Evidence for two-photon generation of propagating bipolaritons in CuCl

Unlike the conventional theory of giant oscillator strength, the recently proposed bipolariton model describes two-photon excitation of an excitonic molecule in terms of resonant polariton-polariton Coulomb scattering. Using angle-resolved spectroscopy, we directly measure the angular dependence of the molecule luminescence under resonant two-photon excitation. We find that the observed photoluminescence is highly directional and well correlated with the propagation direction of the incident laser. This observation unambiguously supports the core idea of this model that replaces the idea of a biexciton by that of a mutually bound propagating polariton molecule.