Monolithic h-BN waveguides: a simple tool for spectroscopic measurements of out-of-plane properties in 2D materials

Hexagonal boron nitride (h-BN) is a key material which enables the observation of pristine electronic and optical properties of two-dimensional materials, such as graphene and transition metal dichalcogenides (TMDs). We will present experimental measurements showing how h-BN used for encapsulation of TMDs can also perform the role of an optical waveguide. Hyperspectral confocal photoluminescence maps will be presented showing how photoemission from encapsulated TMDs can couple into h-BN slabs and then outcouple to free-space at either naturally cleaved edges or from monolithically defined edges. We will compare waveguiding of light emitted by in-plane and out-of-plane dipoles and demonstrate how h-BN waveguides provide a simple route to characterizing dark excitons and trions in monolayer WSe₂. We will show how h-BN waveguides can be integrated with electronic gates, further expanding their utility. Additionally, we will show edge-on Raman spectroscopy of atomically thin MoS₂, where utilizing waveguided excitation gives us an easy route to realizing edge-plane optical pumping, which enhances the out-of-plane Raman modes. The importance of h-BN slab thickness, substrate choice, and polarization effects will be analyzed. Our results add another tool to the 2D materials toolkit and will be directly applicable to others studying the optical properties of layered materials.