Optical refrigeration of semiconductors is a unique mechanism to remove thermal energy from a semiconductor during emissions of anti-Stokes photoluminescence (PL).Such process is different from a regular Stokes PL when the energy of the exciting photons is higher than the band gap of a semiconductor.During anti-Stokes PL, the energy of the incoming laser photons is lower than the band gap energy, and necessary energy deficiency is supplied by thermal energy. Net cooling is observed if the overall energy removed from the system with the emitted photons is higher than the energy released during non-radiative recombination of electron-hole excitation. The practical realization of such mechanism may potentially lead to a number of exciting applications, such as sub 10 K optically pumped cryocoolers.
Despite the simplicity of the underlying principle, actual realization requires remarkably high external PL quantum efficiency (near unity).This entails very high internal PL quantum efficiency and PL extraction efficiency.Low defect bulk semiconductors such as GaAs historically received a lot of attention in attempts to achieve net laser cooling; however, no cooling was observed to date. A major obstacle with such experiments was generally low extraction efficiency from bulk semiconductor due to high refractive index.Another attractive system that may become a core of future optical semiconductor based cryocooler is perovskite nanocrystals. In CsPbBr3 nanocrystals, recent advances in synthesis have led to the achievement of near unity quantum efficiency. This advance, together with absent light trapping inside of a nanocrystal, may lead to the eventual realization of laser cooling in semiconductors.