基于GaSe和ZnGeP2晶体差频产生可调谐太赫兹辐射的理论研究

基于非线性光学频率变换理论,采用已报道的利用非线性光学差频方法产生可调谐太赫兹波的实验条件作为理论分析的实验模型,计算模拟出在不同相位匹配条件下,GaSe和ZnGeP₂晶体差频的相位匹配角、走离角、允许角和有效非线性系数,并对计算结果进行了分析比较,总结出对应输出不同太赫兹波长的最佳相位匹配方式.计算结果为利用非线性晶体差频产生可调谐太赫兹辐射的实验研究提供深入和全面的理论基础. 关键词： 太赫兹波 GaSe晶体 2晶体')" href="#">ZnGeP₂晶体 差频     

基于GaSe和ZnGeP2晶体差频产生可调谐太赫兹辐射的理论研究

基于非线性光学频率变换理论,采用已报道的利用非线性光学差频方法产生可调谐太赫兹波的实验条件作为理论分析的实验模型,计算模拟出在不同相位匹配条件下,GaSe和ZnGeP2晶体差频的相位匹配角、走离角、允许角和有效非线性系数,并对计算结果进行了分析比较,总结出对应输出不同太赫兹波长的最佳相位匹配方式.计算结果为利用非线性晶体差频产生可调谐太赫兹辐射的实验研究提供深入和全面的理论基础.     

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     

增益饱和对光学差频产生太赫兹辐射的功率和稳定性的影响

数值模拟并分析了以GaSe晶体为例对光学差频产生太赫兹（THz）波的特性.结果表明：当THz波长为227.5 μm,晶体长度为26.3 mm时,产生THz波功率达到增益饱和,在增益饱和点输出最高峰值功率可以达到945 W.由于晶体吸收的影响,THz波的增益饱和区是输出功率的非稳定区,而THz波的输出稳定区位于增益饱和区之后,在稳定区的THz波稳定性取决于抽运光的稳定性.当THz波波长为227.5 μm时,达到稳定区所需晶体长度为37.9 mm,此时THz波输出峰值功率可以达到735 W. 关键词： 光学差频 太赫兹辐射 稳定性     

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     

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     

AgGaSe_2晶体差频产生可调谐太赫兹波的理论研究

基于单轴晶体中三波互作用的共线相位匹配技术,采用CO2激光器泵浦非线性晶体AgGaSe2,数值计算出在oee类和oeo类相位匹配条件下,差频产生太赫兹波的相位匹配角、走离角、允许角和有效非线性系数。计算结果表明:两类相位匹配太赫兹波可调谐的范围都非常宽,而且走离角都在1°以下,允许角足够大。在绝大部分太赫兹波区域,oee类匹配有效非线性系数远大于oeo类匹配,实验中适合采用oee类相位匹配方式。     

GaSe晶体的双光子吸收对太赫兹输出的影响

根据光整流效应,利用超快激光脉冲泵浦GaSe晶体实现了0.2~2.5 THz的宽带太赫兹辐射输出。禁带中的电子在两个800 nm光子的作用下激发到导带中形成自由载流子,进而吸收所产生的太赫兹辐射,最终导致太赫兹的输出随泵浦功率的增加而趋于饱和。为了研究双光子吸收对太赫兹输出的影响,测量了800 nm处的GaSe晶体的双光子吸收系数,结果为 0.165 cm/GW。通过对太赫兹输出实验数据的拟合,得到GaSe晶体中自由载流子对太赫兹输出的吸收截面为1×10^-15 cm²。本文的研究结果可用于优化GaSe晶体在强激光泵浦下的太赫兹转换效率。     

新型有机晶体及超宽带太赫兹辐射源研究进展

非线性光学晶体是非线性光学频率变换技术中的核心器件。近些年,为进一步提高基于非线性光学频率变换技术产生太赫兹波的输出能量、转换效率,拓宽产生太赫兹波的带宽,多种新型有机晶体得以发展,并凭借其更加出色的非线性光学性质,成为产生太赫兹波的理想材料。本文按照晶体类型介绍了目前可产生THz波的多种有机晶体的性质,并总结了基于多种有机晶体的超宽带太赫兹辐射源的国内外研究进展,同时结合THz光谱检测技术的应用需求分析了基于有机晶体宽带THz辐射源的发展趋势以及所面临的关键科学问题。     

基于闪锌矿晶体中受激电磁耦子散射产生可调谐太赫兹波的理论研究

基于受激电磁耦子散射原理,采用已报道的利用非线性光学参量振荡方法产生可调谐太赫兹波的实验条件作为理论分析的实验模型,以GaAs,GaP, InP,ZnTe晶体为代表,计算分析了在闪锌矿晶体中参量振荡产生太赫兹波的吸收、增益特性,对输出THz波的调谐特性给出了详尽分析.分析太赫兹波高效耦合输出的腔型结构,并与掺氧化镁铌酸锂晶体组成的太赫兹波参量振荡器做对比. 关键词： 太赫兹波 太赫兹波参量振荡 电磁耦子 闪锌矿晶体     

Simultaneous observation of CO isotopomer absorption by broadband difference-frequency generation using a direct-bonded quasi-phase-matched LiNbO3 waveguide

Rovibration absorption lines both of 12CO and 13CO are observed simultaneously with the output of a 2 microm broadband difference frequency generated in a direct-bonded quasi-phase-matched LiNbO3 waveguide, which is a 50 mm device with a single quasi-phase-matching period that is operated at a constant temperature. The wavelength conversion efficiency and the difference-frequency generation bandwidth reach 100%/W and 100 nm, respectively. The idler output bandwidth in the 2 microm region is obtained by group-velocity matching or phase-mismatch minimization when a 0.94 microm pump laser diode and a 1.55 microm tunable signal source are used.     

Evolution of Dark Spatial Soliton in Quasi-phase-matched Quadratic Media

We theoretically investigate the evolvement of dark spatial soliton with cascading quadratic nonlinearity in quasi-phase-matched second harmonic generation. It is shown that the dark solitary wave can propagate stably when background intensity is large enough, in which diffraction of beam can be balanced by the cascading quadratic nonlinearity. We also analyze the influence of phase-mismatch on the stability of dark soliton propagation.     

基于准周期光学超晶格的高效电光调制差频转换

为获得尽可能大的差频转换效率,基于准周期极化铌酸锂(QPPLN)光学超晶格,提出了级联电光和差频理论,用于高效的差频转换。其方法是沿QPPLN光学超晶格的y方向施加一个外加电场,用来控制能量在抽运光、信号光、o偏振的差频光和e偏振的差频光四个光波之间的转移。计算结果表明,在一个100℃,40mm长的QPPLN光学超晶格中,当1550nm信号光与1064nm抽运光光强比值r<0.324时,对光强超过特定值的任意抽运光都可以通过施加一个适当的外加电场将抽运光完全转化为1550nm信号光和3393.4nm差频光;当r≥0.324,只当抽运光光强落在一定范围内时,才可以通过施加外加电场使抽运光完全转化为信号光和差频光;超过该范围,外加电场不能增加差频光转换效率。计算结果还表明,电光调制差频转换效率对温度和畴构造误差都不敏感。     

Stimulated emission at the difference-frequency generation

 We consider the stimulated emission process that occurs at the difference-frequency generation of a bichromatic field interacting with a three-level atom, where one of the laser fields is strong while the other is weak. It is shown that at the difference-frequency generation an induced peak occurs which can exhibit both gain and attenuation. Conditions under which this takes place are established and discussed herein. Received: 29 April 1996 / Accepted: 17 July 1996     

Dispersive shock waves in phase-mismatched second-harmonic generation

We investigate wave-breaking and dispersive shock wave formation driven by a pulse undergoing second-harmonic generation in a quadratic medium. We show that the process is accessible in the regime of high phase-mismatch (cascading limit) and weak dispersion. Insight into the phenomenon is obtained by means of a suitable hydrodynamic reduction of the equations that govern the mixing.     

Bi-directional spatial soliton emission at engineered nonlinear waveguide interfaces

We observed tunable bi-directional emission of spatial solitons at a quadratically nonlinear interface in periodically poled lithium niobate planar waveguides. The interface consists of the boundary between two quasi-phase-matched regions with a different poling period. We show the intensity and phase-mismatch (temperature or wavelength) dependence of the phenomena.     

超短脉冲倍频中三阶非线性效应的影响及补偿

 为了提高超短脉冲二倍频的转换效率和改善出射倍频光的脉冲形状，对超短脉冲倍频中三阶非线性效应的影响进行了理论分析和数值模拟，并采用初始相位失配的方法来补偿三阶非线性效应的影响。结果表明：用KDP晶体二倍频中心波长为800 nm的超短脉冲，当入射功率密度大于100 GW/cm²时，三阶非线性效应是倍频转换效率的主要影响因素。对脉宽为50 fs，入射功率密度为250 GW/cm²的超短脉冲在KDP晶体（2 mm）中的二次谐波变换，当初始相位失谐0.9 mrad时，转换效率提高了10%，同时由三阶非线性效应引起的强度调制得到明显抑制，出射基频光和倍频光的脉冲形状得到明显改善。     

Squeezing in downconversion in a quasi-phase-matched medium

We consider the generation of squeezed light in the process of degenerate downconversion in a quasi-phase-matched periodically poled nonlinear medium. The squeezing in the downconverted mode is analyzed for different orders of quasi-phase matching and for phase-mismatch parameter as well as varying pump input powers. The results show that a high degree of squeezing and high powers can be obtained for well-matched interactions.     

Four-wave mixing of picosecond pulses in hollow fibers: Phase matching and the influence of high-order waveguide modes

The processes of third-harmonic and difference-frequency generation through the four-wave mixing of picosecond pulses in gas-filled hollow fibers are experimentally studied. Due to the improvement of phase-matching conditions with an appropriate choice of the gas pressure and optimal parameters of the hollow fiber, we were able to use hollow fibers with a large length (up to 30 cm) for difference-frequency generation, which resulted in a considerable increase in the power of the difference-frequency signal at the output of the fiber. Our experimental data reveal a considerable influence of high-order waveguide modes on four-wave mixing processes in a hollow fiber. It is shown that the waveguide regime of nonlinear optical interactions implemented in hollow fibers removes the limitations on the efficiency of third-harmonic and sum-frequency generation, which are characteristic of the tight-focusing regime in media with normal dispersion and which are due to the geometric phase shift arising in tightly focused light beams.     

Collinear second harmonic generation of 20 wavelengths in a single two-dimensional decagonal nonlinear photonic quasi-crystal

We report on the observation of quasi-phase matched collinear second harmonic generation (SHG) at 20 wavelengths in a two-dimensional nonlinear photonic quasi-crystal with decagonal lattice. We show that at some wavelengths the second harmonics are generated via standard quasi-phase matching, namely a reciprocal vector exists that equals the phase-mismatch vector, while at others the SHG is as a result of the projection-based quasi-phase matching in which the momentum conservation is satisfied up to a projection of a reciprocal vector onto the direction of propagation. In spite of different generation mechanisms, the reciprocal vectors (or their projections) involved in the collinear QPM SHG can be described by a generalized equation.