Laser cooling of a solid by 21??K starting from room temperature

A fluorozirconate glass doped with trivalent ytterbium ions has been laser cooled in vacuo from 298 to 277 K by optical pumping of the ions at 1015 nm. The cooling effect arises from anti-Stokes fluorescence of the laser-excited ions and by virtue of the near-unit fluorescence quantum efficiency for ${\mathrm{Yb}}^{3+}$ in sufficiently pure solid hosts. The sample temperatures in the experiment are determined by measurement of the ${\mathrm{Yb}}^{3+}$ emission spectrum; the value of the observed temperature change from room temperature as a function of pump wavelength is successfully explained in terms of a simple two-level model that includes the effect of optical saturation.