Anomalous exciton–radiation coupling in the nano-to-bulk crossover regime

Abstract

Anomalous exciton–radiation coupling in the sample-size regime beyond the long-wavelengthapproximation (LWA) is discussed. In high-quality samples fabricated using thelatest technologies, the long-range excitonic coherence and non-LWA effect cause apeculiar interplay between the spatial structures of the radiation and excitonicwaves, which emerge as an anomalous level structure of the exciton–radiationcoupled system including an extremely large radiative correction. We theoreticallydemonstrated such a situation for a thin film by a microscopic nonlocal theoryin which a size-resonant enhancement of the radiative damping constant andsuccessive interchanges of the confined polariton states can be observed. In order toverify the predicted level structure of confined polaritons, we present the results ofnon-degenerate two-photon scattering spectroscopy investigations and a correspondinganalysis based on the nonlocal theory for the second-order nonlinear process. Thisscheme has successfully elucidated peculiar level structures of confined polaritonsin the non-LWA regime. In the last half of the paper, a new type of resonanthyper-parametric scattering (RHPS) spectroscopy with a non-degenerate inducedexcitation is theoretically proposed as another method to study confined polaritons in thenon-LWA regime. Some calculations for thin films are demonstrated to reveal thepotentiality of this method that provides rich information on confined biexcitons andpolaritons.