Advances in extreme ultraviolet light source technology

In recent years, extreme ultraviolet high harmonic sources have attracted wide attention in the field of electron dynamics due to their strong coherence, short pulse duration and high photon energy, and have been used in various spectral and imaging studies. With the advancement of technology, this light source is developing towards higher repetition frequency, higher photon flux, higher photon energy and shorter pulse width. This advance not only optimizes the measurement resolution of extreme ultraviolet light sources, but also provides new possibilities for future technological development trends. Therefore, the in-depth study and understanding of high repetition frequency extreme ultraviolet light source is of great significance for mastering and applying cutting-edge technology.

For electron spectroscopy measurements on femtosecond and attosecond time scales, the number of events measured in a single beam is often insufficient, making low refrequency light sources insufficient to obtain reliable statistics. At the same time, the light source with low photon flux will reduce the signal-to-noise ratio of microscopic imaging during the limited exposure time. Through continuous exploration and experiments, researchers have made many improvements in the yield optimization and transmission design of high repetition frequency extreme ultraviolet light. The advanced spectral analysis technology combined with the high repetition frequency extreme ultraviolet light source has been used to achieve the high precision measurement of material structure and electronic dynamic process.

Applications of extreme ultraviolet light sources, such as angular resolved electron spectroscopy (ARPES) measurements, require a beam of extreme ultraviolet light to illuminate the sample. The electrons on the surface of the sample are excited to the continuous state by the extreme ultraviolet light, and the kinetic energy and emission Angle of the photoelectrons contain the band structure information of the sample. The electron analyzer with Angle resolution function receives the radiated photoelectrons and obtains the band structure near the valence band of the sample. For low repetition frequency extreme ultraviolet light source, because its single pulse contains a large number of photons, it will excite a large number of photoelectrons on the sample surface in a short time, and the Coulomb interaction will bring about a serious widening of the distribution of photoelectron kinetic energy, which is called the space charge effect. In order to reduce the influence of space charge effect, it is necessary to reduce the photoelectrons contained in each pulse while maintaining the constant photon flux, so it is necessary to drive the laser with high repetition frequency to produce the extreme ultraviolet light source with high repetition frequency.

Resonance enhanced cavity technology realizes the generation of high order harmonics at MHz repetition frequency
In order to obtain an extreme ultraviolet light source with a repetition rate of up to 60 MHz, the Jones team at the University of British Columbia in the United Kingdom performed high order harmonic generation in a femtosecond resonance enhancement cavity (fsEC) to achieve a practical extreme ultraviolet light source and applied it to time-resolved angular resolved electron spectroscopy (Tr-ARPES) experiments. The light source is capable of delivering a photon flux of more than 1011 photon numbers per second with a single harmonic at a repetition rate of 60 MHz in the energy range of 8 to 40 eV. They used a ytterbium-doped fiber laser system as a seed source for fsEC, and controlled pulse characteristics through a customized laser system design to minimize carrier envelope offset frequency (fCEO) noise and maintain good pulse compression characteristics at the end of the amplifier chain. To achieve stable resonance enhancement within the fsEC, they use three servo control loops for feedback control, resulting in active stabilization at two degrees of freedom: the round trip time of the pulse cycling within the fsEC matches the laser pulse period, and the phase shift of the electric field carrier with respect to the pulse envelope (i.e., carrier envelope phase, ϕCEO).

By using krypton gas as the working gas, the research team achieved the generation of higher-order harmonics in fsEC. They performed Tr-ARPES measurements of graphite and observed rapid thermiation and subsequent slow recombination of non-thermally excited electron populations, as well as the dynamics of non-thermally directly excited states near the Fermi level above 0.6 eV. This light source provides an important tool for studying the electronic structure of complex materials. However, the generation of high order harmonics in fsEC has very high requirements for reflectivity, dispersion compensation, fine adjustment of cavity length and synchronization locking, which will greatly affect the enhancement multiple of the resonance-enhanced cavity. At the same time, the nonlinear phase response of the plasma at the focal point of the cavity is also a challenge. Therefore, at present, this kind of light source has not become the mainstream extreme ultraviolet high harmonic light source.