Laser cooling of a solid by 65 K starting from room temperature

In vacuo laser cooling of a solid from 301 to 236 K has been demonstrated. The sample consists of a cladded, multimode optical fiber fabricated from the fluorozirconate glass ZBLANP; the fiber’s waveguiding core is doped with 1 wt. % ${\mathrm{Yb}}^{3+}$ ions. Cooling of the host glass results from anti-Stokes fluorescence of ${\mathrm{Yb}}^{3+}$ following optical pumping in the long-wavelength tail of the ion’s absorption spectrum $\left(\lambda =1015 \mathrm{nm}\right)$. Measurement of the time constant for equilibration of the sample temperature is in excellent agreement with an analytic calculation of this quantity.