Anomalous mode structure of a radiation–exciton coupled system beyond the long-wavelength approximation regime

We study the mode structure of a size-dependent exciton–radiation coupled system by means of nonlocal response theory. Considering excitons whose center-of-mass motion is confined in a thin film, we calculate complex eigenmodes including the radiative correction. Results show peculiar thickness dependence of the level structure and the radiative width for the region beyond the long-wavelength approximation, such as the transition from the lower to the upper polariton nature and an enhancement of the width at a particular thickness. Recently observed peculiar nonlinear responses of nondipole-type excitonic state can be explained by these results.