Second Harmonic Conversion-efficiency of Aberrated Laser Beam

How aberrations of pump wave affect second harmonic generation is studied. The solution is applied to type I second harmonic generation in KDP crystal. The results of conversion-efficiency and transverse intensity distribution in second harmonic generation of Gaussian beam and super-Gaussian beam with aberrations of Zernike polynomials are presented.     

小瓣数贝塞尔声束的二次谐波

采用高斯展开法,研究了具有三个瓣的小瓣数贝塞尔声束的基波和二次谐波的传播性质.根据准线性近似下Khokhlov-Zabolotskaya-Kuznetsov方程的积分形式解,分析了媒质的声吸收(声衰减)对波束形状的影响.结果表明:吸收参量对二次谐波径向分布有很大的影响.当瓣数较少时,实际的有限孔径贝塞尔声束二次谐波仍然具有理想无限大孔径贝塞尔声束的主要特征,在近场无衍射区,径向几乎无衍射. 关键词： 二次谐波 贝塞尔声束 声吸收 有限孔径声束     

利用椭圆高斯光束产生266nm紫外连续激光

本文采用商用532 nm激光器作为基频光源,利用偏硼酸钡(β-BBO)晶体进行外腔倍频,实现了266 nm连续激光的高效输出.文中详细模拟了BBO晶体中的束腰形状对倍频效率的影响,仿真和实验结果均表明椭圆高斯光束可以有效改善走离效应,提高倍频转换效率.通过优化蝶形倍频腔,可以使椭圆高斯光束在腔内共振,当1 W基频光输入时可输出约180 mW的266 nm紫外连续激光,倍频转换效率达到18%.     

高斯光束在偏硼酸钡单晶和孪晶中的倍频现象

理论探讨了高斯光束在非线性单轴晶体中的倍频效应。基于麦克斯韦方程组并考虑晶体的各向异性推导出负单轴I型晶体相位匹配下的倍频耦合波方程组。建立了具体的关于偏硼酸钡（BBO）单晶以及孪晶的物理模型并做数值求解。对求解所得到的单晶以及孪晶的谐波能流分布、晶体出射面探测点处谐波电场时域分布、总输出能量、输出光束频谱和输入光束角谱分别进行分析讨论,从束腰宽度、晶体结构以及材料性质对倍频的影响等方面做出具体合理的解释。研究结果对于单轴晶体的非线性光学性质的研究以及相应的光学晶体器件的研发等具有一定的参考价值。     

飞秒BBO光参量放大中闲频光二次谐波的产生

由于相位匹配条件和非线性晶体透光范围的限制，400nm蓝光抽运的飞秒β-BaB₂O₄（BBO）光参量放大（OPA）输出的参量光调谐范围有限，很难得到波长小于460nm的蓝光和近紫外光.实验采用1kHz钛宝石九通啁啾脉冲放大器的倍频蓝光作抽运光，超连续白光 作种子光，在Ⅰ类非共线相位匹配条件下，利用宽带的飞秒BBO OPA，在一定的实验参数下 获得了530—810nm放大的信号光，以及810nm—17μm波段范围的闲频光.与此同时 ，还获得了410—700nm连续可调的闲频光的二次谐波，其与闲频光层叠分布，单脉冲能量 为26μJ，转换效率大于5%.仅利用单块晶体的飞秒BBO OPA就可以获得410—810nm连 续可调的飞秒脉冲输出，从而为更多研究和应用的需要提供了重要的光源.对飞秒光参量放 大中闲频光二次谐波产生的条件也进行了理论分析. 关键词： 二次谐波 闲频光 非共线相位匹配 飞秒光参量放大     

Time domain bidirectional beam propagation method for second harmonic generation in multilayers

We propose a time domain bidirectional beam propagation method based on scattering operators for wide band pulse propagation in nonlinear multilayers. The reported numerical examples demonstrate the applicability of the algorithm for the analysis of temporal effects related to the process of second harmonic generation.     

Higher-order harmonics of general limited diffraction Bessel beams

In this paper, we extensively study the higher-order harmonic generation of the general limited diffraction m-thorder Bessel beam. The analysis is based on successive approximations of the Khokhlov–Zabolotskaya–Kuznetsov(KZK)equation. Asymptotic expansions are presented for higher-order harmonic Bessel beams in near and far fields. The validity of asymptotic approximation is also analyzed. The higher-order harmonic of the Bessel beam with the lowest zero-order is taken as a special example.     

Effects of transverse profile of pump field on second harmonic generation in periodic nonlinear materials

We report on a theoreticalanalysis of the effects of a converging pump field of Gaussian transverse profile on second harmonic generation in a periodic nonlinear material with quasi-phase-matching. The outputs of the centre intensity and the intensity flux for second harmonic generation are derived by simulation, based on the parameters of quasi-phase-mismatch, the waist and focus positions of the input pump beam. The results show that when the transverse profile of the pump field is taken into account, the quasi-phase-match value and focus position of input beam for maximal second harmonic generation flollow new criteria.     

Vortex–edge dislocation interaction in second-order nonlinear media

Experimental and theoretical results on second harmonic generation with fundamental beams containing both a vortex and an edge phase dislocations are reported. The 3D trajectories in the fundamental and second harmonic beams are calculated for different distances between the dislocations in the input beam. It is shown that each phase dislocation in the second harmonic beam appears as a result of the joint actions of all phase dislocations in the fundamental beam.     

Second-order optical nonlinearity in Sb2S3 microcrystal doped glasses by electron beam irradiation

With Maker fringe measurements, the prominent second harmonic generation was observed in the sol-gel-derived Sb₂S₃ doped silica glasses irradiated by electron beam, which was related to the space-charge electrostatic field that makes the glasses poled and broken the centrosymmetry of the glasses. By sides, the interface of nanoparticles contributed to the second harmonic intensity as well. The second harmonic intensity increased with the increasing of the irradiating current, accelerating voltage and the concentration of the dopant due to the enhanced built-in electrostatic field. The poled region was located in the surface of the sample about several microns by the TSDC measurements. The second harmonic intensity is almost 10 times larger than that of the base glass due to the existence of microcrystal.     

SECOND HARMONIC GENERATION WITH PHOTON NUMBER SQUEEZED LIGHT AS INPUT

We investigate the quantum statistics of the fields produced in second harmonic generation when photon number squeezed light ia used as input. It is found that the squeezed properties of the pump beam can be transferred to a certain extent to the second harmonic beam. The normalized photon fluctuations, or Fano factors, of the fundamental and second harmonic output fields are derived for various values of the interaction parameters.     

Dynamics of resonant self-focusing on second harmonic generation of Gaussian laser beam in a rippled density plasma

Resonant second harmonic generation by a Gaussian laser beam in a rippled density plasma is studied using the moment theory approach. The nonlinearity arises through the relativistic mass effect and ponderomotive forces. The laser beam creates a plasma channel and gives rise to electron density perturbation at the laser frequency. The density perturbation beats with electron quiver velocity to produce second harmonics. The ripple provides phase matching and makes the process a resonant one. The second harmonic power efficiency is increased effectively with density ripple. Self-focusing causes enhancement in the efficiency of harmonic generation.     

Second harmonic generation of high power Cosh-Gaussian beam in cold collisionless plasma

The purpose of this study is to explore the second harmonic generation (SHG) of a high power Cosh-Gaussian beam in cold collisionless plasma. The ponderomotive force causes carrier redistribution from high field to low field region in presence of a Cosh-Gaussian beam thereby producing density gradients in the transverse direction. The density gradients so produced the results in electron plasma wave (EPW) generation at the frequency of the input beam. The EPW interacts with the input beam resulting in the production of 2nd harmonics. WKB and paraxial approximations are employed for obtaining the 2nd order differential equation describing the behavior of the beam's spot size against normalized distance. The impact of well-established laser-plasma parameters on the behavior of the beam's spot size and SHG yield are also analyzed. The focusing behavior of the beam and SHG yield is enhanced with an increase in the density of plasma, the radius of the beam and the decentred parameter, and with a decrease in the intensity of the beam. The results of the current problem are really helpful for complete information of laser-plasma interaction physics.     

Study of second harmonic generation by non-linear crystals with phase conjugation

We present the correction of the phase distortion which occurs during the second harmonic generation by non-linear crystals, such as KDP and KTP at high average power laser. This is due to the optical quality and thicknesses of the crystals which in turn influence the quality of the incident laser beam. This phase distortion is corrected by reflecting back the laser beam into the crystal using a phase conjugate mirror. It is found that the conversion efficiency of second harmonic generation without phase conjugation is more than that with phase conjugation. Far field pattern shows that the distortion of the laser beam can be corrected by using the phase conjugate mirror. Fidelity of the beam profile increases significantly with phase conjugation in the case of KDP crystal.     

On the theory of second harmonic generation

It is shown how the theory of non-linear interaction between the modes of the open resonator, developed previously [1] and used to study resonant second harmonic generation [2], can be applied to second harmonic generation in the absence of a resonator. The conditions under which the second harmonic output, in the small conversion approximation, can be approximated by a Gaussian beam are derived.The optimum value of the focusing parameter I/z₀ (hereI is the length of the crystal, andz ₀ is one-half the confocal parameter of the fundamental beam) for the total output is shown to be 5.68 agreeing precisely with the value given by Boyd and Kleinman [3].     

Enhanced harmonic generation by breaking the phase-matching symmetry

We introduce two static methods to break the phase-matching symmetry in third harmonic generation with a focused Gaussian beam in the tight focusing limit, dramatically increasing the conversion efficiency and mode quality. Both rely on inhibiting harmonic generation immediately after the beam waist, preventing the near perfect cancellation of the third harmonic generation (THG) from before and after the focus. The first method involves placing a thin metal septum at the waist: the laser drills a small pinhole, which, in turn, disrupts the beam focus after the pinhole. The second method is based on placing a large χ⁽³⁾ gas before the focus and a small χ⁽³⁾ gas after the focus.     

腔内倍频调Q激光器中脉冲光场的时间不稳定性研究

以腔内倍频调Q激光器工作原理为基础,研究了多种形式抽运光分布的情况下倍频光的脉冲形状以及脉冲持续过程中光场分布的时间不稳定性,分析了倍频光光束质量随时间的变化规律.发现腔内倍频调Q激光器输出的倍频光光场分布在脉冲的持续过程中并不稳定,呈持续变化.对高斯型抽运的情况研究发现,减小腔长会使脉冲光场分布的不稳定性减小;而增加...     

Noncollinear second harmonic generation in KDP

This paper reports experimental and analytical results on optical second harmonic generation when a noncollinear arrangement is employed. The dependence of second harmonic power on crystal length, absorption, beam radius and the angle between the two fundamental waves is obtained. Double refraction of the crystal is taken into account, but the divergence and depletion of the fundamental waves are neglected. The results of the calculations are confirmed by experiments carried out with a KDP crystal in the near field of two intersecting TEM₀₀ ruby laser beams.An extension of the theory to nonlinear interactions other than second harmonic generation - e.g. four-wave interaction - is possible.     

Evidence of high harmonics from echo-enabled harmonic generation for seeding x-ray free electron lasers

Echo-enabled harmonic generation free electron lasers hold great promise for the generation of fully coherent radiation in x-ray wavelengths. Here we report the first evidence of high harmonics from the echo-enabled harmonic generation technique in the realistic scenario where the laser energy modulation is comparable to the beam slice energy spread. In this experiment, coherent radiation at the seventh harmonic of the second seed laser is generated when the energy modulation amplitude is about 2-3 times the slice energy spread. The experiment confirms the underlying physics of echo-enabled harmonic generation and may have a strong impact on emerging seeded x-ray free electron lasers that are capable of generating laserlike x rays which will advance many areas of science.     

光合色素分子吸附膜的非线性增强效应

吸附在光学表面上的细菌叶绿素和藻兰素生物分子膜.在基波(λ=1.06μm)激光作用下,观察到谐波效应.文中对藻兰素只产生三次谐波而不产生二次谐波做了分析.