高次谐波产生阿秒极紫外和X光脉冲研究新进展

Recent progress of attosecond XUV and X-ray pulses formed by high-harmonic generation

近年来在可见光谱范围内已经把激光脉冲压缩到接近一个光学周期（2～3fs）的物理极限，几fs的时间分辨精度可以描述分子化学反应过程，但是要探测远小于可见光周期的电子跃迁过程则需要阿秒（as）量级的光脉冲。利用脉冲间具有相同载波包络相位的阿秒脉冲序列能把可见光波段的光学频率梳向极紫外波段扩展；利用电子和离子碰撞复合过程短于一个光周期这个时间窗，通过测量激光场椭圆极化率对电子轨迹的微扰实现了as精度的分辨率；通过测量碰撞复合过程中的高能电子的辐射谱可以重构阿秒X光脉冲以及探测强场下束缚态和连续态电子动力学。

The laser pulses with several femtosecond duration close to its natural limit of the wave cycle in the visible spectral range are able to trace chemical processes of molecules. However, probing ultrafast transition process of electrons much faster than the wave cycle needs attosecond scale pulses. The experiments of attosecond XUV and X-ray pulses formed by high-harmonic generation have made much progress. The attosecond pulse train which are consecutive pulses with same carrier envelope phase can push optical frequency combs in the visible and near infrared wavelength to extreme ultraviolet regions. Temporal resolation of attosecond level is realized through measuring lateral distribution of the electron wave packet perturbed by laser ellipticity within attosecond temporal gate of electron-ion collision. The attosecond X-ray pulses strong field electronic dynamics of both bound and continuum electrons can be characterized by measuring the high energy photoelectron spectra yielded by recollision.