有机电光晶体4-(4-二甲基氨基苯乙烯基)甲基吡啶对甲基苯磺酸盐的太赫兹光谱研究

采用色散校正的密度泛函理论(dispersion-corrected density functional theory,DFT-D2)对有机电光晶体4-(4-二甲基氨基苯乙烯基)甲基吡啶对甲基苯磺酸盐(4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate,DAST)进行结构优化和太赫兹光谱计算,通过逐步提高精度进行几何优化的方法寻找DAST收敛的基态稳定结构,获得与DAST初始结构相一致的基态稳定结构.在此结构的基础上,在0—4 THz范围的太赫兹计算光谱与实验测量结果一致,说明采用DFT-D2进行优化的合理性.重要的是,首次通过计算的太赫兹光谱对DAST在0—4 THz范围的太赫兹吸收峰的振动模式进行了详细归属.结果表明:1.12 THz处的振动是DAST阴阳离子的光学声子模式,1.46 THz和1.54 THz两处的振动主要与磺酸盐有关,而2.63 THz和3.16 THz两处的振动则分别源于阳离子的扭转振动和阴离子的转动.该结果不仅很好地说明了阴阳离子分别在太赫兹响应中的贡献,而且为今后通过取代阴阳离子基团获取具有更高二阶非线性效应的DAST衍生物的新合成提供了重要的参考和指导.本文结果说明密度泛函理论在太赫兹光子学上的重要应用,对探究有机电光晶体的太赫兹响应物理原理、性能控制等具有重要的指导价值.     

Difference-frequency terahertz-wave generation from 4-dimethylamino-N-methyl-4-stilbazolium-tosylate by use of an electronically tuned Ti:sapphire laser

Among nonlinear materials, the organic ionic salt crystal 4-dimethylamino- N -methyl-4-stilbazolium-tosylate (DAST) is known for its large nonlinearity. We generated a coherent terahertz (THz) wave, using DAST, from the difference frequency between two oscillating wavelengths of an electronically tuned Ti:sapphire laser. In LiNbO(3), LiTaO(3), KTiOPO(4), and GaP crystals, THz-wave generation was not observed under the same experimental conditions. This result proves the high efficiency of DAST crystals for generation of difference-frequency THz waves.     

非线性有机材料DAST的研究及应用

随着光通信、光信息处理技术等的快速发展,非线性光学材料越来越受到学术界与工业的关注。与无机非线性光学材料相比,有机非线性光学材料具有响应时间短、易于加工和高非线性系数等优点。其中,4-(4-二甲氨基苯乙烯基)甲基吡啶对甲苯磺酸盐(DAST)是人工设计的一种具有非中心对称性和较强极化率的有机非线性材料。大量理论和实验结果表明,DAST具有高二阶光学非线性系数、高电光系数、大双折射率差和低介电常数等特性,能够产生更快、更强的光学非线性响应,还具有优异的太赫兹波发射及吸收性能,是目前综合性能最优、应用最广、最具研究价值的非线性有机材料。近年光谱研究显示,DAST还表现出各向异性的太赫兹光谱特性。文章系统地总结了DAST晶体在太赫兹波发射、二次谐波产生、电光探测和电光调制等众多领域的国内外研究成果,还概括了我们团队近期在DAST基薄膜、太赫兹光谱、光电特性、超材料等新领域的研究进展。此外,还提出将准相位匹配法用于DAST晶体研究、利用电场或自组装膜诱导生长DAST晶体等新思路。对DAST成果的总结和梳理,将促进非线性DAST有机材料在电光调制器、频率转换器、THz探测器等重要领域的应用与理论研究,使其能够更加广泛地应用在光通信、光信息处理、军事技术等重要领域。     

Design and analysis of a metallic waveguide with a DAST cap for continuously phase-matched terahertz difference frequency generation

We present a novel source for continuous terahertz (THz) wave generation using an organic ionic salt, 4-dimethylamino-N-methyl-4-stilbazolium-tosylate (DAST). THz waves are generated based on difference frequency generation (DFG) in the device. Phase matching condition and THz generation between 1.3 THz and 2.7 THz, for optical pump around 1.6 μm, are investigated. Our calculations predict that the device produces a relatively high THz output power of 11.07 μW from a 4 cm long waveguide at 2 THz.     

Widely tunable broadband terahertz radiation generation using a configurationally locked polyene 2-[3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene] malononitrile crystal via difference frequency generation

Widely tunable terahertz (THz) waves were successfully generated from 0.5 to 10 THz via difference frequency generation (DFG) in a configurationally locked polyene 2-[3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene] malononitrile (OH1) crystal. Potassium titanium oxide phosphate optical parametric oscillator pumped by nanosecond Q-switched Nd:YAG laser was used to generate two waves, which were then used to irradiate OH1 crystal. The maximum energy of the generated THz wave was about 461 pJ/pulse. We investigated the dependency of generated THz energy to the excitation pump power density and OH1 crystal thickness. In addition, we compared the THz energy generated by OH1 crystal to 4-N,N-dimethylamino-4′-N′-methyl-4-stilbazolium tosylate (DAST) crystal using DFG, and we achieved 560 times higher energy using OH1 crystal than DAST crystal at around 1.1 THz.     

基于有机吡啶盐晶体的太赫兹频率上转换探测

利用激光泵浦国产有机吡啶盐4-(4-二甲基氨基苯乙烯基)甲基吡啶对甲基苯磺酸盐(4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate,DAST)晶体,通过非线性频率上转换方法实现了室温运转的高灵敏、快响应、宽频段太赫兹探测.高效生成了近红外上转换光,采集到其脉冲包络和光谱,获得了ns量级的时间分辨率,并换算太赫兹波的频率,实现了对太赫兹信息的全面表征.与商用高莱探测器相比,上转换方法在19 THz频点的探测灵敏度高4个数量级;在可探测频率3.15—29.82 THz范围内,响应度普遍高2—3个数量级.结果表明:室温下的光泵频率上转换探测方法在时间分辨率和响应度方面远优于传统的热探测器,极大地提高了差频有源太赫兹系统的动态范围,使差频源在太赫兹波谱分析和成像等领域具有更大的应用潜力.     

利用国内生长的有机晶体DAST作为THz源的研究

利用时域光谱技术分别测量了通过光整流法在国内新生长的有机晶体DAST和国际常用的高效能THz晶体ZnTe中产生的THz脉冲的时域光谱,也测量和分析了有机晶体DAST在THz波段的色散和吸收特性,并通过热辐射探测计(4.2 K Si-bolometer)测量了THz脉冲的绝对能量。结果表明,国内新生长的有机晶体DAST的确是一种高效率的THz产生晶体,优化生长条件后将可得到更高的产生效率。     

Use of the organic crystal DAST for terahertz beam applications

We report the use of the organic crystal DAST as a freely propagating electromagnetic-wave sensor at terahertz (THz) frequency. We also report the result of using a DAST crystal as a mid-IR THz emitter. Compared with the optical rectification from our best ZnTe emitter, that obtained by use of DAST demonstrates a sixfold enhancement of radiation at 7-20 THz.     

Terahertz generation with tandem seeded optical parametric generators

A simple difference frequency generation (DFG) scheme based on two seeded optical parametric generators is presented as a tunable terahertz (THz) source. Using the nonlinear optical crystal 4-dimethylamino-N-methyl-4-stilbazolium-tosylate (DAST) as the DFG crystal, our system has demonstrated continuous and seamless tunable operation from 1.6 to 4.5 THz. The output bandwidth of the THz source is 2.4 GHz. The utility of the source over this spectral range is demonstrated by measuring a high-resolution transmission spectrum of water vapor in air.     

Far-infrared properties of DAST

Using terahertz time-domain spectroscopy, we have measured the index of refraction and the absorption coefficient of the organic ionic salt 4-N, N-dimethylamino-4?-N?-methyl-stilbazolium tosylate (DAST). This promising organic electro-optic material exhibits strong absorption and dispersion for frequencies above 1 THz at both room temperature and 83 K. No reduction in the absorption strength is observed when DAST is cooled, suggesting a single-phonon origin of the resonances. A simple vibration of the anion and cation of the salt is suggested as the origin of the exceptionally strong far-infrared absorption and the high-frequency electro-optic properties of DAST.     

Generation of coherent terahertz phonons by sharp focusing of a femtosecond laser beam in the bulk of crystalline insulators in a regime of plasma formation

The generation of coherent terahertz phonons in a regime of plasma formation by a femtosecond laser radiation with an intensity of 10¹³ W/cm² in the bulk of crystalline quartz has been detected by the method of probing by a probe pulse of the third harmonic. A smooth increase in the frequency of coherent terahertz phonons from 2.2 to 5.5 THz has been detected, along with its subsequent sharp decrease down to 2.2 THz due to an α-β phase transition in crystalline quartz. The generation of 1-THz coherent phonons has been detected in BaF₂ crystals. A smooth variation of the frequency of coherent phonons from 2 to 2.5 THz has been detected in leucosapphire. The generation of coherent phonons during local laser excitation in CaF₂ and LiF crystals develops at the frequencies of 2.3 and 0.1 THz, respectively.     

Nonlinear radiation response of n-doped indium antimonide and indium arsenide in intense terahertz field

The nonlinear radiation responses of two different n-doped bulk semiconductors: indium antimonide(In Sb) and indium arsenide(In As) in an intense terahertz(THz) field are studied by using the method of ensemble Monte Carlo(EMC)at room temperature. The results show that the radiations of two materials generate about 2-THz periodic regular spectrum distributions under a high field of 100 k V/cm at 1-THz center frequency. The center frequencies are enhanced to about 7 THz in In Sb, and only 5 THz in In As, respectively. The electron valley occupancy and the percentage of new electrons excited by impact ionization are also calculated. We find that the band nonparabolicity and impact ionization promote the generation of nonlinear high frequency radiation, while intervalley scattering has the opposite effect. Moreover, the impact ionization dominates in In Sb, while impact ionization and intervalley scattering work together in In As. These characteristics have potential applications in up-convension of THz wave and THz nonlinear frequency multiplication field.     

High-performance fiber-laser-based terahertz spectrometer

We have developed a rapid scanning terahertz (THz) spectrometer based on a synchronized two-fiber-laser system. When the system is set to the asynchronous optical sampling mode, THz spectra extending to 3 THz can be acquired within 1 μs at a signal-to-noise ratio of the electric field of better than 20. Signal averaging results in a dynamic range of more than 60 dB, and frequency components of more than 4 THz can be detected. When the lasers are set to the same repetition rate, electronically controlled optical sampling at a rate of 2.5 kHz is demonstrated, making the system versatile for different spectroscopic applications. Finally, we compare the THz emission spectra of a photoconductive switch that is pumped at 780 nm and a nonlinear DAST crystal excited at 1550 nm. We find that the spectral range of the spectrometer is significantly enhanced at higher frequencies, while the dynamic range remains constant.     

Cherenkov terahertz radiation from Dirac semimetals surface plasmon polaritons excited by an electron beam

We demonstrate a physical mechanism for terahertz(THz) generation from surface plasmon polaritons(SPPs). In a structure with a bulk Dirac semimetals(BDSs) film deposited on a dielectric substrate, the energy of the asymmetric SPP mode can be significantly enhanced to cross the light line of the substrate due to the SPP-coupling between the interfaces of the film. Therefore, the SPPs can be immediately transformed into Cherenkov radiation without removing the wavevector mismatch. Additionally, the symmetric SPP mode can also be dramatically lifted to cross the substrate light line when a buffer layer with low permittivity relative to the substrate is introduced. In this case, dual-frequency THz radiation from the two SPP modes can be generated simultaneously. The radiation intensity is significantly enhanced by over two orders due to the field enhancement of the SPPs. The radiation frequency can be tuned in the THz frequency regime by adjusting the beam energy and the chemical potential of the BDSs. Our results could find potential applications in developing room temperature, tunable, coherent, and intense THz radiation sources to cover the entire THz band.     

Generation of THz radiation using bulk, periodically and aperiodically poled lithium niobate – Part 1: Theory

Optical rectification of femtosecond pulses in nonlinear materials is an efficient method to generate ultra short terahertz (THz) pulses over a wide frequency range extending from 100 GHz to well above 10 THz. Lithium niobate is particularly well suited for such purposes and can be used both in bulk and periodically poled forms. Different optical techniques for the generation of THz pulses are presented and compared theoretically. The whole discussion is performed for the interaction of gaussian beams using the radiating antenna approach that takes into account the diffraction of the THz wave, and therefore may predict the THz emission in a direction that differs from the optical pulse propagation. PACS 42.65.Ky; 42.70.Mp; 42.72.Ai     

Manipulating Nonlinear Photocurrent from Interlayer Coupling in Bilayer Metamaterials for Polarized Terahertz Generation

Polarized terahertz (THz) wave generation is of great significance for chiral and anisotropic sensing applications. However, how to manipulate amplitude, polarization, and ellipticity of the THz generation is still a fundamental challenge. Herein, polarized THz wave generation is achieved from a bilayer metamaterial consisting of T-shaped structure (TSS) and split resonator rings (SRRs) by combining Maxwell and hydrodynamic equations. The elliptically polarized THz wave can be synthetized directly from horizontally and vertically polarized THz components due to the orthogonal nonlinear photocurrents along the arm-directions of TSS and SRRs, respectively. Besides, the ellipticity and the orientation angle of the THz polarization ellipse can be modulated by the twist angle between the SRRs and TSS layers. The maximum ellipticity can reach 0.34 while the orientation angle is tunable from −0.45 to 0.48π by tuning the twist angle. This work proposes an interlayer coupling method for the polarized THz sources based on metamaterials in potential circular dichroism and chiral sensing applications.     

Broadband terahertz pulse emission from ZnGeP2

Optical rectification is demonstrated in (110)-cut ZnGeP(2) (ZGP) providing broadband terahertz (THz) generation. The source is compared to both GaP and GaAs over a wavelength range of 1150 nm to 1600 nm and peak-intensity range of 0.5 GW/cm(2) to 40 GW/cm(2). ZGP peak-to-peak field amplitude is larger than in the other materials due to either lower nonlinear absorption or larger second-order nonlinearity. This material is well suited for broadband THz generation across a wide range of infrared excitation wavelengths.     

Simultaneous all-solid-state multi-wavelength lasers --- a promising pump source for generating highly coherent terahertz waves

A diode-end-pumped Nd:YAG dual-wavelength laser operating at 1319 and 1338 nm is demonstrated. The maximum average output power of the quasi-continuous wave linearly polarized dual-wavelength laser is obtained to be 2.1 W at a repetition rate of 50 kHz with an output power instability of less than 0.38% and beam quality factor M² of 1.45. Using the two lines, the highly coherent and narrow linewidth terahertz radiation of 3.23 THz can be generated in an organic 4-N, N-dimethylamino--methyl-stilbazolium tosylate (DAST) crystal. Meanwhile, the multi-wavelength red laser at 659.5, 664 and 669 nm is generated by frequency doubling and sum frequency processes in a lithium triborate (LBO) crystal. The average red laser output power is enhanced up to 1.625 W at a repetition rate of 15 kHz with an output power instability of better than 0.53% and beam quality factor M² of 6.05. Using the three lines, it is possible to generate the multi-wavelength THz radiation of 3.3, 3.43 and 6.73 THz in an appropriate difference frequency crystal.     

Widely tunable terahertz-wave generation using an N-benzyl-2-methyl-4-nitroaniline crystal

Widely tunable terahertz (THz)-wave generation using difference frequency generation (DFG) in an organic N-benzyl-2-methyl-4-nitroaniline (BNA) crystal was demonstrated. To our knowledge, this is the first report of THz-wave generation by BNA DFG. Large, high-quality single crystals of BNA (phi 8 mm x 30 mm) were grown using the vertical Bridgman method. The nonlinear optical (NLO) coefficient d(33) of the BNA crystal is approximately 234 pm/V, which is the largest value reported for any yellow NLO material. The collinear phase-matching condition of the type-0 configuration is satisfied using a 0.7-1 microm band pump wavelength. We generated THz waves using an organic BNA crystal; the generation range is 0.1-15 THz.     

Water vapor: An extraordinary terahertz wave source under optical excitation

In modern terahertz (THz) sensing and imaging spectroscopy, water is considered a nemesis to be avoided due to strong absorption in the THz frequency range. Here we report the first experimental demonstration and theoretical implications of using femtosecond laser pulses to generate intense broadband THz emission from water vapor. When we focused an intense laser pulse in water vapor contained in a gas cell or injected from a gas jet nozzle, an extraordinarily strong THz field from optically excited water vapor is observed. Water vapor has more than 50% greater THz generation efficiency than dry nitrogen. It had previously been assumed that the nonlinear generation of THz waves in this manner primarily involves a free-electron plasma, but we show that the molecular structure plays an essential role in the process. In particular, we found that THz wave generation from H₂O vapor is significantly stronger than that from D₂O vapor. Vibronic activities of water cluster ions, occurring naturally in water vapor, may possibly contribute to the observed isotope effect along with rovibrational contributions from the predominant monomers.