Room-Temperature Strong Coupling of Hexane-Dispersed Colloidal CdSe Nanoplatelets in a Microcavity Composed of Two Bragg Reflectors

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. Here, we report the facile fabrication and optical characterization of a λ/2 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 results in a thin and flat layer, even under dense NPL conditions, almost without stressing or charging the surface of the NPLs. 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, 25.1 meV (=E_LO: longitudinal optical (LO) phonon energy) below the energy of the dark states, i.e., the exciton reservoir, indicating that relaxation from the exciton reservoir occurs efficiently in this microcavity due to LO-phonon-assisted relaxation. We describe the reflectance and PL properties using a model in which a cavity photon couples to a one-exciton state delocalized over nonuniformly oriented NPLs. This model contributes to an intuitive, quantitative understanding of the microcavity containing colloidal NPLs.