A fluorozirconate glass doped with trivalent ytterbium ions has been laser cooled <i>in vacuo</i> 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 Yb<sup>3+</sup> in sufficiently pure solid hosts. The sample temperatures in the experiment are determined by measurement of the Yb<sup>3+</sup> 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.
© 1998 Optical Society of America
X. Luo, M. D. Eisaman, and T. R. Gosnell, "Laser cooling of a solid by 21K starting from room temperature," Opt. Lett. 23, 639-641 (1998)