Optical refrigeration under laser and solar irradiation
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Abstract
The concept of optical refrigeration dates back to 1929, when Pringsheim recognized that thermal energy associated with the translational degrees of freedom of isolated atoms could be reduced by the process of anti-Stokes fluorescence. Optical refrigeration of a solid was first experimentally demonstrated in 1995 with the Ytterbium-doped fluorozirconate glass by Epstein and his team and since then this invigorating field has garnered much scientific interest for development of an all optical refrigerator. The present works discusses the recent candidate materials including crystals, semiconductors, and ionically doped glasses. Cooling processes and necessary conditions for cooling are outlined, and general thermodynamic limitations are discussed.
10% wt. Ytterbium doped Yttrium Lithium Fluoride (Yb+3:YLF) is chosen as the candidate active material. The Carnot efficiency for laser and sun-light as a pump source is evaluated using a narrow-band approximation outlined by Stephen and his team. A quantum-mechanical cooling model based on Epstein and his team, is developed. In the proposed system, the candidate material is placed on a magnetically suspended platform inside a vacuum chamber and illuminated with laser light with the appropriate wavelength in the near infrared region. The dynamics of important cooling parameters are simulated and studied. The cooling effects due to radiative relaxation compete with the heating effects due to parasitic absorption and non-radiative relaxation but net cooling is observed confirming validity of light source and material parameter selection.
In addition to laser, the conventional source of pump radiation, sun-light as a pump input to the quantum-mechanical model is simulated and the effects on the cooling power and efficiency are studied. To enhance the energy efficiency of the system, fluorescence recovery schemes using photovoltaics are built and studied. Suggestions for experimental realization are given. The developed model can be base for designing a practical optical refrigeration system for laser and sun-light based optical sources.