Laser

1. Laser Two-Photon Generation of Propagating Bipolaritons in Copper(I) ChlorideR. P. H. Chang, PI, Northwestern University,DMR-0520513 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. The solid black line is the angular divergence of the incident laser for |θ| < 25o, corrected for collection efficiency. Circles and dots define the angular distribution (corrected) of over-the-gap (OTGE) and resonant two-photon excited (RTPE) PL from excitonic molecules. The constant fit (dashed blue line) for the circles implies that the molecule PL arises from biexcitons under OTGE. The red line is a best fit for the dots, showing highly directional PL arising from propagating bipolaritons. Jang, et al., Physical Review B, 76, (2007).