Advancing time- and angle-resolved photoemission spectroscopy: The role of ultrafast laser development

In the last decade, there has been a proliferation of laser sources for time- and angle-resolved photoemission spectroscopy (TR-ARPES), building on the proven capability of this technique to tackle important scientific questions. In this review, we aim to identify the key motivations and technologies that spurred the development of various laser sources, from frequency up-conversion in nonlinear crystals to high-harmonic generation in gases. We begin with a historical overview of the field in Sec.1, framed by advancements in light source and electron spectrometer technology. An introduction to the fundamental aspects of the photoemission process and the observables that can be studied is given in Sec.2, along with its dependencies on the pump and probe pulse parameters. The technical aspects of TR-ARPES are discussed in Sec.3. Here, experimental limitations such as space charge and resultant trade-offs in certain source parameters are discussed. Details of various systems and their approach to these trade-offs are given in Sec.4. Within this discussion, we present a survey of TR-ARPES laser sources; a meta-analysis of these source parameters showcases the advancements and trends in modern systems. Lastly, we conclude with a brief discussion of future directions for TR-ARPES and its capabilities in elucidating equilibrium and non-equilibrium observables, as well as its integration with micro-ARPES and spin-resolved ARPES (Sec.5).