trARPES

trARPES, or time-resolved angle-resolved photoemission spectroscopy, is a spectroscopy technique that extends angle-resolved photoemission spectroscopy (ARPES) into the time domain by using ultrafast pump-probe pulses to study the evolution of a material's electronic structure after excitation. It measures how the electronic bands and their occupation change in response to a stimulus on femtosecond to picosecond timescales, providing both energy and momentum information as a function of delay.

In a typical trARPES experiment, a femtosecond pump pulse excites the system, creating non-equilibrium populations. After a controlled delay, a second ultrafast probe pulse photoemits electrons from the sample. The kinetic energy and emission angle of the photoelectrons are measured, giving binding energy and crystal momentum as a function of delay, thereby mapping E(k,t) and transient occupation. Probes are often in the ultraviolet or extreme ultraviolet, produced by high-harmonic generation, which enables access to the near-Fermi-level electronic structure.

Time resolution is determined by the cross-correlation of pump and probe pulses and the instrument response, while energy resolution is set by the photoemission spectrometer. Space-charge effects and surface sensitivity can complicate measurements. Experiments generally require ultra-high vacuum conditions and clean, well-ordered surfaces, sometimes at low temperatures, due to the surface-specific nature of photoemission.

Applications include investigation of ultrafast carrier dynamics, electron-phonon coupling, superconductivity dynamics, charge-density waves, and transient states such as Floquet-engineered band structures. trARPES has provided direct insight into how electronic structure responds to photoexcitation on femtosecond timescales.