Exciton-polaritons in multilayer WSe₂ in a tunable planar microcavity

Full text: Due to high exciton biding energy and oscillator strength transition metal dichalcogenides (TMDs) have recently gained significant attention in a field of light-matter interactions. A unique characteristic of this group of materials is strong dependence of optical properties on their thickness. Single layer flakes are of special interest to study light-matter interactions due to their direct bandgap. Significantly less explored are multi-layer flakes, which, in contrast to monolayers, exhibit an indirect bandgap. We investigated 1 to 4 WSe₂ monolayer flakes, which were encapsulated in high quality hBN to ensure the best optical properties. In case of the monolayer, excitonic transition is visible in both luminescence and reflectance spectra. For thicker layers, weak emission comes mostly from the indirect transition between Q and K points in reciprocal space. However, strong absorption at energies corresponding to direct excitonic transition in K point in the Brillouin zone are still observable in the reflectance spectra. Few-layer flakes under investigation were embedded inside a tunable cavity made of two SiO₂/TiO₂ distributed Bragg reflectors (DBRs). Angle-resolved reflectance spectra, performed at 10 K, revealed strong light-matter coupling regime for all of the investigated WSe₂ flakes. When cavity mode energy approaches direct excitonic resonance in given WSe₂ flake, spectrum consists of two anticrossing branches: lower and upper polariton. Observed Rabi splitting increases with thickness of WSe₂ flake, which is well reproduced with our transfer matrix simulations, based on measurements of the flakes outside the cavity. (author)