Optimum conditions for the generation of the second harmonic of intense laser radiation

A generalized approximation of strong interactions of waves is suggested for the theoretical analysis of second-harmonic generation under conditions of self-action. On the basis of the method suggested, approximate solutions for the efficiency of second-harmonic generation are obtained with regard to the influence of higher nonlinearities, depletion of pumping radiation, and linear phase mismatch. The effects of the phase mismatch and the spatial distribution of the amplitude of the fundamental harmonic on the efficiency of second harmonic generation by intense laser radiation is analyzed. The results obtained with the approximate method developed are shown to be in good agreement with known experimental data and numerical calculations. Optimum conditions for second harmonic generation are determined in a wide range of laser radiation intensity and at different spatial distributions of the fundamental harmonic amplitude.     

Strong-interaction approximation in the theory of second harmonic generation of intense short laser pulses

The method of strong interaction of nonlinear waves has been developed to analyse the second harmonic generation of intense laser radiation in the transient regime. Analytic solution for frequency conversion efficiency – taking into account the pump radiation depletion, influence of phase mismatch, dispersion of group velocities and higher nonlinearities – has been first obtained. A comparison of theoretically derived results and known experimental data has been conducted, and a general agreement of these results has been obtained. The optimal conditions of the second harmonic generation of neodymium laser radiation into femtosecond pulse duration range have been determined.     

Generalized Approximation of Strong Interaction of Waves in the Theory of High-Power Radiation Frequency Doubling

The approximation of strong interaction of waves is developed to analyze the generation of the second harmonic under the conditions of self-action. Based on this method, an approximate expression is obtained for the generation efficiency of the second harmonic with allowance for the effect of higher nonlinearities, depletion of pumping radiation, and linear phase detuning. The effect of phase detuning and change in the spatial distribution of the amplitude of the fundamental radiation on the efficiency of second-harmonic generation has been analyzed. Good agreement of the results of the developed approximate method with experimental data and numerical calculations is shown. The optimum conditions of the second-harmonic generation in a wide range of the laser system radiation intensities are determined.     

Transmission second harmonic generation in CdTe at 1.064 μm

Transmission geometry measurements of the efficiency of second harmonic generation in various thicknesses of CdTe samples were made to determine the conversion efficiency dependence on material thickness. Neglecting pump depletion, it is found that for samples of well-defined symmetry, the second harmonic conversion efficiency scales with film thickness, with no observed enhancement owing to coherence length effects. The angular dependence of the observed second harmonic light in films of well-defined symmetry is consistent with second harmonic generation originating in the bulk.     

KNbO_3高效倍频产生427～470nm蓝紫光

厚1.5 mm的KNbO_ 3晶体在室温下角度相位匹配倍频脉冲参量激光获得427～470nm蓝紫光,能量转换效率超过40％,输出单脉冲蓝紫光的峰功率达60kW.对相位匹配曲线、倍频效率与基波光强的关系和相应匹配角允差作了计算和讨论.     

Generation of ultraviolet radiation with wide angular tolerance in cesium lithium borate crystal

Tangential phase-matching has been realised in cesium lithium borate (CLBO) crystal for the first time for the generation of fourth harmonic (266 nm) of Nd:YAG and third harmonic (226.7 nm) of a dye laser radiation by second harmonic generation and sum-frequency mixing with the angular tolerance as large as 22 mrad and 21 mrad respectively, over one of the interacting beams. An energy conversion efficiency of 15% for fourth harmonic generation is obtained with a 5.5 mm thick crystal and with the average pump powers only 170 and 70 mW. A set of Sellmeier dispersion equations for the CLBO crystal have also been formulated.     

Theoretical studies on frequency doubling in glass optical fibers in constant-intensity approximation

Theoretical consideration of the second harmonic generation in the constant-intensity approximation with account of the inverse effect of the excited wave on a stimulation wave through account of the phase change of interaction waves is presented. The behavior of harmonic wave intensity in a fiber for different parameters of the problem has been studied, the self-action of the light both in glass optical fiber and inside the homogeneous medium has been analyzed. In this approximation a decrease of conversion efficiency and a change of parameters of the curves of synchronism versus the pump intensity, in particular, the change of positions of intensity minima that does not take place in the constant-field approximation, are observed. The propagation of a plane wave packet in the homogeneous medium with quadratic nonlinearity and in an optical fiber accompanied by the self-phase modulation and leading to the change of the Gaussian pulse spectrum is analyzed with account of the phase changes of all the interacting waves.     

Third harmonic generation in the field of a backward pump wave

Third harmonic generation in a nonlinear medium the refractive index of which is negative at the pump frequency and positive at the harmonic frequency with allowance for self-action effects is considered theoretically. An exact solution of equations that describe the parametric interaction of continuous waves has been obtained analytically. The behavior of the solution is analyzed depending on the degree of violation of the phase matching condition. It is shown that, in a definite domain of values of nonlinear susceptibilities of the third order, there exists a critical value of the phase detuning at which the monotonic transform of the pump into the harmonic is changed by a periodical one. This phenomenon does not occur when the third harmonic is generated in ordinary media with a positive refractive index.     

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.     

采用光栅角色散的宽带二倍频及聚焦特性分析

针对带宽为20 nm左右、中心波长为1 053 nm的线性啁啾宽带激光,分析了采用光栅角色散方式宽带二倍频的原理,讨论了光栅角色散性能及光栅加工误差引入的基频光位相扰动对二倍频光转换效率及聚焦特性的影响.研究结果表明:采用光栅角色散方式的宽带二倍频光的转换效率、脉冲宽度和带宽均明显增大;在0～22 nm基频光带宽范围内,二倍频光均可保持80％左右的转换效率;对于入射基频光带宽为22 nm的情况,光栅角色散率在30～80 μrad/nm范围内变化时,宽带二倍频转换效率均可保持在70％以上;基频光有位相扰动时,二倍频光带宽对二倍频聚焦光斑的主瓣影响不大,但对旁瓣有一定的匀滑作用,采用光栅角色散方式宽带二倍频的聚焦光斑旁瓣的匀滑效果更为明显.     

受激旋转拉曼散射效应对基频光光束质量的影响

 采用瞬态受激旋转拉曼散射(SRRS)和空间相位扰动模型，定量分析了泵浦光强度和脉冲宽度对基频光在空气长程传输过程中产生的SRRS效应的阈值距离和转换效率的影响，讨论了泵浦光空间相位畸变对长程传输后的基频光光束质量的影响。研究结果表明：SRRS效应的阈值距离随着泵浦光强度和脉宽的增大而变短；空间相位畸变对斯托克斯光和泵浦光的光强分布存在一定影响，对斯托克斯光相位分布的影响比对泵浦光相位的影响大；SRRS效应的存在将导致基频光光束质量明显变差，对谐波转换效率产生明显影响。     

Efficient second-harmonic generation of continuous-wave Yb fiber lasers coupled with an external resonant cavity

Efficient second-harmonic generation of continuous-wave Yb fiber lasers is reported. A simple bow-tie external resonant cavity incorporating a type I LBO nonlinear optical crystal was employed for second harmonic frequency conversion of a multi-longitudinal-mode Yb fiber laser. It is shown that strong coupling was formed between the Yb fiber laser and the external cavity and, as a result, the laser produced 9.1 W of green output at 535 nm for 43?W of absorbed diode pump power at 975 nm corresponding to an optical conversion efficiency of 21?% with respect to absorbed diode pump power. The prospects for further improvement are discussed.     

Linear and nonlinear Biot waves in a noncohesive granular medium slab: transfer function, self-action, second harmonic generation

Experimental results are reported on second harmonic generation and self-action in a noncohesive granular medium supporting wave energy propagation both in the solid frame and in the saturating fluid. The acoustic transfer function of the probed granular slab can be separated into two main frequency regions: a low frequency region where the wave propagation is controlled by the solid skeleton elastic properties, and a higher frequency region where the behavior is dominantly due to the air saturating the beads. Experimental results agree well with a recently developed nonlinear Biot wave model applied to granular media. The linear transfer function, second harmonic generation, and self-action effect are studied as a function of bead diameter, compaction step, excitation amplitude, and frequency. This parametric study allows one to isolate different propagation regimes involving a range of described and interpreted linear and nonlinear processes that are encountered in granular media experiments. In particular, a theoretical interpretation is proposed for the observed strong self-action effect.     

Nanoparticle formation during laser ablation of solids in liquids

The experimental data on the generation of metal and semiconductor nanoparticles during their laser ablation in liquids is reviewed. The dependence of the morphology of noble metal nanoparticles on the liquid type and laser parameters is discussed. The data on the kinetics of the formation of alloyed Au-Ag nanoparticles under laser irradiation of a mixture of colloid solutions of individual nanoparticles are presented. The effect of femtosecond laser beam self-action during metal ablation in liquids via the second harmonic generation at Ag nanoclusters is discussed. The data on the generation of core-shell nanoparticles during laser ablation of alloys and in the presence of the chemical interaction of formed nanoparticles with surrounding liquid are presented. It was shown that laser ablation of CdS and ZnSe crystals leads to the formation of quantum dots of these semiconductors in solution. The parameters controlling the properties of nanoparticles during ablation in liquids and possible applications of the method are discussed.     

Second harmonic generation by femtosecond laser pulses in the Laue scheme

Results of the study of second harmonic generation by femtosecond laser pulses in a lithium niobate crystal with a regular domain structure in the Laue scheme are presented. The angular distribution of the intensity and the polarization properties of the second harmonic radiation are studied. The orders of the corresponding phase-matching conditions and the angular width of the second harmonic maxima are calculated theoretically. The theoretical results are compared with experimental data.     

Analysis of broadband third harmonic generation with non-collinear angular dispersion in KDP crystals

A new scheme with non-collinear sum frequency generation (SFG) process of broadband third harmonic generation (THG) with two KDP crystals (one doubler and one mixer) using angular dispersion was proposed. The principle of broadband harmonic generation with non-collinear angular dispersion was presented. The comparison between the schemes of non-collinear and collinear SFG process was performed. The effects of the angular dispersion on the conversion efficiency, the pulse shape and the spectrum of the third harmonic field were discussed. The results show that, if proper angular dispersion is added to the fundamental and the second harmonic field outside the mixing KDP crystal, respectively, the broadband third harmonic conversion efficiency can be improved significantly. However, the difficulty of this scheme arises due to the requirement of two gratings with different angular dispersion outside the mixer. With the new scheme, it can be simpler that only one grating is needed to realize the broadband phase matching in the non-collinear SFG process. Although the fundamental and the second harmonic field inside the mixing crystal are non-collinear except the center wavelength, a blazing grating with proper angular dispersion for the fundamental field (twice as that for second harmonic field) can yield the well compensation for phase mismatching in the SFG process. Consequently, the conversion efficiency and the characteristics of the third harmonic field for the broadband third harmonic generation can be improved significantly.     

飞秒时间分辨实验中相关函数和时间零点的确定

结合飞秒激光在研究分子激发态弛豫动力学中的应用,介绍了几种飞秒时间分辨实验中确定泵浦激光脉冲与探测激光脉冲的相关函数和时间零点的方法.对于波长在可见波段的泵浦和探测激光脉冲,我们可以利用非线性光学的技术手段,探测泵浦光与探测光的和频光的强度随二者间的时间延迟的变化来确定相关函数和时间零点;对于波长在紫外甚至更短的波段的泵浦和探测激光脉冲,由于单脉冲能量比较低,目前还很难利用技术手段来测定泵浦激光与探测激光的相关函数及时间零点,可以利用某些原子气体(如Xe)或某些具有短寿命态的分子作平行实验进行间接测量.     

Phase effects at second harmonic generation in the layer media

An analysis is made of quasi-phase-matched interaction at second harmonic generation in a regular domain structure with accounts for losses and change of phases of all the interacting waves. The constant-intensity approximation of basic radiation is applied for this purpose, not to regular domain structure as a whole, but to each separately taken domain. It allows to carry out more strict analysis of quasi-synchronous interaction during the frequency conversion to the polydomain consisting of n layers, forming “grating” periods of modulation of the nonlinear susceptibility. With this, the values of complex amplitudes of basic radiation and second harmonic wave at the outlet of each domain are entrance values of the corresponding complex amplitudes to the following domain. The analytical expression is given for the case of n domains and the factors limiting the efficiency of the process of frequency conversion are analyzed. In the constant-intensity approximation, in contrast to the constant field approximation, the coherent length of domain depends on pump intensity. With increasing pump intensity the optimum length decreases. In a regular domain structure at frequency conversion from a layer to a layer, intensity of the basic radiation changes. Also the optimum length of domains at which conversion efficiency is maximal therefore changes. Thus, it is possible to obtain the high values of conversion frequency at the outlet of a regular domain structure by choice of optimum parameters of a task (length of domains, phase mismatch, pump intensity), as well as using the layers-domains of high quality.     

Influence of third-order nonlinear and phase variations on third harmonic generation

A theoretical analysis is presented for third-harmonic generation in KDP for type I/type II angle-detuning scheme of high-intensity laser to produce third harmonic radiation near 0.35 μm. The effects of the third-order nonlinearity and the phase variations on the frequency conversion have been discussed. The results shown that the third-order nonlinear interactions decreases the tripling efficiency, and increases of the modulate strength of the output intensity of 3ω radiation. However, adjusting the angular detuning can compensate effectively the effects of third-order nonlinearity. Furthermore, 3ω conversion efficiency will drop with the increase of the degree of phase variations, and the increase 3ω conversion efficiency can decrease the influence of third-order nonlinearity on the conversion of third harmonics.