Optical Gain in MoS₂ via Coupling with Nanostructured Substrate : Fabry-Perot Interference and Plasmonic Excitation

Despite the direct band gap of monolayer transition metal dichalcogenides (TMDs), their optical gain remains limited because of the poor light absorption in atomically thin, layered materials. Most approaches to improve the optical gain of TMDs mainly involve modulation of the active materials or multilayer stacking. Here, we report a method to enhance the optical absorption and emission in MoS₂ simply through the design of a nanostructured substrate. The substrate consisted of a dielectric nanofilm spacer (TiO₂) and metal film. The overall photoluminescence intensity from monolayer MoS₂ on the nanostructured substrate was engineered based on the TiO₂ thickness and amplified by Fabry-Perot interference. In addition, the neutral exciton emission was selectively amplified by plasmonic excitations from the local field originating from the surface roughness of the metal film with spacer thicknesses of less than 10 nm. We further demonstrate that the quality factor of the device can also be engineered by selecting a spacer material with a different refractive index.