Resonant two-photon absorption by exciton polaritons in evaporated CuCl films

Biexcitons in CuCl decay into $Z_3$ excitons (longitudinal or transverse) and photons ($M_L$ or $M_T$ luminescence). Here an intense, tunable-laser pulse is used to study this luminescence in thin CuCl films. As a function of laser frequency (3.209-3.219 eV) the luminescence line shapes in the $M_L$ and $M_T$ bands show absorption which we interpret as a resonant two-photon (laser plus luminescence) transition from the ground to biexciton state. The polariton nature of the intermediate state is explicitly considered and shown to bring about a resonant condition, greatly enhancing the absorption process. Reabsorption involving the upper and lower transverse polariton branches has been reported previously. Here we report a new transition which uses the longitudinal exciton as an intermediate state. From laser frequencies and these three reabsorption dip positions we determine a portion of the $Z_3$ exciton-polariton dispersion curve.