Visible Cryogenic Near-field Imaging of Single Photon Emitters in Transition Metal Dichalcogenide Structures

Excitonic properties in transition metal dichalcogenides (TMDs), such as resonance energies and lifetimes, can be engineered by various external stimuli. Common manipulation techniques include electric and magnetic fields, material stacking, moiré superlattice potentials, and strain. Here, we investigate strain effects on the local excitonic spectra of WSe₂. Reports of single photon emission (SPE) in TMD materials around sample edges and naturally occurring, highly strained bubbles and wrinkles inspired various strain engineering studies towards realizing SPE capabilities in 2-dimensional materials. In this work, both AFM tip indentation and nano-patterned pillars are used to elastically deform and strain WSe₂ monolayers to create SPEs. Previous measurements of excitons in highly strained TMDs have primarily relied on far-field optical spectroscopy techniques which are diffraction limited to several hundred nanometers in the visible spectrum. To precisely image nanoscale spatial details requires an order of magnitude increase in resolution capabilities. Here, we present a study of the exciton spectra of strained TMDs using a cryogenic scattering-type scanning near-field optical microscope (s-SNOM). At 9 K, we map the variation in exciton resonance energy due to local strain magnitude. Using simultaneous near-field optical response and sample topography measurements, we extract excitonic resonance shifts in response to the strain distribution with 30 nm resolution to provide additional insights towards the origin of observed SPEs.