Laser coupling in indirect bandgap Ge nanocrystals

In this study, we present the first observation of laser cooling in an indirect bandgap semiconductor. Utilizing precise Raman measurements, we detected dominant anti-Stokes photoluminescence in germanium nanocrystals with average particle sizes ranging from 16 nm to 30 nm. These measurements were consistently reproducible in two distinct experimental setups: germanium nanocrystals in isopropyl alcohol (IPA) solution and dispersed germanium nanocrystal powder. From these observations, we inferred lattice temperatures as low as approximately 50K. This cooling effect can be attributed to several converging factors: ultra-high purity of the germanium nanocrystals, generation of a high-density electron-hole plasma, inherent degeneracy of longitudinal and transverse optical phonons in non-polar indirect bandgap semiconductors, and corresponding confinement effects. Our findings underscore the critical role of laser power density in solid-state cooling, identifying a threshold level that induces dominant anti-Stokes emission.