少周期激光脉冲与气体作用产生的离化电流和THz波辐射

Ionization currents and terahertz emission from the interaction of few-cycle laser pulses with gas targets

本文通过理论和数值模拟,研究少周期激光脉冲电离气体原子产生的离化电流以及相应的THz波辐射.研究表明,少周期激光脉冲离化气体后能产生较大的离化电流,因而可以产生较强的THz辐射.不同的少周期激光脉冲相位导致电离出的电子初始速度和电离起始时刻不同,从而产生的离化电流有所不同,辐射的THz波随激光脉冲的相位成周期性变化.该理论得到一维PIC数值模拟的验证.对于给定的激光脉冲相位,离化电流和THz辐射振幅并没有随入射激光振幅的增加而单调增加,而是存在一些极值点.与均匀分布气体相比,当气体分布具有一定梯度时,辐射表现相似的规律,但频谱会发生一定的变化.

Based on a theoretical model and numerical simulations, the ionization currents and subsequent terahertz （THz） emission induced by the interaction of a few-cycle laser pulses with He gas targets are studied. It is shown that owing to the large transverse current generated by field ionization with few-cycle laser pulses, strong THz emission can be generated. The change of the carder phase of the few-cycle laser pulses leads to the variation of the ionization currents. Correspondingly, the THz emission amplitude shows the characteristic as a periodic function of the carder phase, which is also confirmed by one-dimensional particle-in-cell simulations. For a given carrier phase, the THz emission amplitude is not proportional to the laser amplitude. It shows at least two peaks at certain laser amplitudes. When the gas density profile is not uniform, the emission amplitude has a similar dependence on laser amplitude and carder envelope phase, but the THz pulse duration and spectrum are quite different.