Imaging exciton interactions in two-dimensional materials and heterostructures with spectroscopic microscopy

Abstract Exciton interactions are at the heart of carrier dynamics and applications of two-dimensional materials derived from their strong light-matter interactions. Exciton-phonon interactions affect the dephasing time of excitons in few-layer transition metal dichalcogenides (TMDs) and excitons can couple to each other coherently and incoherently in TMD heterostructures. This coupling introduces a mechanism for quantum coherent control and fast and efficient solar cells and detectors. Exciton-magnon interactions bear the potential for control of spintronics application, while exciton-continuum interactions can lead to new exciton states in twisted bilayer graphene. Linear spectroscopic techniques, including photoluminescence and spontaneous Raman, are ubiquitous yet limited in their ability to study interactions. Nonlinear, ultrafast spectroscopic techniques such as multidimensional coherent spectroscopy and transient absorption spectroscopy are uniquely suited to study exciton interactions with other quasiparticles and give unprecedented access to the most fundamental physics in two-dimensional materials. Because many high-quality samples are still based on exfoliated monolayers, properties change drastically across a sample and in between samples. Therefore, combining spectroscopic techniques with microscopic imaging is an important advance in experimental studies of two-dimensional materials. Spectroscopic microscopy gives statistical information as much as it allows researchers to understand how exciton interactions are affected by changes in the surrounding potential, whether due to strain, doping, or atomic alignment. In this review, we will highlight the progression of experimental techniques alongside the exciton interactions of interest, as different exciton interactions require different tools to study them.