高平均功率高次谐波光源的研究进展(特邀)

Research Progress of High Average Power High Harmonic Source(Invited)

阿秒激光脉冲(10-18 s)具有超高的时间分辨率,可以用于实现原子分子内的电子动力学的探测,这使阿秒科学成为超快光学领域里最受关注的研究前沿。传统的钛宝石飞秒激光驱动的高次谐波阿秒脉冲平均功率较低。最近,高平均功率的掺镱激光发展迅速,为产生高功率高次谐波阿秒脉冲提供了良好的驱动源。本文回顾了近二十年高重频的高次谐波光源的平均功率和最高光子能量逐步提高的研究进展,并介绍了高重频高次谐波阿秒脉冲在泵浦探测实验和相干成像等方面的应用和前景。

The advent of attosecond pulses via High-order Harmonic Generation(HHG)allows the study of electron dynamics with unprecedented temporal resolution.It provides a valuable research tool for ultrafast science,chemistry and so on.However,the low conversion efficiency of HHG restricted the development of attosecond measurement.Titanium-sapphire(Ti∶Sa)femtosecond lasers are the usually adopted to produce HHG.But the Ti∶S laser is typically limited to the about 10 watt level of average power due to the thermal lens effect introduced during the amplification process and thus the average power of the produced HHG is also limited.This limitation has led to the exploration of alternative laser sources to drive HHG,such as high-power ytterbium-doped lasers.Over the last two decades,there has been significant progress in the development of HHG,with a gradual increase in average power and maximum photon energy.This paper reviews some representative work about high-average power HHG sources and discuss the applications and prospects of high-repetition-rate HHG in various experimental scenarios.First,we discuss the Femtosecond Enhancement Cavity(fsEC)technology.It uses coherent interference to enhance laser intensity within the cavity and has enabled the production of HHG sources at a repetition rate of hundreds of megahertz.Then we introduce the HHG driving scheme based on high power Yb-doped laser,including the direct drivers,nonlinear post-compression drivers,Optical Parametric Chirped Pulse Amplification(OPCPA)drivers and short wavelength drivers.So far,the HHG source has average power levels exceeding 10 mW.On the other hand,the high-energy cutoff has been extended up to 620 eV at a repetition rate of 100 kHz.Furthermore,the development of HHG sources expands the capabilities of attosecond science by enabling high-repetition-rate attosecond pulse generation and facilitating attosecond time-resolved spectroscopy.We introduce the generation and measurement of attosecond pulses at high repetition rates.Finally we briefly discuss the applications of high-repetition-rate HHG in coherent diffraction imaging,XUV-IR pump-probe experiments and coincidence detection measurement.In summary,this paper provides a comprehensive overview of the advancements in high-average-power HHG sources,including the transition to high-power ytterbium-doped lasers as driving sources and the development of high-repetition-rate HHG driving schemes.Then we also discuss the applications of the high-repetition-rate HHG in various experiments.It is expected to provide powerful research tools for condensed matter physics,biomedicine and material chemistry.