Room-temperature strong coupling of hexane-dispersed colloidal CdSe nanoplatelets in a microcavity composed of two Bragg reflectors

Abstract: CdSe nanoplatelets (NPLs) are suitable for exploring strong light-matter coupling in semiconductor nanocrystal systems due to their giant oscillator strength and large exciton binding energy. Herein, we report on the facile fabrication and optical characterization of a half-wavelength planar microcavity, which consists of two distributed Bragg reflectors with a hexane layer containing concentrated colloidal CdSe NPLs. Using a hexane solution layer instead of the typically used dried active layers makes the layer thin and flat, even under dense NPL conditions, without stressing or charging of the NPLs' surfaces. Reflectance spectra showed that strong light-matter coupling can be realized at room temperature and that the vacuum Rabi splitting energy is 53.5 meV. Intense photoluminescence (PL) emerges at the lower polariton branch where 25.1 meV (longitudinal optical (LO)-phonon energy) below the energy of the polariton dark states, indicating that the relaxation from the dark states occurs efficiently in this microcavity owing to LO-phonon-assisted relaxation. We describe the reflectance and PL properties using the model that a cavity photon couples to a one-exciton state delocalized over nonuniformly orientated NPLs. This model contributes to an intuitive and quantitative understanding of the microcavity containing colloidal NPLs.