  Laser cooling of a solid is achieved when a coherent laser illuminates the
material in the red tail of its absorption spectrum, and the heat is carried
out by anti-Stokes fluorescence of the blue-shifted photons. Solid-state laser
cooling has been successfully demonstrated in several materials, including
rare-earth-doped crystals and glasses. Silica glass, being the most widely used
optical material, has so far evaded all laser cooling attempts. In addition to
its fundamental importance, many potential applications can be conceived for
anti-Stokes fluorescence cooling of silica. These potential applications range
from the substrate cooling of optical circuits for quantum information
processing and cryogenic cooling of mirrors in high-sensitivity interferometers
for gravitational wave detection to the heating reduction in high-power fiber
lasers and amplifiers. Here we report the net cooling of high-purity Yb-doped
silica glass samples that are primarily developed for high-power fiber laser
applications, where special care has been taken in the fabrication process to
reduce their impurities and lower their parasitic background loss. The
non-radiative decay rate of the excited state in Yb ions is very small in these
glasses due to the low level of impurities, resulting in near-unity quantum
efficiency. We report the measurement of the cooling efficiency as a function
of the laser wavelength, from which the quantum efficiency of the silica glass
is calculated.
