Laser Cooling of a Solid by 16 K Starting from Room Temperature

An ${\mathrm{Yb}}^{3+}$-doped optical fiber is laser cooled in vacuo from 298 to 282 K. Cooling results from anti-Stokes fluorescence of the ytterbium ions after optical pumping at a wavelength of 1015 nm. The sample temperature is deduced from the emission spectrum, which is sensitive to the populations in the excited-state multiplet of the ions. The temperature change is limited by the coupling between the fiber and ambient blackbody radiation, as confirmed when samples suddenly exposed to the pump laser are found to exponentially relax towards thermal steady state with the expected time constants.