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.Comment: 34 pages, 4 figures (6 figures in Supporting Information