AFRL-AFOSR-JP-TR-2016-0067 Laser Cooling of II-VI Semiconductors

The breakthrough of laser cooling in semiconductor has stimulated strong interest in further scaling up towards practical optical refrigeration. The challenge is the stoichiometric defect in bulk crystal which introduces mid-gap states that manifest as broad-band long-wavelength emission. In this project, we have developed optical floating zone method to grown millimeter-scale CdS bulk crystals. The steady-state spectroscopy measurement suggested that such defect emission prevailing in commercial CdS wafer is absent in our as-grown crystal, attributing to the low-temperature synthesis enabled by the optical heating method compared with conventional Bridgeman method. Further research is on-going to investigate the decay dynamics of the exciton emission. In addition, this project touched upon the laser cooling of inorganic-organic perovskite crystals, which are more immune to mid-gap defects.