Post-solitons and electron vortices generated by femtosecond intense laser interacting with uniform near-critical-density plasmas

Generation of nonlinear structures, such as stimulated Raman side scattering waves, post-solitons and electron vortices, during ultra-short intense laser pulse transportation in near-critical-density (NCD) plasmas is studied by using multi-dimensional particle-in-cell (PIC) simulations. In two-dimensional geometries, both P- and S-polarized laser pulses are used to drive these nonlinear structures and to check the polarization effects on them. In the S-polarized case, the scattered waves can be captured by surrounding plasmas leading to the generation of post-solitons, while the main pulse excites convective electric currents leading to the formation of electron vortices through Kelvin-Helmholtz instability (KHI). In the P-polarized case, the scattered waves dissipate their energy by heating surrounding plasmas. Electron vortices are excited due to the hosing instability of the drive laser. These polarization dependent physical processes are reproduced in two different planes perpendicular to the laser propagation direction in three-dimensional simulation with linearly polarized laser driver. The current work provides inspiration for future experiments of laser-NCD plasma interactions.     

Nonlinear propagation of an intense Laguerre–Gaussian laser pulse in a plasma channel

The nonlinear propagation of an intense Laguerre–Gaussian(LG) laser pulse in a parabolic preformed plasma channel is analyzed by means of the variational method. The evolution equation of the spot size is derived including the effects of relativistic self-focusing, preformed channel focusing, and ponderomotive self-channeling. The parametric conditions of the LG laser pulse and plasma channel for propagating with constant spot size, periodically focusing and defocusing oscillation,catastrophic focusing, and solitary waves are obtained. Compared with the laser pulse with fundamental Gaussian(FG)mode, it is found that the effect of vacuum diffraction is reduced by half and the effects of relativistic and wakefield focusing are decreased by a quarter due to the hollow transverse intensity profile of the LG laser pulse, while the effect of channel focusing is the same order of magnitude with that of the FG laser pulse. Thus, the matched condition for the intense LG laser pulse with constant spot size is released obviously, while the parameters of the laser and plasma for the existence of solitary waves nearly coincide with those of the FG laser pulse.     

Acoustic field excited by a pulsed laser line source in a cylinder

The excitation and propagation of the acoustic waves in an elastic cylinder are studied by laser ultrasonics both theoretically and experimentally. The theoretical analysis of the two-dimensional acoustic field excited by a pulsed laser line source impacting on the generatrix of an elastic cylinder is presented. The dispersive properties for both cylindrical Rayleigh wave and the higher modes--whispering gallery (WG) modes are analyzed in detail. The numerical transient displacement waveforms for a detecting point located another terminal of the cylinder diameter opposite the source are calculated. The experimental excitation and detection of the acoustic waves in an aluminum cylinder are carried out on a laser ultrasonic system, which mainly consists of a Q-switched Nd:YAG laser and a laser interferometer. The wave components of bulk waves and surface waves (cylindrical Rayleigh waves and WG modes) are analyzed by comparing the numerical and experimental waveforms. The results are in good agreement.     

Effect of laser beam incidence angle on the thermoelastic generation in semi-transparent materials

When a laser beam is absorbed in a semi-transparent material, a volume acoustic source is created owing to penetration of the laser beam inside the material and to thermoelastic transduction. Many experimental and theoretical studies have been conducted to better understand this ultrasound generation process with normal laser light incidence on the material surface. The purpose of this paper is to analyze the effects of the asymmetry caused by oblique incidence of a laser line source on the generation of acoustic waves in semi-transparent isotropic materials. Experiments on a glass plate demonstrate that such an obliquely incident laser light strongly affects bulk acoustic waves generation. Compressional and shear waves are enhanced and the loss of symmetry of the acoustic source causes asymmetrical behavior of the acoustic waves. Surprisingly, compressional-wave amplitude decreases whereas shear-wave amplitude increases in the region where the electromagnetic energy is refracted. This feature is explained by semi-analytical calculations.     

Nonlinear Dynamics of Wave Beams and Packets in an Ionizable Medium

We present a review of the studies on nonlinear dynamics of the plasma–field system formed in the processes of breakdown of a gas by high-intensity laser or microwave radiation. The ionization instability dominating these processes significantly modifies the known effects of self-action of waves in a medium and gives rise to a number of new effects which are absent for other nonlinearity mechanisms. We describe the most important among these effects, such as the ionization–field instability of a plane wave, the self-channeling of radiation in the form of surface or leaky waves, and the self-conversion of the spectrum of the ionizing radiation. The results of numerical simulations of the dynamics of nonequilibrium freely-localized discharges created by focused microwave and laser pulses are presented.     

Simulation on thermoelastic stress field and laser ultrasound waveform in non-metallic materials by using FEM

The thermoelastically generated stress and ultrasound fields in non-metallic materials by laser illumination are presented by using finite element method (FEM). The model accounts for the effects of thermal diffusion and optical penetration, as well as the finite width and duration of the laser source. The numerical results show that the strength and feature of the force source can be related to the heat input and certain material properties. The relationships between the stress waves and the ultrasound waveforms, particularly the precursor, are analyzed. The typical surface acoustic waves (SAWs) and the bulk waveforms at the epicenter are presented to illustrate the generated field and provide insight to the relevance of different mechanism taken into account in the model. PACS 02.70.Dh; 43.35.+d; 42.62.-b     

Numerical analysis of bulk conical waves in anisotropic cylinders; application to stiffness tensor measurement

A point source-point receiver technique, based on laser generation and laser detection of acoustic waves, allows determination of mechanical properties of an anisotropic cylinder. The nature of the material and the geometry of the sample give a dispersive behaviour to the diffracted waves and make the acoustic signature difficult to interpret. To overpass the intricacies, wave fronts (conical waves in the volume and helical waves on the surface) are synthesized from signals provided by scanning the primitive line of the cylinder with a laser point source. In order to distinguish between direct bulk conical waves and other contributions in the acoustic response, some considerations on line surface waves and on reflected bulk conical waves are supplied. The identification of the stiffness tensor components, based on the inversion of the bulk waves phase velocities, is applied to signals simulated for a composite material.     

脉冲激光在镀层圆柱表面激发的表面波

分别从理论和实验上研究了脉冲激光在快、慢镀层圆柱表面激发的表面波声场。脉冲激光线源在镀层圆柱周向上激发超声波信号并利用激光干涉仪检测。对钢基底锌镀层圆柱和锡基底铜镀层圆柱试样表面波的理论频散和实验信号估计频散进行了比较,瞬态位移场则利用留数定理与FFT技术数值仿真得到并与实验波形进行对比。结果显示:理论频散和仿真波形与实验信号都具有极好的一致性,而快、慢镀层圆柱之间的表面波性质有显著差异。文中所用理论分析和数值方法为使用激光超声技术反演和定量表征镀层材料参数等无损检测技术奠定了理论基础。     

空气中激光支持爆轰波实验及理论分析

为了研究激光击穿空气产生的等离子体爆轰波形成机制和传播规律,利用高能量CO2激光器产生强激光,进行了空气中产生激光支持等离子体爆轰波实验。实验中:设置了诱导靶板,用于诱发和定位空气中的激光支持爆轰波;以激光器升压过程球隙放电产生的光信号作为触发源,触发高时间分辨率(纳秒级)的高速相机,记录了激光支持爆轰波的成长和传播全过程。分析了激光支持爆轰波的形成机理和传播规律。采用C-J爆轰理论,计算了激光支持爆轰波的压力和温度。研究结果表明:激光支持等离子体爆轰波形成初期,等离子体爆轰波发光体为球形;随着时间增加,等离子体爆轰波发光体的形状类似流星,且头部为等离子体前沿吸收层,亮度较高,而尾部等离子体温度较低,亮度较弱。等离子体爆轰波高速向激光源的方向移动,爆轰波速度高达18 km/s,温度约为107K。随着激光强度的减弱,爆轰波速度迅速按指数规律衰减,当爆轰波吸收的激光能量不能有效支持爆轰波传播时,爆轰波转变为冲击波。     

Electron acceleration in the inverse free electron laser with a helical wiggler by axial magnetic field and ion-channel guiding

Electron acceleration in the inverse free electron laser (IFEL) with a helical wiggler in the presence of ion-channel guiding and axial magnetic field is investigated in this article. The effects of tapering wiggler amplitude and axial magnetic field are calculated for the electron acceleration. In free electron lasers, electron beams lose energy through radiation while in IFEL electron beams gain energy from the laser. The equation of electron motion and the equation of energy exchange between a single electron and electromagnetic waves are derived and then solved numerically using the fourth order Runge-Kutta method. The tapering effects of a wiggler magnetic field on electron acceleration are investigated and the results show that the electron acceleration increases in the case of a tapered wiggler magnetic field with a proper taper constant.     

激光协同聚焦超声辐照生物样品增强热效应的实验和理论研究

对中等强度聚焦超声在生物样品中产生的热效应以及激光协同超声增强热效应进行了实验和理论研究。实验上,对生物和仿生样品在超声作用和激光协同超声作用下加热情况进行测量,通过对比表明,激光协同超声作用于生物样品,引起空化效应以及温度升高更为明显。同时,理论上对聚焦超声在生物样品中衰减产生的热效应、超声空化以及激光协同超声增强空化及其产生热效应进行机理分析。通过对机理的分析表明,激光引起的光致核化使超声空化更易于产生,有效的增强空化效应,进而增强热效应。为对具体实验给出量化分析和估算,通过理论与实验结果相拟合,对超声传播引起的温度升高进行计算,并估算超声和激光协同超声产生空化微泡对加热效应的不同贡献,为空化效应在超声治疗中的贡献提供参考数据。     

Laser ultrasonic characterization of adhesive bonds between epoxy coating and aluminum substrate

Nanosecond pulsed laser irradiation through transparent epoxy bonded to aluminum substrate excites wide-band ultrasonic waves at the bonded interface. The normal displacements on the rear surface of aluminum produced by the direct and multi-reflected longitudinal waves in the coating layer are detected by a laser interferometer. The amplitude of the reflected signal depends on the properties of the coating/substrate interface, which is described by terms of the interfacial stiffness using a spring boundary model. The waveforms at the epicenter versus interfacial stiffness are simulated and found to be in good agreement with experimental results. The relation between the interfacial stiffness and the amplitude ratio of the reflected and direct waves is thus established. An image of amplitude ratio of a specimen (null 10 mm) is obtained from the epicenter waveforms recorded by a laser ultrasonic scanning system, which shows the distribution of bond quality on the bonding interface.     

Near threshold nucleation and growth of cavitation bubbles generated with a picosecond laser

The nucleation and growth of cavitation bubbles few micrometers in size in water generated by a 60 ps 515 nm fiber laser is observed and visualized near nucleation threshold. The study is performed by monitoring the plasma size, the cavitation bubble size and the emitted shock waves. The latter two aspects are supported by the Gilmore model using a Noble-Abel-stiffened-gas (NASG) equations of state. For the first time, two types of cavitation events are identified and visualized that exhibit a difference of more than two orders of magnitude in the excitation energy converted to mechanical effects with minimal change in excitation laser pulse energy. The result is localized cavitation and reduced mechanical stress on water-based media with potentially positive implications for laser treatments of biological tissue.     

Characteristics and application of laser-generated acoustic shock waves in air

When a high-power laser beam is focused at a point, the air at the focal point is heated to temperatures of thousands of degrees within several nanoseconds and breaks down. This generates a spark that, in turn, is accompanied by an acoustic shock wave. The acoustic shock waves generated by focussing the beam from a pulsed laser with a 1064 nm wavelength and a power of 800 mJ per pulse have been measured using 1/4″ and 1/8″ B&K microphones. Nonlinear sound levels are observed up to 1.5 m from the laser-induced sparks. Beyond a certain region close to the source, levels are found to decrease in a manner consistent with spherical spreading plus nonlinear hydrodynamic losses. Analysis of the waveforms shows that the acoustic pulses associated with the laser-induced sparks are more repeatable and have higher intensity than those from an electrical spark source. Laser-generated acoustic shock waves are ideal for simulating a blast wave or a sonic boom in the laboratory and for studying the associated propagation effects. To illustrate this application, the propagation of the laser generated shock waves over a series of different hard, rough surfaces has been investigated. The results show the distinctive influences of ground roughness on the propagation of the shock wave.     

Scattering of light waves by electron electrostatic waves in laser produced plasmas

The propagation of light waves in an underdense plasma is studied using one-dimensional Vlasov-Maxwell numerical simulation.It is found that the light waves can be scattered by electron plasma waves as well as other heavily and weakly damping electron wave modes,corresponding to stimulated Raman and Brilluoin-like scatterings.The stimulated electron acoustic wave scattering is also observed as a high scattering level.High frequency plasma wave scattering is also observed.These electron electrostatic wave modes are due to a non-thermal electron distribution produced by the wave-particle interactions.The collision effects on stimulated electron acoustic wave and the laser intensity effects on the scattering spectra are also investigated.     

约束靶面黑漆涂层对激光冲击波的影响

 介绍了约束靶表面的黑漆涂层对激光冲击波的影响。激光功率密度 10⁹W/cm²量级，脉宽33ns，波长1.06μm。靶材选用两种不同厚度的铜和铝，靶表面用有机玻璃约束。通过对冲击波压力的直接测量发现，涂层不仅可以增加激光冲击波压力，而且还影响冲击波的演化过程。比较有无涂层的靶面SEM照片发现，黑漆涂层能有效地保护激光辐照表面，使之不受激光烧蚀。     

Ellipsometric measurement of magnetooptical nonreciprocal effects

To detect small rotation rates with a ring laser the two counter-propagating beams must be uncoupled. This lock-off can be realized using nonreciprocal optical effects yielding different optical path lengths for the two waves. Usually magnetooptical effects are employed for nonreciprocity. To measure, instead of calculating from optical constants, the magnitude of this nonreciprocal effects an ellipsometric method is proposed. Lock-off elements using Faraday or Kerr effect (polar and transverse, respectively) are treated. First measurements were made on FeNi films.     

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。     

Laser-enhanced thermal effects during focused ultrasound radiation on biological tissues

The thermal effects induced by a moderate intensity focused ultrasound and enhanced by combined laser pulses for bio-tissues and tissue-phantom are studied experimentally and theoretically. At first, the heating effects of bio-tissues and tissue-phantom induced by ultrasound and enhanced by laser are measured experimentally. The heating processes induced by attenuations of focused ultrasonic waves and cavitation effects of the focused ultrasound and combined laser are analyzed theoretically. By analyzing the mechanisms of these effects, it is found that the laser nucleation makes the cavitation bubble generation more easily, which can effectively enhance the ultrasonic cavitation effects, and then enhance the thermal effects of the samples. On the other hand, to evaluate quantitatively the heating processes induced by the focused ultrasound and enhanced by the pulsed laser, by fitting the theoretical calculations to the experimental results, the corresponding cavitation bubbles and rising temperatures induced by the focused ultrasound with and without laser can be estimated approximately.