High-performance time- and angle-resolved photoemission spectroscopy studies on quantum materials

Exciting solid materials with femtosecond lasers enables the study of many-body interactions in quantum materials during non-equilibrium processes for the time scales of different degrees of freedom in materials may vary, and such nonthermal methods can lead to the realization of new or hidden states. As one of the frontier research areas in condensed matter physics, ultrafast electronic dynamics in quantum materials after photoexcitation can be studied using time-and angle-resolved photoemission spectroscopy as the key experimental tool. In this review, we focus on the use of nonlinear optical crystals and high-harmonic generation for time-and angle-resolved photoemission spectroscopy. We cover recent advances in the field, including (1) the probing of unoccupied electronic states in quantum materials, (2) the observation of light-induced ultrafast electronic phase transitions, (3) the measurement of microscopic electron-phonon coupling strength, and (4) the discovery of light-induced new states.