High quality femtosecond HHG light source for Time-resolved Photoemission Spectroscopy

High-quality femtosecond light sources are critical for developing advanced time- and angle-resolved photoemission spectroscopy (TrARPES), which is powerful in capturing the electronic dynamics in solid state materials. Over the past decades, femtosecond high harmonic generation (HHG) light sources using inert gas as nonlinear media have been increasingly applied as probe light sources in TrARPES measurements. While major improvement in the energy resolution and photon flux has been achieved, the optimization of the HHG probe beam size and selective control of the light polarization, which are important for TrARPES measurements, still call for further investigation.

Recently, a research team from Tsinghua University developed a high-quality HHG light source for TrARPES measurements. The light source has several advantages: a small beam size, selective control of light polarization, and compatibility with mid-infrared (MIR) pumping, which are suitable for investigating light-induced electronic structure engineering. The work has been published in Ultrafast Science.

The HHG light source is systematically optimized to achieve a few advantages. First of all, the HHG beam size is optimize to sub-hundred micrometer. The typical beam size of HHG source (>100 μm) makes it unsuitable for measuring small samples. Here by developing a guiding principle for optimizing HHG focusing, the authors have been able to achieve a focus HHG beam size down to 57μm x 90μm, making it possible to investigate small samples down to the monolayer limit. Secondly, the HHG light source has selective polarization control between s-polarization (light polarization perpendicular to the scattering plane) and p-polarization (light polarization parallel to the scattering plane), which can be easily switched by rotating the polarization of the driving laser. Such HHG polarization control is critical for obtaining high quality data quality, as demonstrated by measurements on materials such as topological materials, graphene, transition metal dichalcogenides. In addition to advantages in the HHG probe source, the implementation of mid-infrared (MIR) pumping with a high time resolution of 130 fs is demonstrated on graphene, makes such TrARPES system possible to investigate the ultrafast dynamics upon selective or tailored pumping.

The development of high quality femtosecond HHG source makes it possible to investigate the ultrafast dynamics of a wider range of solid state materials such as graphene, transition metal dichalcogenides (TMDCs) down to monolayer limit. When further combined with the MIR pumping, this would open up new opportunities for Floquet engineering of solid state materials where the time-periodic light-field can be used as an ultrafast control knob to manipulate the transient electronic structure and material properties.