Characterization of a high harmonics beam line for the application of time-resolved spectroscopy

这个项目的任务是表征高次谐波脉冲，以应用于时间分辨光电子能谱。我们所完成的研究工作包括：1)利用镀金属中空光纤将脉冲有效压缩到5fs；2)发展了一种新的高次谐波脉冲表征技术。提出了这些问题：1)从5-fs激光脉冲中高效地产生高次谐波脉冲；2)更方便地将高次谐波脉冲应用到时间分辨光谱中。为了产生强阿秒脉冲，需要提高几个周期的激光脉冲的脉冲能量。因此，在实验条件相同的情况下，我们采用镀铝中空纤维来提高光纤的透过率，使透过率从40%提高到70%。最后，我们得到了经啁啾反射镜压缩后的5fS、660μ的J脉冲。这对于产生阿秒脉冲将是非常有用的。我们提出了一种新的基于光谱剪切干涉的高次谐波脉冲表征技术。在该方法中，利用光电效应将光脉冲复制到光电子波包上。以Ne或Ar原子为靶，由于Ne和Ar具有两个电离能，产生了一对光谱剪切光电子波包。从光谱剪切后的光电子波包中可以提取光谱色散。该方法操作简单，灵敏度高。我们将这种方法应用于钛宝石激光器的19次谐波，在激光脉冲宽度为36fs的情况下，仍能产生3.7fS的脉冲。激光电场的非绝热上升可以解释较短的脉冲宽度。

The mission of this project is to characterize high harmonic pulses to apply to time-resolved photoelectron spectroscopy. The research issues we have accomplished are 1) the efficient pulse compression to 5 fs using metal-coated hollow fiber and 2) the development of a new characterization technique of high harmonic pulses. These issues were raised 1) to generate high harmonic pulses efficiently from 5-fs laser pulses and 2) to apply high harmonic pulses to time-resolved spectroscopy more conveniently.Few-cycle laser pulses are used to generate attosecond high harmonic pulses. To generate intense attosecond pulses, the pulse energy of few-cycle laser pulses should be boosted. Therefore, we used an aluminum coated hollow fiber in the fiber compression to improve the transmittance efficiency, improved from 40% to 70% under the same experimental condition. Finally, we could obtain the 5-fs, 660-μJ pulses after compressed by chirped mirrors. These will be very useful to generate attosecond pulses.We propose a new characterization technique of high harmonic pulses based on the spectral shearing interferometry. In this method, optical pulses are copied on the photoelectron wave packet by the photoelectric effect. Using neon or argon atoms as a target, a pair of spectral sheared photoelectron wave packets is generated because neon and argon have two ionization energies. From the spectral sheared photoelectron wave packets, the spectral dispersion can be extracted. This method is quite simple and has high sensitivity. We applied this method to the 19th harmonic of Ti:sapphire laser and could generate 3.7-fs pulses, although the pulse duration of the laser pulse was 36 fs. The nonadiabatic rising of the laser electric field can explain the relatively short pulse duration.