Self-action of laser radiation in cluster plasma

We have analytically and numerically studied the self-action dynamics of laser radiation in a plasma with ionized gas clusters. Based on the simplified model of a cluster in the form of a superposition of two charged (electron and ion) bunches, we analyze the nonlinearity mechanisms. We refine the electrodynamic cluster model by the molecular dynamics method. The polarization behavior of the plasma bunch in the main part of the laser pulse is shown to be the same as that in the simplified model. We investigate the self-action dynamics of laser radiation under conditions when the nonlinearity of the stratified medium is determined by the anharmonicity of the electron motion in the cluster, while the group velocity dispersion is determined by both the background plasma and the ionized clusters. Since the characteristic field for the electron nonlinearity depends strongly on the cluster size, the peculiarities of the self-action dynamics result from plasma bunch expansion. The spatiotemporal evolution of the wave field is shown to be accompanied by pulse self-compression near the trailing edge.     

Structural features of the self-action dynamics of ultrashort electromagnetic pulses

Self-focusing dynamics of electromagnetic pulses of arbitrary duration is analyzed numerically and analytically. The wave-field evolution is considered by the wave equation in the reflectionless approximation under quite general assumptions about the dispersion of the medium. Methods for qualitative investigation of the self-focusing dynamics of quasimonochromatic radiation are generalized to the case of wave packets with the length of a few oscillation periods. In particular, sufficient conditions for collapse and many other integral relations are obtained by the momentum method. A self-similar-type transformation is used to show that new structural features are primarily associated with the nonlinear dispersion of the medium (with the dependence of the group velocity of a wave packet on its amplitude). Numerical analysis confirms that the self-focusing of radiation is preceded by an increase in the steepness of the longitudinal profile.     

Relativistic Electron Acceleration in a Wake Field Generated by the Intense Femtosecond Laser Interaction with a Mixture Jet of Deuterium Clusters and Molecules

High energy electron acceleration in a wake field generated in the intense ultrashort （30fs） laser pulse cluster gas jet interaction is experimentally demonstrated. Relativistic electrons with energy of 60 MeV were observed. These high energy electrons split into two beams due to the relativistic self-focusing of the laser.     

Self-action of Bessel wave packets in a system of coupled light guides and formation of light bullets

The self-action of two-dimensional and three-dimensional Bessel wave packets in a system of coupled light guides is considered using the discrete nonlinear Schrödinger equation. The features of the self-action of such wave fields are related to their initial strong spatial inhomogeneity. The numerical simulation shows that for the field amplitude exceeding a critical value, the development of an instability typical of a medium with the cubic nonlinearity is observed. Various regimes are studied: the self-channeling of a wave beam in one light guide at powers not strongly exceeding a critical value, the formation of the “kaleidoscopic” picture of a wave packet during the propagation of higher-power radiation along a stratified medium, the formation of light bullets during competition between self-focusing and modulation instabilities in the case of three-dimensional wave packets, etc. In the problem of laser pulse shortening, the situation is considered when the wave-field stratification in the transverse direction dominates. This process is accompanied by the self-compression of laser pulses in well enough separated light guides. The efficiency of conversion of the initial Bessel field distribution to two flying parallel light bullets is about 50%.     

Plasma effects and the modulation of white light spectra in the propagation of ultrashort, high-power laser pulses in barium fluoride

We report the results of a study of white light generation in a 7.5 cm long crystal of a high band-gap material, barium fluoride, using ultrashort (<42 fs) laser pulses over a range of values of incident laser power that extend up to more than three orders of magnitude larger than the critical power for self-focusing (P_cr). We explore white light generation and the intensity and spectral distributions within filaments that are formed as a result of the interplay of self-focusing and plasma-induced defocusing. The onset of plasma effects occurs at power levels in excess of 7 GW for ultrashort pulses in BaF₂. For incident power levels that are three orders of magnitude larger than the critical power, blue-shifting of the incident laser wavelength is observed in addition to asymmetric continuum generation. The blue shift enables us to estimate the temporal variation of the electron density in the plasma that is generated within the BaF₂ crystal. PACS 52.38.Hb; 42.65.Jx; 42.65.Tg; 33.80.Wz; 52.35.Mw     

Wideband modulation instability at combination frequencies in wave beams

We study theoretically wideband modulation instability at combination frequencies in media having cubic nonlinearity of self-focusing type along with the higher-order defocusing nonlinearity. It is assumed that in a medium with a purely cubic nonlinearity, the medium dispersion does not permit modulation instability. In this case, a collapse of the wave field exists if the beam power is higher than the critical power of self-focusing. The higher-order nonlinearity limits the field at the nonlinear focus, and the instability at combination frequencies becomes possible. It turns out that the field at the nonlinear focus increases with increasing excess of the beam power over the critical power of self-focusing. The obtained values of the nonlinear dielectric permittivity are used for determination of the growth rates of instability at combination frequencies. These growth rates ensure an increase in the combination fields from noise levels up to values comparable with the field of the high-power beam. Such an increase takes place if the beam power is severalfold higher than the critical one. The developed theory can be used for explanation of spectrum superbroadening during self-focusing of sufficiently short laser pulses and high-harmonic generation. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 6, pp. 522–532, June 2007.     

超短强激光脉冲在等离子体隧道中传输的理论与数值模拟研究

研究了超短强激光脉冲在非扰动抛物型部分电离的预等离子体隧道中的传输特性.从Maxwell方程出发得到了两个包含衍射、三阶强度非线性、等离子体散焦、等离子体隧道聚焦以及相对论自聚焦等效应在内的激光场演化方程，即折射率方程和哈密顿-雅可比方程.在此基础 上得到了激光在等离子体隧道中传输的包络方程以及光斑半径与传输距离、隧道宽度等初始 参量的关系. 关键词： 等离子体隧道聚焦 相对论自聚焦 势阱     

Structural features of self-action of laser radiation in the electromagnetic induced transparency mode

Structural features of the laser radiation self-focusing dynamics in the electromagnetic induced transparency (EIT) band are studied for an atomic system with a Λ-type energy level diagram. Effective nonlinearity of an EIT medium is manifested primarily as nonlinear dispersion (dependence of the group velocity on the wave amplitude). Qualitative analysis of the dynamics of self-action of laser pulses, which is confirmed by numerical simulation, shows that nonlinear evolution of a 3D wave packet follows the scenario of self-focusing, which serves as the background on which the envelope profile turnover and the formation of a shock wave occur at an advanced rate.     

超短强激光脉冲激励甲烷团簇的内电离机理

采用三维粒子动力学模拟方法研究了甲烷团簇在超短强激光脉冲激励下的爆炸动力学行为,重点讨论了几种典型的内电离机理对团簇爆炸过程中离子的价态和动能的影响.研究表明,在激光脉冲强度比较小的情况下,团簇中的原子主要是在光场作用下通过隧道电离的方式发生电离.当激光场进一步增强时,势垒压低电离是电离的主要方式.在相同的较高激光强度下,团簇更容易通过势垒压低电离达到高的电离价态.团簇发生电离后,其内部库仑电场的点火电离效应和内部滞留自由电子的碰撞电离效应也将增强团簇的再次电离过程. 关键词： 超短强激光脉冲 甲烷团簇 内电离     

Propagation of the high power laser pulse in multicomponent cluster targets

We present the results of 3D PIC and 2D PIC simulations of the ultra short high irradiance laser pulse interaction with targets where the plasma containing multicomponent cluster targets and multicluster cloud is imbedded in an underdense plasma. In both cases the laser radiation expels the electrons from the clusters and ejects them into the wake plasma wave generated by the ultrashort laser pulse in the underdense plasma. This provides a novel mechanism for the electron injection into the wake field for acceleration.     

高功率激光在非均匀大气中传输的自聚焦效应研究进展

地基激光空间碎片清除和利用激光辐射把转换的太阳能从空间轨道输运到地面等应用中,不可避免地遇到高功率激光在非均匀大气中的传输问题。由于激光功率已远远超过大气非线性自聚焦临界功率,大气自聚焦效应是影响光束质量的一个重要物理因素。概述了近年来国内外高功率激光在非均匀大气中上行或下行传输的自聚焦效应研究进展,主要介绍了高功率激光在非均匀大气中的传输模型、理论基础、数值和解析研究方法,着重介绍了自聚焦效应对激光传输特性和光束质量的影响,并总结了优化靶面光束质量的方案。此外,还介绍了大气群速度色散效应和大气湍流效应等物理因素对激光光束质量的影响。最后,还提出了该领域值得进一步深入研究的一些问题。     

共振自聚焦理论

本文应用光学混频和自聚焦理论,研究了两束频率差等于等离子体振荡频率的激光在均匀等离子体中的共振自聚焦行为,解析地导出了非线性介电常数和共振自聚焦的阈值功率,并讨论了它对激光等离子体参数的定标关系,并且与普通的有质动力自聚焦作了比较。此外,本文还研究了偏离频率匹配条件对共振自聚焦阈值功率的影响。 关键词：     

Distributions of ions in a cluster plasma created by a laser pulse

Energy and charge distributions of ions are calculated for a cluster beam irradiated by a high-power ultrashort laser pulse. It is shown that the self-consistent field of a cluster ionized by the laser beam strongly affects the characteristics of the ion distributions obtained after the cluster explodes. The mean concentration of atoms bound into clusters in a beam, the cluster size distribution, and the focal-spot diameter are found to have a weak effect on both energy and charge distributions of the ions, whereas the energy spectrum of the produced ions is determined by the mean cluster size.     

Relativistic nonlinearity and wave-guide propagation of rippled laser beam in plasma

In the present paper we have investigated the self-focusing behaviour of radially symmetrical rippled Gaussian laser beam propagating in a plasma. Considering the nonlinearity to arise from relativistic phenomena and following the approach of Akhmanov et al, which is based on the WKB and paraxial-ray approximation, the self-focusing behaviour has been investigated in some detail. The effect of the position and width of the ripple on the self-focusing of laser beam has been studied for arbitrary large magnitude of nonlinearity. Results indicate that the medium behaves as an oscillatory wave-guide. The self-focusing is found to depend on the position parameter of ripple as well as on the beam width. Values of critical power has been calculated for different values of the position parameter of ripple. Effects of axially and radially inhomogeneous plasma on self-focusing behaviour have been investigated and presented here.     

Dipole resonances of an ionized cluster

We study the resonance properties of an ionized spherical cluster that interacts with an optical radiation field in terms of linear hot-plasma polarizability models. Based on a generalization that includes spatial dispersion in the well-known Mie problem of the diffraction of a plane wave by a small-size plasma sphere, we calculate the eigenfrequencies, the radiative and collisional damping constants, and the resonance amplitudes of the cluster surface and bulk plasmon fields. The role of collisionless dissipation processes is analyzed in terms of a one-dimensional kinetic model. The latter allows the corresponding damping constants for both types of plasmons to be determined as functions of the characteristic electron-plasma boundary collision frequency. We show that both types of plasmons in certain domains of cluster and external-radiation parameters can undergo a strong resonance that causes both the amplitude of the scattered wave and the absorbed power and the field inside the cluster to increase significantly.     

Self-Focusing and de-Focusing of Intense Left- and Right-Hand Polarized Laser Pulse in Hot Magnetized Plasma in the Presence of an External Non-Uniform Magnetized Field

In this paper, self-focusing of an intense circularly polarized laser beam in the presence of a non-uniform positive guide magnetic field with slope constant parameter δ in hot magnetized plasma, using Maxwell’s equations and relativistic fluid momentum equation is investigated. An envelope equation governing the spot-size of laser beam for both of left- and right-hand polarizations has been derived, and the effects of the plasma temperature and magnetic field on the electron density distribution of hot plasma with respect to variation of normalized laser spot-size has been studied. Numerical results show that self-focusing is better increased in the presence of an external non-uniform magnetic field. Moreover, in plasma density profile, self-focusing of the laser pulse improves in comparison with no non-uniform magnetic field. Also, with increasing slope of constant parameter of the non-uniform magnetic field, the self-focusing increases, and subsequently, the spot-size of laser pulse propagated through the hot magnetized plasma decreases.     

超短脉冲激光空间调制下小尺度自聚焦的实验研究

实验研究了超短脉冲激光的空间小尺度自聚焦的动态变化过程.具体实验过程是使经过正交细丝的衍射来实现激光的空间调制,然后通过非线性Kerr介质二硫化碳,随着输入功率的增大,超短脉冲开始出现小尺度自聚焦增长现象.实验发现,每个小尺度自聚焦峰值并不是随着功率的增大而无限地增长下去,而是在不同位置出现了新的调制增长点,这些新出现的增长与原有的增长相互竞争,最终导致了分裂成丝.通过数值模拟相应的传输过程,发现数值模拟结果与实验结果符合很好. 关键词： 小尺度自聚焦 高功率激光 超短脉冲     

Comparison between geometrically focused pulses versus filaments in femtosecond laser ablation of steel and titanium alloys

Kerr self-focusing of high-power ultrashort laser pulses in atmosphere may result in a structure or structures of high intensity that can propagate over long distances with little divergence. Filamentation has garnered significant interest in the nonlinear optics community due to its unique properties. Salient features of filaments include a central region of intense laser power (greater than the ionization threshold of the propagation medium) and a low temperature plasma column that lasts up to nanoseconds in duration after the passage of the laser pulse. Steel and titanium samples are ablated by filaments and by sharply focused sub-picosecond laser pulses. We then performed metrology on the samples to compare the ablation features in addition to modeling of the plasma ablation process. Ablation with filaments leads to a wider range of material responses as compared to ablation with sharply focused pulse. This results in potential complications for applications of filament ablation that depends on the rate of material removal and spectroscopic analysis.     

A study of balancing the competing effects of ultrashort laser induced plasma for optimal laser machining

We report that plasma generated during processing of materials with ultrashort pulse lasers and the associated high intensity optical beam have both favourable and unfavourable impact on the machined surface quality. Intensity of the optical beam propagating through ambient air medium enhanced further by self-focusing is sufficiently high to cause gas breakdown forming air plasma. The generated plasma reduces the effect of self-focusing but also distorts the beam profile. Duration of the pulse being too short for thermal equilibrium to establish, ablation occurs largely by direct removal of the material forming another plasma plume. Normally, the scattering effect of plasma results in distortions of the fabricated features. However, for certain parameter ranges, the competing self-focusing and gas plasma plumes supplemented with the material plasma can combine to cause filamentation, eliminating the distortions. Filament of hot plasma also acts as a well-shaped energy source. In the present study, brass is taken as an example for the investigation. Experiments were conducted to capture the spectrum of the light scattered by plasma using a spectrometer. Analysis was done to estimate the material plasma. Theoretical calculation on the intensity distribution in an optical beam propagating through air was then followed for a range of parameter values taking the self-focusing effect of the medium and the impact of the plasma generated by its breakdown. Approximate values of the machining parameters for clean fabrication are deduced from the calculations, which were used to conduct a laser machining test on brass.