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.     

Photoemission of a single-electron wave packet in a strong laser field

The radiation emitted by a single-electron wave packet in an intense laser field is considered. A relation between the exact quantum formulation and its classical counterpart is established via the electron's Wigner function. In particular, we show that the wave packet, even when it spreads to the scale of the wavelength of the driving laser field, cannot be treated as an extended classical charge distribution, but rather behaves as a pointlike emitter carrying information on its initial quantum state. We outline an experimental setup dedicated to put this conclusion to the test.     

The initiation of superfluorescence

The early stages of a superfluorescent pulse are described with full account of both quantum and propagation effects. The pulse is triggered by zero point fluctuations of the atomic polarization field. If the number of atoms N is large these zero point fluctuations, and the electric field radiated at early times as well, have a gaussian distribution with width 4/N. An equivalent classical stochastic process is found in terms of which the later, intrinsically nonlinear part of the radiation problem can be analyzed.     

弥散补偿飞秒激光量子理论研究

本文根据我们新建的激光器模型,导出了非线性成形飞秒光脉冲的量子理论锁模方程。在薛定谔表象中,用含时哈特利近似方法求得了脉冲电场及光子数分布的形式解。最后讨论了量子锁模方程过渡到经典情形下稳态时的脉冲能量、脉宽及啁啾参量与飞秒激光器运转参量间的关系,得到了理论与实验一致的结果。     

Electron-positron pair creation by powerful laser-ion interaction

Presently available high-power laser pulses of ponderomotive energy U _p ? 2mc ² should permit the fundamental processes of quantum electrodynamics in such fields, in particular, the formation of electron-positron pairs in impacts of laser pulses with highly charged ions, to be observed. We evaluate the highly nonlinear production rates of this process and investigate the most favorable conditions of pair production, in particular, either along the direction of linear polarization or in the propagation direction of the laser pulse. For femtosecond radiation pulses, it is possible to represent the laser beam by a monochromatic and linearly polarized electromagnetic plane wave. This approximation considerably simplifies the calculations required.     

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     

Compton scattering and electron-atom scattering in an elliptically polarized laser field of relativistic radiation power

Presently available laser sources can yield powers for which the ponderomotive energy of an electrons can be equal to or even larger than the rest energy mc ² of an electron. Therefore it has become of interest to consider fundamental radiation-induced or assisted processes in such powerful laser fields. In the present work we consider laser-induced Compton scattering and laser-assisted electron atom scattering in such fields, assuming that the laser beam has arbitrary elliptic polarization. We investigate in detail the angular and polarisation dependence of the differential cross-sections of the two laser-induced or laser-assisted nonlinear processes as a function of the order N of absorbed or emitted laser photons . The present work is a generalization of our previous analysis of Compton scattering and electron-atom scattering in a linearly polarized laser field [Phys. Rev. A 65, 022712 and 033408 (2002)].Received: 3 November 2002, Published online: 29 April 2003PACS: 34.50.Rk Laser-modified scattering and reactions - 34.80.Qb Laser-modified scattering - 32.80.Wr Other multiphoton processes     

Fluorescence versus laser effect: Towards a unified theory

A single evolution equation is established to treat the mechanisms of fluorescence and the laser effect, using a mixed representation, classical for the electromagnetic field and quantum for the dipoles. The model approach takes advantage of the principle of conservation of energy for the system of electromagnetic field and dynamic dipole. A resulting nonlinear differential equation is derived and is shown to sustain two fixed points associated with fluorescence and laser emission. The existence of a pumping threshold is confirmed for the laser effect. In particular the pumping rate determines whether light will be emitted by fluorescence or by the laser effect, and there is no pumping that engenders fluorescence and laser emission simultaneously. The initial physical conditions, chosen to integrate the evolution equation in time, assume a nonvanishing electric polarization of the emitting dipole rather than a nonvanishing electromagnetic field. The distribution function accounting for the thermal fluctuations of the random initial polarization is also calculated.     

任意椭圆偏振激光场非线性汤姆逊散射的一般表述与X射线产生的优化条件

基于相对论条件的电动力学,解析求解了任意椭圆偏振条件下激光场的非线性汤姆逊散射的一般表示.利用解析结果得到了背向非线性汤姆逊散射高次谐波的极值条件.结果表明对于基频背向汤姆逊散射,在相同条件下,圆偏振激光具有最大值而线偏振是最小值,如果激光偏振态从圆偏振连续的过渡到线偏振,背向汤姆逊散射的角功率随之单调递减.这一效应对高强度入射激光尤为重要,当 a² >5时,圆偏振情形的贡献几乎是线偏振的2倍.这对基于汤姆逊散射机制的X射线源实验研究具有重要参考意义. 关键词： 非线性汤姆逊散射 偏振度 极值     

Peak intensity measurement of relativistic lasers via nonlinear Thomson scattering

The measurement of peak laser intensities exceeding 10(20) W/cm(2) is in general a very challenging task. We suggest a simple method to accurately measure such high intensities up to about 10(23) W/cm(2), by colliding a beam of ultrarelativistic electrons with the laser pulse. The method exploits the high directionality of the radiation emitted by ultrarelativistic electrons via nonlinear Thomson scattering. Initial electron energies well within the reach of laser wake-field accelerators are required, allowing in principle for an all-optical setup. Accuracies of the order of 10% are theoretically envisaged.     

Surface-Enhanced Hyper-Raman Spectroscopy

Abstract

Hyper-Raman scattering (HRS), first theoretically predicted by Decius and Rauch in 1959 [l] and experimentally demonstrated by Terhune et al. in 1965 [2], is a nonlinear optical process involving two incident photons (ω₀) and one emitted photon (ω). The emitted hyper- Raman photon frequencies are Raman-shifted relative to the second harmonic frequency (2ω₀) of the incident laser radiation [3–6]. The energy difference (2ω₀ – w) corresponds to one of the characteristicvibrational frequencies of the scattering medium or molecule. In Fig. 1 is given a schematic illustration of resonant and nonresonant HRS. The primary advantage of this nonlinear optical technique lies in its more relaxed selection rules compared with IR and Raman [7,8]. AlllR-active vibrational modes are hyper-Raman allowed, and those modes inactive in both IR and Raman (i.e., the “silent” modes) may be active in hyper-Raman scattering.     

Manipulating Photoelectron Distributions for Rare Gas Atoms Ionized with Polarized Femtosecond Laser Pulses

Above-threshold ionizations of rare gas atoms excited by polarized femtosecond laser pulses are investigated. The photoelectron momentum spectra are obtained applying the strong-field approximation (SFA) theory. It is found that, distribution of the emitted photoelectrons varies with different polarizations of laser pulses. We have interpreted the relationship between the observed distribution and the laser polarization taking advantage of tunneling ionization theory and simple-man model. The polarization sensitivity indicates that one can easily manipulate the photoelectron distribution by controlling the polarization of the exciting pulse.     

30 ps pulses source based on a fiber-coupled passively Q-switched microchip laser

We report the design of low-cost and compact short-pulse source based on a fiber-coupled Q-switched microchip laser. The combination of stimulated Raman scattering and nonlinear polarization rotation effects in the fiber associated with appropriate filtering makes it possible to tune pulse duration down to 32 ps with peak power above 3 kW. Pulse to pulse peak power fluctuation is below 4%.     

The polarization effect of a laser in multiphoton Compton scattering

The multiphoton Compton scattering in a high-intensity laser beam is studied by using the laser-dressed quantum electrodynamics(QED) method, which is a non-perturbative theory for the interaction between a plane electromagnetic field and a charged particle. In order to analyze in the real experimental condition, a Lorentz transformation for the cross section of this process is derived between the laboratory frame and the initial rest frame of electrons. The energy of the scattered photon is analyzed, as well as the cross sections for different laser intensities and polarizations and different electron velocities. The angular distribution of the emitted photon is investigated in a special velocity of the electron, in which for a fixed number of absorbed photons, the electron energy will not change after the scattering in the lab frame.We obtain the conclusion that higher laser intensities suppress few-laser-photon absorption and enhance more-laser-photon absorption. A comparison between different polarizations is also made, and we find that the linearly polarized laser is more suitable to generate nonlinear Compton scattering.     

Propagation of the ultrashort laser pulses through the quantum nonlinear medium with resonant properties

The propagation of ultrashort few-cycle laser pulses through a quantum nonlinear medium with resonant properties is studied using the method of slowly varying amplitudes with allowance for dispersion effects. A self-consistent system of nonlinear wave equation for the evolution of laser field and the nonstationary Schrödinger equation that determines the material polarization response is numerically solved. Specific features of the propagation of laser pulse caused by a relatively large spectral width and the nonadiabatic character of the laser pulse are established. The rise of the slowly dumping polarization at the eigenfrequency of the medium and the consequent stretching of the laser pulse are observed. The role of the resonant absorption of the laser energy in the medium is analyzed. The nonlinear spectral broadening is demonstrated for the laser pulse propagating through the medium.     

极强激光场驱动超亮伽马辐射和正负电子对产生的研究进展

高功率超短超强激光脉冲的诞生开启了相对论非线性光学、高强场物理、新型激光聚变、实验室天体物理等前沿领域.近年来,随着数拍瓦级乃至更高峰值功率激光装置的建成,超强激光与等离子体相互作用进入到一个全新的高强场范畴.这种极强激光场与等离子体相互作用蕴含着丰富的物理过程,除了经典的波与粒子作用、相对论效应、有质动力效应等非线性物理过程外,量子电动力学(QED)效应变得格外重要,例如辐射阻尼效应、正负电子对产生、强伽马射线辐射、QED级联、真空极化等.本文主要介绍我们近年来在极端强激光场与等离子体相互作用中激发的QED效应以及伴随的超亮强伽马射线辐射和稠密正负电子对产生等方面的研究进展.     

Nonstationary two-flux model of radiation transport for optical tomography of scattering media

A nonstationary two-flux model is formulated for the transport of radiation in an inhomogeneous scattering medium and is applied to the situation where such a medium is irradiated by the narrow beam of a pulsed laser. It is shown that when the time distribution of the transmitted photons is measured it is possible simultaneously to reconstruct the two spatial functions (the coefficients of absorption coefficient and of scattering of the radiation by the medium) by means of an inverse Radon transformation and the solution of a system of nonlinear differential equations on the projection lines. An analytic solution is obtained in quadratures for these differential equations. The results constitute a method of solving problems in optical tomography in an inhomogeneous scattering medium Zh. Tekh. Fiz. 67, 61–65 (May 1997)     

Above-threshold detachment of negative ions by circularly polarized few-cycle laser fields

In accordance with nonperturbative quantum scattering theory,we investigate photoelectron angular distributions(PADs) from above-threshold detachment(ATD) of negative ions irradiated by circularly polarized few-cycle laser fields.Electrons ejected on the polarization plane demonstrate distinct anisotropies in angular distributions which distinctly vary with the carrier-envelope(CE) phase.The anisotropy is caused by interference between transition channels;it also depends strongly on laser frequency,pulse duration,and kinetic energy of photoelectrons.Optimal emission of photoelectrons,which varies with CE phase,makes it possible to control photoelectron motion.     

Evolution of optical pulses in the presence of third-order dispersion

We model the propagation of femtosecond pulses through optical fibres by a nonlinear Schrödinger (NLS) equation involving a perturbing term arising due to third-order dispersion in the medium. The perturbative effect of this higher-order dispersion causes the usual NLS soliton to emit a radiation field. As a result, the given initial pulse propagating through the fibre exhibits nonsolitonic behaviour. We make use of a variational method to demonstrate how an initial pulse by the interaction with the emitted radiation can evolve into a soliton. We also demonstrate that the effect of interaction between the initial pulse and radiation field can be accounted for by including, in the evolution equation, terms associated with self-steepening and stimulated Raman scattering that characterize the optical medium.     

基于激光等离子体的X/γ辐射研究进展

随着激光技术的不断发展,激光功率突破10 PW量级,激光与物质相互作用进入近量子电动力学（QED）范畴。从弱相对论激光到相对论激光再到强相对论激光,激光场与物质的耦合可以产生能量从keV到MeV甚至GeV的X/γ射线。这些辐射具有通量大、亮度高、能量高和脉宽短等特点,在核物理、高能量密度物理、天体物理等基础研究以及材料科学、成像、医学等领域具有广泛应用前景。系统梳理了近年来相对论强激光与气体、近临界密度等离子体及固体靶相互作用,通过诸如同步辐射、betatron和类betatron辐射、Thomson散射和非线性Compton散射过程等产生高能X/γ射线的最新研究进展,总结了各种方案产生的X/γ射线的品质因子和潜在应用,并为下一步基于强激光大科学装置的实验研究提供理论参考。