影响单电子非线性汤姆孙散射因素的研究

应用电子汤姆孙散射的经典理论，通过理论分析和计算机模拟，研究了超短超强激光脉冲作用下电子产生的辐射脉冲的性质.计算表明，在这种情况下，电子的辐射通常以阿秒脉冲列的形式出现.讨论了不同激光场参数（包括激光强度、脉宽、初相位和偏振态）、不同电子初始状态（初始速度和位置）对辐射脉冲的时间和空间特性的影响.通常在相对论光强条件下，激光强度越大，电子辐射越强，脉宽越窄，中心频率越大，并且方向性越好；电子在线偏振激光中产生的辐射效率，比在同样强度下圆偏振激光中产生的效率更高；无论入射光是线偏振光，还是圆偏振光,辐射场呈现较复杂的偏振态, 并且它与辐射方向有关.当电子具有一定的初始能量时，通常辐射场的振幅随电子初始能量的增大而增大.不管电子的初始能量以及运动方向如何，做相对论运动的电子产生的辐射趋向于出现在靠近电子运动方向的角度区域.     

Stimulated Raman scattering pulses. I. Weak scattering

Rate equations are used to compute a weak stimulated Raman scattering pulse in the case where the scattered radiation energy can be neglected in the over-all energy balance. Expressions for the shape, position and height of the maximum, duration, and spectral width of the scattering pulse as functions of the parameters of the driving laser, scattering material, and observation conditions are derived.     

激光脉冲在各向异性散射介质内的瞬态热效应

本文考察了激光脉冲在吸收、发射、各向异性散射介质内引起的瞬态热效应。将激光脉冲辐射在介质内的传递过程分为两个子过程：发射－衰减－反射过程和吸收－散射过程。用光线踪迹法结合节点分析导出辐射传递系数和辐射源项,用控制容积法解瞬态能量方程。检验结果表明,本文的计算方法准确。在此基础上,考察了散射特性、初始温度对激光脉冲响应的影响。     

Vectorial stimulated Raman scattering resolution on the semi-line

Stimulated Raman scattering between a laser pump pulse and a Stokes pulse is considered in a two-level medium with vectorial optical fields. The model on the semi-line is proved to be solvable by inverse scattering transform scheme. Among solutions, soliton generation is discussed. Then, it is shown how rotation of the Stokes wave leads to a spike of pump radiation in the time domain.     

Thermal signatures of a localized inhomogeneity in a 2-D participating medium subjected to an ultra-fast step-pulse laser wave

The transport of a train of short-pulse radiation through a 2-D rectangular participating medium consisting of local inhomogeneities is investigated in this article. A collimated beam of step temporal profile is considered. The pulse wave consists of 1 and 4 pulses. The absorbing and scattering participating medium contains a square-shaped local inhomogeneity. The inhomogeneity differs from the rest of the medium by its scattering albedo. The pulse width and the period of the laser wave are of the order of a nano-second. Transmittance and reflectance signals are analyzed for the effects of the extinction coefficient and the scattering albedo. Heat flux distributions inside the medium are also studied. The finite-volume method is used to solve the problem.     

初始相位对周期量级激光脉冲Thomson散射特性的影响

 用电子Thomson散射的经典理论,研究了周期量级激光脉冲作用下电子Thomson散射的特性,讨论了不同激光强度下,激光脉冲的初始相位对电子辐射的空间分布以及特定方向上频谱分布特性的影响。计算表明:对弱激光脉冲,电子辐射的空间分布类似于偶极天线的对称双叶结构,初始相位对电子的辐射几乎没有影响;而对强激光脉冲,电子辐射的空间分布出现了三叶结构,初始相位对电子的辐射影响非常显著。     

Interaction between ultra short pulses and a dense scattering medium by Monte Carlo simulation: consideration of particle size effect

With an enough short-pulse incident to an individual particle, elementary scattering modes can be observed: internal or external reflection, refraction and diffraction. Simulation of pulse propagation in dense scattering medium is usually computed for large observation time, so that time delays of pulse interaction with the particles are negligible compared to propagation times between particle. A Monte Carlo method is proposed to compute the propagation of an incident 100 fs laser pulse in dense medium taking into account time-dependent scattering characteristics of particle: observation time of scattered light is less than 5000 fs. Two extreme cases are exemplified: predominance of direct and single-scattered photons appears in a thin time window for small particles (1 μm). On the contrary multiple scattering is always predominant and scrambles the transmitted signal for large particles (100 μm).     

对受激布里渊散射激光进行组束的数值模拟及方案设计

 提出了时间上串行的多路激光脉冲通过受激布里渊散射(SBS)池进行组束的方法,并对其进行了数值模拟研究。利用6束每束能量为45J的KrF激光进行组束, 可获得能量为141.89J,脉宽为670ps的Stokes输出光。根据模拟结果设计出了时间上串行的SBS激光组束的两个方案, 对其进行了讨论。数值模拟还发现在介质的增益系数更大、声子寿命更短的情况下,输出激光脉冲的脉宽可以压缩得更窄。     

Determination of optical characteristics of biological tissues by temporal distribution of an ultrashort laser pulse passed through homogeneous scattering layer

The determination of optical characteristics of biological tissues by the temporal distribution of an ultrashort laser pulse passed through a homogeneous scattering layer is considered. Four nonstationary models used to describe passing of laser pulses through a highly scattering medium are compared for the first time. The domains of applicability for the diffusion models are determined. It has been shown that the determination of optical characteristics of highly scattering media by experimental temporal distributions leads to different numerical values depending on the used model. Nevertheless, any of these models can be used because the performed study does not permit one to give preference to any of them. However, in determining optical characteristics of the scattering samples, it is necessary to specify the used model, as well as the conditions under which the initial experimental data (in particular, the thickness of the layer of a studied sample) are obtained.     

Determination of nonlinear optical characteristics of transparent materials using single laser pulses

A new method of express analysis of nonlinear optical properties of materials by determining their nonlinear refraction and nonlinear absorption upon irradiation by a single laser pulse is proposed. The nonlinear optical characteristics of media are determined based on an analysis of variations in the spatial distribution of radiation focused by a cylindrical lens and passed through the medium. The method proposed is tested by the example of optical nonlinearities of CS₂.     

超短激光脉冲驱动的Thomson散射空间分布特性

研究了超短激光脉冲驱动的Thomson散射的辐射空间分布. 发现辐射的空间分布对称性显著地依赖于超短脉冲的载波相位η₀，在η₀＝0，π时，辐射的空间分布呈现出二重或一重对称性；而在其他载波相位时，这些空间分布对称性遭到破坏. 辐射光的准直性也依赖于驱动脉冲的载波相位，在η₀＝±π/2时辐射光的准直性最好. 这些结果表明可以通过改变驱动激光脉冲的载波相位来控制辐射光的空间分布，也可以利用辐射空间分布对相位的依赖特性来测量超短激光脉冲的载波相位. 关键词： Thomson散射 空间分布 载波相位     

Theory of stimulated concentration scattering of light in a liquid suspension of transparent microspheres

A theory of stimulated light scattering in a nonlinear liquid suspension of transparent microspheres—an artificially created medium whose nonlinearity is caused by modulation of the concentration of microspheres by gradient forces in a field of spatially inhomogeneous laser radiation—is constructed. The threshold, angular, and spectral characteristics of the scattering are studied in the diffusion-limit approximation based on the solution of the system of wave equations in combination with the Planck-Nernst two-dimensional equation for the concentration of microspheres. The transient regime of scattering in the field of a specified step-like pump pulse is considered. A sharp angular dependence of the scattering efficiency on the microsphere radius is predicted and proposed for use in optical diagnostics of liquid suspensions of dielectric microspheres—highly efficient wideband nonlinear media.     

Thomson and Compton scattering with an intense laser pulse

Our paper concerns the scattering of intense laser radiation on free electrons and it is focused on the relation between nonlinear Compton and nonlinear Thomson scattering. The analysis is performed for a laser field modeled by an ideal pulse with a finite duration, a fixed direction of propagation and indefinitely extended in the plane perpendicular to it. We derive the classical limit of the quantum spectral and angular distribution of the emitted radiation, for an arbitrary polarization of the laser pulse. We also rederive our result directly, in the framework of classical electrodynamics, obtaining, at the same time, the distribution for the emitted radiation with a well defined polarization. The results reduce to those established by Krafft et al. [Phys. Rev. E 72, 056502 (2005)] in the particular case of linear polarization of the pulse, orthogonal to the initial electron momentum. Conditions in which the differences between classical and quantum results are visible are discussed and illustrated by graphs.     

紧聚焦激光束作用于电子实现单个阿秒脉冲输出

研究了电子在聚焦的圆偏振高斯激光束中的非线性汤姆孙散射过程，在此基础上提出了实现单个阿秒脉冲输出的新机理.通过计算机模拟，发现利用紧聚焦的激光脉冲可以有效地增大辐射脉冲链的最高峰和次高峰的峰值强度比即信噪比，从而将阿秒脉冲链变为单个阿秒脉冲输出.紧聚焦情形下，随着驱动激光强度的增大，辐射信号的脉宽变短，信噪比变大；同时当减小激光束腰半径时，辐射信号信噪比也能得到有效改善.研究还发现，利用几个光周期的极短激光脉冲与电子的相互作用也能实现单个阿秒脉冲输出. 关键词： 阿秒脉冲 紧聚焦 信噪比     

Detection of the nonballistic component of scattered light in the process of acousto-optic visualization

For determining the optimum conditions of detecting the signal during acousto-optic visualization, the modulation characteristics of radiation of a laser beam crossed by a focused acoustic beam in a medium and scattered by this medium were studied. The characteristics were obtained by measuring the amplitude of the alternating-photocurrent component at the ultrasonic frequency as a function of the geometry of the experiment, the parameters of the scattering medium, and the laser radiation power. Special attention was given to the magnitude of the signal-to-noise ratio. A photodetector recording the field of the nonballistic component of the scattered radiation was positioned outside the geometrical sizes of the laser beam. The photocurrent component corresponding to this radiation at the ultrasonic frequency was used as a parameter of the acousto-optic visualization. Images of optically opaque objects immersed into a medium whose scattering parameters corresponded to the transition from the regime without scattering to the multiple-scattering regime were obtained.     

Nonlinear interaction of a circularly polarized electromagnetic wave with a dense laser plasma

The interaction of an intense circularly polarized laser pulse with a layer of plasma of supercritical density is studied. The nonlinear skin effect for the electromagnetic field and the coefficient of collisionless absorption of the laser pulse were calculated analytically. It is shown that, in the process of interaction with the plasma, the laser pulse generates solitons propagating through the plasma layer and transferring the radiation through the opaque medium. The coefficient of transparency of the plasma layer for the soliton-like penetration of the laser radiation was calculated. The plasma parameters at which the collisionless absorption is small as compared to the transformation of the laser energy into solitons were found.     

Numerical optimization of the extracavity Raman laser with barium nitrate crystal

The radiation transfer equations of the extracavity Raman laser including up to the third Stokes beams and backward Raman scattering terms were deduced in detail from the wave equation and material equations of stimulated Raman scattering. The radiation transfer equations were solved numerically to optimize the performance of the extracavity Raman lasers with barium nitrate crystal as the nonlinear medium. The optimum reflectivity of the output coupler at the first Stokes was figured out numerically to achieve the maximum conversion efficiency of the first Stokes, and found to be closely related to the pump pulse duration, peak intensity of the pump pulse, and Raman crystal length. With the resonator mirrors highly reflective at the first Stokes, the highest conversion efficiency of the second Stokes was obtained when the input mirror was highly reflective at second Stokes, whereas the output coupler was highly transmissive at the second Stokes. It was found that too high intracavity intensity of the second Stokes would impede the efficient energy extraction from the pump pulse to the first Stokes, and consequently, limit the conversion efficiency of the second Stokes.     

Transient radiative transfer in participating media with pulse-laser irradiation—an approximate Galerkin solution

An assessment is made of the Galerkin technique as an effective method of solution for transient radiative transfer problems in participating media. A one-dimensional absorbing and isotropically scattering plane-parallel gray medium irradiated with a short-pulse laser on one of its boundaries is considered for the application of the method. The medium is non-emitting and the boundaries are non-reflecting and non-refracting. In the integral formulation of the problem for the source function, the time-wise variation of the radiation intensity at any point and in any direction in the medium is assumed to be the same as the time-wise variation of the average intensity at the same point as an approximation for the application of the method. The transient transmittance and reflectance of the medium are evaluated for various values of the optical thickness, scattering albedo and pulse duration. The results are in agreement with those available in the literature. It is demonstrated that the method is relatively simple to implement and yields accurate results.