  Laser cooling of matter through anti-Stokes photoluminescence, where the
emitted frequency of light exceeds that of the impinging laser by virtue of
absorption of thermal vibrational energy, has been successfully realized in
condensed media, and in particular with rare earth doped systems achieving
sub-100K solid state optical refrigeration. Studies suggest that laser cooling
in semiconductors has the potential of achieving temperatures down to ~10K and
that its direct integration can usher unique high-performance nanostructured
semiconductor devices. While laser cooling of nanostructured II-VI
semiconductors has been reported recently, laser cooling of indirect bandgap
semiconductors such as group IV silicon and germanium remains a major
challenge. Here we report on the anomalous observation of dominant anti-Stokes
photoluminescence in germanium nanocrystals. We attribute this result to the
confluence of ultra-high purity nanocrystal germanium, generation of high
density of electron-hole plasma, the inherent degeneracy of longitudinal and
transverse optical phonons in non-polar indirect bandgap semiconductors, and
commensurate spatial confinement effects. At high laser intensities, laser
cooling with lattice temperature as low as ~50K is inferred.
