Control of Rabi-splitting energies of exciton polaritons in CuI microcavities

We have investigated the active-layer-thickness dependence of exciton-photon interactions in CuI microcavities. The active layer thickness was changed from lambda/2 to 2 lambda, where lambda corresponds to an effective resonant wavelength of the lowest-lying exciton. In the CuI active layer, thermal strain removes the degeneracy of the heavy-hole (HH) and light-hole (LH) excitons at the G point. Angle-resolved reflectance spectra measured at 10 K demonstrate the strong coupling between the HH and LH excitons and cavity photon, resulting in the formation of three cavity-polariton branches: the lower, middle, and upper polariton branches. The energies of the three cavity-polariton modes as a function of incidence angle are reasonably explained using a phenomenological Hamiltonian to describe the exciton-photon strong coupling. It is found that the interaction energies of the cavity-polariton modes, the so-called vacuum Rabi-splitting energies, are systematically controlled from 29 (50) to 48 (84) meV for the LH (HH) exciton by changing the active layer thickness from lambda/2 to 2 lambda. The active-layer-thickness dependence of the Rabi-splitting energies is semi-quantitatively explained by a simple model.