Bipolariton coupling in biexciton optical decay: Degenerate and nondegenerate polariton emissions in CuCl

Abstract

Radiative decay of biexcitons wave-vector (formula presented) selectively generated in a low-(formula presented) region (formula presented) is studied in CuCl, where (formula presented) is the biexciton wave number of degenerate two-photon excitation, and the bipolariton nature of low-(formula presented) biexcitons is experimentally demonstrated. All the radiative relaxation channels of low-(formula presented) biexcitons are clarified, which dissociate into two lower-branch polaritons and into upper- and lower-branch polaritons yielding nearly degenerate and nondegenerate pair emission lines, respectively. The three-dimensional scattering geometry allows for simultaneous observation of both degenerate and nondegenerate lines. Nearly degenerate two-polariton emissions have larger intensities than nondegenerate emissions. Completely degenerate emissions from (formula presented) biexcitons have the largest intensity, reflecting singularity of the joint density of polariton states. These features are in remarkable contrast with the resonant two-photon absorption process of biexcitons, which shows resonance enhancement not at the degenerate two-photon energy but at the exciton resonance. In spite of the larger transition probability, nearly degenerate emissions have a negligible contribution to the emission band arising from thermally distributed biexcitons. The dispersion curve of the (formula presented)-exciton lower(upper)-branch polariton is determined from the K dependence of the emission energies. The observed emission lines are all unambiguously assigned and the relative intensities of the simultaneously observed lines are compared with a biexciton radiative decay theory based on the bipolariton model. The energy dependence of the transition probability of the observed biexciton radiative decay channels is quantitatively reproduced by the theory, verifying that the biexciton radiative decay rate is determined by the exciton components in final-state polaritons (Hopfield coefficients) through the exciton-exciton Coulomb interaction. The radiative lifetimes of (formula presented) biexcitons are found to be shorter than those of (formula presented) biexcitons as expected from the theory. The lifetimes of low-(formula presented) biexcitons are strongly affected by the coupling with final two-polariton states, leading to a 20% reduction of the lifetimes compared with the case of no polariton effects. © 2001 The American Physical Society.