Laser intensity effects in photoemission and photoelectron spectra produced in laser-induced autoionisation

Strong laser-field-induced autoionisation in the presence of both photoionising and radiative decay of the autoionising state (AS) is investigated, focusing on the laser intensity dependence of the photoemission and photoelectron spectra. In contrast to previous predictions, power broadening and increasing reduction of the doublet peak heights with field strength are found in the photoemission spectrum. Similar effects leading to considerable suppression and even complete disappearance of the lowest-order peaks in the photoelectron spectrum, together with peak switching, are also demonstrated, which are closely related to above-threshold ionisation. In addition, it is suggested that the total number of energetic photoelectrons may serve as an alternative to measuring the atomic parameters of the AS. All these effects are attributed to the presence of the strong `probe': laser-induced decay of the AS.

Plasma channeling by multiple short-pulse lasers

Channeling by a train of laser pulses into homogeneous and inhomogeneous plasmas is studied using particle-in-cell simulation. When the pulse duration and the interval between the successive pulses are appropriate, the laser pulse train can channel into the plasma deeper than a single long-pulse laser of similar peak intensity and total energy. The increased penetration distance can be attributed to the repeated actions of the ponderomotive force, the continuous between-pulse channel lengthening by the inertially evacuating ions, and the suppression of laser-driven plasma instabilities by the intermittent laser-energy cut-offs.

Electron acceleration by an intense short-pulse laser in underdense plasma

Electron acceleration from the interaction of an intense short-pulse laser with low density plasma is considered. The relation between direct electron acceleration within the laser pulse and that in the wake is investigated analytically. The magnitude and location of the ponderomotive-force-caused charge separation field with respect to that of the pulse determine the relative effectiveness of the two acceleration mechanisms. It is shown that there is an optimum condition for acceleration in the wake. Electron acceleration within the pulse dominates as the pulse becomes sufficiently short, and the latter directly drives and even traps the electrons. The latter can reach ultrahigh energies and can be extracted by impinging the pulse on a solid target. (C) 2003 American Institute of Physics.

Electron dynamics and harmonics emission spectra due to electron oscillation driven by intense laser pulses

The dynamics and harmonics emission spectra due to electron oscillation driven by intense laser pulses have been investigated considering a single electron model. The spectral and angular distributions of the harmonics radiation are numerically analyzed and demonstrate significantly different characteristics from those of the low-intensity field case. Higher-order harmonic radiation is possible for a sufficiently intense driving laser pulse. A complex shifting and broadening structure of the spectrum is observed and analyzed for different polarization. For a realistic pulsed photon beam, the spectrum of the radiation is redshifted for backward radiation and blueshifted for forward radiation, and spectral broadening is noticed. This is due to the changes in the longitudinal velocity of the electron during the laser pulse. These effects are much more pronounced at higher laser intensities giving rise to even higher-order harmonics that eventually leads to a continuous spectrum. Numerical simulations have further shown that broadening of the high harmonic radiation can be limited by increasing the laser pulse width. The complex shifting and broadening of the spectra can be employed to characterize the ultrashort and ultraintense laser pulses and to study the ultrafast dynamics of the electrons. (c) 2006 American Institute of Physics.

Space-frequency coupling, conical waves, and small-scale filamentation in water

Numerical simulations of fs laser propagation in water have been made to explain the small-scale filaments in water we have observed by a nonlinear fluorescence technique. Some analytical descriptions combined with numerical simulations show that a space-frequency coupling mainly from the interplay among self-phase modulation, dispersion and phase mismatching will reshape the laser beam into a conical wave which plays a major role of energy redistribution and can prevent laser beam from self-guiding over a long distance. An effective group velocity dispersion is introduced to explain the pulse broadening and compression in the filamentation. (c) 2005 American Institute of Physics.

High-power extracavity pulse compression in solid materials

A novel technique for high-power extracavity pulse compression with a nonlinear solid material is demonstrated. Before spectral broadening by self-phase modulation in the solid material, a short filament generated in argon is used as a spatial filter, which works for a uniform spectrum broadening over the spatial profile. Compensated by chirped mirrors, a 15-fs pulse is generated from a 32-fs input laser pulse. A total transmission larger than 80% after the solid material is achieved.

A scaling law of photoelectron angular distributions in one-cycle laser pulses

The photoelectron angular distributions (PADs) from above-threshold ionization of atoms irradiated by one-cycle laser pulses satisfy a scaling law. The scaling law denotes that the main features of the PADs are determined by four dimensionless parameters: (1) the ponderomotive number u(p) = U-p/hw, the ponderomotive energy U-p in units of laser photon energy; (2) the binding number E-b = E-b/h(w), the atomic binding energy E-b in units of laser photon energy; (3) the number of absorbed photons q; (4) the carrier-envelope phase phi(0), the phase of the carrier wave with respect to the envelope. We verify the scaling law by theoretical analysis and numerical calculation, compared to that in long-pulse case. A possible experimental test to verify the scaling law is suggested.

紧聚焦的超短超强激光脉冲在真空中加速斜入射的相对论电子

研究了紧聚焦的线偏振飞秒超强高斯激光脉冲俘获并剧烈加速斜入射低能相对论电子的效应，发现被俘获的电子在激光脉冲纵向有质动力的强大加速作用下，可以获得GeV量级的能量，并详细研究了入射电子的初能量、斜入射角、电子与激光脉冲的相对延迟时间和激光脉冲宽度等条件对电子能量增益的影响，发现当激光脉宽超过10λ时，脉宽对电子能量增益影响不大．

非相对论线偏振激光下的J×B加热

利用解析的方法研究了非相对论线偏振激光与等离子体相互作用中的J×B加热吸收机理,建立了一种包含两类有质动力效应在内的自洽理论.探讨了密度轮廓修正下的J×B加热机理,给出了相应的吸收系数随激光场强度变化的关系曲线.研究发现,当激光场强度A0=20时,J×B加热所导致的吸收系数fabs约为2.8%.

负啁啾高强度飞秒脉冲在正常色散材料中传输特性研究

利用块状正常色散材料作非线性介质，实验研究了负啁啾强激光飞秒脉冲在正常色散材料的传输过程中，脉冲波形及频谱随入射脉冲能量的变化特性．实验发现，负啁啾脉冲经过材料后激光光谱在被整形和压缩的同时，脉冲宽度也被压缩．实验获得的压缩脉冲宽度短于相等光谱宽度下的双曲正割转换极限脉冲宽度。数值模拟结果显示，在实验条件下模拟结果具有同样的变化特性．

利用块状介质进行飞秒强激光脉冲的腔外压缩

高强度飞秒激光脉冲的腔外压缩是获得高次谐波阿秒脉冲驱动源的必要手段。实验研究了超强超短飞秒激光脉冲在经过块状介质后的光谱展宽和色散补偿压缩现象。单脉冲能量0．26mJ，脉宽50fs的激光脉冲经透镜在空气中聚焦后再入射到块状材料上，出射脉冲光谱被展宽到接近40nm。由于在块状材料中的自聚焦效应，出射光束质量变好并保持较小的空间啁啾。利用熔融石英棱镜对补偿带有正色散的出射脉冲，最后得到〉0．1mJ，19fs的压缩脉冲。利用SPIDER装置测量了出射脉冲的脉宽和光谱相位。整个系统的能量效率大约为35％，压缩后的

飞秒强激光作用下电子振荡导致的谐波辐射频谱的展宽和红移

采用单电子模型研究了圆偏振飞秒脉冲激光作用下电子振荡导致的谐波辐射频谱的特性。研究发现随着激光强度的增加，电子在激光场中运动的相对论效应可以导致谐波辐射，并且发现谐波辐射频谱随着激光强度的增加发生了展宽和红移。电子与强激光脉冲相互作用，电子除了在激光场的作用下做横向振荡运动之外，激光脉冲的纵向有质动力对电子还有推动作用，这是产生谐波频谱红移的原因，而谐波辐射频谱展宽是由电子纵向速度的变化引起的。分析激光场中电子在不同方向的辐射频谱表明：随着谐波阶数的升高，红移在有规律地变大；在θ=3π／4方向上电子频谱的

Active phase control and frequency chirp effects on supercontinuum generation in high birefringence photonic crystal fiber

An acoustic-optics programmable dispersive filter (AOPDF) was first employed to actively control the linearly polarized femtosecond pump pulse frequency chirp for supercontinuum (SC) generation in a high birefringence photonic crystal fiber (PCF). By accurately controlling the second order phase distortion and polarization direction of incident pulses, the output SC spectrum can be tuned to various spectral energy distributions and bandwidths. The pump pulse energy and bandwidth are preserved in our experiment. It is found that SC with broader bandwidth can be generated with positive chirped pump pulses except when the chirp value is larger than the optimal value, and the same optimal value exists for the pump pulses polarized along the two principal axes. With optimal positive chirp, more than 78% of the pump energy can be transferred to below 750 nm. Otherwise, negative chirp will weaken the blue-shift broadening and the SC bandwidth. (C) 2007 Elsevier B.V. All rights reserved.

Pondermotive shift of photoelectron spectra in intense laser field

In a recent experimental work on the excess photon detachment (EPD) of H- ions [Phys. Rev. Lett. 87 (2001) 243001] it has been found that the ponderomotive shift of each EPD peak increases with the order of the EPD channel. By using a nonperturbative quantum scattering theory, we obtain the kinetic energy spectra for the differential detachment rate along the laser polarization for several laser intensities. It is demonstrated that higher order EPD peaks are produced mainly at relatively higher laser intensities. By calculating the overall EPD spectra with varying laser intensities, it is found that the ponderomotive shift of each EPD peak increases with the order of the EPD channel. Our calculations are in good agreement with the experimental observation. It is found that different EPD channels occur mainly when the laser field reaches some values, thus the intensity distribution of the laser field is responsible for the varying ponderomotive shifts.

Radiation damping effects on the ultrashort x-ray pulses generated by nonlinear Thomson backscattering

Nonlinear Thomson backscattering of an intense Gaussian laser pulse by a counterpropagating energetic electron is investigated by numerically solving the electron equation of motion taking into account the radiative damping force. The backscattered radiation characteristics are different for linearly and circularly polarized lasers because of a difference in their ponderomotive forces acting on the electron. The radiative electron energy loss weakens the backscattered power, breaks the symmetry of the backscattered-pulse profile, and prolongs the duration of the backscattered radiation. With the circularly polarized laser, an adjustable double-peaked backscattered pulse can be obtained. Such a profile has potential applications as a subfemtosecond x-ray pump and probe with adjustable time delay and power ratio. (c) 2006 American Institute of Physics.