Nanoscale-resolved spectroscopy of excitons in atomically-thin transition metal dichalcogenides

Excitons play a dominant role in opto-electronic properties of atomically thin van der Waals semiconductors, such as transition metal dichalcogenides (TMDs). Further, excitons can be engineered by dielectric screening, interlayer hybridization and moiré potentials. These engineering knobs inherently lead to heterogeneous properties at the nanoscale. However, probing the optical response from excitons at the nanoscale is a formidable task. In this talk, we couple a tunable continuous-wave laser to a scattering-type scanning near-field optical microscope (s-SNOM), and measure the near-infrared optical spectra of a MoSe₂ monolayer, WSe₂ monolayer, MoSe₂/WSe₂ heterobilayer and WSe₂ trilayer at nanoscale. The observed spectra, in all cases, reveal resonances that we attribute to excitonic optical responses and are resolved with 20 nm spatial resolution. Nanoscale-resolved dielectric functions can be extracted quantitatively from s-SNOM spectra. Further, the evolution of both exciton resonance energies and radiation rates in a MoSe₂/WSe₂ heterobilayer and a WSe₂ trilayer relative to their isolated monoayers is investigated. Our work brings opportunities for locally exploring excitons with nanoscale spatial inhomogeneity, such as moiré exciton and exciton polarition.