A scaling law of photoionization in ultrashort pulses

By means of the numerical solution of time-dependant Schrodinger equation, we verify a scaling law of photoionization in ultrashort pulses. We find that for a given carrier-envelope phase and duration of the pulse, identical photoionizations are obtained provided that when the central frequency of the pulse is enlarged by k times, the atomic binding potential is enlarged by k times, and the laser intensity is enlarged by k(3) times. The scaling law allows us to reach a significant control over direction of photoemission and offers exciting prospects of reaching similar physical processes in different interacting systems which constitutes a novel kind of coherent control.

周期量级超短激光脉冲作用下导带电子的光吸收与碰撞电离

激光照射下光学材料的损伤过程中，导带电子的加热和碰撞电离是非常重要的过程，影响着导带电子的产生、晶格能量的沉积和破坏．分析了Drude模型的局限性，从经典力学出发求解了周期量级激光场中导带电子的运动方程，计算了导带电子的光吸收和碰撞电离，分析了激光强度、载波相位等对碰撞电离的影响．

相对论飞秒激光脉冲在真空中对预加速电子的加速

通过求解电子运动的相对论方程，发现预加速电子在超强超短激光脉冲的作用下可以获得很高的能量增益．飞秒激光脉冲的上升沿在焦点附近的区域有效加速电子后，电子和光脉冲一起传播一段距离（远大于瑞利长度）后，激光强度变得很弱，从而使脉冲下降沿对电子的减速作用可以忽略不计，因此电子只经历加速过程而没有被减速，当电子和光脉冲分离时，电子获得了很高的能量增益．当光强为10^19W／cm^2，电子的初始能量为MeV量级时，电子的能量增益可以达到0．1GeV．进一步讨论了电子的能量增益与电子的初始条件与激光脉冲的参数之间的关系

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%.

μ^-子催化核聚变中强脉冲激光对介原子μ^3He的电离

数值求解了一维含时的Schroedinger方程，研究了μ子催化核聚变反应中激光强度和波长对介原子μ^3He电离的影响．发现当激光强度为10^19-10^23W/cm^2量级时，介原子μ^3He有2．7％左右的电离率；当激光强度达到6．0×10^24W/cm^2时，对介原子μ^3He有显著的电离，并且电离率随着激光的强度、波长而递增，进而会有效提高μ子的催化效率．

双色激光场中激光强度和脉宽对线性多原子分子离子电离的影响

利用强激光场电离和离解分子来研究分子激发态的波包结构是强场物理的重要研究方向。利用短时指数传播子对称分割法和快速傅里叶变换技术。数值求解了一维含时Schr（oe）dinger方程，探讨了双色激光场中激光的基波和谐波强度之间的不同配比以及脉宽对线性多原子分子离子电离的影响。理论计算结果表明：基波和谐波的相对相位为π时，尽管随着激光的基波和谐波强度之间配比的变化，电离几率随原子间距变化的趋势基本保持不变，但在一定的激光基波强度下(1．2×10^13～1．2×10^15W／cm^2)，激光基波强度的变化可以明显

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

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．讨沦了不同激光场参数(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

Pure Coulomb explosions of highly charged methane clusters investigated by a simple electrostatic model

The pure Coulomb explosions of the methane clusters (CA(4))(n), (light atom A = H or D) have been investigated by a simplified electrostatic model for both a single cluster and an ensemble of clusters with a given cluster size distribution. The dependence of the energy of ions produced from the explosions on cluster size and the charge state of the carbon ions has been analysed. It is found that, unlike the average proton energy which increases with the charge q of the carbon ions, the average deuteron energy tends to saturate as q becomes larger than 4. This implies that when the laser intensity is sufficiently high for the (CD4)(n) to be ionized to a charge state of (C4+D4+)(n), the neutron yield from a table-top laser-driven Coulomb explosion of deuterated methane clusters (CD4)(n) could be increased significantly by increasing the interaction volume rather than by increasing the laser intensity to produce the higher charge state (C6+D4+)(n). The flight-time spectra of the carbon ions and the light ions have also been studied.

Dynamics of rotationally resolved multiphoton ionization processes in molecules

This dissertation presents the results of studies of several rotationally- resolved resonance enhanced multiphoton ionization (REMPI) processes in some simple molecular systems. The objective of these studies is to quantitatively identify the underlying dynamics of this highly state-specific process which utilizes the narrow bandwidth radiation of a laser to ionize a molecule by first preparing an excited state via multiphoton absorption and subsequently ionizing that state before it can decay. Coupled with high-resolution photoelectron spectroscopy, REMPI is clearly an important probe of molecular excited states and their photoioniza tion dynamics.

A key feature of our studies is that they are carried out using accurate Hartree-Fock orbitals to describe the photoelectron orbitals of the molecular ions. The use of such photoelectron orbitals is important in rotationally-resolved studies where the angular momentum coupling in the photoelectron orbital plays a significant role in the photoionization dynamics. In these studies the Hartree-Fock molecular molecular photoelectron orbitals are obtained by numerical solution of a Lippmann-Schwinger integral equation.

Studies reported here include investigations of (i) ionic rotational branching ratios and their energy dependence for REMPI via the A^2Σ^+(3sσ) and D^2Σ^+(3pσ)states of NO, (ii) the influence of angular momentum constraints on branching ratios at low photoelectron energies for REMPI via low-J levels of the resonant intermediate state, (iii) the strong dependence of photoelectron angular distributions on final ionic rotational state and on the alignment in REMPI of the A^2Σ^+ state of NO, (iv) vibrational state dependence of ionic rotational branching ratios arising from rapid orbital evolution in resonant states (E'^2Σ^+(3pσ) of CH), (v) the influence of rovibronic interactions on the rotational branching ratios seen in REMPI via the D^2Σ^+(3pσ) state of NO, and (vi) effects of laser intensity on the photoionization dynamics of REMPI.

Carrier-envelope phase control of carrier-wave Rabi flopping in asymmetric semiparabolic quantum well

We investigate the carrier-wave Rabi flopping effects in an asymmetric semiparabolic semiconductor quantum well (QW) with few-cycle pulse. It is found that higher spectral components of few-cycle ultrashort pulses in the semiparabolic QW depend crucially on the carrier-envelope phase (CEP) of the few-cycle ultrashort pulses: continuum and distinct peaks can be achieved by controlling the CEP. Our results demonstrate that by adjusting the CEP of few-cycle ultrashort pulses, the intersubband dynamics in the asymmetric semiparabolic QW can be controlled in an ultrashort timescale with moderate laser intensity. (c) 2008 Optical Society of America.

Polarization-dependent supercontinuum generation from light filaments in air

We investigate polarization-dependent properties of the supercontinuum emission generated from filaments produced by intense femtosecond laser pulses propagating through air over a long distance. The conversion efficiency from the 800-nm fundamental to white light is observed to be higher for circular polarization than for linear polarization when the laser intensity exceeds the threshold of the breakdown of air. (C) 2005 Optical Society of America.

Temperature dependence of dark lines in coherent population trapping

We measure the signal amplitude and linewidth of a dark line in coherent population trapping in the Rb vapour cell filled with mixed buffer gas N-2 and Ar as a function of cell temperature. We find that the dark line signal amplitude increases with temperature up to a maximum at 49 degrees C and then drops at higher temperatures due to quenching effects of N-2. The linewidth of the dark line remains basically constant, at 1080 Hz. We also measure the linewidth of the dark line as a function of laser intensity. The linewidth increases linearly with laser intensity. An intrinsic linewidth (FWHM=896 Hz at 3.4 GHz) of the Rb cell is obtained.

激光脉冲宽度对有质动力加速电子的影响

基于真空中单电子运动模型，编程计算得到了高斯激光脉冲与初始位于激光传播轴上电子的相互作用结果。不同激光参鼍条件下，得到了电子的能量增益与激光强度、焦斑大小和脉冲宽度关系。结果表明，高斯激光脉冲焦斑较大时，电子没有明显的能量增益，高斯激光脉冲焦斑太小时，电子也没有明显的能量增益。电子的能量增益有一个最佳焦斑大小。在相同激光强度下，电子能量增益的最佳焦斑大小随脉冲宽度的增大而增大，但最佳焦斑大小与脉冲宽度的比值基本上是不变的。

激光功率密度对Al膜靶后表面快电子发射的影响

报道了在20 TW皮秒激光器上完成的p偏振激光与等离子体相互作用过程中产生的快电子的角分布和能谱测量结果.实验得到：当激光功率密度小于1017 W/cm^2时,电子发射没有明显定向性,在激光入射面内多峰发射;当激光功率密度大于1017 W/cm^2,小于1018 W/cm^2时,电子主要沿靶面法线方向发射;当激光功率密度达到相对论强度时,电子主要沿激光传播方向发射;激光功率密度未达到相对论强度时,靶后表面法线方向快电子能谱拟合平均温度符合共振吸收温度定标率;激光功率密度达相对论强度以上时,靶后表面法线方向