Spherical cavity-mode laser with self-organized CuCl microspheres

We report lasing of exciton–polaritons in semiconductor microspheres with spherical cavity modes under subpicosecond ultraviolet laser excitation at low temperature. Small bulk crystals of CuCl are melted and formed into fused microspheres with diameters ranging from a few to several tens of micrometers. The threshold for lasing is 1 nJ/pulse, which corresponds to ${10}^9$ photons/pulse. We found that the biexciton-to-longitudinal-exciton transition is responsible for the lasing.