Near-field Imaging of Excitons in Transition Metal Dichalcogenides

Atomically thin monolayer transition metal dichalcogenides (TMDs) exhibit remarkable physical properties resulting from their reduced dimensionality. These materials give rise to an especially promising platform for fundamental studies of two-dimensional (2D) systems with wide reaching applications in optoelectronics. A direct consequence of this reduced dimensionality is the formation of strongly bound electron-hole pairs, or excitons, which govern the material's optical properties. Previous measurements of excitons in these systems have primarily relied on far-field optical spectroscopy techniques which are diffraction limited to several hundred nanometers. Here, we present a study of the exciton spectra of TMD heterostructures using a cryogenic scattering-type scanning near-field optical microscope (s-SNOM). Using a tunable visible source, we map the exciton resonances in the TMD materials with sub 100 nm spatial resolution at both room temperature and 10 K. As the temperature is lowered to 10 K, the exciton resonance spectrally blueshifts and narrows by at least an order of magnitude. These preliminary results demonstrate cryogenic visible s-SNOM to be an effective nanoscale excitonic probe.