Abstract
In previous work,1 we demonstrated the laser cooling of an impurity-doped insulator by 0,3 K below room temperature. The cooling effect takes advantage of anti-Stokes photoluminescence of laser-excited Yb3+ impurities and relies on the near-unit fluorescence quantum efficiency for this ion in a highly purified ZrF4-BaF2-LaF3-AlF3-NaF-PbF2 (ZBLANP) glass host. More specifically, long-wavelength optical pumping between high-lying levels of the Yb3+ ion’s 2F7/2 ground electronic-state manifold and low-lying levels of the 2F5/2 excited-state manifold (cf. Fig. 1) yields short-wavelength emission following the rapid non-radiative rethermalization of the population within both manifolds. In this experiment, the sample consisted of a rectangular parallelepiped of dimensions 2.5 mm × 2.5 mm × 6.9 mm suspended in a vacuum chamber. In more recent experiments,2 use of a 250-μm-diameter optical fiber reduces the surface area of the sample by more than a factor of 10, thereby yielding a commensurate decrease in the net radiative heat load arising from room-temperature Planck radiation. Temperature differences between the sample and the room- temperature environment as high as 21 K have now been measured for a pump power of ~1.3 W at 1015 nm.
© 1998 Optical Society of America
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