Abstract
Laser cooling of a solid is achieved when a coherent laser illuminates the material in the red tail of its absorption spectrum, and the heat is carried out by anti-Stokes fluorescence of the blue-shifted photons. Solid-state laser cooling has been successfully demonstrated in several materials, including rare-earth-doped crystals and glasses. Here we show the net cooling of high-purity Yb-doped silica glass samples that are fabricated with low impurities to reduce their parasitic background loss for fiber laser applications. The non-radiative decay rate of the excited state in Yb ions is very small in these glasses due to the low level of impurities, resulting in near-unity quantum efficiency. We report the measurement of the cooling efficiency as a function of the laser wavelength, from which the quantum efficiency of the Yb-doped silica is calculated.
Highlights
Laser cooling of a solid is achieved when a coherent laser illuminates the material in the red tail of its absorption spectrum, and the heat is carried out by anti-Stokes fluorescence of the blue-shifted photons
Where λf is the mean wavelength of the escaped fluorescence, λp is the laser pump wavelength, ηext is the external quantum efficiency, and ηabs is the absorption efficiency; they are defined as ηext ηeWr W tot
ΑrðλpÞ αrðλpÞ þ αb where Wr, Wnr, and Wtot are radiative, non-radiative, and total decay rates of the excited state, respectively, and ηe is the fluorescence extraction efficiency. αb is the background absorption coefficient and αr is the resonant absorption coefficient. Both ηext and ηabs must be very close to unity to observe laser cooling, because λp cannot be much longer than λf to keep αr(λp) sufficiently large for a near-unity value of ηabs
Summary
Laser cooling of a solid is achieved when a coherent laser illuminates the material in the red tail of its absorption spectrum, and the heat is carried out by anti-Stokes fluorescence of the blue-shifted photons. The perennial failure in the laser cooling of RE-doped silica glass made one wonder whether it would ever be possible for the Yb-doped silica glass to have a sufficiently small non-radiative decay rate of the Yb excited-state population to achieve a nearunity internal quantum efficiency. Advancements in solid-state laser cooling may eventually lead to all-optical compact and vibration-free cryocoolers that can reduce the thermal noise in semiconductor-based single-photon detectors or quantum information processing circuits[5] Another important application is for radiation-balanced fiber lasers (RBFLs), where the cooling from anti-Stokes fluorescence offsets the waste heat generation in the laser[22,23,24,25,26,27]. We will determine the wavelength dependence of the cooling efficiency of our Ybdoped silica glass samples as a function of the pump laser wavelength to observe their transition from the heating to cooling regime
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