强激光啁啾脉冲中原子的高次谐波辐射

High-Order Harmonic Emission From a Single Atom in the Intense Chirped Laser Pulses

数值计算了强激光啁啾脉冲中原子的谐波辐射。啁啾效应使谐波发生移动和展宽，即相对于无啁啾脉冲情形，负啁啾脉冲中谐波平台的前半部分发生兰移，而后半部分谐波谱则发生红移。当正啁啾脉冲时、谱移动则正好相反。无论正负啁啾，所有的谐波谱均被展宽。若正确的相位条件被满足，个别谐波谱宽支持亚飞秒脉冲的产生。这些结果与最近的啁啾谐波实验符合得很好，但后半部分谐波谱的反向移动在该实验中并没有观察到。本文的计算表明，人们可以利用激发脉冲的频率啁啾进行高次谐波的调谐、并有可能获得超短的软Ｘ射线辐射。

Highorder optical harmonic emission from a single atom exposed in the intense chirped laser pulses has been calculated. Harmonic spectral shifting and broadening are found. Namely, for the negatively chirped pulses, blueshifting can be observed in the front part (11 st^51 st order) of the harmonic plateau. But the back part from 53rd to 91st harmonic orders occur to redshift. When the sign of chirp is changed to be positive, the spectral shifting is just reverse. Regardless the sign of chirp every harmonic order is broadened in different degree for different order number. The bandwidths of individual peaks are broad enough to support subfemtosecond pulses if the pertinent phase conditions can be met. These calculation results agree well with the recent experiment performed by Zhou et al. although the opposite spectral shifting in the back part of harmonic plateau is not reported in their experiment. It indicates that a tunable, ultrashort soft Xray source can be obtained by controlling the degree and sign of chirp.