Coherent wake emission of high-order harmonics from overdense plasmas

We present a new mechanism for high-order harmonic generation by reflection of a laser beam from an overdense plasma, efficient even at moderate laser intensities (down to Igamma2 approximately 4x10(15) W cm-2 microm2). In this mechanism, a transient phase matching between the electromagnetic field and plasma oscillations within a density gradient leads to the emission of harmonics up to the plasma frequency. These plasma oscillations are periodically excited in the wake of attosecond electron bunches which sweep across the density gradient. This process leads to a train of unevenly spaced chirped attosecond pulses and, hence, to broadened and chirped harmonics. This last effect is confirmed experimentally.     

[Russian Text Ignored]

A nonlinear theory of the interaction of an external pump field with ion-acoustic and h.f. surface waves in a plasma layer with inhomogeneous boundary regions is presented for the case, when the pump field frequency is less than the electron plasma frequency. The evolution of the parametrically excited turbulence and its stationary state are both analytically treated and numerically demonstrated. Dependent on the pump field strength, two mechanisms are to be distinguished: (i) nonlinear generation of damped ion-acoustic harmonics, (ii) cascade excitation of secondary eigenmodes by growing surface oscillations. At only slight overthreshold values, mechanism (i) is responsible for the occurence of the steady state, whereas, for higher pump field strength, the wave amplitudes as well as their saturated levels are mainly determined by mechanism (ii).     

超短脉冲强激光与固体靶作用产生的高次谐波红移

用一维粒子模拟程序研究了超短脉冲强激光与超临界密度等离子体平板作用产生的高次谐波。分析了振荡镜面模型所不能解释的伴随高次谐波出现的频率红移现象。通常激光与固体靶作用中的高次谐波是由于激光从振荡靶面的反射产生的.除此之外，相对论光强的激光与固体靶作用还应考虑光压对靶的烧蚀推进作用，这时激光相当于从一个移动的振荡靶面反射，所以产生了带有红移的高次谐波. 关键词： 红移 超短脉冲强激光 高次谐波 粒子模拟     

空间非均匀啁啾双色场驱动下氦离子的高次谐波以及孤立阿秒脉冲的产生

本文理论上研究了初态为基态与第一激发态等权叠加的一维氦离子在空间非均匀啁啾双色场驱动下氦离子的高次谐波发射及孤立阿秒脉冲的产生. 研究表明, 一维氦离子在空间非均匀啁啾双色场驱动下发射的高次谐波相对于均匀场情况截止位置得到明显扩展, 得到了光滑的超连续谱,并应用半经典三步模型解释了高次谐波发射的物理机理. 通过小波变换的方法对连续谱进行了时频分析, 并且与电子的经典运动轨迹进行了对比分析, 结果显示在空间非均匀场中长量子轨道消失, 短量子轨道加强. 讨论了空间非均匀啁啾双色场中时间延迟对谐波和孤立阿秒脉冲产生的影响, 发现适当调整时间延迟值可以得到较大延展的光滑的超连续谐波谱, 本方案中时间延迟为t₀=1.6πup/ω₁时得到了最大延展, 通过对谐波中600次到680次(80次)谐波合成得到32 as的孤立脉冲.     

Nonlinear Evolution of Driven Electron Plasma Oscillations in Inhomogeneous Plasmas

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Plasma radiation spectra in the presence of static electric and high-frequency radiation fields

Harmonics generation of high-frequency radiation in a plasma embedded in a constant electric field is investigated theoretically. It is shown that the electron directed motion due to the static electric field yields the appearance in the plasma emission spectrum of high-frequency radiation even harmonics. The conditions are established when the even harmonics generation is as effective as that of the odd ones. At variance with the odd harmonics, the even harmonics polarization plane is found to rotate with respect to that of the fundamental field. The basic dependencies concerning the rotation angle and the generation efficiency on the plasma and field parameters are established.Received: 3 May 2004, Published online: 14 September 2004PACS: 52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.) - 52.38.Dx Laser light absorption in plasmas (collisional, parametric, etc.)     

Microwave ponderomotive action on the inhomogeneous collisionless and collisional plasmas

The nonlinear interaction of a high-power microwave (MW) with an unmagnetized inhomogeneous plasma is investigated in collisionless and collisional regimes. The electron density distribution and the nonlinear wave equation in an inhomogeneous plasma are obtained by taking into account the ponderomotive force due to the high-power MW. It is shown that the electron density distribution becomes very steepened in the presence of the ponderomotive force. In the collisional regime, the expression for electron temperature is also found by considering ohmic heating. It is indicated that the amplitude of oscillations of the electron temperature and dielectric permittivity increases and the wavelength of these oscillations decreases with increasing energy flux, hence modulation occurs.     

Nonlinear dynamics of a generator on a virtual cathode with modulated emission

A numerical study of a low-voltage vircator with controlled emissions from a thermocathode is performed for when an external signal effects on the electron beam and modulates the emission. The strong influence of the modulation parameters on characteristics of oscillations of the beam with virtual cathode is noted. It is shown that when the modulation frequency is tuned to the one of harmonics of a virtual cathode’s free oscillations, there is a considerable increase in the power of high-frequency harmonics of the virtual cathode’s free oscillations in the output spectrum.     

Stable generation of high-order harmonics of femtosecond laser radiation from laser produced plasma plumes at 1 kHz pulse repetition rate

We present a method for the creation of stable weakly ionized plasmas from laser ablation of solid targets using a 1 kHz pulse repetition rate laser, which can be used for stable high-order harmonic generation from plasma plumes. The plasma plumes were generated from cylindrical rotating targets. Without target rotation the intensity of harmonics in the 40-80 nm range drops by more than one order of magnitude during less than 10(3) shots, while, with rotation of the target at typically 30 revolutions per minute, stable emission of high-order harmonics from aluminum plasma plumes with variation of less than 10% was maintained for >10(6) laser shots.     

少周期非均匀场下原子空间位置对高次谐波发射的影响

本文通过数值求解含时薛定谔方程,研究了少周期非均匀激光脉冲作用下氦原子选取不同的空间位置时高次谐波的发射情况.通过对原子在纳米结构空隙中心附近区域不同空间位置上的高次谐波发射情况的研究发现,非均匀激光场中,即使原子处在中心附近的很小的区域内,原子的空间位置也直接影响着高次谐波发射.随着空间内原子坐标的增大,高次谐波的截止得到扩展,且谐波平台区并非单向扩展,而是向两端同时的扩展.通过时频分析和半经典三步模型研究了高次谐波发射的物理机制,并对获得的物理现象给出合理的解释.     

Odd harmonic generation of ultra-intense laser pulses reflectedfrom an overdense plasma

A method of generating odd harmonics from an intense laser incident upon a sharp vacuum-overdense plasma interface is presented. One- and two-dimensional simulations are used to investigate the interaction of ultra-intense laser pulses with a sharp vacuum-plasma interface. With an intensity greater than 10¹⁸ W/cm² , these pulses have a pressure greater than 10³ Mbar creating large density oscillations and relativistic electron velocities at the surface. This results in efficient odd harmonic generation. The author present is a physical model for this harmonic generation mechanism, along with some scaling of the power in the third harmonic. This is compared with relativistic particle-in-cell simulations that include mobile and immobile ions, as well as a variety of upper shelf densities. A discussion on how ion profile modifications influence the harmonics is also presented     

相对传播的双脉冲激光与薄膜靶作用产生的强单色谐波

研究了相对传播的双脉冲激光与薄膜靶的作用,观察到很强的谐波产生.其物理图像是:圆偏振高对比度强激光脉冲作用于薄膜靶,由光压推动产生的高密度等离子体靶向前运动,同时由于电荷分离场的作用,使得离子束和电子束在纵向上都有好的聚束,从而产生以相对论速度向前运动的等离子体镜;反向入射一个探测光到已被加速的等离子体镜上,由多普勒频移产生强的单色N次谐波,探测光脉冲被"压缩"至原来的1/N.还讨论了激光和等离子体参数对等离子体镜的运动和谐波级次的影响,以及相对论运动等离子体镜的稳定性对谐波的影响.     

Flow oscillations in radial expansion of an inhomogeneous plasma layer

The cylindrically symmetric radial evolution of an inhomogeneous plasma layer expanding into vacuum is investigated nonperturbatively by first determining the spatial structure of the plasma flow structure. The evolution is then governed by a set of ordinary differential equations. The effect of the plasma inhomogeneity on the nonlinear coupling among the electron and ion flow components and oscillations is investigated.     

Generation and Radiation of Waves at Combined Frequencies in a Warm Inhomogeneous Plasma Layer

We investigate generation and radiation of waves at combined frequencies from an arbitrary inhomogeneous, isotropic plasma layer, when electromagnetic waves are obliquely incident on it to interact with a surface wave at the plasma boundary. This interaction has been described for both S- and P-polarized waves. We consider a warm plasma layer with thickness very small as compared to the wavelengths of oscillations. It is shown that generated waves are strongly amplified, compared to cold plasma, when phase velocities of generated waves approaches the electron thermal velocity. Waves are not emitted when P-polarized waves are incident perpendicular onto the plasma layer.     

Nonlinear second harmonic generation by light wave-plasma interaction in oscillating magnetic field

The nonlinear generation of second harmonic electromagnetic waves in a thin inhomogeneous (dense and rarefied) plasma layer (of lengthd) by obliquely and normally incident light waves is analyzed. We consider the effect of an external time-dependent magnetic field on the generation and amplification of waves. Two cases are considered, when the magnetic field oscillates at a frequency (i) equal to and (ii) double that of the incident wave. For normal incidence, waves are not radiated in case (i), while in case (ii) the second harmonics are radiated equally from the plasma boundaries atx=0 andx=d. For a rarefied plasma, the second harmonics are radiated with equal amplitudes in both cases.     

Arc spot ignition caused by sheath instability

The transition from the stationary and homogeneous state of the collision-free space charge sheath between plasma and wall to a strongly inhomogeneous and instationary state with are spots is initiated by local enhancement of power supply from the plasma, connected with increasing electron emission. The general relations between surface emissivity, net current density, sheath potential, and power density in such “active” boundary layers are derived, taking the feedback of emitted electrons on the plasma parameters into account, as far as necessary. Joule heat generation is taken into account. The decisive effect of the transition is the rapidly increasing yield of thermo-field electron emission. Conditions and possible paths to sheath instability and arc spot formation are discussed     

Surface Plasma Waves at a Rough Interface of a Solid

The dispersion law for surface polaritons in an inhomogeneous plasma is obtained and studied. The plasma is formed by surface electron states at the interface of a solid under conditions where the irregularity scale is much greater than the wavelength. It is shown that the irregularity of the plasma results in spatial dispersion of surface electrostatic oscillations.     

Half-Integer Harmonics Generation in Laser-Produced Plasma

Half integer (3/2ω₀, 1/2ω₀) harmonics generation under oblique incidence of the pump-wave on the spatially inhomogeneous plasma is theoretically investigated. Harmonics generation is connected with the two-plasmon convective instability excitation near the quarter-critical density. Harmonics spectra and intensities depend essentially on the pump wave polarization. In particular, significant harmonics spectra broadening arises under the p-polarized pump wave.     

On nonlinear oscillations in an inhomogeneous electron flux

This work considers nonlinear oscillations in an inhomogeneous electron flux moving in a high-frequency field between two parallel planes placed perpendicular to the beam motion. It is shown with the help of the Hamiltonian formalism in the threewave approximation that under specific conditions there occurs a periodic exchange of energy between the high-frequency field and the electron flux and also between the long-wave and short-wave harmonics of the flux.The authors express thanks to N. M. Filipenko who participated in the discussion of the obtained results.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 88–93, June, 1981.     

Influence of surface waves on plasma high-order harmonic generation

The influence of surface plasma waves on high-order harmonic generation from the interaction of intense lasers with overdense plasma is analyzed. It is shown that the surface waves lead to the emission of harmonics away from the optical axis, whereas the high-order on-axis harmonics are lowered in intensity. Our simulation results indicate that surface plasma wave generation plays a crucial role in surface high-order harmonic generation experiments. Furthermore, a novel surface plasma wave generation process different from the well-known two-surface wave decay is observed in the highly relativistic regime.