Observation of Exciton‐Polaritons in a Topless 2D‐Semiconductor Microcavity

Abstract 2D transition metal dichalcogenides (TMDs) and planar Fabry–Pérot microcavities have emerged as a promising system for investigating exciton‐photon interactions at room temperature, in which the structural simplification of these 2D‐semiconductor microcavities is highly desired for directly probing bosonic quasiparticles and developing on‐chip integrated polariton devices. Here, room‐temperature exciton‐polariton formation is reported in a topless 2D‐semiconductor microcavity, in which the λ/2‐thick 2D‐semiconductor/spacer heterostructure with relatively high refractive indexes is dedicatedly employed to form the electromagnetic‐field antinode at the top surface. The 2D‐semiconductor layer is simultaneously employed as the gain medium and the quasi‐top reflector, contributing to the effective longitudinal optical confinement. The Rabi splitting energies of 44, 65, and 84 meV are respectively achieved in the topless monolayer‐, bilayer‐, and trilayer‐WS 2 microcavities, where the splitting energy is proportional to the square root of the number of layers. Moreover, the role of exciton reservoirs is recognized in angle‐resolved photoluminescence studies, which leads to the population increase of the lower polariton branch. This work provides a robust strategy to simplify the microcavity structure and generate exciton‐polaritons, which will be beneficial to directly studying bosonic condensation phenomena of exciton‐polaritons, such as superfluidity and quantum vortices, at the device surface.