Through the Lens of a Momentum Microscope: Viewing Light‐Induced Quantum Phenomena in 2D Materials

Van der Waals (vdW) materials at their 2D limit are diverse, flexible, and unique laboratories to study fundamental quantum phenomena and their future applications. Their novel properties rely on their pronounced Coulomb interactions, variety of crystal symmetries and spin‐physics, and the ease of incorporation of different vdW materials to form sophisticated heterostructures. In particular, the excited state properties of many 2D semiconductors and semi‐metals are relevant for their technological applications, particularly those that can be induced by light. In this paper, the recent advances made in studying out‐of‐equilibrium, light‐induced, phenomena in these materials are reviewed using powerful, surface‐sensitive, time‐resolved photoemission‐based techniques, with a particular emphasis on the emerging multi‐dimensional photoemission spectroscopy technique of time‐resolved momentum microscopy. The advances this technique has enabled in studying the nature and dynamics of occupied excited states in these materials are discussed. Then, the future research directions opened by these scientific and instrumental advancements are projected for studying the physics of 2D materials and the opportunities to engineer their band‐structure and band‐topology by laser fields. In this article, time‐resolved momentum‐microscopy is highlighted as a powerful new technique for characterizing photoexcited two‐dimensional materials in momentum‐space. The recent scientific progress that is made thanks to this technique is surveyed. The future directions in which it will be impactful are outlined: the dynamics of photoexcitations, and the control of the band‐structure by Floquet band engineering.