The extreme-ultraviolet high-harmonic light source has attracted significant attention in electron dynamics because of its strong coherence, short pulse duration, and high photon energy. It has been applied in various spectroscopy and imaging studies. Using a high repetition rate, high photon flux, narrow linewidth, femtosecond extreme-ultraviolet-light source (1 fs=10-15 s) enables direct observation of conduction band structures and femtosecond-scale electron dynamics in materials. Furthermore, processes such as electron tunneling and molecular dissociation can be investigated extensively using a broadband attosecond light source. In recent years, the development of water window spectral-range wide-spectrum attosecond light sources has facilitated the detection of reaction pathways between molecules and the motion of charge carriers on material surfaces. By applying electron and ion detection techniques, time-resolved coherent measurements and other attosecond transient spectroscopy studies have been conducted. For attosecond-scale electron spectroscopy measurements, the number of events in a single-shot measurement is often insufficient, making low-repetition-rate light sources inadequate for obtaining reliable statistical data. Therefore, it is necessary to use high-repetition-rate extreme-ultraviolet-light sources.
The efficiency and photon number per pulse of high-repetition-rate high-harmonic generation are significantly lower than those of low-repetition-rate high-harmonic generation. The single-pulse energy of high-repetition-rate driving lasers is lower than that of low-repetition-rate driving lasers; therefore, tight focusing is required to achieve the high-intensity field necessary for high harmonic generation. However, the interaction region of the laser and gas is small during tight focusing, making it relatively difficult to achieve the required phase-matching conditions. Consequently, the conversion efficiency of high-repetition-rate high-harmonic generation is low. Various methods have been developed in terms of driving laser, high-harmonic-generation methods, and beamline design to improve the photon flux of high-repetition-rate high-harmonic generation.
The use of extreme-ultraviolet high-harmonic light sources during experiments requires the presence of a monochromator or spectrometer. A spectrometer with high acquisition efficiency and high resolution is required to optimize high harmonic generation by adjusting the interaction configuration and characterize energy level transitions using attosecond transient absorption spectroscopy. In pump-probe experiments for dynamics research, a femtosecond extreme-ultraviolet-light source with good energy and time resolutions is required. A monochromator is required to select individual orders of high harmonics, achieving energy resolution control and minimizing the time broadening caused by the monochromator. Significant efforts have been made in beamline design to use extreme-ultraviolet high-harmonic-light sources in physics experiments.
Currently, extreme-ultraviolet high-harmonic light sources are advancing towards higher photon fluxes and repetition rates, which places higher demands on the repetition rate and single-pulse energy of femtosecond lasers. Chiang et al. used a long-cavity titanium sapphire laser to increase the repetition rate of the driving laser to 4 MHz and achieved high harmonic output. In 2015, H?drich et al. used a fiber laser to increase the repetition rate of high harmonics to 10.7 MHz. The highest average power of femtosecond lasers has now exceeded 10 kW.
The generation and optimization of high harmonics have been ongoing research topics. Using high-performance and high-repetition-rate lasers with high energy and few-cycle pulse length can prevent tight focusing and achieve high harmonic generation efficiency using fundamental-frequency-driving light. Csizmadia et al. directly generated high harmonics using a few-cycle 1030 nm driver laser with a repetition rate of 100 kHz and obtained a high-photon-flux extreme-ultraviolet-light source. By applying pulse compression, shorter pulse high-repetition-rate driving light can more easily achieve the peak power density required for high harmonic generation, thus achieving higher efficiency than long-pulse driving light. Wang et al. applied dual-color field-assisted pulse compression to obtain a high-photon-flux extreme-ultraviolet-light source. High-repetition-rate high-harmonic light sources above the MHz level require field-enhanced methods for generation. Among them, resonant enhancement cavities have been applied to time- and angle-resolved photoemission spectroscopy (Tr-ARPES) beamlines. Mills et al. used a fiber laser with a repetition rate of 60 MHz to obtain a high-photon-flux extreme ultraviolet-light source.
Monochromators and spectrometers are essential instruments for applying extreme ultraviolet-light sources. Rohde et al. used a metal film as a monochromatizing device while compressing the pump light, and the comprehensive performance of the compressed light source can approach the Fourier transform limit of extreme ultraviolet (XUV). Wang et al. developed an approach to reduce the pulse front tilt by adding slits at the defocused plane, taking advantage of the spatial distribution characteristics of forward-tilted pulses. Csizmadia et al. designed a transmission scheme using two off-plane mount (OPM) monochromators, with the first monochromator used to adjust the line width of XUV and the second monochromator used to compensate for the pulse front tilt generated by the grating. This design is used to almost completely compensate for the pulse front tilt generated by the grating.
High-repetition-rate extreme-ultraviolet-light sources have been widely used in electron dynamics research and have potential for applications in attosecond spectroscopy and microscopic imaging. These light sources are evolving towards increased repetition rates, photon fluxes, photon energies, and decreased pulse durations. This review summarizes the generation and control of high-repetition-rate extreme-ultraviolet-light sources and the optimization of their resolving capability for applications. Future development trends of such light sources are also discussed.
