium; SPF, shortpass filter; Spec, spectrometer. Reprinted with permission wave mixing in a gas medium; SPF, shortpass filter; Spec, spectrometer. Reprinted with permission wave mixing in a gas medium; SPF, shortpass filter; Spec, spectrometer. Reprinted with permission from [68]. from [68]. from [68]. Figure 22. (a) A chirped adiabatic frequency conversion scheme employed the uncompressed broad- Figure Figure 22. 22.(a() aA ) chirped A chirped adiaba adiabatic tic frequ frequency ency conversion conversion schem scheme e employed t employed he uncompressed broad- the uncompressed band output of a near-IR OPCPA mixed with a longer-wavelength picosecond pump pulse in a band output of a near-IR OPCPA mixed with a longer-wavelength picosecond pump pulse in a periodica broadband lly po output led qu ofasi-phas a near-IR e-matching grating, OPCPA mixed with a bu a longer lk sili -wavelength con post-com picosecond pressor and pump a frequ pulse ency in-a periodically poled quasi-phase-matching grating, a bulk silicon post-compressor and a frequency- resolved optical gating (FROG) characterization device. Measurement of a single-cycle MIR pulse. periodically poled quasi-phase-matching grating, a bulk silicon post-compressor and a frequency- resolved optical gating (FROG) characterization device. Measurement of a single-cycle MIR pulse. (b) Retrieved pulse as a function of time (blue, temporal intensity; gray, temporal phase) showing a resolved optical gating (FROG) characterization device. Measurement of a single-cycle MIR pulse. (b) Retrieved pulse as a function of time (blue, temporal intensity; gray, temporal phase) showing a 10.7-fs full-width at half-maximum (FWHM) pulse duration (1.2 optical cycles at the central wave- (b) Retrieved pulse as a function of time (blue, temporal intensity; gray, temporal phase) showing 10.7-fs full-width at half-maximum (FWHM) pulse duration (1.2 optical cycles at the central wave- length of 2.8 μm), which is 1.15 times the transform-limited duration. (c) Spectral intensity (red, a 10.7-fs full-width at half-maximum (FWHM) pulse duration (1.2 optical cycles at the central length of 2.8 μm), which is 1.15 times the transform-limited duration. (c) Spectral intensity (red, measured; blue, retrieved) and retrieved spectral phase (gray). Reprinted with permission from [69]. wavelength of 2.8 m), which is 1.15 times the transform-limited duration. (c) Spectral intensity (red, measured; blue, retrieved) and retrieved spectral phase (gray). Reprinted with permission from [69]. measured; blue, retrieved) and retrieved spectral phase (gray). Reprinted with permission from [69]. 4.2. MIR Single-Cycle Pulse Generation via FWM 4.2. MIR Single-Cycle Pulse Generation via FWM 4.2. MIR Single-Cycle Pulse Generation via FWM In 2012, Y. Nomura et. al. investigated MIR sub-cycle pulse generation via FWM. The In 2012, Y. Nomura et. al. investigated MIR sub-cycle pulse generation via FWM. The fundamental mode and second-harmonic pulses of a 25 fs Ti:sappire amplifier output In 2012, Y. Nomura et al. investigated MIR sub-cycle pulse generation via FWM. The fundamental mode and second-harmonic pulses of a 25 fs Ti:sappire amplifier output with the wavelength of 800 nm and energy of 0.9 mJ at 1 kHz were focused into argon gas, fundamental mode and second-harmonic pulses of a 25 fs Ti:sappire amplifier output with with the wavelength of 800 nm and energy of 0.9 mJ at 1 kHz were focused into argon gas, producing a phase-stable sub-cycle MIR pulse through FWM assisted by filament [70]. As the wavelength of 800 nm and energy of 0.9 mJ at 1 kHz were focused into argon gas, producing a phase-stable sub-cycle MIR pulse through FWM assisted by filament [70]. As producing a phase-stable sub-cycle MIR pulse through FWM assisted by filament [70]. As shown in Figure 23, a phase-stable 250 nJ, 7.4 fs (0.57 cycles) MIR conical emission centered at 3.9 m with its spectrum coverage of 2–20 m was created. Photonics 2021, 8, x FOR PEER REVIEW 18 of 24 Photonics 2021, 8, x FOR PEER REVIEW 18 of 24 Photonics 2021, 8, 290 18 of 24 shown in Figure 23, a phase-sTable 250 nJ, 7.4 fs (0.57 cycles) MIR conical emission cen- shown in Figure 23, a phase-sTable 250 nJ, 7.4 fs (0.57 cycles) MIR conical emission cen- tered at 3.9 μm with its spectrum coverage of 2–20 μm was created. tered at 3.9 μm with its spectrum coverage of 2–20 μm was created. Figure 23. (a) Experimental and (b) retrieved XFROG traces. The retrieved pulse in (c) time and Figure 23. (a) Experimental and (b) retrieved XFROG traces. The retrieved pulse in (c) time and (d) (d) frequency domain. The spectrum measured with Fourier transform spectrometer (brown solid frequency domain. The Figure 23. (a) Experimental and ( spectrum measured with b) retrieved XFROG Fourier transform spectr traces. The retrieved ometer (brown sol pulse in (c) time id and (d) cu curve) frequency domain. The rve) is is al also so sshown. hown. Reprinted with Reprinted spectrum measured with with permis permission sion from [70] from Fourier transform spectr [70.]. ometer (brown solid curve) is also shown. Reprinted with permission from [70]. 4.3. MIR Single-Cycle Pulse Generation via OPA 4.3. MIR Single-Cycle Pulse Generation via OPA In 2017, H. K. Liang et al. generated a MIR sub-cycle pulse from an OPA with the 4.3. MIR Single-Cycle Pulse Generation via OPA In 2017, H. K. Liang et. al. generated a MIR sub-cycle pulse from an OPA with the signal and idler pulses at 3.2 m and 6.4 m [35]. It was demonstrated that with the stable In 2017, H. K. Liang et. al. generated a MIR sub-cycle pulse from an OPA with the signal and idler pulses at 3.2 μm and 6.4 μm [35]. It was demonstrated that with the stable carrier-envelope phase for both the signal and idler pulses and the careful control of the signal and idler pulses at 3.2 μm and 6.4 μm [35]. It was demonstrated that with the stable carrier-envelope phase for both the signal and idler pulses and the careful control of the relative delay, the signal and idler pulses were synthesized without extra coherent control. carrier-envelope phase for both the signal and idler pulses and the careful control of the relative delay, the signal and idler pulses were synthesized without extra coherent control. As shown in Figure 24, the synthesized pulse had a spectral coverage from 2.5 to 9.0 m, relative delay, the signal and idler pulses were synthesized without extra coherent control. As shown in Figure 24, the synthesized pulse had a spectral coverage from 2.5 to 9.0 μm, and and a pulse width of 12.4 fs which corresponded to 0.88 cycles for a central wavelength As shown in Figure 24, the synthesized pulse had a spectral coverage from 2.5 to 9.0 μm, and a pulse width of 12.4 fs which corresponded to 0.88 cycles for a central wavelength of 4.2 μm. of 4.2 m. a pulse width of 12.4 fs which corresponded to 0.88 cycles for a central wavelength of 4.2 μm. Figure 24. Temporal characterisation of the synthesised MIR pulse. The retrieved spectral (a) and temporal ( Figure 24. b) in Temporal characterisation of the tensity profiles of the synthesised pulse. synthesised M The dotted curves IR pulse. The re are the retrie trieved spectra ved phase l (a) and . Figure 24. Temporal characterisation of the synthesised MIR pulse. The retrieved spectral (a) and Pulse width (12.4 fs) at full width at half maximum is measured with a centre wavelength at 4.2 μm. temporal (b) intensity profiles of the synthesised pulse. The dotted curves are the retrieved phase. temporal (b) intensity profiles of the synthesised pulse. The dotted curves are the retrieved phase. It c Pulse orresponds width (12 to 0.88 .4 fs opti ) at full width at ha cal cycle. Reprinted with permission from [35]. lf maximum is measured with a centre wavelength at 4.2 μm. Pulse width (12.4 fs) at full width at half maximum is measured with a centre wavelength at 4.2 m. It corresponds to 0.88 optical cycle. Reprinted with permission from [35]. It corresponds to 0.88 optical cycle. Reprinted with permission from [35]. 4.4. MIR Single-Cycle Pulse Generation via IPDFG 4.4. MIR Single-Cycle Pulse Generation via IPDFG 4.4. MIR Single-Cycle Pulse Generation via IPDFG In 2018, H. Timmers et. al. presented a scheme for generating super-octave spanning In 2018, H. Timmers et. al. presented a scheme for generating super-octave spanning MIR frequenc In 2018, y combs with a band H. Timmers et al. presented width sp aannin scheme g fro for mgenerating 4 to 12 μm super through IPDFG in a -octave spanning n MIR frequency combs with a bandwidth spanning from 4 to 12 μm through IPDFG in an OP-Ga MIR frP equency crystal driven combs by with a a few-c bandwidth ycle Er-pump spanning infr fr aom struct 4 to ure 12 [28 m ]. As through shown IPDFG in Figure in an OP-GaP OP-GaP crystal crystadriven l driven by by a few-cycle a few-cycle Er-pump Er-pump infrastr infructur astruct e [u 28 re ]. [As 28]. As shown shown in Figu inr Fi e 25 gure a, 25a, the output of an Er mode-locked laser was amplified using an Er-doped fiber ampli- the 25a, the o outputu of tput of an Eran Er m mode-locked ode-loc laser ked laser was amplified was ampliusing fied us an ing an Er-doped fibe Er-doped fiber amplifier r ampl.i- fier. The amplified pulses then undergo nonlinear broadening in a nonlinear fiber and were compre The fieramplified . The amp ssed to pulses lified p a few then ulses -cycle pulse under then undergo go width se nonlinear nornline v br inoadening ga ar br s thoaden e pu in ming in a p o nonlinear f IPD a nonl FG. M fiber ine IRa rr f and adib iaer and wer tione compr were compre essed to ssed to a few-cycle a few-cycle pulse pulse width width se serving rvas ingthe as tpump he pum of p o IPDFG. f IPDFG MIR . MIR radiation radiation spanning from 4 to 12 μm was generated from an OP-GaP crystal. Subsequently, in 2019, spanning spanning fr fro om m 4 4 to to 12 12 μ m m was was generated generated from from an an OP-GaP OP-GaP crystal. crystal.Subsequently Subsequently , in , in 2019, 2019, from the same group, A. S. Kowligy et. al. measured the temporal profile of the MIR pulse from the same group, A. S. Kowligy et al. measured the temporal profile of the MIR pulse from the same group, A. S. Kowligy et. al. measured the temporal profile of the MIR pulse via electro-optical sampling using an ultra-short near-infrared reference pulse. A 1.2-cycle MIR pulse oscillating at a 7.6 m centre wavelength was obtained [73]. Photonics 2021, 8, x FOR PEER REVIEW 19 of 24 Photonics 2021, 8, 290 19 of 24 via electro-optical sampling using an ultra-short near-infrared reference pulse. A 1.2-cycle Photonics 2021, 8, x FOR PEER REVIEW 19 of 24 MIR pulse oscillating at a 7.6 μm centre wavelength was obtained [73]. via electro-optical sampling using an ultra-short near-infrared reference pulse. A 1.2-cycle MIR pulse oscillating at a 7.6 μm centre wavelength was obtained [73]. Figure 25. (a) Experimental layout for IPDFG comb generation. (b) The spectrum of this few-cycle Figure 25. (a) Experimental layout for IPDFG comb generation. (b) The spectrum of this few-cycle driver. The inset of (b) displays the measured intensity profile of the pump pulse, corresponding driver. The inset of (b) displays the measured intensity profile of the pump pulse, corresponding Figure 25. (a) Experimental layout for IPDFG comb generation. (b) The spectrum of this few-cycle to a pulse duration of 10.6 fs. (c) Super-octave longwave infrared (LWIR) spectra containing up to to a pulse duration of 10.6 fs. (c) Super-octave longwave infrared (LWIR) spectra containing up to driver. The inset of (b) displays the measured intensity profile of the pump pulse, corresponding 0.25 mW of power. Reprinted with permission from [24]. 0.25 mW of power. Reprinted with permission from [24]. to a pulse duration of 10.6 fs. (c) Super-octave longwave infrared (LWIR) spectra containing up to 0.25 mW of power. Reprinted with permission from [24]. In 2019, T. P. Butler et. al. reported a phase-stable source with watt-scale average In 2019, T. P. Butler et al. reported a phase-stable source with watt-scale average power and broad bandwidth (6–18 μm) via the IPDFG in GaSe crystal pumped by thu- power In 2 and 019 br , T. oad P. But bandwidth ler et. al(6–18 . reported m)a p viahthe ase-IPDFG stable so in urce w GaSeitcrystal h watt-sc pumped ale averby age thulium- lium-doped fiber-laser system at 2 μm [71]. The 2 μm pump pulses from the fiber chirped- power and broad bandwidth (6–18 μm) via the IPDFG in GaSe crystal pumped by thu- doped fiber-laser system at 2 m [71]. The 2 m pump pulses from the fiber chirped-pulse p lium ulse -doped amplif fiber-lase ier had r syst an ave em at 2 rage pμm o [ wer 71]. The 2 of 100 μW an m pump pulse d were com s from t preh ssed b e fibery chirped- grating pairs to amplifier had an average power of 100 W and were compressed by grating pairs to the pulse amplifier had an average power of 100 W and were compressed by grating pairs to the output of 40 W. The compressed 2 μm output was then divided into two parts and output of 40 W. The compressed 2 m output was then divided into two parts and further the output of 40 W. The compressed 2 μm output was then divided into two parts and further compressed nonlinearly through photonic crystal fibers to 13 fs with 4.5 W power compressed nonlinearly through photonic crystal fibers to 13 fs with 4.5 W power and 32 fs further compressed nonlinearly through photonic crystal fibers to 13 fs with 4.5 W power and 32 fs with a 30 W power, respectively. The 32 fs pulse was focused into a 1 mm GaSe with a 30 W power, respectively. The 32 fs pulse was focused into a 1 mm GaSe crystal for and 32 fs with a 30 W power, respectively. The 32 fs pulse was focused into a 1 mm GaSe crystal for MIR IPDFG. As shown in Figure 26, after a 6 mm of bulk germanium for com- MIR IPDFG. As shown in Figure 26, after a 6 mm of bulk germanium for compression, a crystal for MIR IPDFG. As shown in Figure 26, after a 6 mm of bulk germanium for com- pression, a MIR transient with a broad spectral range (6–18 μm) and ultra-short pulse MIR transient with a broad spectral range (6–18 m) and ultra-short pulse duration of 43 fs pression, a MIR transient with a broad spectral range (6–18 μm) and ultra-short pulse duration of 43 fs (~1 cycle) at 50 MHz was characterized through electro-optical sampling (~1 cycle) at 50 MHz was characterized through electro-optical sampling with the help of duration of 43 fs (~1 cycle) at 50 MHz was characterized through electro-optical sampling the with the hel 13 fs near p -infrar of the ed 13r fs efer ne ence. ar-infrared reference. with the help of the 13 fs near-infrared reference. Figure 26. (a) Measured (left)and retrieved (right) FROG spectrograms of the longer pulse PCF com- Figure 26. (a) Measured (left)and retrieved (right) FROG spectrograms of the longer pulse PCF pression channel. (b) Retrieved FROG temporal intensity and phase. (c) Retrieved FROG spectral compression channel. (b) Retrieved FROG temporal intensity and phase. (c) Retrieved FROG spectral Figure 26. (a) Measured (left)and retrieved (right) FROG spectrograms of the longer pulse PCF com- intensity and phase compared to independently measured spectrum obtained using a NIR grating pression channel. (b) Retrieved FROG temporal intensity and phase. (c) Retrieved FROG spectral spectrometer. (d–f) shows the same information as (a–c), this time measured for the shorter pulse PCF channel. Reprinted with permission from [71]. Photonics 2021, 8, 290 20 of 24 5. Prospects of High-Power Broad-Band Few-Cycle MIR Lasers The development of high-power broadband few-cycle MIR lasers has been driven by a number of applications in the field of strong-field physics, high-fidelity molecule detection, and cold tissue ablation applications. In strong-field physics, high-order harmonic genera- tion (HHG) with excellent spatial coherence is probably one of the biggest driving forces of strong MIR OPCPA. Extreme ultra-violet harmonics with the photon energy exceeding the water absorption window have been generated via the MIR OPCPA pump [17]. The famous 3.9 m OPCPA has enabled the generation of soft X-ray HHG with the photon energy extended to 1500 eV. Besides HHG, femtosecond hard X-ray covering tens of keV photon energy has been excited through plasma generation in a metallic target, pumped by the MIR OPCPA [13]. Attosecond pulse generation is another main application of MIR OPCPA in the field of strong-field physics. A total of 40–50 as isolated attosecond pulses have been generated pumped by 1.8 m OPCPA/OPA [1]. Moreover, terahertz generation with a high conversion efficiency of 2.36% has been achieved via MIR two-colour filamentation in air pumped by MIR OPCPA centred at 3.9 m. Besides the peak power of the MIR OPCPA system, which sets the threshold and cut-off of the aforementioned strong-field applications, the average power is another important parameter to pursue, which accounts for photon flux. Therefore, it is suggested high-average power MIR OPCPAs with decent pulse energy be the next research phase focus serving as the enablers to reveal a more uncharted continent in the field of strong-field physics. Molecule detection is another important application of MIR OPCPA. Laser filamen- tation in air pumped by energetic MIR OPCPAs has been realized at the 3.9 and 2 m wavelengths [74,75]. Stand-off detection of ambient air molecules such as CO have been demonstrated via air filamentation pumped using MIR OPCPA [22]. By using the atmo- spheric transparent windows in the MIR wavelength region, namely the 2–5.5 m and 8–14 m bands, more molecules in the air could be detected with MIR OPCPAs, especially at longer wavelengths such as 5 and 9 m. In addition, dual-frequency combs (DFCs) based on broadband MIR lasers have been developed for sensitive and precision molecule sensing. With DFCs at the 3–5 m wavelength range, the detection of molecular species in 13 18 17 15 34 a gas mixture, including isotopologues containing isotopes such as C, O, O, N, S, S, and deuterium, with part-per-billion sensitivity and sub-Doppler resolution has been demonstrated [23]. At longer wavelengths covering 4 to 12 m, DFCs have enabled the high-precision vapor detection of methanol and ethanol. With the high-average power of the MIR broadband laser sources at a 100 MHz repetition rate, a good signal-to-noise ratio 1/2 (67 Hz ) has been achieved with a sub-ms acquisition time. Besides strong-field physics and spectroscopic applications, high-power, broadband, few-cycle lasers have also been used in minimally invasive surgery. However, limited by the available femtosecond laser wavelengths, its current applications in bio-medical micro processing/surgery are only limited to cataract surgery [76,77] and myopia correction surgery [78–80]. An MIR wavelength at 3–10 m coincides with strong molecular resonant peaks, which results in strong and sharp absorption peaks for various molecules. The strong absorption resonances of water, protein, and lipids have been investigated using MIR femtosecond laser exposure from a free-electron laser facility in ocular, brain, and dermis tissues. A new mechanism for tissue ablation was proposed. It was found that when a MIR femtosecond laser at the 6.2–6.7 m wavelength is chosen, the laser output power is absorbed by both water and proteins. Reaching ~60 C, collagen undergoes structural transitions from highly ordered arrays to amorphous gelatin with less resilience, which enables better tissue ablation efficiency and less lateral damages. With the emerging and development of high-power MIR femtosecond lasers at more flexible wavelengths, we foresee promising prospects for watt-level MIR femtosecond lasers in the soft and hard tissue cold ablation applications. Photonics 2021, 8, 290 21 of 24 6. Conclusions In this paper, we have shown that the field of high-energy, high-power, few-cycle MIR lasers has experienced rapid development over the last 10 years that has been driven by the demands of strong-field physics (such as in HHG, attosecond generation, and terahertz generation experiments), high-fidelity molecule detection (such as in MIR dual- comb spectroscopy), and tissue cold ablation. Possible trends for the next phase would include increasing the high average power (high repetition rate), for example, by a few to a few tens watts of MIR few-cycle pulses with mJ pulse energy output, would be highly desired to boost the photon flux, enhance the signal-to-noise ratio, and reduce detection time. This puts requirements and challenges in place to improve on both the high-power pump source and techniques to raise the parametric conversion efficiency, for example, by using a quasi-parametric amplifier [81,82], cascaded extraction from OPA [83], or a flat-top pumped parametric process. Author Contributions: Writing—original draft preparation, K.T., L.H. and X.Y.; writing—review and editing, H.L. All authors have read and agreed to the published version of the manuscript. Funding: National Natural Science Foundation of China (62075144) and the Engineering Featured Team Fund of Sichuan University (2020SCUNG105). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: No new data were created or analyzed in this study. Data sharing is not applicable to this article. Conflicts of Interest: The authors declare no conflict of interest. References 1. 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Journal

PhotonicsMultidisciplinary Digital Publishing Institute

Published: Jul 21, 2021

Keywords: mid-infrared; few-cycle pulse; optical parametric amplification; optical parametric chirped-pulse amplification; intra-pulse difference-frequency generation

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