超短激光脉冲在等离子体中的分裂以及类孤子结构的形成

用一维粒子模拟研究了超短脉冲在等离子体中传播时产生的光孤子结构以及由此形成的脉冲分裂现象，比较了不同峰值强度和脉冲宽度对形成光孤子以及脉冲传播方式的影响.研究表明: 脉冲宽度在若干个到十几个振荡周期的超短脉冲在等离子体中可能形成高速传播的光孤子；脉冲宽度增加和强度增大两种方式都可以使得孤子结构的传播速度减慢,且由于高阶孤子的衰变使得初始激光脉冲在等离子体中发生分裂，形成两个以不同速度向前传播的孤子结构.由孤子阶数的理论计算可较好地预言激光脉冲在等离子体中分裂的子脉冲数. 关键词： 光孤子 超短激光脉冲 等离子体 粒子模拟     

强激光与等离子体相互作用中低频电磁场孤子波的产生及其捕获

用二维粒子模拟程序研究了超短脉冲强激光与等离子体相互作用中局域低频电磁场的产生现象.这种低频电磁场在超短脉冲激光激发尾波场、脉冲后沿产生频率下移的过程中形成.通常它们的振荡频率接近于或低于电子等离子体振荡频率，因而被捕获在等离子体中(即传播速度接近于零).在演化过程中，通常它们以孤子场的形式出现.这种孤子波的形成及其演化与离子运动有极大关系.用相对论强激光脉冲可以产生达到相对论振幅的电磁场孤子波，后者可以把离子加速到非常高的能量.研究还表明，在二维几何位形下，孤子波产生与入射光的偏振态有很大关系.     

交叉传播激光脉冲与等离子体相互作用产生的等离子体密度光栅

理论上研究了两束交叉传播的激光束与等离子体相互作用产生的电子和离子密度调制. 用一维粒子模拟程序(particle-in-cell,PIC)研究了两束激光脉冲产生的干涉场激发的等离子体布拉格光栅. 研究表明等离子体初始密度、脉冲强度和宽度共同影响等离子体布拉格光栅的演化. 光栅的密度峰值可以达到初始等离子体密度的8倍以上,并且可以维持几皮秒的时间. 等离子体布拉格光栅可以囚禁由受激拉曼散射形成的电磁孤子,从而形成准稳态的孤子结构,很大程度上降低了形成电磁孤子所要求的激光脉冲强度. 关键词： 等离子体布拉格光栅 电磁孤子 交叉传播激光束 粒子模拟     

激光聚变快点火机理研究

对超强超短激光等离子体相互作用和快点火机理及其物理可行性进行了理论和数值模拟研究.研制了二维和二维半粒子云模拟程序,数值模拟了超强超短激光脉冲在非均匀等离子体中的传播.提出快点火穿孔速度和深度以及用于快点火的超强超短激光器的初步指标 关键词： 超强超短激光等离子体相互作用 快点火机理 粒子云模拟     

激光尾流场的2.5维粒子模拟

 用2D3V PIC粒子模拟方法分析了超短脉冲超强激光在稀薄等离子体中激发尾流场的产生过程及电子在尾流场中的加速过程。“前向Raman散射”使得激光脉冲沿传播方向拉长，脉冲的尾部变陡，它导致静电场的相速度和饱和时超热电子的最大动能明显减小，也使得激发尾流场的最佳脉冲宽度变小。     

超短激光脉冲在共振介质中传播的时空特性

 用有限差分方法求解Maxwell和 Bloch 的联立方程，计算得到共振介质中超短光传播的时空特性。结果表明，在考虑空间效应时，峰面积为2π 的超短光脉冲不再以孤子的形式传播，光场的形状将发生变化，变形的程度和光与物质相互作用的强度有关：强度较小时，光场包络在时间和空间两方面都展宽；强度较大时，则发生分裂。光与物质相互作用的强度与光场大小、光场频率、偶极矩、失谐以及共振粒子浓度等因素有关。     

近共振区超短强激光脉冲激发的等离子体尾波场

 用一维相对论粒子模拟研究了相对论超短强激光脉冲在等离子体中传播时激发的尾波场,初步获得了近共振区尾波场的峰值幅度随激光脉冲宽度变化的特点,发现在近共振区等离子体波激发出现增强。通过准静态近似下尾波激发的一维非线性方程数值求解,并与粒子模拟结果比较,得到了该非线性方程的适用范围：当激光脉冲宽度小于等离子体波波长的4倍时,该方程所得结果与粒子模拟结果一致;而当激光脉冲宽度大于该数值时,该方程不再适用。     

稀薄等离子体中的超短强激光脉冲孤子及尾波孤子

在弱相对论条件下，给出了超短强激光脉冲在稀薄等离体中传播的孤子解及其所激发的尾波场孤子解。     

激光与固体靶相互作用中低密度预等离子体对高能电子产生的影响

 对线极化、圆极化的超短超强激光脉冲与靶前有一段低密度预等离子体的固体靶的相互作用进行了理论和粒子模拟研究。激光通过有质动力加速机制加速预等离子体中的电子，研究了电子获得的最大能量随激光强度和预等离子体密度的变化。当激光脉冲与靶直接作用时，靶中的电子由于J×B机制而得到加速，所获得的能量比预等离子体中电子低。研究表明，在超短超强激光脉冲与固体靶相互作用中，预等离子体的存在有利于高能电子的产生。     

利用光孤子机制实现超宽范围波长调谐的理论和实验研究

以获得光参量啁啾脉冲放大系统主激光与抽运光两种波长、宽带、超短脉冲的产生及精密同步为目标,探索采用光孤子机制实现超宽带宽范围的可调谐超短脉冲产生. 数值模拟了孤子传输过程中的时域-频域演化过程及与其他非线性效应的相互作用过程和特性. 实验验证了利用光孤子机制产生可调谐脉冲的方法. 同时还观察到孤子的形成、分裂、自频移等现象,在可见光到近红外波段都演示了波长可调谐的输出特性,并且与理论分析结果符合较好. 关键词： 光参量啁啾脉冲放大 可调谐脉冲产生 非线性薛定谔方程 孤子机制     

The formation of relativistic electromagnetic solitons in plasma Bragg gratings induced by two counter-propagating laser pulses

The generation of relativistic electromagnetic solitons in plasma with spatiotemporal density modulation is investigated. When two counter-propagating laser pulses overlap in underdense plasma, the interaction between the pulses and plasma modulates the electron and ion densities resulting in localized, stable, long-living relativistic electromagnetic solitons. They are caused by the Stimulated Raman Scattering instability. The dependence of the formation of relativistic electromagnetic solitons on the ion motion, plasma parameters, and laser parameters is studied by particle-in-cell simulations as well.     

Soliton dynamics of megawatt ultrashort light pulses in a hollow photonic-crystal fiber: Effect of high-order dispersion and retarded nonlinearity

The properties of megawatt optical solitons in hollow photonic-crystal fibers (PCFs) are studied. We demonstrate regimes where high-power laser pulses can be compressed to a few cycles of light field and identify the main tendencies in the soliton dynamics of ultrashort laser pulses in hollow PCFs originating from dispersion properties and limited transmission band of such waveguides. The influence of retarded nonlinearity on the soliton dynamics of high-power laser pulses in a hollow PCF is analyzed. In a multisoliton regime, retarded nonlinearity and high-order dispersion are shown to result in the formation of high-power pulses displaying no oscillations of their temporal envelope, which would be typical of standard high-order solitons.     

高强度激光与等离子体相互作用中的受激Raman级联散射、光子凝聚以及大振幅电磁孤立子的产生与加速

应用一维相对论电磁粒子模拟程序，详细研究了线性极化强激光入射到无碰撞稀疏密度长等离子体中引起的受激Raman散射、Raman级联散射、级联散射到光子凝聚、以及大振幅电磁孤立子的产生与加速. 通过研究发现：在适当的激光振幅和等离子体状态下，强的光子凝聚现象会导致大振幅电磁孤立子的产生，电磁孤立子可以以静止、向后以及向前加速的形式存在；在密度均匀的等离子体中，电磁孤立子的加速不仅依赖于激光振幅而且依赖于等离子体的长度；电磁孤立子的电磁频率大约为未扰动电子等离子体振荡频率的二分之一左右，孤立子内电磁场的电场具有半周期结构，相应电磁场的磁场以及静电场则具有一个完整的周期结构. 关键词： 粒子模拟 受激Raman散射 Raman级联散射 光子凝聚 电磁孤立子     

Chirping and spreading of ultrashort laser pulses in underdense plasmas

A linear theory on the propagation of ultrashort pulses including only a few cycles in underdense plasmas is presented. It is shown that the dispersion in plasmas causes severe distortions in the pulse shape, including pulse chirping and spreading. The analytical calculations coincide very well with those obtained by particle-in-cell (PIC) simulations. The upper limit of the peak amplitude of the pulses, above which the linear theory breaks down due to the setting in of nonlinear effects of both the relativistic electron-mass increase and ponderomotive force, is also examined by PIC simulations. At certain high amplitudes, it is found that the ultrashort laser pulses can propagate like solitons.     

Evolution of dissipative-soliton laser from molecule to multipulse

We report on the evolution of dissipative-soliton laser from single pulse to soliton molecule, lastly to multiple pulses. The experimental observations show that the pulse separation of soliton molecules is oscillating stochastically. It is found that the proposed fiber laser delivers pulses from a soliton to a soliton molecule, two solitons, a soliton molecule together with a soliton, and three solitons, respectively, when, the pump strength is enhanced gradually.     

High density plasmas for recombination X-ray lasers

The scaling of recombination XUV lasers to shorter wavelengths requires laser plasmas produced at initial electron densities close to solid. With pump laser pulses longer than a few tens of picoseconds the hydrodynamic motion of the plasma during the interaction makes this difficult to achieve. In contrast, when picosecond laser pulses are used the laser energy is absorbed close to solid density since the plasma expansion is insignificant during the laser pulse. This results in hot near solid density plasmas which are needed for hydrogenic recombination X-ray lasers operating in the water window. Experimental observations have shown that a fully ionized aluminium plasma with a temperature of about 400 eV and a density well above 10²³ cm^–3 is produced when an aluminium target is irradiated with a single 3.5 ps high power KrF laser pulse.     

激光脉冲诱导的等离子体密度调制及其产生的相位反射

理论研究和数值模拟发现入射光和反射光在低密度等离子体中形成的干涉场可以产生深度的等离子体密度调制. 对于中等强度的入射光，譬如10¹⁵W／cm² ，产生密度调制的时间尺度在几十个光周期的范围. 这样的等离子体密度调制可以起类似布拉格反射镜的 作用，使得后面的入射光在临界面以下的区域产生相位反射. 因为密度调制的周期是光在等 离子体中波长的一半，其产生的反射率可以接近100％. 相位反射也可以在不均匀的低密度 等离子体中产生，它可以极大地减少等离子体对光的吸收，因此在惯性约束核聚变中需要考 虑到它的影响. 关键词： 相位反射 密度调制 激光等离子体 粒子模拟     

Stochastic heating and acceleration of electrons in colliding laser fields in plasma

We propose a mechanism that leads to efficient acceleration of electrons in plasma by two counterpropagating laser pulses. It is triggered by stochastic motion of electrons when the laser fields exceed some threshold amplitudes, as found in single-electron dynamics. It is further confirmed in particle-in-cell simulations. In vacuum or tenuous plasma, electron acceleration in the case with two colliding laser pulses can be much more efficient than with one laser pulse only. In plasma at moderate densities, such as a few percent of the critical density, the amplitude of the Raman-backscattered wave is high enough to serve as the second counterpropagating pulse to trigger the electron stochastic motion. As a result, even with one intense laser pulse only, electrons can be heated up to a temperature much higher than the corresponding laser ponderomotive potential.