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。     

周期极化非线性晶体中太赫兹波辐射研究

 从理论上详细研究了飞秒激光在周期极化非线性晶体中由光整流效应产生的太赫兹（THz）波辐射。利用天线辐射原理和光栅衍射理论,着重研究THz波辐射的频域场和时域场的分布。讨论和分析了THz波辐射的中心频率、频谱宽度和电场随辐射角的变化。研究表明,THz波的带宽反比于晶体的长度或光栅数,电场随辐射角呈准谐波变化。     

Pool-Frenkel thermoelectric modulation of exciton photoluminescence in GaSe crystals

Effect of external field on the exciton photoluminescence of GaSe crystals has been investigated and it has been observed that the PL is quenched with the applied field. The changes observed in the PL spectra have been analyzed with impact exciton, Franz-Keldysh and Pool-Frenkel effects. From the analyses of the experimental data, it has been found that the intensity of direct free, indirect free and bound exciton peaks decreased exponentially with the square root of applied field as I∼exp-β√E. The energy positions of emission peaks were found to shift to longer wavelength with the applied field as ΔE∼β√E. From these findings, the Pool-Frenkel thermoelectric field effect is seen to be the dominant mechanism in the variation of exciton PL with the applied field even though the impact exciton and Franz-Keldysh effects contribute.     

Characteristics of efficient few-cycle terahertz radiation generated in as-grown nonlinear organic single crystals

We investigated efficient terahertz wave generation by optical rectification in as-grown nonlinear organic single crystal HMQ-T (2-(4-hydroxy-3-methoxystyryl)-1-methylquinolinium 4-methylbenzenesulfonate). Optimal thickness of crystals directly available by a slow cooling method in methanol solution enabled us to achieve high-field few-cycle THz waves at 800-nm pumping. With 95-mW pumping at 1-kHz repetition rate, an optical-to-THz conversion efficiency of 2.7 × 10⁻⁴ was achieved and the THz electric field strength, measured by electro-optic sampling, was as high as 110.1 kV/cm. Such an efficient THz source based on as-grown HMQ-T crystals can be used for investigation of various nonlinear phenomena in the THz spectral region.     

ZnTe晶体中光学整流产生的THz辐射及其电光探测研究

借助抽运-探测技术研究了ZnTe晶体中光学整流产生的太赫兹(THz)辐射，利用ZnTe晶体的线性电光效应探测THz辐射场分布，观察到了较窄(约为0.2 ps)的THz场分布及相应较宽(响应超过4 THz，半峰宽约为2.4 THz)的THz频谱，并运用琼斯矩阵对实验结果进行了理论拟合. 研究了飞秒激光脉冲波长(750—850 nm)、脉冲宽度(56—225 fs)和晶体旋转与THz辐射产生的关系. 同时改变探测光偏振方向进行偏振调制，并从理论上分析了偏振调制对THz辐射探测的影响. 关键词： THz辐射 光学整流 电光探测 ZnTe     

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     

Effect of thermal blooming in second-harmonic generation in nonlinear crystals with a regular domain structure

Analytical expressions are obtained for the power of the second harmonic in nonlinear crystals having a regular domain structure with allowance for thermal blooming for a Gaussian beam of basic radiation in the quasi-stationary plane-wave approximation. Calculations were carried out in both the approximation of an assigned field of basic radiation and a substantially nonlinear regime of exhaustion of basic radiation. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp,. 257–260, March–April, 2000.     

Investigation of the water-vapor continuum in the THz region using a multipass cell

Laboratory absorption measurements of the water-vapor continuum in the far infrared region from 12 to (0.4 - 1.83 THz) were obtained using a multipass absorption cell, a Fourier transform spectrometer and a liquid-He-cooled bolometer detector. Measurements were made at a temperature, with water vapor and nitrogen pressures up to 2.2 and , respectively. The effects of the choice of lineshape function and far-wing cut-off factors on the reported continuum absorption are analyzed by modeling the resonant water-vapor spectrum using van Vleck-Weisskopf and Lorentzian lineshapes. Comparisons with available microwave data and model calculations are also presented.     

飞秒激光在晶体中传输的级联非线性过程

系统地回顾了有关级联过程导致的各种非线性效应的工作,指出了级联效应可导致不同光谱成分间出现能量转移现象,且二阶级联过程中产生的白光还能够提供种子光进而得到基于参量放大过程的宽带锥形辐射。在此基础上,进一步研究了入射的基频波长对于超连续白光能量转移效果及产生倍频效率的影响以及二阶级联过程引起的偏振调制现象,指出当入射波长越靠近相位匹配波长时倍频效率越高,发现二阶级联效应还可以引起显著的基频光偏振调制。     

飞秒激光在晶体中传输的级联非线性过程

系统地回顾了有关级联过程导致的各种非线性效应的工作,指出了级联效应可导致不同光谱成分间出现能量转移现象,且二阶级联过程中产生的白光还能够提供种子光进而得到基于参量放大过程的宽带锥形辐射。在此基础上,进一步研究了入射的基频波长对于超连续白光能量转移效果及产生倍频效率的影响以及二阶级联过程引起的偏振调制现象,指出当入射波长越靠近相位匹配波长时倍频效率越高,发现二阶级联效应还可以引起显著的基频光偏振调制。     

Sum-frequency mixing of optical vortices in nonlinear crystals

Results of theoretical and experimental investigation of the influence of walk-off on sum-frequency mixing of optical vortices in nonlinear crystals are presented. Various phenomena of vortex interaction such as decay of higher order vortices into singly charged vortices, formation of aligned arrays of vortices perpendicular to walk-off direction, particle-like pulling and pushing of vortices, and appearance of pairs of vortices having opposite charges were observed.     

聚苯乙烯芪盐L—B膜的二阶非线性光学性质研究

首次合成了一种称之为聚苯乙烯芪盐的有机高分子材料.此材料可以在水面上形成稳定的单分子层膜,并且所形成的单分子层膜可以转移到固体衬底上去.对聚苯乙烯芪盐单分子层膜进行光学二次谐波产生测量,得到聚苯乙烯芪盐对应于每个发色团的二阶非线性极化率为1.2×10~(-28)esu.制备了该材料两种不同类型的多层膜,讨论了多层膜二次谐波产生信号不随层数增加而增加的可能原因.     

Optimization of type-II frequency doubling of spatial and temporal limited laser light in nonlinear crystals

Type-II frequency doubling of short Gaussian laser pulses is studied theoretically. The situation is analysed when the group velocity mismatch and the walk-off angle of the interacting waves limit the frequency conversion efficiency. It is shown that with collinear ‘o’ and ‘e’ polarized fundamental waves a temporal and spatial separation of the fundamental beams is necessary to compensate for both effects. Results of the presented model are discussed for KDP crystals.     

Analysis of femtosecond optical pulse propagation in one-dimensional nonlinear photonic crystals using finite-difference time-domain method

In this paper, we have used the finite-difference time-domain (FDTD) method to analyze the optical pulse propagation in a nonlinear, one-dimensional photonic crystal (1DPC). Hyperbolic secant pulses with various carrier wavelengths are utilized in this study. In a nonlinear regime, a 1DPC introduces a photonic band-gap whose central wavelength and width depend on the input pulse intensity. In the present work, three different cases are considered. These correspond to the carrier wavelengths of the incident pulses being out of, near to, and partially in the band-gap. For each case, the effect of nonlinearity on pulse propagation is investigated. Also, we have analyzed the two-frequency regime, in which each of the two pulses has a different carrier frequency (wavelength). This kind of study can be done directly with FDTD without any further computational burden but it is somewhat complicated using nonlinear coupled-mode equations (NLCME) and nonlinear Schrödinger equation (NLSE), which require separate treatments for each carrier wavelength.     

Investigation of thermal evolution of nanodomain structures in nonlinear barium sodium niobate crystals

By the 90°elastic light scattering investigation and far field observation in the range of 20-800℃,the relation between behavior of light scattering anomalies and evolution of nanodomain structures in lattice of barium sodium niobate(Ba2NaNb5O15,BSN)crystal was clarified.The correlation between anomalies on the temperature curves of the elastic light scattering intensity and temperature transformations of nanodomains was studied by X-ray and electron microscope methods.Phase transition near 500℃ and movement in field of scattering light could be explained by appearance of a new incommensurate phase.     

Allowance for angular divergence in the generation of the second harmonic in crystals with a regular domain structure

We obtained basic analytical expressions for the efficiency of the second harmonic in nonlinear crystals with a regular domain structure; in these expressions we took into account the angular divergence of the laser beam. The calculation was done in the geometric-optics approximation of the given field of laser radiation. We show that the expressions obtained are similar to the corresponding expressions for homogeneous nonlinear crystals. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 1, pp. 134–138, January–February, 2000.     

Change of optimum conditions of second harmonic generation of spatial limited laser beams in nonlinear crystals

Frequency doubling of spatially limited laser beams in nonlinear crystals is investigated theoretically. It was shown that the major factor, which reduces the second harmonic generation (SHG) efficiency in this case, is a diaphragm aperture effect, caused by influence of extraordinary polarized laser beams. In that case a spatial shift of mutually orthogonally polarized beams of the incident waves relative to each other in perpendicular direction of wave transmission at the entry to crystals lead to increase the efficiency of SHG of limited beams.     

Investigation of nonlinear absorption in YAP and YAG single crystals doped with Co and co-doped with Si

Since our attempts to grow YAP:Co,Si non-linear absorbers have failed, we switched to Czochralski grown YAGs doped with the same ions. This time, however, the new material was found to exhibit excellent, non-linear absorption, which can be used directly in Q-switching elements in laser systems. In this work we report on technology of YAG single crystals doped with Co and Si, their optical properties, and, finally, on their non-linear absorption. An explanation on why the YAPs were found to be inefficient in non-linear absorption was also provided.     

Detection of a new SRS‐promoting phonon mode and cross‐cascaded χ(3)‐nonlinear lasing in single crystals of calcite (trigonal CaCO3)

For calcite (CaCO₃), one of the pioneer crystals in nonlinear optics, new results of stimulated Raman scattering (SRS) spectroscopy are presented. Among them are the discovery of a new SRS‐promoting vibration mode with ω_SRS2 ≈︁ 282 cm^‐1 and its participation, together with the main SRS mode ω_SRS1 ≈︁ 1086.5 cm^‐1, in cross‐cascaded (χ⁽³⁾ ↔ χ⁽³⁾) nonlinear‐lasing generation, as well as the observation of efficient self‐upconversion via cascaded parametric four‐wave processes of one‐micron Stokes and anti‐Stokes χ⁽³⁾‐lasing into the UV‐region of third harmonic generation. The investigations show that calcite is able to generate a χ⁽³⁾‐lasing comb of more than two octaves bandwidth. The article also gives a brief review on the discovery and study of the SRS‐effect in natural crystals (minerals), which have expanded our ability to study the photon‐phonon nonlinear‐laser interactions in crystalline materials. A short summary of information about χ⁽³⁾‐lasing properties of the triangular planar structure units in SRS‐active crystals is included.