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Generation of Ultrabroad and Intense Supercontinuum in Mixed Multiple Thin Plates

Generation of Ultrabroad and Intense Supercontinuum in Mixed Multiple Thin Plates hv photonics Communication Generation of Ultrabroad and Intense Supercontinuum in Mixed Multiple Thin Plates Jing Li, Wenjiang Tan *, Jinhai Si, Zhen Kang and Xun Hou Key Laboratory for Physical Electronics and Devices of the Ministry of Education, Shaanxi Key Lab of Information Photonic Technique, School of Electronic Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China; lijing525@stu.xjtu.edu.cn (J.L.); jinhaisi@mail.xjtu.edu.cn (J.S.); kangzhen@stu.xjtu.edu.cn (Z.K.); houxun@mail.xjtu.edu.cn (X.H.) * Correspondence: tanwenjiang@mail.xjtu.edu.cn Abstract: Supercontinuum (SC) generation using multiple thin plates is demonstrated with a fem- tosecond laser pulse. We propose an improved technique to obtain larger spectrum broadening and higher spectral intensity by employing mixed multiple thin plates with different thicknesses and materials. Furthermore, the spectrum has good stability, which is superior to that of the spectrum induced by the traditional single filament in bulk material. Our approach offers a route towards simple and stable SC generation for potential applications. Keywords: femtosecond laser; supercontinuum generation; spectral broadening; multiple thin plates 1. Introduction SC generation is a universal phenomenon produced by the nonlinear propagation of intense femtosecond laser pulses in a transparent medium [1]. SC is characterized by an extremely broad spectrum with frequencies ranging from infrared to ultraviolet. The underlying physics of filament-induced SC generation appears to be a complex process Citation: Li, J.; Tan, W.; Si, J.; Kang, Z.; Hou, X. Generation of Ultrabroad that involves a dynamic interplay of spatial and temporal effects: self-focusing, self-phase and Intense Supercontinuum in modulation, self-steepening, group velocity dispersion, and multiphoton absorption [2–6]. Mixed Multiple Thin Plates. Photonics SC has attracted significant research interest in diverse fields owing to its advantages of 2021, 8, 311. https://doi.org/ ultrabroadband radiation, high spectral brightness, and high spatial coherence, which 10.3390/photonics8080311 are equivalent to those of white-light lasers. It has found wide application in optical frequency combs [7], biomedical imaging [8], molecular fingerprint spectroscopy [9], and Received: 21 June 2021 the generation of few-cycle femtosecond pulses [10]. Accepted: 27 July 2021 In recent years, there have been many ways to generate an ultrabroadband SC. For Published: 3 August 2021 instance, a femtosecond laser pulse is injected into inert-gas-filled hollow-core fibers. This method could enable the compression of pulses with high compression ratios and better Publisher’s Note: MDPI stays neutral spatial modes [11,12]. However, it has some inherent drawbacks, such as low efficiency of with regard to jurisdictional claims in fiber coupling and complexity of adjustment of the experimental apparatus. In addition, published maps and institutional affil- SC can be efficiently generated in a solid-state material due to its higher nonlinear index iations. of refraction. It is a simple and robust method to broaden the spectrum. However, when the peak power of the laser pulse is sufficiently high, self-focusing and multiphoton ionization result in optical breakdown and permanent damage to the material. More recently, a novel technique that generates SCs with multiple thin plates has attracted Copyright: © 2021 by the authors. much attention [13–16]. This technique effectively circumvents energy loss and optical Licensee MDPI, Basel, Switzerland. breakdown. It has been reported in nearly one octave-spanning spectrum ranging and a This article is an open access article few-millijoule pulse energy [17,18]. Different applications place differing demands on the distributed under the terms and characteristics of the SC spectrum. For example, the spectral coherence and a broad spectral conditions of the Creative Commons range are the key factors in pulse compression. In the application of femtosecond transient Attribution (CC BY) license (https:// absorption spectroscopy, high intensity of shortwave components will provide a broad creativecommons.org/licenses/by/ spectral detection range, and better spectral stability will improve the detection sensitivity. 4.0/). Photonics 2021, 8, 311. https://doi.org/10.3390/photonics8080311 https://www.mdpi.com/journal/photonics Photonics 2021, 8, x FOR PEER REVIEW 2 of 8 transient absorption spectroscopy, high intensity of shortwave components will provide a broad spectral detection range, and better spectral stability will improve the detection sensitivity. In addition to supercontinuum generation, the use of thin plates makes it pos- sible to perform more complex manipulations with high-energy ultrashort pulses. A new concept of using thin plates was proposed for spectral broadening with subsequent tem- poral compression of ultrashort pulses having energy as high as a few hundred Joules [19]. An experiment was reported in which the successful compression of high-energy laser pulses was achieved [20]. The measured spectrum broadening in thin plates together with the fundamental spectrum also allows the reconstruction of the pulse intensity and phase [21]. In this paper, we experimentally investigated the generation of SC by mixing multi- Photonics 2021, 8, 311 2 of 8 ple thin plates. The broadening and intensity of the SC spectrum in the short-wavelength region was further enhanced through a set of multiple thin plates with different thick- In addition to supercontinuum generation, the use of thin plates makes it possible to nesses and materials. perform more complex manipulations with high-energy ultrashort pulses. A new concept of using thin plates was proposed for spectral broadening with subsequent temporal compression of ultrashort pulses having energy as high as a few hundred Joules [19]. An 2. Experiments experiment was reported in which the successful compression of high-energy laser pulses was achieved [20]. The measured spectrum broadening in thin plates together with the The experimental apparatus of SC generation in multiple thin plates is illustrated in fundamental spectrum also allows the reconstruction of the pulse intensity and phase [21]. Figure 1. A Ti:sapphire femtosecond laser system operating with a pulse duration of 50 fs In this paper, we experimentally investigated the generation of SC by mixing multiple and central wavelength of 800 nm at a repetition rate of 1 kHz was used in our experiment. thin plates. The broadening and intensity of the SC spectrum in the short-wavelength region was further enhanced through a set of multiple thin plates with different thicknesses A neutral density filter placed in the beginning was used to control the average power of and materials. the laser beam to obtain a suitable energy per pulse of 520 µJ. The beam diameter was 2. Experiments approximately 10 mm. The output pulse was loosely focused onto a set of fused silica thin The experimental apparatus of SC generation in multiple thin plates is illustrated in plates with a convex lens (f = 2000 mm). The diameter of the focal point was approximately Figure 1. A Ti:sapphire femtosecond laser system operating with a pulse duration of 50 fs 41 and 0 µm central at wavelength the 1/eof width 800 nm atmeasured by a repetition rate of the knife 1 kHz was used -ed ingour e method. The peak inte experiment. nsity was A neutral density filter placed in the beginning was used to control the average power 12 2 approximately 7.9 × 10 W/cm at the focal point. The fused silica thin plates (Beijing of the laser beam to obtain a suitable energy per pulse of 520 J. The beam diameter was Zhong Cheng Quartz Glass Co., Ltd., BJ, CHN) used in our paper were squares with sides approximately 10 mm. The output pulse was loosely focused onto a set of fused silica thin plates with a convex lens (f = 2000 mm). The diameter of the focal point was approximately of 20 mm and thicknesses of 120 µm and 200 µm. These plates were placed at the waist of 410 m at the 1/e width measured by the knife-edge method. The peak intensity was the beam at the Brew 12 ster 2angle (55.5°) to reduce the Fresnel reflection loss at the surfaces. approximately 7.9  10 W/cm at the focal point. The fused silica thin plates (Beijing Zhong Cheng Quartz Glass Co., Ltd., Beijing, China) used in our paper were squares The generated SC spectrum was collected by a lens on a fiber-coupled spectrometer with sides of 20 mm and thicknesses of 120 m and 200 m. These plates were placed at (Ocean Optics HR 4000). A filter was used in front of the spectrometer to suppress the the waist of the beam at the Brewster angle (55.5 ) to reduce the Fresnel reflection loss at the surfaces. The generated SC spectrum was collected by a lens on a fiber-coupled fundamental frequency light at a center wavelength of 800 nm. spectrometer (Ocean Optics HR 4000). A filter was used in front of the spectrometer to suppress the fundamental frequency light at a center wavelength of 800 nm. Figure 1. Schematic of the experimental setup (NDF: neutral density filter). Figure 1. Schematic of the experimental setup (NDF: neutral density filter). 3. Results and Discussion Firstly, seven 120 m thin fused silica plates were applied for SC generation. The first thin plate was placed 20 cm before the focus of the lens, and the average power was 3. Results and Discussion approximately 489 mW after the plate. The spacing between these thin plates was 8 cm, Firstly, seven 120 µm thin fused silica plates were applied for SC generation. The first 10 cm, 5 cm, 2.5 cm, 2.5 cm, 2.5 cm, respectively. The positions of the plates were determined by obtaining the broadest transmitted spectrum after each plate without optical damage. thin plate was placed 20 cm before the focus of the lens, and the average power was ap- The thin plates should be placed in sequence, resulting in the best spectrum broadening. As proximately 489 mW after the plate. The spacing between these thin plates was 8 cm, 10 the laser passes through the thin plate, the self-focusing effect causes the laser beam to form a focal point in the air. It should be noted that the next thin plate should be placed away cm, 5 cm, 2.5 cm, 2.5 cm, 2.5 cm, respectively. The positions of the plates were determined from the focal point to effectively avoid material damage. The beam diameter was increased by obtaining the broadest transmitted spectrum after each plate without optical damage. after the seventh plate, leading to a reduction in peak power. When more 120-m-thin fused The thin plates should be placed in sequence, resulting in the best spectrum broadening. silica plates were added, the width of the spectrum ceased to broaden. The average power after the second to seventh plates was 480 mW, 473 mW, 458 mW, 448 mW, 437 mW, and As the laser passes through the thin plate, the self-focusing effect causes the laser beam to 428 mW, respectively. To understand the process of spectrum broadening, the fundamental form a focal point in the air. It should be noted that the next thin plate should be placed spectrum and the SC spectrum after each plate were recorded, as shown in Figure 2. It was observed that the broadened spectra in the first three plates were symmetric to away from the focal point to effectively avoid material damage. The beam diameter was the fundamental spectrum, which coincided with the process of self-phase modulation. increased after the seventh plate, leading to a reduction in peak power. When more 120- However, the spectra were significantly broadened from the fourth to seventh thin plates, as shown in Figure 2b. The primary mechanism responsible for spectral broadening was µm-thin fused silica plates were added, the width of the spectrum ceased to broaden. The the nonlinear phase accumulation pass through the plate, leading to pulse self-steepening. average power after the second to seventh plates was 480 mW, 473 mW, 458 mW, 448 mW, 437 mW, and 428 mW, respectively. To understand the process of spectrum broadening, the fundamental spectrum and the SC spectrum after each plate were recorded, as shown Photonics 2021, 8, x FOR PEER REVIEW 3 of 8 in Figure 2. It was observed that the broadened spectra in the first three plates were sym- metric to the fundamental spectrum, which coincided with the process of self-phase mod- ulation. However, the spectra were significantly broadened from the fourth to seventh thin plates, as shown in Figure 2b. The primary mechanism responsible for spectral broad- ening was the nonlinear phase accumulation pass through the plate, leading to pulse self- Photonics 2021, 8, 311 3 of 8 steepening. As a comparison, the supercontinnum generation in an 840 µm fused silica plate without filamentation was also investigated. The thickness of this plate was equiva- As a comparison, the supercontinnum generation in an 840 m fused silica plate without lent to the sum of the seven thin plates mentioned above. To avoid the filamentation, this filamentation was also investigated. The thickness of this plate was equivalent to the sum single plate was placed 25 cm before the focal point, and the SC spectrum is shown by the of the seven thin plates mentioned above. To avoid the filamentation, this single plate was red dotted line in Figure 2b. The output average power was approximately 455 mW. The placed 25 cm before the focal point, and the SC spectrum is shown by the red dotted line in Figure 2b. The output average power was approximately 455 mW. The spectral width spectral width induced by the single plate was narrower than that of the multiple thin induced by the single plate was narrower than that of the multiple thin plates due to the plates due to the lower input laser intensity. lower input laser intensity. (a) (b) Figure 2. Spectra passing through the fused silica plates. The “0 piece” indicates fundamental spectrum. Red dotted line Figure 2. Spectra passing through the fused silica plates. The “0 piece” indicates fundamental spec- represents the spectrum generated from the single 840 m plate. Each spectrum was normalized by its maximum value. trum. Red dotted line represents the spectrum generated from the single 840 µm plate. Each spec- trum was normalized by its maximum value. We compared the effect of different thicknesses in multiple thin plates on spectral broadening. The spectrum of SC generation with 120 m and 200 m thin fused silica plates is shown in Figure 3. Seven plates with the same thickness were strategically placed at the We compared the effect of different thicknesses in multiple thin plates on spectral waist position of the incident laser beam. It was observed that a broader spectrum could broadening. The spectrum of SC generation with 120 µm and 200 µm thin fused silica be obtained with seven pieces of 120 m thin fused silica plates, and the final spectrum plates is shown in Figure 3. Seven plates with the same thickness were strategically placed extended to approximately 490 nm in the short-wavelength region. However, the cutoff wavelength in the short-wavelength region was approximately 520 nm for 200 m thin at the waist position of the incident laser beam. It was observed that a broader spectrum plates. In the course of the experiment, it was found that 200 m thin plates were easily could be obtained with seven pieces of 120 µm thin fused silica plates, and the final spec- damaged and had poor stability. Essentially, there was a longer optical path inside the trum extended to approximately 490 nm in the short-wavelength region. However, the thicker plates, which was more prone to energy loss from multiphoton ionization and optical damage, resulting in less spectral broadening. However, a thinner plate has a cutoff wavelength in the short-wavelength region was approximately 520 nm for 200 µm shorter optical path, which avoids these disadvantageous factors but provides very limited thin plates. In the course of the experiment, it was found that 200 µm thin plates were self-phase modulation. Self-phase modulation is usually characterized by the B-integral. easily damaged and had poor stability. Essentially, there was a longer optical path inside 2p It is defined as B = n Idz, where l is the central wavelength, n is the nonlinear 2 2 l 0 the thicker plates, which refraction wa index, s more pron I is the intensity e to ener of incident gy loss light, from m z is the ultiphoton ionization an coordinate along the beam d direction, and L is the thickness of Kerr medium. When the optical intensity is constant, optical damage, resulting in less spectral broadening. However, a thinner plate has a the B-integral increases with the propagation distance. The nonlinear refractive index for shorter optical path, which avoids these disadvantageous factors but provides very lim- 16 2 fused silica is 2.4  10 cm /W. The estimated value of the B-integral is approximately ited self-phase modu 3.0lfor atithe on. S 200e lf-ph m thin ase plate modul and 1.8 afor tion is usual the 120 m thin ly cha platerin acour teriz experiment. ed by the B-i Thus, the nte- accumulation of self-phase modulation for the 200 m thin plate is 1.67 times that of the 2π gral. It is defined as , where λ is the central wavelength, n2 is the nonlinear Bn = Idz 120 m plate. W 2 e infer that if a set of thin plates with different thicknesses was employed, the SC spectrum would have a more expanded range. refraction index, I is the intensity of incident light, z is the coordinate along the beam di- rection, and L is the thickness of Kerr medium. When the optical intensity is constant, the B-integral increases with the propagation distance. The nonlinear refractive index for −16 2 fused silica is 2.4 × 10 cm /W. The estimated value of the B-integral is approximately 3.0 for the 200 µm thin plate and 1.8 for the 120 µm thin plate in our experiment. Thus, the accumulation of self-phase modulation for the 200 µm thin plate is 1.67 times that of the 120 µm plate. We infer that if a set of thin plates with different thicknesses was employed, the SC spectrum would have a more expanded range. Photonics 2021, 8, 311 4 of 8 Photonics 2021, 8, x FOR PEER REVIEW 4 of 8 Photonics 2021, 8, x FOR PEER REVIEW 4 of 8 Figure 3. SC spectra after the laser beam propagated through the fused silica thin plates. Each Figure 3. SC spectra after the laser beam propagated through the fused silica thin plates. Each spec- Figure 3. SC spectra after the laser beam propagated through the fused silica thin plates. Each spectrum was normalized by its maximum value. spectrum was normalized by its maximum value. trum was normalized by its maximum value. Normally, the diameter of the spot gradually increased as the number of insetting Normally, the diameter of the spot gradually increased as the number of insetting plates increased, while the peak power density of the laser decreased. Therefore, it is Normally, the diameter of the spot gradually increased as the number of insetting plates increased, while the peak power density of the laser decreased. Therefore, it is advantageous to inset a thicker plate in the back. This can not only accumulate a greater plates increased, while the peak power density of the laser decreased. Therefore, it is ad- advantageous to inset a thicker plate in the back. This can not only accumulate a greater effect of self-phase modulation but also avoid some adverse nonlinear influences, and it effect of self-phase modulation but also avoid some adverse nonlinear influences, and it vantageous to inset a thicker plate in the back. This can not only accumulate a greater can finally contribute to further spectrum broadening. To provide a better comparison, can finally contribute to further spectrum broadening. To provide a better comparison, the effect of self-phase modulation but also avoid some adverse nonlinear influences, and it the seventh thin plate was replaced with a 200 μm thin plate based on the above seventh thin plate was replaced with a 200 m thin plate based on the above experiments. can finally contribute to further spectrum broadening. To provide a better comparison, experiments. It is necessary to adjust the position of the new inset plate to broaden the It is necessary to adjust the position of the new inset plate to broaden the spectrum as spectrum as much as possible. After passing through all the plates, the spectra were the seventh thin plate was replaced with a 200 µm thin plate based on the above experi- much as possible. After passing through all the plates, the spectra were measured, as measured, as shown in Figure 4a. The spectra were broader than those of the seven plates shown ments. It in Figur is ne ecess 4a. ary The to spectra adjuwer st the e br posit oader ion than of the new i those of thenseven set pla plates te to with broa the den the spectrum with the same thickness, which extended to 470 nm in the short-wavelength region. In same thickness, which extended to 470 nm in the short-wavelength region. In addition, the as much as possible. After passing through all the plates, the spectra were measured, as addition, the spectral intensity was significantly increased in the range from 470 nm to spectral intensity was significantly increased in the range from 470 nm to 650 nm compared shown in Figure 4a. The spectra were broader than those of the seven plates with the same 650 nm compared to that of the seven 120 μm plates. Figure 4b shows a color photographic to that of the seven 120 m plates. Figure 4b shows a color photographic image of the beam thickness, which extended to 470 nm in the short-wavelength region. In addition, the spec- image of the beam profile in the far field that was taken by a digital camera. The most profile in the far field that was taken by a digital camera. The most striking feature is a red striking feature is a red ring surrounding a white circular central area. tral intensity was significantly increased in the range from 470 nm to 650 nm compared to ring surrounding a white circular central area. that of the seven 120 µm plates. Figure 4b shows a color photographic image of the beam profile in the far field that was taken by a digital camera. The most striking feature is a red ring surrounding a white circular central area. (a) (b) Figure 4. (a) SC spectrum after the laser beam propagated through a set of thin plates with mixed Figure 4. (a) SC spectrum after the laser beam propagated through a set of thin plates with mixed thickness. (b) The image of output beam profile. thickness. (b) The image of output beam profile. We further analyzed the influence of mixed thin plates with different materials and We further analyzed the influence of mixed thin plates with different materials and thicknesses thicknesses on onspectral spectralbr bro oadening. adening. Usually Usually, , aathin thinplate plate of offused fusedsilica silicais is used usedto togenerate generate SC. SC.The The material materia of l the of th thin e th plates in plis ates noticonfined s not con to fifused ned to silica. fused Because silica. fluori Becad use e materials fluoride (a) (b) have a large bandgap, it is possible to further broaden the spectrum to the short-wavelength materials have a large bandgap, it is possible to further broaden the spectrum to the short- Figure 4. (a) SC spectrum after the laser beam propagated through a set of thin plates with mixed side. However, it is susceptible to optical damage owing to the low damage threshold wavelength side. However, it is susceptible to optical damage owing to the low damage of thick fluoride ness. ( material b) The image of ou [22]. We surmised tput beam profile. that a plate with fluoride materials placed at the threshold of fluoride material [22]. We surmised that a plate with fluoride materials placed at the back of multiple thin plates would expand the spectrum to a greater degree. Then, we continued to insert a 1-mm-thick CaF2 plate ((111), Union Optic Inc., WUH, We further analyzed the influence of mixed thin plates with different materials and thicknesses on spectral broadening. Usually, a thin plate of fused silica is used to generate SC. The material of the thin plates is not confined to fused silica. Because fluoride materi- als have a large bandgap, it is possible to further broaden the spectrum to the short-wave- length side. However, it is susceptible to optical damage owing to the low damage thresh- old of fluoride material [22]. We surmised that a plate with fluoride materials placed at the back of multiple thin plates would expand the spectrum to a greater degree. Then, we continued to insert a 1-mm-thick CaF2 plate ((111), Union Optic Inc., WUH, CHN) based Photonics 2021, 8, 311 5 of 8 Photonics 2021, 8, x FOR PEER REVIEW 5 of 8 back of multiple thin plates would expand the spectrum to a greater degree. Then, we on the broadened spectrum of the combined plates with different thicknesses. It should continued to insert a 1-mm-thick CaF plate ((111), Union Optic Inc., Wuhan, China) based be noted that the Ca onFthe 2 plbr atoadened e was placed spectr aum t a dist of the anc combined e of apprplates oximat with ely 1 dif 0 cm ferent from thicknesses. the last It should be noted that the CaF plate was placed at a distance of approximately 10 cm from the fused silica plate to avoid optical damage. The position of the new inset plate was adjusted last fused silica plate to avoid optical damage. The position of the new inset plate was so that the spectrum was as broad as possible. The recorded spectra are shown in Figure adjusted so that the spectrum was as broad as possible. The recorded spectra are shown 5. The spectrum was obviously broadened to approximately 430 nm in the short-wave- in Figure 5. The spectrum was obviously broadened to approximately 430 nm in the length region. Moreover, the intensity of short-wavelength components in the range of short-wavelength region. Moreover, the intensity of short-wavelength components in the 430 nm to 600 nm was greatly increased. As a comparison, a 1-mm-thick fused silica plate range of 430 nm to 600 nm was greatly increased. As a comparison, a 1-mm-thick fused was inserted. The spectrum no longer broadened, as shown by the green line in Figure 5. silica plate was inserted. The spectrum no longer broadened, as shown by the green line in In fact, CaF2 had a higher nonlinear refractive index and lower critical power of self-fo- Figure 5. In fact, CaF had a higher nonlinear refractive index and lower critical power of cusing [23]. In terms self-focusing of the resu [23 lts ]. sho In terms wn in of Fi the gurr esults e 2, mor shown e sel in f-p Figur hase e modu 2, morla e self-phase tion mighmodulation t might be accumulated through multiple thinner CaF plates as opposed to those that are be accumulated through multiple thinner CaF2 plates as opposed to those that are 1-mm- 1-mm-thick. If there are thinner CaF plates, further broadening to wavelengths less than thick. If there are thinner CaF2 plates, further broadening to wavelengths less than 400 nm 400 nm could be achieved by inserting multiple CaF plates. could be achieved by inserting multiple CaF2 plates. (a) (b) Figure 5. SC spectrum after the laser beam propagated through a set of mixed thin plates with dif- Figure 5. SC spectrum after the laser beam propagated through a set of mixed thin plates with different materials and ferent materials and thickness. (a) Spectra on a log scale; (b) Spectra on a linear scale. thickness. (a) Spectra on a log scale; (b) Spectra on a linear scale. The stability of the SC spectrum is an important factor in many applications. We The stability of the SC spectrum is an important factor in many applications. We in- investigated the stability of the SC spectrum in our experiment, which is crucial in transient vestigated the stability of the SC spectrum in our experiment, which is crucial in transient absorption spectroscopy. In general, SC is generated by a single filament in bulk media in absorption spectroscopy. In general, SC is generated by a single filament in bulk media in most experiments. Thus, a piece of fused silica with the size of 15 mm  15 mm  10 mm most experiments. Thus, a piece of fused silica with the size of 15 mm × 15 mm × 10 mm was used to induce a single filament for comparison. The spectra of SC obtained from was used to induce a single filament for comparison. The spectra of SC obtained from the the bulk fused silica and the mixed multiple thin plates are shown in Figure 6a. It was bulk fused silica and the mixed multiple thin plates are shown in Figure 6a. It was ob- observed that the spectral intensity and broadening for the bulk fused silica was inferior to served that the spectr thatal int for the ensity mixed and multiple broaden thin ing plates. for the bu A color lkphotographic fused silica wa image s inf of eri the or to SC generated from the bulk fused silica was recorded by a digital camera and is shown in Figure 6b. that for the mixed multiple thin plates. A color photographic image of the SC generated The transverse section of the beam image reveals a typical conical emission pattern. A from the bulk fused silica was recorded by a digital camera and is shown in Figure 6b. central white core was surrounded by colored Newton’s rings appearing in an order The transverse section of the beam image reveals a typical conical emission pattern. A opposite to diffraction. Furthermore, the spectral stability that we measured in multiple central white core was surrounded by colored Newton’s rings appearing in an order op- thin plates was compared to the case of a single filament generated in bulk fused silica. posite to diffraction. Furthermore, the spectral stability that we measured in multiple thin A series of spectra were collected by the spectrometer for 10 min with an acquisition plates was compared to the case of a single filament generated in bulk fused silica. A series interval time of 6 s. The spectral integral intensity was calculated and normalized by its of spectra were collected by the spectrometer for 10 min with an acquisition interval time maximum value. The fluctuations of SC are represented by the difference between the of 6 s. The spectrnormalized al integral int spectral ensitintegral y was ca intensity lculated an and its d norm mean a value. lized b The y spectral its maxim fluctuations um with collection time are shown in Figure 7. The spectrum induced by a single filament exhibited value. The fluctuations of SC are represented by the difference between the normalized small fluctuations, while, in the case of multiple thin plates, it was relatively stable. In spectral integral intensity and its mean value. The spectral fluctuations with collection addition, the standard deviation (SD) was used to quantitatively evaluate the stability of time are shown in Figure 7. The spectrum induced by a single filament exhibited small the SC spectrum. The SD was defined as s = I I /N, where N is the number fluctuations, while, in the case of multiple thin plates, it was relåatively stable. In addition, i=1 the standard deviation (SD) was used to quantitatively evaluate the stability of the SC spectrum. The SD was defined as , where N is the number of spectra σ=− I IN ()  i i =1 collected, Ii indicates each normalized spectral integral intensity, and Ī is the average value of the normalized spectral integral intensity. As the value of SD is smaller, the spectrum is more stable. The SDs of the spectra induced by bulk media, multiple thin plates, and Photonics 2021, 8, 311 6 of 8 Photonics 2021, 8, x FOR PEER REVIEW 6 of 8 of spectra collected, I indicates each normalized spectral integral intensity, and I is the the spectrum is more stable. The SDs of the spectra induced by bulk media, multiple thin Photonics 2021, 8, x FOR PEER REVIEW 6 of 8 average value of the normalized spectral integral intensity. As the value of SD is smaller, plates, and the fundamental frequency light were calculated to be 0.0398, 0.0285, and the spectrum is more stable. The SDs of the spectra induced by bulk media, multiple 0.0149, respectively. The results indicate that the spectrum induced by multiple thin plates thin plates, and the fundamental frequency light were calculated to be 0.0398, 0.0285, and shows better stability, which is 40 percent higher than that of the spectrum induced by a 0.0149, respectively. The results indicate that the spectrum induced by multiple thin plates the fundamental frequency light were calculated to be 0.0398, 0.0285, and 0.0149, respec- single filament in bulk media. The reason is as follows. As is well known, SC generation shows better stability, which is 40 percent higher than that of the spectrum induced by a tively. The results indicate that the spectrum induced by multiple thin plates shows better single is a co fil m apl ment ex n in on bulk linea media. r optica The l pr reason ocessis . The as follows. intensiAs ty st isawell bility known, of SC SC wil generation l be influenced by stability, which is 40 percent higher than that of the spectrum induced by a single filament is a complex nonlinear optical process. The intensity stability of SC will be influenced by the nonlinear amplification of the input pulse fluctuations. In terms of the SC generation in bulk media. The reason is as follows. As is well known, SC generation is a complex the nonlinear amplification of the input pulse fluctuations. In terms of the SC generation mechanism, the SC induced by the thin plates almost originated from pure self-phase nonlinear optical process. The intensity stability of SC will be influenced by the nonlinear mechanism, the SC induced by the thin plates almost originated from pure self-phase modulation, while the SC induced by bulk media might be affected by many nonlinear amplification of the input pulse fluctuations. In terms of the SC generation mechanism, modulation, while the SC induced by bulk media might be affected by many nonlinear the SC induced by the thin plates almost originated from pure self-phase modulation, optical processes besides the self-phase modulation. Thus, the stability of the SC optical processes besides the self-phase modulation. Thus, the stability of the SC generated while the SC induced by bulk media might be affected by many nonlinear optical pro- generated from the multiple thin plates might be shot-noise-limited and better than that from the multiple thin plates might be shot-noise-limited and better than that of the SC cesses besides the self-phase modulation. Thus, the stability of the SC generated from the of the SC generated from bulk media. generated from bulk media. multiple thin plates might be shot-noise-limited and better than that of the SC generated from bulk media. (a) (b) (a) (b) Figure 6. (a) SC spectra generated from the bulk fused silica and the mixed multiple thin plates. (b) Figure 6. (a) SC spectra generated from the bulk fused silica and the mixed multiple thin plates. Figure 6. (a) SC spectra generated from the bulk fused silica and the mixed multiple thin plates. (b) Image of the SC beam profile in the bulk fused silica. (b) Image of the SC beam profile in the bulk fused silica. Image of the SC beam profile in the bulk fused silica. Figure 7. Fluctuation of normalized spectral integral intensity with collection time. σ is the standard Figure 7. Fluctuation of normalized spectral integral intensity with collection time.  is the standard deviation of normalized spectral integral intensity. deviation of normalized spectral integral intensity. Figure 7. Fluctuation of normalized spectral integral intensity with collection time. σ is the standard 4. Conclusions deviation of normalized spectral integral intensity. In summary, we have demonstrated a technique using multiple thin plates to gener- 4. Conclusions ate a more intense and broader SC spectrum. This is achieved by a set of solid thin plates with different thicknesses and materials. The spectrum intensity and spectrum broaden- In summary, we have demonstrated a technique using multiple thin plates to generate a more intense and broader SC spectrum. This is achieved by a set of solid thin plates with different thicknesses and materials. The spectrum intensity and spectrum Photonics 2021, 8, 311 7 of 8 4. Conclusions In summary, we have demonstrated a technique using multiple thin plates to generate a more intense and broader SC spectrum. This is achieved by a set of solid thin plates with different thicknesses and materials. The spectrum intensity and spectrum broadening in the short-wavelength region were significantly increased compared to those of traditional thin plates with identical thicknesses and materials. Furthermore, the spectrum exhibited excellent stability, which is superior to that of the spectrum induced by the single filament. Our results indicate that mixing multiple thin plates might be an interesting substitute for other SC generation techniques. For instance, when SC was generated in bulk material, the spectral broadening and intensity were insufficient. The results can be utilized in a wide range of applications, including femtosecond transient absorption spectroscopy, high-resolution investigation of ultrafast phenomena, and the generation of bright isolated attosecond pulses. Author Contributions: Conceptualization, J.S. and W.T.; validation, J.S. and W.T.; investigation, J.L and Z.K.; resources, W.T.; writing—original draft preparation, J.L.; writing—review and editing, W.T. and J.L.; visualization, J.L.; supervision, X.H. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant number 62027822 and 61690221; the National Key Research and Development Program of China, grant number 2019YFA0706402; the Natural Science Basic Research Plan in Shaanxi Province of China, grant number 2018JM6012, and the Fundamental Research Funds for the Central Universities, grant number xzy012019039. Data Availability Statement: Data sharing is not applicable. Conflicts of Interest: The authors declare no conflict of interest. References 1. Alfano, P.R. The Supercontinuum Laser Source; Springer: New York, NY, USA, 1989. 2. Rothenberg, J.E. Space-time focusing: Breakdown of the slowly varying envelope approximation in the self-focusing of femtosec- ond pulses. Opt. Lett. 1992, 17, 1340–1342. [CrossRef] [PubMed] 3. Alfano, P.R. The Supercontinuum Laser Source: Fundamentals with Updated References, 2nd ed.; Springer: New York, NY, USA, 2006. 4. Ward, H.; Bergé, L. Temporal shaping of femtosecond solitary pulses in photoionized media. Phys. Rev. Lett. 2003, 90, 53901. [CrossRef] [PubMed] 5. Couairon, A.; Mysyrowicz, A. Femtosecond filamentation in transparent media. Phys. Rep. 2007, 411, 47–189. [CrossRef] 6. Bloembergen, N. The influence of electron plasma formation on superbroadening in light filaments. Opt. Commun. 1973, 8, 285–288. [CrossRef] 7. Brabec, T.; Krauz, F. 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Sub-4 fs laser pulses at high average power and high repetition rate from an all-solid-state setup. Opt. Express 2018, 26, 8941–8956. [CrossRef] Photonics 2021, 8, 311 8 of 8 17. Canhota, M.; Weigand, R.; Crespo, H.M. Simultaneous measurement of two ultrashort near-ultraviolet pulses produced by a multiplate continuum using dual self-diffraction dispersion-scan. Opt. Lett. 2019, 44, 1015–1018. [CrossRef] 18. Lu, C.H.; Wu, W.H.; Kuo, S.H.; Guo, J.Y.; Chen, M.C.; Yang, S.D.; Kung, A.H. Greater than 50 times compression of 1030 nm Yb:KGW laser pulses to single-cycle duration. Opt. Express 2019, 27, 15638–15648. [CrossRef] [PubMed] 19. Mourou, G.; Mironov, S.; Khazanov, E.; Sergeev, A. Single cycle thin film compressor opening the door to Zeptosecond-Exawatt physics. Eur. Phys. J. Spec. Top. 2014, 223, 1181–1188. [CrossRef] 20. Farinella, D.M.; Wheeler, J.; Hussein, A.E.; Nees, J.; Stanfield, M.; Beier, N.; Ma, Y.; Cojocaru, G.; Ungureanu, R.; Pittman, J.; et al. Focusability of laser pulses at petawatt transport intensities in thin-film compression. J. Opt. Soc. Am. B 2019, 36, A28–A32. [CrossRef] 21. Anashkina, E.A.; Ginzburg, V.N.; Kochetkov, A.A.; Yakovlev, I.V.; Kim, A.V.; Khazanov, E.A. Single-shot laser pulse reconstruction based on self-phase modulated spectra measurements. Sci. Rep. 2016, 6, 33749. [CrossRef] [PubMed] 22. Jia, T.Q.; Li, X.X.; Feng, D.H.; Cheng, C.F.; Li, R.X.; Chen, H.; Xu, Z.Z. Theoretical and experimental study on femtosecond laser induced damage in CaF2 crystals. Appl. Phys. A 2005, 81, 645–649. [CrossRef] 23. DeSalvo, R.; Sheik-Bahae, M.; Said, A.A.; Hagan, D.J.; Van Stryland, E.W. Z-scan measurements of the anisotropy of nonlinear refraction and absorption in crystals. Opt. Lett. 1993, 18, 194–196. [CrossRef] [PubMed] http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Photonics Multidisciplinary Digital Publishing Institute

Generation of Ultrabroad and Intense Supercontinuum in Mixed Multiple Thin Plates

Photonics , Volume 8 (8) – Aug 3, 2021

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hv photonics Communication Generation of Ultrabroad and Intense Supercontinuum in Mixed Multiple Thin Plates Jing Li, Wenjiang Tan *, Jinhai Si, Zhen Kang and Xun Hou Key Laboratory for Physical Electronics and Devices of the Ministry of Education, Shaanxi Key Lab of Information Photonic Technique, School of Electronic Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China; lijing525@stu.xjtu.edu.cn (J.L.); jinhaisi@mail.xjtu.edu.cn (J.S.); kangzhen@stu.xjtu.edu.cn (Z.K.); houxun@mail.xjtu.edu.cn (X.H.) * Correspondence: tanwenjiang@mail.xjtu.edu.cn Abstract: Supercontinuum (SC) generation using multiple thin plates is demonstrated with a fem- tosecond laser pulse. We propose an improved technique to obtain larger spectrum broadening and higher spectral intensity by employing mixed multiple thin plates with different thicknesses and materials. Furthermore, the spectrum has good stability, which is superior to that of the spectrum induced by the traditional single filament in bulk material. Our approach offers a route towards simple and stable SC generation for potential applications. Keywords: femtosecond laser; supercontinuum generation; spectral broadening; multiple thin plates 1. Introduction SC generation is a universal phenomenon produced by the nonlinear propagation of intense femtosecond laser pulses in a transparent medium [1]. SC is characterized by an extremely broad spectrum with frequencies ranging from infrared to ultraviolet. The underlying physics of filament-induced SC generation appears to be a complex process Citation: Li, J.; Tan, W.; Si, J.; Kang, Z.; Hou, X. Generation of Ultrabroad that involves a dynamic interplay of spatial and temporal effects: self-focusing, self-phase and Intense Supercontinuum in modulation, self-steepening, group velocity dispersion, and multiphoton absorption [2–6]. Mixed Multiple Thin Plates. Photonics SC has attracted significant research interest in diverse fields owing to its advantages of 2021, 8, 311. https://doi.org/ ultrabroadband radiation, high spectral brightness, and high spatial coherence, which 10.3390/photonics8080311 are equivalent to those of white-light lasers. It has found wide application in optical frequency combs [7], biomedical imaging [8], molecular fingerprint spectroscopy [9], and Received: 21 June 2021 the generation of few-cycle femtosecond pulses [10]. Accepted: 27 July 2021 In recent years, there have been many ways to generate an ultrabroadband SC. For Published: 3 August 2021 instance, a femtosecond laser pulse is injected into inert-gas-filled hollow-core fibers. This method could enable the compression of pulses with high compression ratios and better Publisher’s Note: MDPI stays neutral spatial modes [11,12]. However, it has some inherent drawbacks, such as low efficiency of with regard to jurisdictional claims in fiber coupling and complexity of adjustment of the experimental apparatus. In addition, published maps and institutional affil- SC can be efficiently generated in a solid-state material due to its higher nonlinear index iations. of refraction. It is a simple and robust method to broaden the spectrum. However, when the peak power of the laser pulse is sufficiently high, self-focusing and multiphoton ionization result in optical breakdown and permanent damage to the material. More recently, a novel technique that generates SCs with multiple thin plates has attracted Copyright: © 2021 by the authors. much attention [13–16]. This technique effectively circumvents energy loss and optical Licensee MDPI, Basel, Switzerland. breakdown. It has been reported in nearly one octave-spanning spectrum ranging and a This article is an open access article few-millijoule pulse energy [17,18]. Different applications place differing demands on the distributed under the terms and characteristics of the SC spectrum. For example, the spectral coherence and a broad spectral conditions of the Creative Commons range are the key factors in pulse compression. In the application of femtosecond transient Attribution (CC BY) license (https:// absorption spectroscopy, high intensity of shortwave components will provide a broad creativecommons.org/licenses/by/ spectral detection range, and better spectral stability will improve the detection sensitivity. 4.0/). Photonics 2021, 8, 311. https://doi.org/10.3390/photonics8080311 https://www.mdpi.com/journal/photonics Photonics 2021, 8, x FOR PEER REVIEW 2 of 8 transient absorption spectroscopy, high intensity of shortwave components will provide a broad spectral detection range, and better spectral stability will improve the detection sensitivity. In addition to supercontinuum generation, the use of thin plates makes it pos- sible to perform more complex manipulations with high-energy ultrashort pulses. A new concept of using thin plates was proposed for spectral broadening with subsequent tem- poral compression of ultrashort pulses having energy as high as a few hundred Joules [19]. An experiment was reported in which the successful compression of high-energy laser pulses was achieved [20]. The measured spectrum broadening in thin plates together with the fundamental spectrum also allows the reconstruction of the pulse intensity and phase [21]. In this paper, we experimentally investigated the generation of SC by mixing multi- Photonics 2021, 8, 311 2 of 8 ple thin plates. The broadening and intensity of the SC spectrum in the short-wavelength region was further enhanced through a set of multiple thin plates with different thick- In addition to supercontinuum generation, the use of thin plates makes it possible to nesses and materials. perform more complex manipulations with high-energy ultrashort pulses. A new concept of using thin plates was proposed for spectral broadening with subsequent temporal compression of ultrashort pulses having energy as high as a few hundred Joules [19]. An 2. Experiments experiment was reported in which the successful compression of high-energy laser pulses was achieved [20]. The measured spectrum broadening in thin plates together with the The experimental apparatus of SC generation in multiple thin plates is illustrated in fundamental spectrum also allows the reconstruction of the pulse intensity and phase [21]. Figure 1. A Ti:sapphire femtosecond laser system operating with a pulse duration of 50 fs In this paper, we experimentally investigated the generation of SC by mixing multiple and central wavelength of 800 nm at a repetition rate of 1 kHz was used in our experiment. thin plates. The broadening and intensity of the SC spectrum in the short-wavelength region was further enhanced through a set of multiple thin plates with different thicknesses A neutral density filter placed in the beginning was used to control the average power of and materials. the laser beam to obtain a suitable energy per pulse of 520 µJ. The beam diameter was 2. Experiments approximately 10 mm. The output pulse was loosely focused onto a set of fused silica thin The experimental apparatus of SC generation in multiple thin plates is illustrated in plates with a convex lens (f = 2000 mm). The diameter of the focal point was approximately Figure 1. A Ti:sapphire femtosecond laser system operating with a pulse duration of 50 fs 41 and 0 µm central at wavelength the 1/eof width 800 nm atmeasured by a repetition rate of the knife 1 kHz was used -ed ingour e method. The peak inte experiment. nsity was A neutral density filter placed in the beginning was used to control the average power 12 2 approximately 7.9 × 10 W/cm at the focal point. The fused silica thin plates (Beijing of the laser beam to obtain a suitable energy per pulse of 520 J. The beam diameter was Zhong Cheng Quartz Glass Co., Ltd., BJ, CHN) used in our paper were squares with sides approximately 10 mm. The output pulse was loosely focused onto a set of fused silica thin plates with a convex lens (f = 2000 mm). The diameter of the focal point was approximately of 20 mm and thicknesses of 120 µm and 200 µm. These plates were placed at the waist of 410 m at the 1/e width measured by the knife-edge method. The peak intensity was the beam at the Brew 12 ster 2angle (55.5°) to reduce the Fresnel reflection loss at the surfaces. approximately 7.9  10 W/cm at the focal point. The fused silica thin plates (Beijing Zhong Cheng Quartz Glass Co., Ltd., Beijing, China) used in our paper were squares The generated SC spectrum was collected by a lens on a fiber-coupled spectrometer with sides of 20 mm and thicknesses of 120 m and 200 m. These plates were placed at (Ocean Optics HR 4000). A filter was used in front of the spectrometer to suppress the the waist of the beam at the Brewster angle (55.5 ) to reduce the Fresnel reflection loss at the surfaces. The generated SC spectrum was collected by a lens on a fiber-coupled fundamental frequency light at a center wavelength of 800 nm. spectrometer (Ocean Optics HR 4000). A filter was used in front of the spectrometer to suppress the fundamental frequency light at a center wavelength of 800 nm. Figure 1. Schematic of the experimental setup (NDF: neutral density filter). Figure 1. Schematic of the experimental setup (NDF: neutral density filter). 3. Results and Discussion Firstly, seven 120 m thin fused silica plates were applied for SC generation. The first thin plate was placed 20 cm before the focus of the lens, and the average power was 3. Results and Discussion approximately 489 mW after the plate. The spacing between these thin plates was 8 cm, Firstly, seven 120 µm thin fused silica plates were applied for SC generation. The first 10 cm, 5 cm, 2.5 cm, 2.5 cm, 2.5 cm, respectively. The positions of the plates were determined by obtaining the broadest transmitted spectrum after each plate without optical damage. thin plate was placed 20 cm before the focus of the lens, and the average power was ap- The thin plates should be placed in sequence, resulting in the best spectrum broadening. As proximately 489 mW after the plate. The spacing between these thin plates was 8 cm, 10 the laser passes through the thin plate, the self-focusing effect causes the laser beam to form a focal point in the air. It should be noted that the next thin plate should be placed away cm, 5 cm, 2.5 cm, 2.5 cm, 2.5 cm, respectively. The positions of the plates were determined from the focal point to effectively avoid material damage. The beam diameter was increased by obtaining the broadest transmitted spectrum after each plate without optical damage. after the seventh plate, leading to a reduction in peak power. When more 120-m-thin fused The thin plates should be placed in sequence, resulting in the best spectrum broadening. silica plates were added, the width of the spectrum ceased to broaden. The average power after the second to seventh plates was 480 mW, 473 mW, 458 mW, 448 mW, 437 mW, and As the laser passes through the thin plate, the self-focusing effect causes the laser beam to 428 mW, respectively. To understand the process of spectrum broadening, the fundamental form a focal point in the air. It should be noted that the next thin plate should be placed spectrum and the SC spectrum after each plate were recorded, as shown in Figure 2. It was observed that the broadened spectra in the first three plates were symmetric to away from the focal point to effectively avoid material damage. The beam diameter was the fundamental spectrum, which coincided with the process of self-phase modulation. increased after the seventh plate, leading to a reduction in peak power. When more 120- However, the spectra were significantly broadened from the fourth to seventh thin plates, as shown in Figure 2b. The primary mechanism responsible for spectral broadening was µm-thin fused silica plates were added, the width of the spectrum ceased to broaden. The the nonlinear phase accumulation pass through the plate, leading to pulse self-steepening. average power after the second to seventh plates was 480 mW, 473 mW, 458 mW, 448 mW, 437 mW, and 428 mW, respectively. To understand the process of spectrum broadening, the fundamental spectrum and the SC spectrum after each plate were recorded, as shown Photonics 2021, 8, x FOR PEER REVIEW 3 of 8 in Figure 2. It was observed that the broadened spectra in the first three plates were sym- metric to the fundamental spectrum, which coincided with the process of self-phase mod- ulation. However, the spectra were significantly broadened from the fourth to seventh thin plates, as shown in Figure 2b. The primary mechanism responsible for spectral broad- ening was the nonlinear phase accumulation pass through the plate, leading to pulse self- Photonics 2021, 8, 311 3 of 8 steepening. As a comparison, the supercontinnum generation in an 840 µm fused silica plate without filamentation was also investigated. The thickness of this plate was equiva- As a comparison, the supercontinnum generation in an 840 m fused silica plate without lent to the sum of the seven thin plates mentioned above. To avoid the filamentation, this filamentation was also investigated. The thickness of this plate was equivalent to the sum single plate was placed 25 cm before the focal point, and the SC spectrum is shown by the of the seven thin plates mentioned above. To avoid the filamentation, this single plate was red dotted line in Figure 2b. The output average power was approximately 455 mW. The placed 25 cm before the focal point, and the SC spectrum is shown by the red dotted line in Figure 2b. The output average power was approximately 455 mW. The spectral width spectral width induced by the single plate was narrower than that of the multiple thin induced by the single plate was narrower than that of the multiple thin plates due to the plates due to the lower input laser intensity. lower input laser intensity. (a) (b) Figure 2. Spectra passing through the fused silica plates. The “0 piece” indicates fundamental spectrum. Red dotted line Figure 2. Spectra passing through the fused silica plates. The “0 piece” indicates fundamental spec- represents the spectrum generated from the single 840 m plate. Each spectrum was normalized by its maximum value. trum. Red dotted line represents the spectrum generated from the single 840 µm plate. Each spec- trum was normalized by its maximum value. We compared the effect of different thicknesses in multiple thin plates on spectral broadening. The spectrum of SC generation with 120 m and 200 m thin fused silica plates is shown in Figure 3. Seven plates with the same thickness were strategically placed at the We compared the effect of different thicknesses in multiple thin plates on spectral waist position of the incident laser beam. It was observed that a broader spectrum could broadening. The spectrum of SC generation with 120 µm and 200 µm thin fused silica be obtained with seven pieces of 120 m thin fused silica plates, and the final spectrum plates is shown in Figure 3. Seven plates with the same thickness were strategically placed extended to approximately 490 nm in the short-wavelength region. However, the cutoff wavelength in the short-wavelength region was approximately 520 nm for 200 m thin at the waist position of the incident laser beam. It was observed that a broader spectrum plates. In the course of the experiment, it was found that 200 m thin plates were easily could be obtained with seven pieces of 120 µm thin fused silica plates, and the final spec- damaged and had poor stability. Essentially, there was a longer optical path inside the trum extended to approximately 490 nm in the short-wavelength region. However, the thicker plates, which was more prone to energy loss from multiphoton ionization and optical damage, resulting in less spectral broadening. However, a thinner plate has a cutoff wavelength in the short-wavelength region was approximately 520 nm for 200 µm shorter optical path, which avoids these disadvantageous factors but provides very limited thin plates. In the course of the experiment, it was found that 200 µm thin plates were self-phase modulation. Self-phase modulation is usually characterized by the B-integral. easily damaged and had poor stability. Essentially, there was a longer optical path inside 2p It is defined as B = n Idz, where l is the central wavelength, n is the nonlinear 2 2 l 0 the thicker plates, which refraction wa index, s more pron I is the intensity e to ener of incident gy loss light, from m z is the ultiphoton ionization an coordinate along the beam d direction, and L is the thickness of Kerr medium. When the optical intensity is constant, optical damage, resulting in less spectral broadening. However, a thinner plate has a the B-integral increases with the propagation distance. The nonlinear refractive index for shorter optical path, which avoids these disadvantageous factors but provides very lim- 16 2 fused silica is 2.4  10 cm /W. The estimated value of the B-integral is approximately ited self-phase modu 3.0lfor atithe on. S 200e lf-ph m thin ase plate modul and 1.8 afor tion is usual the 120 m thin ly cha platerin acour teriz experiment. ed by the B-i Thus, the nte- accumulation of self-phase modulation for the 200 m thin plate is 1.67 times that of the 2π gral. It is defined as , where λ is the central wavelength, n2 is the nonlinear Bn = Idz 120 m plate. W 2 e infer that if a set of thin plates with different thicknesses was employed, the SC spectrum would have a more expanded range. refraction index, I is the intensity of incident light, z is the coordinate along the beam di- rection, and L is the thickness of Kerr medium. When the optical intensity is constant, the B-integral increases with the propagation distance. The nonlinear refractive index for −16 2 fused silica is 2.4 × 10 cm /W. The estimated value of the B-integral is approximately 3.0 for the 200 µm thin plate and 1.8 for the 120 µm thin plate in our experiment. Thus, the accumulation of self-phase modulation for the 200 µm thin plate is 1.67 times that of the 120 µm plate. We infer that if a set of thin plates with different thicknesses was employed, the SC spectrum would have a more expanded range. Photonics 2021, 8, 311 4 of 8 Photonics 2021, 8, x FOR PEER REVIEW 4 of 8 Photonics 2021, 8, x FOR PEER REVIEW 4 of 8 Figure 3. SC spectra after the laser beam propagated through the fused silica thin plates. Each Figure 3. SC spectra after the laser beam propagated through the fused silica thin plates. Each spec- Figure 3. SC spectra after the laser beam propagated through the fused silica thin plates. Each spectrum was normalized by its maximum value. spectrum was normalized by its maximum value. trum was normalized by its maximum value. Normally, the diameter of the spot gradually increased as the number of insetting Normally, the diameter of the spot gradually increased as the number of insetting plates increased, while the peak power density of the laser decreased. Therefore, it is Normally, the diameter of the spot gradually increased as the number of insetting plates increased, while the peak power density of the laser decreased. Therefore, it is advantageous to inset a thicker plate in the back. This can not only accumulate a greater plates increased, while the peak power density of the laser decreased. Therefore, it is ad- advantageous to inset a thicker plate in the back. This can not only accumulate a greater effect of self-phase modulation but also avoid some adverse nonlinear influences, and it effect of self-phase modulation but also avoid some adverse nonlinear influences, and it vantageous to inset a thicker plate in the back. This can not only accumulate a greater can finally contribute to further spectrum broadening. To provide a better comparison, can finally contribute to further spectrum broadening. To provide a better comparison, the effect of self-phase modulation but also avoid some adverse nonlinear influences, and it the seventh thin plate was replaced with a 200 μm thin plate based on the above seventh thin plate was replaced with a 200 m thin plate based on the above experiments. can finally contribute to further spectrum broadening. To provide a better comparison, experiments. It is necessary to adjust the position of the new inset plate to broaden the It is necessary to adjust the position of the new inset plate to broaden the spectrum as spectrum as much as possible. After passing through all the plates, the spectra were the seventh thin plate was replaced with a 200 µm thin plate based on the above experi- much as possible. After passing through all the plates, the spectra were measured, as measured, as shown in Figure 4a. The spectra were broader than those of the seven plates shown ments. It in Figur is ne ecess 4a. ary The to spectra adjuwer st the e br posit oader ion than of the new i those of thenseven set pla plates te to with broa the den the spectrum with the same thickness, which extended to 470 nm in the short-wavelength region. In same thickness, which extended to 470 nm in the short-wavelength region. In addition, the as much as possible. After passing through all the plates, the spectra were measured, as addition, the spectral intensity was significantly increased in the range from 470 nm to spectral intensity was significantly increased in the range from 470 nm to 650 nm compared shown in Figure 4a. The spectra were broader than those of the seven plates with the same 650 nm compared to that of the seven 120 μm plates. Figure 4b shows a color photographic to that of the seven 120 m plates. Figure 4b shows a color photographic image of the beam thickness, which extended to 470 nm in the short-wavelength region. In addition, the spec- image of the beam profile in the far field that was taken by a digital camera. The most profile in the far field that was taken by a digital camera. The most striking feature is a red striking feature is a red ring surrounding a white circular central area. tral intensity was significantly increased in the range from 470 nm to 650 nm compared to ring surrounding a white circular central area. that of the seven 120 µm plates. Figure 4b shows a color photographic image of the beam profile in the far field that was taken by a digital camera. The most striking feature is a red ring surrounding a white circular central area. (a) (b) Figure 4. (a) SC spectrum after the laser beam propagated through a set of thin plates with mixed Figure 4. (a) SC spectrum after the laser beam propagated through a set of thin plates with mixed thickness. (b) The image of output beam profile. thickness. (b) The image of output beam profile. We further analyzed the influence of mixed thin plates with different materials and We further analyzed the influence of mixed thin plates with different materials and thicknesses thicknesses on onspectral spectralbr bro oadening. adening. Usually Usually, , aathin thinplate plate of offused fusedsilica silicais is used usedto togenerate generate SC. SC.The The material materia of l the of th thin e th plates in plis ates noticonfined s not con to fifused ned to silica. fused Because silica. fluori Becad use e materials fluoride (a) (b) have a large bandgap, it is possible to further broaden the spectrum to the short-wavelength materials have a large bandgap, it is possible to further broaden the spectrum to the short- Figure 4. (a) SC spectrum after the laser beam propagated through a set of thin plates with mixed side. However, it is susceptible to optical damage owing to the low damage threshold wavelength side. However, it is susceptible to optical damage owing to the low damage of thick fluoride ness. ( material b) The image of ou [22]. We surmised tput beam profile. that a plate with fluoride materials placed at the threshold of fluoride material [22]. We surmised that a plate with fluoride materials placed at the back of multiple thin plates would expand the spectrum to a greater degree. Then, we continued to insert a 1-mm-thick CaF2 plate ((111), Union Optic Inc., WUH, We further analyzed the influence of mixed thin plates with different materials and thicknesses on spectral broadening. Usually, a thin plate of fused silica is used to generate SC. The material of the thin plates is not confined to fused silica. Because fluoride materi- als have a large bandgap, it is possible to further broaden the spectrum to the short-wave- length side. However, it is susceptible to optical damage owing to the low damage thresh- old of fluoride material [22]. We surmised that a plate with fluoride materials placed at the back of multiple thin plates would expand the spectrum to a greater degree. Then, we continued to insert a 1-mm-thick CaF2 plate ((111), Union Optic Inc., WUH, CHN) based Photonics 2021, 8, 311 5 of 8 Photonics 2021, 8, x FOR PEER REVIEW 5 of 8 back of multiple thin plates would expand the spectrum to a greater degree. Then, we on the broadened spectrum of the combined plates with different thicknesses. It should continued to insert a 1-mm-thick CaF plate ((111), Union Optic Inc., Wuhan, China) based be noted that the Ca onFthe 2 plbr atoadened e was placed spectr aum t a dist of the anc combined e of apprplates oximat with ely 1 dif 0 cm ferent from thicknesses. the last It should be noted that the CaF plate was placed at a distance of approximately 10 cm from the fused silica plate to avoid optical damage. The position of the new inset plate was adjusted last fused silica plate to avoid optical damage. The position of the new inset plate was so that the spectrum was as broad as possible. The recorded spectra are shown in Figure adjusted so that the spectrum was as broad as possible. The recorded spectra are shown 5. The spectrum was obviously broadened to approximately 430 nm in the short-wave- in Figure 5. The spectrum was obviously broadened to approximately 430 nm in the length region. Moreover, the intensity of short-wavelength components in the range of short-wavelength region. Moreover, the intensity of short-wavelength components in the 430 nm to 600 nm was greatly increased. As a comparison, a 1-mm-thick fused silica plate range of 430 nm to 600 nm was greatly increased. As a comparison, a 1-mm-thick fused was inserted. The spectrum no longer broadened, as shown by the green line in Figure 5. silica plate was inserted. The spectrum no longer broadened, as shown by the green line in In fact, CaF2 had a higher nonlinear refractive index and lower critical power of self-fo- Figure 5. In fact, CaF had a higher nonlinear refractive index and lower critical power of cusing [23]. In terms self-focusing of the resu [23 lts ]. sho In terms wn in of Fi the gurr esults e 2, mor shown e sel in f-p Figur hase e modu 2, morla e self-phase tion mighmodulation t might be accumulated through multiple thinner CaF plates as opposed to those that are be accumulated through multiple thinner CaF2 plates as opposed to those that are 1-mm- 1-mm-thick. If there are thinner CaF plates, further broadening to wavelengths less than thick. If there are thinner CaF2 plates, further broadening to wavelengths less than 400 nm 400 nm could be achieved by inserting multiple CaF plates. could be achieved by inserting multiple CaF2 plates. (a) (b) Figure 5. SC spectrum after the laser beam propagated through a set of mixed thin plates with dif- Figure 5. SC spectrum after the laser beam propagated through a set of mixed thin plates with different materials and ferent materials and thickness. (a) Spectra on a log scale; (b) Spectra on a linear scale. thickness. (a) Spectra on a log scale; (b) Spectra on a linear scale. The stability of the SC spectrum is an important factor in many applications. We The stability of the SC spectrum is an important factor in many applications. We in- investigated the stability of the SC spectrum in our experiment, which is crucial in transient vestigated the stability of the SC spectrum in our experiment, which is crucial in transient absorption spectroscopy. In general, SC is generated by a single filament in bulk media in absorption spectroscopy. In general, SC is generated by a single filament in bulk media in most experiments. Thus, a piece of fused silica with the size of 15 mm  15 mm  10 mm most experiments. Thus, a piece of fused silica with the size of 15 mm × 15 mm × 10 mm was used to induce a single filament for comparison. The spectra of SC obtained from was used to induce a single filament for comparison. The spectra of SC obtained from the the bulk fused silica and the mixed multiple thin plates are shown in Figure 6a. It was bulk fused silica and the mixed multiple thin plates are shown in Figure 6a. It was ob- observed that the spectral intensity and broadening for the bulk fused silica was inferior to served that the spectr thatal int for the ensity mixed and multiple broaden thin ing plates. for the bu A color lkphotographic fused silica wa image s inf of eri the or to SC generated from the bulk fused silica was recorded by a digital camera and is shown in Figure 6b. that for the mixed multiple thin plates. A color photographic image of the SC generated The transverse section of the beam image reveals a typical conical emission pattern. A from the bulk fused silica was recorded by a digital camera and is shown in Figure 6b. central white core was surrounded by colored Newton’s rings appearing in an order The transverse section of the beam image reveals a typical conical emission pattern. A opposite to diffraction. Furthermore, the spectral stability that we measured in multiple central white core was surrounded by colored Newton’s rings appearing in an order op- thin plates was compared to the case of a single filament generated in bulk fused silica. posite to diffraction. Furthermore, the spectral stability that we measured in multiple thin A series of spectra were collected by the spectrometer for 10 min with an acquisition plates was compared to the case of a single filament generated in bulk fused silica. A series interval time of 6 s. The spectral integral intensity was calculated and normalized by its of spectra were collected by the spectrometer for 10 min with an acquisition interval time maximum value. The fluctuations of SC are represented by the difference between the of 6 s. The spectrnormalized al integral int spectral ensitintegral y was ca intensity lculated an and its d norm mean a value. lized b The y spectral its maxim fluctuations um with collection time are shown in Figure 7. The spectrum induced by a single filament exhibited value. The fluctuations of SC are represented by the difference between the normalized small fluctuations, while, in the case of multiple thin plates, it was relatively stable. In spectral integral intensity and its mean value. The spectral fluctuations with collection addition, the standard deviation (SD) was used to quantitatively evaluate the stability of time are shown in Figure 7. The spectrum induced by a single filament exhibited small the SC spectrum. The SD was defined as s = I I /N, where N is the number fluctuations, while, in the case of multiple thin plates, it was relåatively stable. In addition, i=1 the standard deviation (SD) was used to quantitatively evaluate the stability of the SC spectrum. The SD was defined as , where N is the number of spectra σ=− I IN ()  i i =1 collected, Ii indicates each normalized spectral integral intensity, and Ī is the average value of the normalized spectral integral intensity. As the value of SD is smaller, the spectrum is more stable. The SDs of the spectra induced by bulk media, multiple thin plates, and Photonics 2021, 8, 311 6 of 8 Photonics 2021, 8, x FOR PEER REVIEW 6 of 8 of spectra collected, I indicates each normalized spectral integral intensity, and I is the the spectrum is more stable. The SDs of the spectra induced by bulk media, multiple thin Photonics 2021, 8, x FOR PEER REVIEW 6 of 8 average value of the normalized spectral integral intensity. As the value of SD is smaller, plates, and the fundamental frequency light were calculated to be 0.0398, 0.0285, and the spectrum is more stable. The SDs of the spectra induced by bulk media, multiple 0.0149, respectively. The results indicate that the spectrum induced by multiple thin plates thin plates, and the fundamental frequency light were calculated to be 0.0398, 0.0285, and shows better stability, which is 40 percent higher than that of the spectrum induced by a 0.0149, respectively. The results indicate that the spectrum induced by multiple thin plates the fundamental frequency light were calculated to be 0.0398, 0.0285, and 0.0149, respec- single filament in bulk media. The reason is as follows. As is well known, SC generation shows better stability, which is 40 percent higher than that of the spectrum induced by a tively. The results indicate that the spectrum induced by multiple thin plates shows better single is a co fil m apl ment ex n in on bulk linea media. r optica The l pr reason ocessis . The as follows. intensiAs ty st isawell bility known, of SC SC wil generation l be influenced by stability, which is 40 percent higher than that of the spectrum induced by a single filament is a complex nonlinear optical process. The intensity stability of SC will be influenced by the nonlinear amplification of the input pulse fluctuations. In terms of the SC generation in bulk media. The reason is as follows. As is well known, SC generation is a complex the nonlinear amplification of the input pulse fluctuations. In terms of the SC generation mechanism, the SC induced by the thin plates almost originated from pure self-phase nonlinear optical process. The intensity stability of SC will be influenced by the nonlinear mechanism, the SC induced by the thin plates almost originated from pure self-phase modulation, while the SC induced by bulk media might be affected by many nonlinear amplification of the input pulse fluctuations. In terms of the SC generation mechanism, modulation, while the SC induced by bulk media might be affected by many nonlinear the SC induced by the thin plates almost originated from pure self-phase modulation, optical processes besides the self-phase modulation. Thus, the stability of the SC optical processes besides the self-phase modulation. Thus, the stability of the SC generated while the SC induced by bulk media might be affected by many nonlinear optical pro- generated from the multiple thin plates might be shot-noise-limited and better than that from the multiple thin plates might be shot-noise-limited and better than that of the SC cesses besides the self-phase modulation. Thus, the stability of the SC generated from the of the SC generated from bulk media. generated from bulk media. multiple thin plates might be shot-noise-limited and better than that of the SC generated from bulk media. (a) (b) (a) (b) Figure 6. (a) SC spectra generated from the bulk fused silica and the mixed multiple thin plates. (b) Figure 6. (a) SC spectra generated from the bulk fused silica and the mixed multiple thin plates. Figure 6. (a) SC spectra generated from the bulk fused silica and the mixed multiple thin plates. (b) Image of the SC beam profile in the bulk fused silica. (b) Image of the SC beam profile in the bulk fused silica. Image of the SC beam profile in the bulk fused silica. Figure 7. Fluctuation of normalized spectral integral intensity with collection time. σ is the standard Figure 7. Fluctuation of normalized spectral integral intensity with collection time.  is the standard deviation of normalized spectral integral intensity. deviation of normalized spectral integral intensity. Figure 7. Fluctuation of normalized spectral integral intensity with collection time. σ is the standard 4. Conclusions deviation of normalized spectral integral intensity. In summary, we have demonstrated a technique using multiple thin plates to gener- 4. Conclusions ate a more intense and broader SC spectrum. This is achieved by a set of solid thin plates with different thicknesses and materials. The spectrum intensity and spectrum broaden- In summary, we have demonstrated a technique using multiple thin plates to generate a more intense and broader SC spectrum. This is achieved by a set of solid thin plates with different thicknesses and materials. The spectrum intensity and spectrum Photonics 2021, 8, 311 7 of 8 4. Conclusions In summary, we have demonstrated a technique using multiple thin plates to generate a more intense and broader SC spectrum. This is achieved by a set of solid thin plates with different thicknesses and materials. The spectrum intensity and spectrum broadening in the short-wavelength region were significantly increased compared to those of traditional thin plates with identical thicknesses and materials. Furthermore, the spectrum exhibited excellent stability, which is superior to that of the spectrum induced by the single filament. Our results indicate that mixing multiple thin plates might be an interesting substitute for other SC generation techniques. For instance, when SC was generated in bulk material, the spectral broadening and intensity were insufficient. The results can be utilized in a wide range of applications, including femtosecond transient absorption spectroscopy, high-resolution investigation of ultrafast phenomena, and the generation of bright isolated attosecond pulses. Author Contributions: Conceptualization, J.S. and W.T.; validation, J.S. and W.T.; investigation, J.L and Z.K.; resources, W.T.; writing—original draft preparation, J.L.; writing—review and editing, W.T. and J.L.; visualization, J.L.; supervision, X.H. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant number 62027822 and 61690221; the National Key Research and Development Program of China, grant number 2019YFA0706402; the Natural Science Basic Research Plan in Shaanxi Province of China, grant number 2018JM6012, and the Fundamental Research Funds for the Central Universities, grant number xzy012019039. 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Journal

PhotonicsMultidisciplinary Digital Publishing Institute

Published: Aug 3, 2021

Keywords: femtosecond laser; supercontinuum generation; spectral broadening; multiple thin plates

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