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Domain-Reversed Lithium Niobate Single-Crystal Fibers are Potentially for Efficient Terahertz Wave Generation

Domain-Reversed Lithium Niobate Single-Crystal Fibers are Potentially for Efficient Terahertz... Hindawi Publishing Corporation Advances in OptoElectronics Volume 2008, Article ID 208458, 5 pages doi:10.1155/2008/208458 Research Article Domain-Reversed Lithium Niobate Single-Crystal Fibers are Potentially for Efficient Terahertz Wave Generation 1 2 Yalin Lu and Kitt Reinhardt The Physics Department, Laser and Optics Research Center (LORC), 2354 Fairchild Dr. 2A31, United States Air Force Academy, CO 80840, USA Air Force Office of Scientific Research (AFOSR/NE), 875 North Randolph Street, Suite 326, Arlington, VA 22203, USA Correspondence should be addressed to Yalin Lu, yalin.lu@usafa.edu Received 16 May 2008; Accepted 31 August 2008 Recommended by Hiroshi Murata Nonlinear frequency conversion remains one of the dominant approaches to efficiently generate THz waves. Significant material absorption in the THz range is the main factor impeding the progress towards this direction. In this research, a new multicladding nonlinear fiber design was proposed to solve this problem, and as the major experimental effort, periodic domain structure was introduced into lithium niobate single-crystal fibers by electrical poling. The introduced periodic domain structures were nondestructively revealed using a crossly polarized optical microscope and a confocal scanning optical microscope for quality assurance. Copyright © 2008 Y. Lu and K. Reinhardt. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. Introduction extreme, continuous-wave (cw) THz generation has been realized in free-electron lasers and quantum cascade lasers. The terahertz (THz) frequency range (0.1∼30 THz) lies The former offers high output power, but they are bulky in the gap between microwave and infrared of the elec- and inflexible. The latter, however, provides the potential tromagnetic spectrum. THz technology lags behind both for good system compactness, high efficiency, and suitable microwave and infrared technologies, mainly because of frequency tunability, but with very limited output power availability and short wavelength coverage. the limitations in both THz generation and detection. Development of new THz sources has been recently receiving Using periodically poled nonlinear optical crystals considerable interest in many applications such as security for efficient THz generation is becoming an alternative inspection, spectroscopy, medical imaging, and sensing. The approach. Conventional techniques such as difference fre- application requirements for such THz sources are versatile, quency generation (DFG), which uses two laser sources and it will be reasonable to classify them according to the (either nanosecond-pulsed or cw), are attractive in inducing THz sources’ compactness, frequency tunability, emission coherent THz waves with a suitable frequency tunability. linewidth, coherence, and output power. Phase matching among the three interactive waves (two opti- The most common approach used to generate THz waves cal and one THz) can be realized by artificially introducing is to rectify a femotosecond (fs) laser pulse using an electro- reversed domain structures (so-called quasiphase matching optic (EO) crystal. Efficient ultrabroad band, single-cycle (QPM) method if periodically or quasiperiodically poled THz wave generation has been realized in a few crystals such [2]). Unfortunately, such techniques’ generation efficiency as ZnTe or GaP at a wavelength around 800 nm, under the is low due to the strong absorption of THz waves in those condition of matching both optical and THz pulses’ group commonly used nonlinear crystals such as LiNbO (LN), velocities [1]. Tradeoff for this approach is that the majority KTiOPO (KTP) [3]. of such EO crystals have strong material dispersion, which Optical rectification of fs laser pulses using artificially limits the output wave’s bandwidth and power. On the other poled nonlinear optical crystals is used to generate multicycle 2 Advances in OptoElectronics or arbitrary wave forms [4]. When an fs optical pulse discussed here, should be highly transparent for a wide THz propagates through a poled lithium niobate (PLN) crystal frequency range. (2) with its second-order nonlinear susceptibility (χ ) reversing The energy and the momentum conversion laws for its sign between neighboring domains, a THz nonlinear generating the THz frequencies via the DFG, for example, polarization is generated via DFG or optical rectification. can be described as Due to the group velocity mismatch between optical and E = E + E , ω1 ω2 ωTHz THz waves, the optical pulse will lead the THz by the optical (1) → → → → pulse duration τ after a walkoff length l = cτ /(n − p w p THz K = K + K + K , ω1 ω2 ωTHz Λ n ). If the domain length of the poled nonlinear crystal is optical comparable to the walk-off length, each domain in the crystal where E is the photon energy, K is the wave vector at each contributes a half cycle to the radiated THz field. Similar frequency, and K is the grating’s reciprocal vector. The to the above DFG approach, in this case, high material generated THz frequency can be then determined by absorption to THz waves will be still the major reason for the c 1 poor generation efficiency. Apparently, either a significant ω = · ,(2) THz improvement on the material’s transparency over those THz Λ n − n · sin Φ O THz wavelengths or a new device design able to significantly where Λ is the domain-poled period, n and n are minimize the THz absorption issue will be pressingly in O THz refractive indices for optical and THz waves, respectively, and demand in order to bring such devices to the more practical Φ is the internal direction of the generated THz emission, side of the potential THz applications. which will be a key parameter when designing the device for In this article, a new device design relying on the realizing efficient coupling of the generated THz wave from multicladding nonlinear fiber format (MCNF) will be dis- the main core into cladding 3, as shown in Figure 2 before. cussed for potential efficient THz generation application. Figure 3 shows the calculated dependence of forward and This design has the potential to solve the nonlinear material’s backward THz frequencies on the poled domain periods (Λ) absorption issue over those generated THz waves, and it when using lithium niobate single crystals. Listed K , K , maintains the high conversion efficiency that a strongly p s and K are wave vectors for the pump (ω ), the signal THz 1 confined optical fiber may provide. To realize such new (ω ), and the generated THz wave, respectively. Aside each multicladding fiber designs, efficient fabrication of the poled 2 curve, schematic of the corresponding wave-vector diagram nonlinear optical fibers will be the first and major step, is also shown. Inset inside Figure 3 shows a relationship of and this will be discussed with details in Section 3 after THz frequency versus the internal emission angle (Φ)ata introducing the device design. In Section 4, those reversed fixed poling period around 50 μm using the LiNbO crystal. domain structures are nondestructively revealed by both Combining with the general waveguide theories of optical crossly polarized optical microscope (CPOM) and confocal fibers, simulation results obtained here can be further used scanning optical microscope (CSOM). to design the device including both material selection and the dimensional determination of claddings and the main 2. The Multicore Nonlinear Fiber Design core. However, this is a separate research effort that will be published in somewhere else. In the following sections, we Figure 1 shows the schematic of the multicladding nonlinear mainly report the fabrication of a periodically poled LiNbO fiber. The main core inside the design can be a domain- 3 single-crystal fiber, which is the key to further make the inverted lithium niobate single-crystal fiber (1), while the MCNF device. first (2), the second (3), and the third (4) claddings can be made from the polyamide matrix materials with their refractive indices changed by certain dopants. The refractive 3. Poling the LN Single-Crystal Fiber index requirements for such designs are as follows: for the optical wave, n >n , n ,and n ; and for the THz The a-axis-oriented LN single-crystal fibers having cross- O1 O2 O3 O4 wave, n >n and n >n . In this case, the pumping section dimensions ranging from 100 μm to 130 μmand T3 T2 T3 T4 optical wave will be confined inside the main core (1), and lengths from 10 mm to 50 mm were grown using the LHPG the generated THz wave will be coupled into and propagates method [5]. Normally, an a-axis LN single-crystal fiber in cladding 3. Geometrically, cladding 2 should be thin, has an elliptical cross-section with two ridges as shown which should allow the generated THz wave losslessly side- in Figure 4(a). Its c-axis orientation is determined along emitted into cladding 3. In the THz frequency range, LN the short axis of the ellipse and the b-axis along its long crystal has a refractive index around 5.5, and that of those axis. This natural configuration makes it convenient to use polyamide materials is normally around 2.1. For the two contacting electrodes to electrically pole the fiber, simply materials in optical frequencies, their refractive indices are for the reason that the applied electric field is required to around 2.2 and 1.4, respectively. Apparently, the use of be aligned parallel to the c-axis (the polarization direction). such MCNF design has the potential to fully eliminate the During the poling, the fiber is placed on top of a copper block nonlinear optical material absorption issue discussed before and a gold grating is slightly pressed onto the fiber. The gold by separating the generated THz beam from the optical grating, which has the predetermined structures, was made beam into a different path of propagation. Those cladding on a Φ2” sapphire wafer using a standard photolithographic materials to be selected, such as the polyamide materials process (Figure 4(b)). Listed d and d inside the inset are 1 2 Advances in OptoElectronics 3 Optical THz guide guide O1 O3 n n O2 O4 Main core n , n O1 T1 2 3 4 1 T1 T3 n n T2 T4 Axial direction (a) (b) Figure 1: Schematic of the multicladding nonlinear fiber design. (a): the cross-section view, and (b): the axial index profile. 4.5 Λ = 50 μm Forward THz 2.5 Forward 3.5 Forward 1.5 THz Backward Optical 2.5 Backward THz −40 0 40 pulses 2 Φ (deg) 1.5 Backward Figure 2: Schematic of the domain-reversed nonlinear core, the THz generated THz wave coupling, and both forward and backward K propagation THz waves. Λ 0.5 10 20 30 40 50 60 70 Domain period Λ (μm) Figure 3: Thegenerated THzfrequency versus thedomainperiod gold electrode widths, which are changeable by designing, in LiNbO crystal for both forward and backward propagation in order to adjust the poled domain period duty cycle. schemes. K , K , K , K , are wave vectors for the pump (ω ), p s THz Λ 1 the signal (ω ), the generated THz wave, and the reciprocal vector The copper block and gold grating are connected to each of the grating, respectively. Inset shows the relationship between the polarity of a high-voltage power supply. LN crystals have generated THz frequency and the internal emission angle (this angle a coercive field approximately 22 kV/mm. For an LN fiber is to be used for further coupling the THz wave out from the main with a dimension of 120 μm along the c-axis, the applied core). voltage should be close to 4 kV. In our experiments, special measures such as filling with Teflon coatings were taken to prevent the breakdown of air, simply because of the short electrode space and high voltage (this is different from puling LN fiber was simply put between a crossly polarized light. bulk crystal wafers). Most as-grown single-crystal fibers The method can disclose periodic domains in a relatively present a single-domain structure. However, it is difficult to larger view field. Because of this, in this research the discriminate between the +c or −c surfaces without using CPOM was mainly used to evaluate both completeness and destructive methods such as wet etching. In our case, simply uniformity of the periodic domains along the length of switching the applied electrical field’s polarity and observing the fiber, characteristics that can be regarded as the poling the sudden current change during poling can find the right quality at macroscale level. The CSOM, on the other hand, polarization arrangement. provides a means for measuring the electro-optic response Periodic domain structure inside the LN fiber was to a small ac electrical field modulation. With the use of then revealed using both a crossly polarized optical micro- lock-in amplification, the sensitivity of this method can scope (CPOM) and a confocal scanning optical microscope be greatly enhanced, allowing one to obtain high-contrast (CSOM) [6]. The main reason to use the two methods is ferroelectric images using a relatively small ac field. The because of their nature of being nondestructive to samples linear relationship between the electro-optic coefficient and under the measurement. When using the CPOM, the poled the ferroelectric polarization allows this technique to work n n THz optical Frequency (THz) THz 4 Advances in OptoElectronics Sapphire Copper block (a) 10 μm Figure 5: The periodic domains inside a 17 mm long LN single- crystal fiber revealed by a crossly polarized optical microscope. 10 μm Center 180 domain boundary (b) Figure 4: (a) The poling setup used for poling a-axis-grown lithium Duty cycle ∼ 1 niobate single-crystal fiber (inset: the fiber’s elliptic cross-section 10 μm image). (b) One design of the top interdigital electrode. d and d 1 2 inside the inset are gold electrodes’ width, which are changeable by (a) (b) design in order to adjust the poled domain period duty cycle. Figure 6: The confocal scanning optical microscope images under both dc-mode (a) and ac-mode (b). Uniformity of the period duty over a broad range of field amplitudes, frequencies, and cycle across the fiber length is good. orientations of electric field, ferroelectric axis, and light polarization. The CSOM method was used because of its capability to nondestructively study the LN single-crystal dark dots are due to surface defects and possible surface fibers and to disclose the domain structure details including contaminations. Using this operational mode, ferroelectric domain boundary or period duty circle inside the fibers, domains or boundaries are invisible. In Figure 5(b), a characteristics that can be regarded as the poling quality at simultaneously acquired image of the ac field-modulated the microscale level. signal is presented. The dark and bright areas represent In the CSOM, a 632.0 nm HeNe laser is used as a ferroelectric domains having +c and −c polarizations. The light source. The beam then passes a linear polarizer and observed domain boundaries are thin and sharp, and are half-wave plate which provides a capability to select the below the resolution of the CSOM. The image confirms the beam’s polarization direction. The light is focused to a periodicity of ∼20 μm and a period duty circle of about 1 : 1. diffraction-limited spot using a high NA objective. The fiber Uniformity of the period duty cycle across the fiber length was mounted on a three-dimensional piezoelectric scanner. was examined to be quite good. In the measurement, the LN fiber was tightly clamped by two silver-coated copper plates as electrodes. The soft silver coating was used for reaching better electric contact. The 4. Conclusion LN fiber was aligned with its b-axis normal to the plates. The ac voltage was applied onto the two electrodes variable A new multicladding nonlinear fiber design was proposed from 0–100 V, and, therefore, the ac field can vary between to generate THz waves with the potential of reaching high 0∼ 7 kV/cm. efficiency. Inside the design, the main core of the fiber will Figure 5 shows an image of the periodically poled be a domain-reversed nonlinear crystal such as periodically ferroelectric domains inside the LN single-crystal fiber under poled LN crystal and the multiple claddings are from a crossly polarized optical microscope. The periodicity of the those THz transparent materials such as polyamides. This structure was measured to be roughly 20 μm, whichisvery design has the potential to reduce the common material close to the designed value for further generating 2.5 THz absorption issue over the THz frequency and keeps the wavelength. A typically scanned image of linearly polarized high efficiency when using the fiber format. As one of the light reflected from the LN fiber’s surface is shown in major experimental efforts, periodic ferroelectric domain Figure 6(a). This is actually an optical image when observed structures were successfully introduced into lithium niobate using a conventional optical microscope. The stripes and single-crystal fibers. The poling completeness and domain 5 μm 13 μm 5 μm Advances in OptoElectronics 5 uniformity were examined by using a crossly polarized optical microscope. The periodic domains’ boundary and period duty circle were examined by a confocal scanning optical microscope. References [1] Q. Wu and X.-C. Zhang, “Ultrafast electro-optic field sensors,” Applied Physics Letters, vol. 68, no. 12, pp. 1604–1606, 1996. [2] Y.-L. Lu,L.Mao,S.-D. Cheng, N.-B.Ming,and Y.-T.Lu, “Second-harmonic generation of blue light in LiNbO crystal with periodic ferroelectric domain structures,” Applied Physics Letters, vol. 59, no. 5, pp. 516–518, 1991. [3] R. Guo, K. Akiyama, H. Minamide, and H. Ito, “All-solid-state, narrow linewidth, wavelength-agile terahertz-wave generator,” Applied Physics Letters, vol. 88, no. 9, Article ID 091120, 3 pages, [4] Y.-S. Lee, T. Meade, V. Perlin, H. Winful, T. B. Norris, and A. Galvanauskas, “Generation of narrow-band terahertz radiation via optical rectification of femtosecond pulses in periodically poled lithium niobate,” Applied Physics Letters, vol. 76, no. 18, pp. 2505–2507, 2000. [5] M.M.Fejer,J.L.Nightingale, G. A. Magel, andR.L.Byer, “Laser-heated miniature pedestal growth apparatus for single- crystal optical fibers,” Review of Scientific Instruments, vol. 55, no. 11, pp. 1791–1796, 1984. [6] C. Hubert and J. Levy, “Nanometer-scale imaging of domains in ferroelectric thin films using apertureless near-field scanning optical microscopy,” Applied Physics Letters, vol. 73, no. 22, pp. 3229–3231, 1998. 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Domain-Reversed Lithium Niobate Single-Crystal Fibers are Potentially for Efficient Terahertz Wave Generation

Advances in OptoElectronics , Volume 2008 – Oct 30, 2008

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Hindawi Publishing Corporation
Copyright
Copyright © 2008 Yalin Lu and Kitt Reinhardt. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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1687-563X
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1687-5648
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10.1155/2008/208458
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Abstract

Hindawi Publishing Corporation Advances in OptoElectronics Volume 2008, Article ID 208458, 5 pages doi:10.1155/2008/208458 Research Article Domain-Reversed Lithium Niobate Single-Crystal Fibers are Potentially for Efficient Terahertz Wave Generation 1 2 Yalin Lu and Kitt Reinhardt The Physics Department, Laser and Optics Research Center (LORC), 2354 Fairchild Dr. 2A31, United States Air Force Academy, CO 80840, USA Air Force Office of Scientific Research (AFOSR/NE), 875 North Randolph Street, Suite 326, Arlington, VA 22203, USA Correspondence should be addressed to Yalin Lu, yalin.lu@usafa.edu Received 16 May 2008; Accepted 31 August 2008 Recommended by Hiroshi Murata Nonlinear frequency conversion remains one of the dominant approaches to efficiently generate THz waves. Significant material absorption in the THz range is the main factor impeding the progress towards this direction. In this research, a new multicladding nonlinear fiber design was proposed to solve this problem, and as the major experimental effort, periodic domain structure was introduced into lithium niobate single-crystal fibers by electrical poling. The introduced periodic domain structures were nondestructively revealed using a crossly polarized optical microscope and a confocal scanning optical microscope for quality assurance. Copyright © 2008 Y. Lu and K. Reinhardt. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. Introduction extreme, continuous-wave (cw) THz generation has been realized in free-electron lasers and quantum cascade lasers. The terahertz (THz) frequency range (0.1∼30 THz) lies The former offers high output power, but they are bulky in the gap between microwave and infrared of the elec- and inflexible. The latter, however, provides the potential tromagnetic spectrum. THz technology lags behind both for good system compactness, high efficiency, and suitable microwave and infrared technologies, mainly because of frequency tunability, but with very limited output power availability and short wavelength coverage. the limitations in both THz generation and detection. Development of new THz sources has been recently receiving Using periodically poled nonlinear optical crystals considerable interest in many applications such as security for efficient THz generation is becoming an alternative inspection, spectroscopy, medical imaging, and sensing. The approach. Conventional techniques such as difference fre- application requirements for such THz sources are versatile, quency generation (DFG), which uses two laser sources and it will be reasonable to classify them according to the (either nanosecond-pulsed or cw), are attractive in inducing THz sources’ compactness, frequency tunability, emission coherent THz waves with a suitable frequency tunability. linewidth, coherence, and output power. Phase matching among the three interactive waves (two opti- The most common approach used to generate THz waves cal and one THz) can be realized by artificially introducing is to rectify a femotosecond (fs) laser pulse using an electro- reversed domain structures (so-called quasiphase matching optic (EO) crystal. Efficient ultrabroad band, single-cycle (QPM) method if periodically or quasiperiodically poled THz wave generation has been realized in a few crystals such [2]). Unfortunately, such techniques’ generation efficiency as ZnTe or GaP at a wavelength around 800 nm, under the is low due to the strong absorption of THz waves in those condition of matching both optical and THz pulses’ group commonly used nonlinear crystals such as LiNbO (LN), velocities [1]. Tradeoff for this approach is that the majority KTiOPO (KTP) [3]. of such EO crystals have strong material dispersion, which Optical rectification of fs laser pulses using artificially limits the output wave’s bandwidth and power. On the other poled nonlinear optical crystals is used to generate multicycle 2 Advances in OptoElectronics or arbitrary wave forms [4]. When an fs optical pulse discussed here, should be highly transparent for a wide THz propagates through a poled lithium niobate (PLN) crystal frequency range. (2) with its second-order nonlinear susceptibility (χ ) reversing The energy and the momentum conversion laws for its sign between neighboring domains, a THz nonlinear generating the THz frequencies via the DFG, for example, polarization is generated via DFG or optical rectification. can be described as Due to the group velocity mismatch between optical and E = E + E , ω1 ω2 ωTHz THz waves, the optical pulse will lead the THz by the optical (1) → → → → pulse duration τ after a walkoff length l = cτ /(n − p w p THz K = K + K + K , ω1 ω2 ωTHz Λ n ). If the domain length of the poled nonlinear crystal is optical comparable to the walk-off length, each domain in the crystal where E is the photon energy, K is the wave vector at each contributes a half cycle to the radiated THz field. Similar frequency, and K is the grating’s reciprocal vector. The to the above DFG approach, in this case, high material generated THz frequency can be then determined by absorption to THz waves will be still the major reason for the c 1 poor generation efficiency. Apparently, either a significant ω = · ,(2) THz improvement on the material’s transparency over those THz Λ n − n · sin Φ O THz wavelengths or a new device design able to significantly where Λ is the domain-poled period, n and n are minimize the THz absorption issue will be pressingly in O THz refractive indices for optical and THz waves, respectively, and demand in order to bring such devices to the more practical Φ is the internal direction of the generated THz emission, side of the potential THz applications. which will be a key parameter when designing the device for In this article, a new device design relying on the realizing efficient coupling of the generated THz wave from multicladding nonlinear fiber format (MCNF) will be dis- the main core into cladding 3, as shown in Figure 2 before. cussed for potential efficient THz generation application. Figure 3 shows the calculated dependence of forward and This design has the potential to solve the nonlinear material’s backward THz frequencies on the poled domain periods (Λ) absorption issue over those generated THz waves, and it when using lithium niobate single crystals. Listed K , K , maintains the high conversion efficiency that a strongly p s and K are wave vectors for the pump (ω ), the signal THz 1 confined optical fiber may provide. To realize such new (ω ), and the generated THz wave, respectively. Aside each multicladding fiber designs, efficient fabrication of the poled 2 curve, schematic of the corresponding wave-vector diagram nonlinear optical fibers will be the first and major step, is also shown. Inset inside Figure 3 shows a relationship of and this will be discussed with details in Section 3 after THz frequency versus the internal emission angle (Φ)ata introducing the device design. In Section 4, those reversed fixed poling period around 50 μm using the LiNbO crystal. domain structures are nondestructively revealed by both Combining with the general waveguide theories of optical crossly polarized optical microscope (CPOM) and confocal fibers, simulation results obtained here can be further used scanning optical microscope (CSOM). to design the device including both material selection and the dimensional determination of claddings and the main 2. The Multicore Nonlinear Fiber Design core. However, this is a separate research effort that will be published in somewhere else. In the following sections, we Figure 1 shows the schematic of the multicladding nonlinear mainly report the fabrication of a periodically poled LiNbO fiber. The main core inside the design can be a domain- 3 single-crystal fiber, which is the key to further make the inverted lithium niobate single-crystal fiber (1), while the MCNF device. first (2), the second (3), and the third (4) claddings can be made from the polyamide matrix materials with their refractive indices changed by certain dopants. The refractive 3. Poling the LN Single-Crystal Fiber index requirements for such designs are as follows: for the optical wave, n >n , n ,and n ; and for the THz The a-axis-oriented LN single-crystal fibers having cross- O1 O2 O3 O4 wave, n >n and n >n . In this case, the pumping section dimensions ranging from 100 μm to 130 μmand T3 T2 T3 T4 optical wave will be confined inside the main core (1), and lengths from 10 mm to 50 mm were grown using the LHPG the generated THz wave will be coupled into and propagates method [5]. Normally, an a-axis LN single-crystal fiber in cladding 3. Geometrically, cladding 2 should be thin, has an elliptical cross-section with two ridges as shown which should allow the generated THz wave losslessly side- in Figure 4(a). Its c-axis orientation is determined along emitted into cladding 3. In the THz frequency range, LN the short axis of the ellipse and the b-axis along its long crystal has a refractive index around 5.5, and that of those axis. This natural configuration makes it convenient to use polyamide materials is normally around 2.1. For the two contacting electrodes to electrically pole the fiber, simply materials in optical frequencies, their refractive indices are for the reason that the applied electric field is required to around 2.2 and 1.4, respectively. Apparently, the use of be aligned parallel to the c-axis (the polarization direction). such MCNF design has the potential to fully eliminate the During the poling, the fiber is placed on top of a copper block nonlinear optical material absorption issue discussed before and a gold grating is slightly pressed onto the fiber. The gold by separating the generated THz beam from the optical grating, which has the predetermined structures, was made beam into a different path of propagation. Those cladding on a Φ2” sapphire wafer using a standard photolithographic materials to be selected, such as the polyamide materials process (Figure 4(b)). Listed d and d inside the inset are 1 2 Advances in OptoElectronics 3 Optical THz guide guide O1 O3 n n O2 O4 Main core n , n O1 T1 2 3 4 1 T1 T3 n n T2 T4 Axial direction (a) (b) Figure 1: Schematic of the multicladding nonlinear fiber design. (a): the cross-section view, and (b): the axial index profile. 4.5 Λ = 50 μm Forward THz 2.5 Forward 3.5 Forward 1.5 THz Backward Optical 2.5 Backward THz −40 0 40 pulses 2 Φ (deg) 1.5 Backward Figure 2: Schematic of the domain-reversed nonlinear core, the THz generated THz wave coupling, and both forward and backward K propagation THz waves. Λ 0.5 10 20 30 40 50 60 70 Domain period Λ (μm) Figure 3: Thegenerated THzfrequency versus thedomainperiod gold electrode widths, which are changeable by designing, in LiNbO crystal for both forward and backward propagation in order to adjust the poled domain period duty cycle. schemes. K , K , K , K , are wave vectors for the pump (ω ), p s THz Λ 1 the signal (ω ), the generated THz wave, and the reciprocal vector The copper block and gold grating are connected to each of the grating, respectively. Inset shows the relationship between the polarity of a high-voltage power supply. LN crystals have generated THz frequency and the internal emission angle (this angle a coercive field approximately 22 kV/mm. For an LN fiber is to be used for further coupling the THz wave out from the main with a dimension of 120 μm along the c-axis, the applied core). voltage should be close to 4 kV. In our experiments, special measures such as filling with Teflon coatings were taken to prevent the breakdown of air, simply because of the short electrode space and high voltage (this is different from puling LN fiber was simply put between a crossly polarized light. bulk crystal wafers). Most as-grown single-crystal fibers The method can disclose periodic domains in a relatively present a single-domain structure. However, it is difficult to larger view field. Because of this, in this research the discriminate between the +c or −c surfaces without using CPOM was mainly used to evaluate both completeness and destructive methods such as wet etching. In our case, simply uniformity of the periodic domains along the length of switching the applied electrical field’s polarity and observing the fiber, characteristics that can be regarded as the poling the sudden current change during poling can find the right quality at macroscale level. The CSOM, on the other hand, polarization arrangement. provides a means for measuring the electro-optic response Periodic domain structure inside the LN fiber was to a small ac electrical field modulation. With the use of then revealed using both a crossly polarized optical micro- lock-in amplification, the sensitivity of this method can scope (CPOM) and a confocal scanning optical microscope be greatly enhanced, allowing one to obtain high-contrast (CSOM) [6]. The main reason to use the two methods is ferroelectric images using a relatively small ac field. The because of their nature of being nondestructive to samples linear relationship between the electro-optic coefficient and under the measurement. When using the CPOM, the poled the ferroelectric polarization allows this technique to work n n THz optical Frequency (THz) THz 4 Advances in OptoElectronics Sapphire Copper block (a) 10 μm Figure 5: The periodic domains inside a 17 mm long LN single- crystal fiber revealed by a crossly polarized optical microscope. 10 μm Center 180 domain boundary (b) Figure 4: (a) The poling setup used for poling a-axis-grown lithium Duty cycle ∼ 1 niobate single-crystal fiber (inset: the fiber’s elliptic cross-section 10 μm image). (b) One design of the top interdigital electrode. d and d 1 2 inside the inset are gold electrodes’ width, which are changeable by (a) (b) design in order to adjust the poled domain period duty cycle. Figure 6: The confocal scanning optical microscope images under both dc-mode (a) and ac-mode (b). Uniformity of the period duty over a broad range of field amplitudes, frequencies, and cycle across the fiber length is good. orientations of electric field, ferroelectric axis, and light polarization. The CSOM method was used because of its capability to nondestructively study the LN single-crystal dark dots are due to surface defects and possible surface fibers and to disclose the domain structure details including contaminations. Using this operational mode, ferroelectric domain boundary or period duty circle inside the fibers, domains or boundaries are invisible. In Figure 5(b), a characteristics that can be regarded as the poling quality at simultaneously acquired image of the ac field-modulated the microscale level. signal is presented. The dark and bright areas represent In the CSOM, a 632.0 nm HeNe laser is used as a ferroelectric domains having +c and −c polarizations. The light source. The beam then passes a linear polarizer and observed domain boundaries are thin and sharp, and are half-wave plate which provides a capability to select the below the resolution of the CSOM. The image confirms the beam’s polarization direction. The light is focused to a periodicity of ∼20 μm and a period duty circle of about 1 : 1. diffraction-limited spot using a high NA objective. The fiber Uniformity of the period duty cycle across the fiber length was mounted on a three-dimensional piezoelectric scanner. was examined to be quite good. In the measurement, the LN fiber was tightly clamped by two silver-coated copper plates as electrodes. The soft silver coating was used for reaching better electric contact. The 4. Conclusion LN fiber was aligned with its b-axis normal to the plates. The ac voltage was applied onto the two electrodes variable A new multicladding nonlinear fiber design was proposed from 0–100 V, and, therefore, the ac field can vary between to generate THz waves with the potential of reaching high 0∼ 7 kV/cm. efficiency. Inside the design, the main core of the fiber will Figure 5 shows an image of the periodically poled be a domain-reversed nonlinear crystal such as periodically ferroelectric domains inside the LN single-crystal fiber under poled LN crystal and the multiple claddings are from a crossly polarized optical microscope. The periodicity of the those THz transparent materials such as polyamides. This structure was measured to be roughly 20 μm, whichisvery design has the potential to reduce the common material close to the designed value for further generating 2.5 THz absorption issue over the THz frequency and keeps the wavelength. A typically scanned image of linearly polarized high efficiency when using the fiber format. As one of the light reflected from the LN fiber’s surface is shown in major experimental efforts, periodic ferroelectric domain Figure 6(a). This is actually an optical image when observed structures were successfully introduced into lithium niobate using a conventional optical microscope. The stripes and single-crystal fibers. The poling completeness and domain 5 μm 13 μm 5 μm Advances in OptoElectronics 5 uniformity were examined by using a crossly polarized optical microscope. The periodic domains’ boundary and period duty circle were examined by a confocal scanning optical microscope. References [1] Q. Wu and X.-C. Zhang, “Ultrafast electro-optic field sensors,” Applied Physics Letters, vol. 68, no. 12, pp. 1604–1606, 1996. [2] Y.-L. Lu,L.Mao,S.-D. Cheng, N.-B.Ming,and Y.-T.Lu, “Second-harmonic generation of blue light in LiNbO crystal with periodic ferroelectric domain structures,” Applied Physics Letters, vol. 59, no. 5, pp. 516–518, 1991. [3] R. Guo, K. Akiyama, H. Minamide, and H. Ito, “All-solid-state, narrow linewidth, wavelength-agile terahertz-wave generator,” Applied Physics Letters, vol. 88, no. 9, Article ID 091120, 3 pages, [4] Y.-S. Lee, T. Meade, V. Perlin, H. Winful, T. B. Norris, and A. Galvanauskas, “Generation of narrow-band terahertz radiation via optical rectification of femtosecond pulses in periodically poled lithium niobate,” Applied Physics Letters, vol. 76, no. 18, pp. 2505–2507, 2000. [5] M.M.Fejer,J.L.Nightingale, G. A. Magel, andR.L.Byer, “Laser-heated miniature pedestal growth apparatus for single- crystal optical fibers,” Review of Scientific Instruments, vol. 55, no. 11, pp. 1791–1796, 1984. [6] C. Hubert and J. Levy, “Nanometer-scale imaging of domains in ferroelectric thin films using apertureless near-field scanning optical microscopy,” Applied Physics Letters, vol. 73, no. 22, pp. 3229–3231, 1998. 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