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.     

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.     

Nonlinear optical terahertz wave sources

We developed a Cherenkov phase-matching method for monochromatic THz-wave generation using the DFG, process with a lithium niobate crystal, which resulted in both high conversion efficiency and wide tunability. Although THz-wave generation by Cherenkov phase matching has been demonstrated using femtosecond pumping pulses, producing very high peak power, these THz-wave sources are not monochromatic. Our method generates monochromatic and tunable THz, waves using a nanosecond pulsed laser source. We also show that Cherenkov radiation with waveguide structure is an effective strategy for achieving extremely wide tunable THz-wave source. We fabricated MgO-doped lithium niobate slab waveguide and demonstrated difference frequency generation of THz-wave generation with Cherenkov phase matching. Extremely frequency-widened THz-wave generation, from 0.1 to 7 THz was observed.     

利用各向同性半导体晶体差频产生可调谐THz辐射的理论研究

理论研究了利用剩余射线带色散补偿相位匹配原理，在Ⅲ-Ⅴ族和Ⅱ-Ⅳ族光学各向同性的半导体非线性晶体中差频产生可调谐THz波的可行性问题.根据这些半导体材料的色散特性，并以近简并点双共振KTP-OPO的可调谐相干双波长输出作为差频抽运源，对它们的相位匹配能力、差频增益特性、品质因数以及差频过程中的相干长度进行了理论分析和计算，确定了ZnTe晶体是在共线相位匹配情况下较为理想的THz波差频晶体，而InP晶体则更适合用于非共线相位匹配情况. 关键词： 非线性光学 THz辐射 差频 各向同性半导体晶体     

Compact and widely tunable terahertz source based on a dual-wavelength intracavity optical parametric oscillation

In this paper, a continuously tunable terahertz (THz) source is obtained using a compact intracavity pumped dual-wavelength optical parametric oscillation operating around 2.1 μm as difference-frequency generation pump source. The tuning range of the THz-wave frequency covers from 0.147 THz to 3.651 THz. Based on the collinear difference-frequency generation in the GaSe crystal, the experiment result shows that our schematic is a good option to construct a compact and portable terahertz source with widely tunable range.     

Tunable source of terahertz radiation based on the difference-frequency generation in a GaP crystal

A tunable source of terahertz (1–4 THz) radiation with a line width of 12 GHz is created on the basis of difference- frequency generation in a GaP crystal. Radiation of a pulsed Nd:YAG laser with a wavelength of 1064 nm and a tunable optical parametric oscillator is used as the pump. The radiation is generated in 10-ns-duration pulses with a pulse repetition rate of 10 Hz. The pulsed radiation power is about 15 mW.     

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.     

Widely tunable monochromatic THz sources based on phase-matched difference-frequency generation in nonlinear-optical crystals: A novel approach

We review our up-to-date result on the development of widely tunable monochromatic THz sources, implemented based on difference-frequency generation (DFG) in GaSe, ZnGeP₂, and GaP. Using a GaSe crystal, the output wavelength was tuned in the range from 66.5 μm to 5664 μm (from 150 cm^?1 to 1.77 cm^?1) with the highest peak power 389 W. This tuning range is the widest ever produced for a continuously tunable and coherent tabletop THz source. Moreover, the conversion efficiency 0.1% is also the highest ever achieved for a tabletop system. On the other hand, based on DFG in a ZnGeP₂ crystal, the output wavelength was tuned in the ranges 83.1–1642 μm and 80.2–1416 μm for two phase-matching configurations. The output power has reached 134 W so far. Finally, using a GaP crystal, the output wavelength was tuned in the range 71.1–2830 μm, whereas the highest peak power was 15.6 W. The advantage of using GaP over GaSe and ZnGeP₂ is that crystal rotation is no longer required for wavelength tuning. Instead, one just needs to tune the wavelength of one mixing beam within the bandwidth of as narrow as 15.3 nm.     

Enhancement of terahertz wave difference frequency generation based on a compact walk-off compensated KTP OPO

A compact, walk-off compensated dual-wavelength KTP OPO near the degenerate point of 2.128 μm pumped by a Nd:YAG pulsed laser is employed as the pump for terahertz (THz) source based on difference frequency generation (DFG) in a GaSe crystal. Coherent THz radiation that is continuously tunable in the range of 81-1617 μm (0.186-3.7 THz) is achieved. An enhancement of 76.7% in average for the THz energies at different wavelengths is realized using the walk-off compensated KTP OPO than the common one. Using a 8 mm-long GaSe crystal, the maximum output THz pulse energy is 48.9 nJ with the peak power of 11 W, corresponding to the energy conversion efficiency of 5.4 × 10^− 6 and the photon conversion efficiency of about 0.09%.     

光纤型宽带可调连续波差频产生中红外激光器

报道一种新型光纤化宽带可调连续波中红外激光器系统,该激光系统采用准相位匹配(QPM)的差频产生(DFG)技术,非线性晶体为多周期的周期性极化掺镁铌酸锂晶体(PPMgLN),掺镱光纤激光器(YDFL)用作抽运光源,可调激光器经掺铒光纤放大器(EDFA)功率提升后作为信号光源。利用建立的准相位匹配-差频产生系统,获得了连续波中红外差频光输出;实验发现,温度导致相位失谐的半峰全宽(FWHM)为4.5℃;通过优化选择晶体周期,并结合调节信号光波长和控制温度,该准相位匹配-差频产生系统可在3.1~3.6μm内连续调谐。     

Generation of ultrafast mid-infrared laser by DFG between two actively synchronized picosecond lasers in a MgO:PPLN crystal

We report the difference-frequency generation (DFG) of ultrafast mid-infrared laser radiation around 3???m between two picosecond laser pulses with the center wavelengths of 800?nm and 1064?nm in a MgO:PPLN crystal at room temperature. The two laser pulses were generated from the actively synchronized picoseconds Ti:sapphire and Nd:YVO₄ oscillators. We measured the DFG wavelengths tunable from 3.19?C3.29???m and the output power is potential to be several mW. This experiment proves a possible roadmap for ultrafast mid- and far-infrared laser radiation generation and even for the THz radiation.     

光纤型宽带可调连续波差频产生中红外激光器转换效率的研究

以掺镱光纤激光器为抽运源、掺铒光纤激光器后接掺铒光纤放大器为信号源,利用周期极化掺镁铌酸锂晶体,研究了全光纤化差频产生中红外激光器的转换效率特性。结果表明,抽运光和信号光偏振态影响差频产生过程的转换效率,利用偏振控制器,可将抽运光和信号光偏振方向调节到与晶体光轴方向平行,以获得高的转换效率。抽运光和信号光的光束质量既影响差频产生过程的转换效率,又决定晶体纵向位置的容限,当聚焦系统由自聚焦透镜和焦距100mm平凸透镜组成时,相对转换效率达0.717mW-2,晶体纵向位置容限为44mm。此外,差频光在3126.36~3529.6nm范围内调谐时,转换效率基本保持不变。     

磷化镓高功率太赫兹共线差频源的研究

在众多实现太赫兹辐射的方法中, 非线性光学共线差频能够实现高功率、宽波段、连续可调谐的太赫兹波辐射. 理论分析表明, 各向同性磷化镓晶体, 在1064 nm附近波长激光共线差频下具有毫米量级的相干长度, 能够满足高功率宽波段的太赫兹辐射条件.实验证明, 磷化镓晶体共线差频实现高功率宽波段的太赫兹光辐射, 其太赫兹光波长调谐范围为95.9–773.4 μm (0.39–3.13 THz), 最高峰值功率7 W位于频率2.0 THz处.该实验结果与理论计算基本保持一致. 关键词： 太赫兹源 磷化镓 共线差频     

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.     

高性能85mm短腔光学参变振荡器的THz电磁波输出特性分析

报道了以MgO∶LiNbO3为非线性光学介质,采用85 mm长的法布里珀罗单谐振腔结构形式的光学参变振荡器,产生THz电磁波的实验结果。使用波长为1064 nm的Nd∶YAGQ开关脉冲激光器作为抽运光源,通过改变入射角度使参变振荡器的相位匹配条件发生变化。采用Si真空量热器,并利用THz波干涉测量仪;或通过测量闲频光的频率对产生的THz波频率进行了测量。实验表明该参变振荡器输出频率调谐范围为0.9~3.0 THz。在抽运光能量为20 mJ/pulse,脉冲宽度16 ns,重复频率50 Hz条件下得到输出峰值位于1.2 THz,能量为102.5 PJ/pulse的THz波输出。通过引入Si棱镜阵列减小了THz波在晶体中的全反射,从而提高THz波的能量输出。使用金属缝隙探测器,对辐射的THz波的波束水平方向空间分布进行了测量,分析了Si棱镜阵列的衍射效应对THz波束空间分布的影响。     

基于中红外DFG光源的甲烷气体光谱检测方法研究

基于中红外光源的气体光谱检测是新的痕量气体监测与分析方法,在大气监测领域具有重要的应用。构建了一套基于中红外DFG光源的甲烷气体光谱检测系统。该系统以1 550 nm和1 060 nm波段可调谐半导体激光器作为基频光源,采用PPLN晶体作为差频非线性变频器件,实现了3.3 μm处的窄线宽可调谐中红外光源输出。实验结果表明,当PPLN晶体工作温度为99.5℃时,闲频光的输出功率为112 μW,差频转换效率达到1.246 mW/W2。晶体的温度接受带宽为4.3℃,泵浦光波长接受带宽为5.3 nm。在此基础上,分别利用直接吸收法和谐波检测法获得了3 028.751 cm-1处的甲烷气体吸收光谱和二次谐波检测信号。     

基于600—2000 nm抽运源的太赫兹相干光源的最新进展

最近几年,紧凑型、便携式、低廉、可调谐的太赫兹波源由于具有广泛的应用前景因而成为太赫兹领域的研究焦点.波长为600—2000 nm范围的近红外光源由于具有体积紧凑、价格低廉、频率可调谐和工作稳定的特点,所以频繁被选为太赫兹波产生的抽运源.本文着重从双波长可调谐抽运源、内腔光参量振荡器太赫兹发生器两个方面进行了较系统的阐述.分析表明,近红外相对中红外双波长抽运光源由于具有更紧凑的结构而成为研究热点;差频和非线性级联过程由于具有调谐范围宽的特点而成为产生太赫兹辐射的重要手段. 关键词： 太赫兹辐射 近红外波段 调谐宽度 非线性级联过程     

基于飞秒锁模光纤激光脉冲基频光的差频产生红外光梳

报道了一种基于飞秒锁模光纤激光脉冲基频光的光纤型差频产生(DFG)红外光梳及其研制技术.基于自主研制的重频锁定200 MHz飞秒锁模掺铒光纤激光器,经啁啾脉冲光纤放大与超连续谱产生技术,优化近零色散OFS光纤(型号:OFS-980-20)长度,结合可调延时线,获得了精准同步的基频双色脉冲;以GaSe为非线性晶体,利用光整流技术,产生了可在6—10μm范围内宽带调谐的DFG红外光梳,光梳最大光谱宽度可达1.3μm.这种光纤型远红外光梳可望在分子光谱精密测量等领域发挥重要作用.     

Terahertz electrically controlled nematic liquid crystal lens

We report some preliminary results on the realization of terahertz (THz) nematic liquid crystal (LC) lens with electric tunable focal length. A LC cell with a single circular-hole electrode is driven by an AC voltage to yield the desired spatial distribution of the refractive index. The two-dimensional function of the phase delay introduced into THz wave depends on the AC voltage and the electrodes contacting the LC layer. This lens is operated at room temperature and its convergent optical characteristic is demonstrated at 118 μm. We realize the lens with a clear aperture of 1 mm. The lens matched with a bolometer array is capable of obtaining real-time THz images of a sample without any mechanical scanning.     

级联差频产生太赫兹辐射的理论研究

针对差频产生太赫兹(THz)辐射转换效率低的缺点,提出了级联差频的新机理以提高转换效率,并以ZnTe晶体为例,对级联差频产生THz辐射的原理和过程进行了理论研究.通过对级联差频耦合波方程组的求解,得出了ZnTe晶体中级联差频的最佳抽运条件和ZnTe晶体的最佳长度,并且分析了晶体吸收、波矢失配及抽运强度对级联差频的影响.计算结果表明,通过级联差频可以大大提高THz波的转换效率,其光子转换效率甚至可以超过Manley-Rowe关系的限制. 关键词： 太赫兹辐射 差频 级联 ZnTe晶体