Evidence for Optical Cooling of Biexcitons via Polariton-assisted Dissociation Scattering in CuCl

In 1991, a study based on experimental data obtained from a block single crystal of CuCl cooled to 2 K and excited by a strong monochromatic laser beam near the exciton resonance revealed a new emission band. The band was attributed to a scattering process in which a biexciton interacts with an incident photon, producing scattered light while leaving behind a set of a longitudinal and a transverse exciton. Because the spectrum of this scattering band strongly overlapped with the ordinary ML emission band of the biexciton, quantitative separation of the two contributions was not possible at that time. In the present work, modern numerical optimization techniques enabled high-precision separation of the overlapping bands and quantitative verification of the scattering process. In addition, analysis combining the extracted ML band with previously unpublished ML spectra measured simultaneously revealed that the scattering process causes a remarkable reduction in the effective temperature of the biexciton system. This evaporative-cooling-like phenomenon associated with the scattering band demonstrates a new aspect of many-body interactions in dense exciton systems.