电子在超短激光脉冲修正场中的动力学特性研究

给出了一种精确描述超短、紧聚焦激光脉冲的新方法，其思路是根据两个无量纲小量ε=1/(ω0t0)和s=1/(k0w0)（其中ω0=ck0为中心振荡频率，t0为脉冲延迟时间，w0为激光束腰半径）进行展开来计算脉冲的高阶修正场.在激光束近轴近似表达式的基础上，给出了高斯脉冲一阶修正场的解析表达式，并研究发现其振幅和相位相对于零阶修正场（即长脉冲近似）的修正量都在ε的量级甚至更小.另外对电子在超短高斯脉冲一阶修正场中的动力学特性研究发现：对于ω0t0>20的情况，零阶修正场可以正确地描述电子被光场加速的特性；当ω0t0<20时，则需要采用高阶修正场. 关键词： 超短激光脉冲 激光加速     

非傍轴高斯光束传输方程解的探讨

由洛伦兹规范下光场满足的矢势和标势方程出发, 探讨了初始光场具有任意对称性的一般非傍轴光束在空间的传输情况。利用角频谱并结合傅里叶变换处理得到了非傍轴光束的传输方程。利用扰动方法并结合新近似取法得到了非傍轴横向光场的扰动形式解, 该解对于近场弱非傍轴情况下的效果比强非傍轴条件下要好。给出了光束非傍轴传输高阶(八阶)修正解。对于零阶情况, 非傍轴光束退化为傍轴光束, 传输方程也相应退化为傍轴传输的模型方程。高斯解为扰动参量的零阶修正解。     

周期量级激光脉冲的Thomson散射

用经典辐射理论对线偏振周期量级激光脉冲的线性Thomson散射进行分析，从理论上得到它可产生亚阿秒脉冲的结论. 计算显示，在电子相对论因子为50、激光脉冲中心波长为1μm、归一化光场强度为0.01的情况下，用包含1.5个光周期的激光脉冲，可获得0.2as(半高全宽)的散射脉冲输出. 还对光场载波包络初相φ_ce和电子进入光场的初相φ_in对散射脉冲的影响作了分析讨论，结果表明，在适当的φ_ce和φ_in条件下，能实现单个阿秒脉冲输出，并可对脉冲宽度和频率进行调谐. 关键词： 线性Thomson散射 周期量级激光脉冲 载波包络初相 阿秒脉冲     

Group-delay Dispersion in Double-prism Pair and Limitation in Broadband Laser Pulses

A general expression of group-delay dispersion is obtained without any approximation for a pair of double prisms with an arbitrary apex angle. This expression also includes the change of dispersion resulting from change in insertion of the prism material into the beam by translating prism-pairs. The high-order dispersion can be calculated by means of this expression. The limitation for generation of negative group-delay dispersion by use of prism-pairs is presented for ultrashort laser pulses with broadband spectrum.     

Population transfer by femtosecond laser pulses in a ladder-type atomic system

The population transfer in a ladder-type atomic system driven by linearly polarized sech-shape femtosecond laser pulses is investigated by numerically solving Schr6dinger equation without including the rotating wave approximation （RWA）. It is shown that population transfer is mainly determined by the Rabi frequency （strength） of the driving laser field and the chirp rate, and that the ratio of the dipole moments and the pulse width also have a prominent effect on the population transfer. By choosing appropriate values of the above parameters, complete population transfer can be realized.     

Three-dimensional evolution of ultrashort pulses in dispersive media beyond the slowly varying envelope approximation

A new analytical solution for the three-dimensional evolution of femtosecond pulses in a medium with group velocity dispersion is presented. It is found that for Gaussian pulses as short as a few optical cycles, the coupling between spatial and temporal parameters becomes important, which will cause breakdown of the slowly varying envelope approximation (SVEA) equation.     

Group-delay Dispersion in Double-prism Pair and Limitation in Broadband Laser Pulses

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Spatiotemporal Evolution of Intense Short Gaussian Laser Pulses in Weakly Relativistic Magnetized Plasma

The weakly relativistic regime of propagation of a short and intense laser pulse in the magnetized plasma is investigated. By considering relativistic nonlinearity and using non‐linear Schrödinger equation with paraxial approximation, two second‐order coupled differential equations are obtained for the longitudinal pulse width parameter (in time) and for the transverse pulse width parameter (in space). The simultaneous evolution of spot size and length of a relativistic Gaussian laser pulse in a magnetized plasma can be calculated by the numerical solution of the equations. The effect of magnetic field is investigated. It is observed that in the presence of magnetic field both the self‐compression and the self‐focusing can be enhanced. Furthermore, the interplay between the longitudinal self‐compression and the transverse self‐focusing in a magnetized plasma is investigated. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Calculation of radiation acoustical fields from phased arrays with nonparaxial multi-Gaussian beam model

A nonparaxial multi-Gaussian beam model based on the rectangular aperture is proposed in order to overcome the limitation of paraxial Gaussian beam model which losing accuracy in off-axis beam fields. With the method, acoustical field generated by an ultrasonic linear phased array transducer is calculated and compared with the corresponding field obtained by Rayleigh-Sommerfeld integral, paraxial multi-Gaussian beam model, and Fraunhoffer approximation method. Simulation examples show that nonparaxial multi-Gaussian beam model is not limited by the paraxial approximation condition and can predict efficiently and accurately the acoustical field radiated by a linear phased array transducer over a wide range of steering angles.     

Phase relations between focused bichromatic laser pulses in terahertz wave generation from gas plasma

Terahertz emission from air excited by the tightly focused fundamental and second harmonic pulses of femtosecond Ti:Sapphire laser has been analyzed. It has been found that the curved phase fronts of the pump waves cancel each other resulting in the flat phase front of the nonlinear polarization at terahertz frequency. Also, in addition to the phase terms obtained using plane-wave approximation, we have found the yet unreported phase term, which in most cases can be as large as π/2.     

The Spectral Intensity of the Plasma of Single and Double Laser Pulses

We compare radiation of plasma flares produced on exposure of metals to laser radiation in a monopulse generation mode in the case of single and double pulses with change in air pressure. For double pulses, we established an increase in the duration and intensity of the radiation of spectral lines and also an analogy between the change in their intensities and those of single pulses obtained at a reduced air pressure.     

Spectral anomalies of diffracted chirped Gaussian pulses and their potential applications

Starting from the Rayleigh-Sommerfeld diffraction integral and without invoking the paraxial approximation, analytical expressions for the field distribution, far-field power spectrum and temporal far-field distribution of chirped Gaussian pulses diffracted at a circular aperture are derived, which enables us to study the spectral anomalous behavior of diffracted chirped Gaussian pulses in the far field. The potential applications of spectral anomalies of ultrashort pulses are discussed. It is found that at the critical angle the spectral switch appears. The frequency difference between the two equal heights of spectral switches increases and the corresponding critical diffraction angle slightly increases as the chirp parameter increases and pulse duration decreases. In a certain region of the truncation parameter, the critical angle decreases with increasing truncation parameter. By suitably varying the pulse duration, chirp parameter and truncation parameter, information encoding and transmission are achievable in the use of chirped Gaussian pulses.     

Intense THz-radiation from semiconductors under magnetic field irradiated with femtosecond laser pulses

The THz-radiation power from InAs reaches sub-mW level in a 1.7-T magnetic field irradiated with femtosecond laser pulses of 1.5-W average power. The THz-radiation power is related almost quadratically both to the magnetic field and to the excitation laser power. Furthermore, the THz-radiation spectrum is found to be controlled by the excitation pulsewidth, chirp direction of the excitation pulse, and the magnetic field.     

超短飞秒光脉冲产生技术

本文叙述了群速弥散补偿被动锁模环型染料激光器的特性;研究了产生极短飞秒光脉冲的条件和光脉冲强度分布的非对称性。该技术产生的最短光脉冲宽度为21 fs;光脉冲宽度可调范围从最短到500fs以上。实验和理论数据拟合证明光强分布为非对称双曲正割平方,非对称因子r在7到11之间。     

Exact solution of the CPMG pulse sequence with phase variation down the echo train: application to R₂ measurements

An implicit exact algebraic solution of CPMG experiments is presented and applied to fit experiments. Approximate solutions are also employed to explore oscillations and effective decay rates of CPMG experiments. The simplest algebraic approximate solution has illustrated that measured intensities will oscillate in the conventional CPMG experiments and that using even echoes can suppress errors of measurements of R? due to the imperfection of high-power pulses. To deal with low-power pulses with finite width, we adapt the effective field to calculate oscillations. An optimization model with the effective field approximation and dimensionless variables is proposed to quantify oscillations of measured intensities of CPMG experiments of different phases of the π pulses. We show, as was known using other methods, that repeating one group of four pulses with different phases in CPMG experiments, which we call phase variation, but others call phase alternation or phase cycling, can significantly smooth the dependence of measured intensities on frequency offset in the range of ±?γB?. In this paper, a second-order expression with respect to the ratio of frequency offset to π-pulse amplitude is developed to describe the effective R? of CPMG experiments when using a group phase variation scheme. Experiments demonstrate that (1) the exact calculation of CPMG experiments can remarkably eliminate systematic errors in measured R?s due to the effects of frequency offset, even in the absence of phase variation; (2) CPMG experiments with group phase variation can substantially remove oscillations and effects of the field inhomogeneity; (3) the second-order expression of the effective decay rate with phase variation is able to provide reliable estimates of R? when offsets are roughly within ±?γB?; and, most significantly, (4) the more sophisticated optimization model using an exact solution of the discretized CPMG experiment extends, to ±γB?, the range of offsets for which reliable estimates of R? can be obtained when using the preferred phase variation scheme.     

Structure of a nonparaxial gaussian beam near the focus: III. Stability,eigenmodes, and vortices

Exact analytical structurally stable solutions of the Maxwell equations for singular mode beams propagating in free space or a uniform isotropic medium are obtained. Approximate boundary conditions are chosen in the form of the requirement that in the paraxial approximation the fields of nonparaxial mode beams in the waist plane are transformed into the fields of eigenmodes and vortices of weakly guiding optical fibers with the axial symmetry of refractive index. It is shown that optical vortices, in spite of a rather complex structure of field distribution, do not experience substantial changes in the beam form and reproduce, in general features, the field of paraxial vortices. Linear perturbations of the characteristic parameters of mode beams do not change the structure of their electromagnetic field. Nonparaxial singular beams have one more important property, in addition to the fact that the structure of these beams in the paraxial approximation is similar to the structure of the fields of eigenmodes in a fiber. The propagation constants of eigenmodes of a fiber exactly coincide (in the first approximation of perturbation theory) with the projection of the wave vector of a mode beam on the optical axis (an analog of the propagation constant). The possibility of the paraxial transition for nonparaxial mode beams with arbitrary values of azimuthal and radial indices is shown. The properties of nonparaxial modes are illustrated by numerous examples. The solutions obtained and the results of their analysis can be used for exact matching optical fibers and laser beams in various applications.     

圆偏振激光附加太赫兹场作用下的高次谐波发射和孤立阿秒脉冲的产生

本文通过数值求解二维含时薛定谔方程,研究了圆偏振激光和太赫兹组合场作用下,H+2的高次谐波发射和孤立阿秒(1 as=10-18s)脉冲的产生.研究发现,无论在圆偏振激光场的x或y方向附加一个太赫兹场,都可以产生一个比较平滑的连续谐波谱.通过时频分析我们发现,高次谐波的贡献主要来自于短轨道.适当选取一些级次的连续谐波进行叠加,可以得到129 as或83 as的孤立阿秒脉冲.     

圆偏振激光附加太赫兹场作用下的高次谐波发射和孤立阿秒脉冲的产生

本文通过数值求解二维含时薛定谔方程,研究了圆偏振激光和太赫兹组合场作用下,H2+的高次谐波发射和孤立阿秒脉冲的产生。研究发现,无论在圆偏振激光场的 或 方向附加一个太赫兹场,都可以产生一个比较平滑的连续谐波谱。通过时频分析我们发现,高次谐波的贡献主要来自于短轨道。适当选取一些级次的连续谐波进行叠加,可以得到129 as或83 as的孤立阿秒脉冲。     

一类系列脉冲激光照射真空中铝靶产生的LSD波

本文应用一维双温激光靶相互作用简化物理模型对脉宽为10ns级系列脉冲激光照射真空中铝靶产生的LSD波进行了数值研究。对于单个脉冲宽度τ_p=10ns,脉冲间距τ_p=50ns的系列脉冲情况,得到了LSD波点火的脉冲个数N_(LSD)与单脉冲归一化强度(I_s/I_(LSD))的关系。分析了系列脉冲激光LSD波点火的积累效应以及LSD波的屏蔽效应。     

不加腔外展宽器的高重复率钛宝石再生放大器

最近我们在省略腔外展宽器的放大器中实现了自锁模钛宝石脉冲的放大,实验中我们用自制的的振荡器产生脉宽为20f.