Bipolariton coupling in biexciton optical decay: Degenerate and nondegenerate polariton emissions in CuCl - art. no. 045209

Radiative decay of biexcitons wave-vector (K) selectively generated in a lo w-K region (0 less than or equal to \K\less than or similar to 2k(0)) is st udied in CuCl, where 2k(0) is the biexciton wave number of degenerate two-p hoton excitation, and the bipolariton nature of low-K biexcitons is experim entally demonstrated. All the radiative relaxation channels of low-K biexci tons are clarified, which dissociate into two lower-branch polaritons and i nto upper- and lower-branch polaritons yielding nearly degenerate and nonde generate pair emission lines. respectively. The three-dimensional scatterin g geometry allows for simultaneous observation of both degenerate and nonde generate lines. Nearly degenerate two-polariton emissions have larger inten sities than nondegenerate emissions. Completely degenerate emissions from K = 2k(0) biexcitons have the largest intensity, reflecting singularity of t he joint density of polariton states. These features are in remarkable cont rast with the resonant two-photon absorption process of biexcitons, which s hows resonance enhancement not at the degenerate two-photon energy but at t he exciton resonance. In spite of the larger transition probability, nearly degenerate emissions have a negligible contribution to the emission band a rising from thermally distributed biexcitons. The dispersion curve of the Z (1,2)(Z(3))-exciton lower(upper)-branch polariton is determined from the K dependence of the emission energies. The observed emission lines are all un ambiguously assigned and the relative intensities of the simultaneously obs erved lines are compared with a biexciton radiative decay theory based on t he bipolariton model. The energy dependence of the transition probability o f the observed biexciton radiative decay channels is quantitatively reprodu ced by the theory, verifying that the biexciton radiative decay rate is det ermined by the exciton components in final-state polaritons (Hopfield coeff icients) through the exciton-exciton Coulomb interaction. The radiative lif etimes of K similar or equal to 0 biexcitons are found to be shorter than t hose of K similar or equal to 2k(0) biexcitons as expected from the theory. The lifetimes of low-K biexcitons are strongly affected by the coupling wi th final two-polariton states. leading to a 20% reduction of the lifetimes compared with the case of no polariton effects.