  Photothermal heating represents a major constraint that limits the
performance of many nanoscale optoelectronic and optomechanical devices
including nanolasers, quantum optomechanical resonators, and integrated
photonic circuits. Although radiation-pressure damping has been reported to
cool an individual vibrational mode of an optomechanical resonator to its
quantum ground state, to date the internal material temperature within an
optomechanical resonator has not been reported to cool via laser excitation.
Here we demonstrate the direct laser refrigeration of a semiconductor
optomechanical resonator >20K below room temperature based on the emission of
upconverted, anti-Stokes photoluminescence of trivalent ytterbium ions doped
within a yttrium-lithium-fluoride (YLF) host crystal. Optically-refrigerating
the lattice of a dielectric resonator has the potential to impact several
fields including scanning probe microscopy, the sensing of weak forces, the
measurement of atomic masses, and the development of radiation-balanced
solid-state lasers. In addition, optically refrigerated resonators may be used
in the future as a promising starting point to perform motional cooling for
exploration of quantum effects at mesoscopic length scales,temperature control
within integrated photonic devices, and solid-state laser refrigeration of
quantum materials
