激光脉冲在气体中产生的离化波前的演化及其对光脉冲传播的影响

利用带离化子程序的一维粒子模拟程序，对光脉冲与离化波前的相互作用进行了模拟研究，讨论了在原子多度电离的情况下光脉冲与离化波前的相互作用关系.研究表明，由于原子各级离化势的不连续性使得离化波前在空间上出现平台区；离化过程导致激光频率发生蓝移，这个蓝移又对脉冲形状产生调制，使脉冲前沿呈阶跃型增长；阶跃型的光脉冲包络使离化波前的平台区变短，直至整个离化波前分为几个不同梯度的区域.还讨论了不同元素和不同密度的气体中产生的离化波前的特点.同时分析了光脉冲频率随传播距离的关系，指出由于光脉冲宽度的增加，将导致出射的光 关键词： 激光等离子体 光电离 离化波前 激光频率上转换

Evolution of ionization front and its effects on laser pulse propagation in gaseous targets

When an intense laser pulse propagates in a gaseous target, an ionization front is produced at the leading edge of the laser pulse, which moves at the speed of light. By use of a one-dimensional particle-in-cell code with the field ionization included, we simulate the interaction between the laser pulse and its ionization front. It is found that, because of the discontinunities in atomic ionization potentials for different ionization orders, a flat-top region of the electron density appears inside the ionization front at the beginning. The ionization induces blue shif in the laser frequency, which modifies the laser pulse into a step-like profile. This new pulse profile in turn shortens the flat-top region in the ionization front. Generally the ionization front evolves into several parts, corresponding to different ionization orders, with different plasma density gradients. The effects of different gas elements and initial gas densities on the produced ionization fronts have been investigated. The conditions ith regard to the laser intensity and gas species to produce steep density gradients in ionization fronts have been studied for applications in frequency up-conversion by use of counter-propagating light pulses. Finally, the spectrum evolution of the ionizing pulse is investigated and the resulting fine spectrum structure located between the fundamental and the second harmonic is analyzed.