7 March 2007 Localized electrons and phonon-induced laser cooling of solids
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Abstract
Under appropriate conditions absorption of light by a solid can initiate a process by which it is cooled. Here, two schemes for laser cooling via localized electrons are addressed. The first scheme utilizes two states of a localized center. In this two-level scheme, the cooling process is initiated with photon absorption in the low-energy tail of a localized state's strain-broadened absorption spectrum. The subsequent atomic relaxation transfers energy of especially large vibratory atomic strains into electrical energy that is then extracted via photon emission. Cooling can occur at elevated temperatures but is suppressed as the temperature is lowered. The second scheme involves three energy levels of a localized center. Cooling is facilitated when i) the photo-excitation of an electron promotes it to the lower of the two upper levels followed by ii) its electron-phonon-induced promotion to the upper-most level and the subsequent iii) return of the electron to its initial state via emission of a photon of higher energy than that of the absorbed photon. However, competing relaxation processes contribute to heating. The net cooling power is calculated. Heating predominates at low temperatures. Significant cooling at elevated temperatures requires satisfying very restrictive conditions. Among these: i) the energy separation between two highest states must be very small; ii) the degeneracy of the highest state must exceed that of the state below it, and; iii) the effective electron-phonon interaction, responsible for energy levels' Stokes shifts, must be exceptionally weak. Different avenues to promising systems to achieve laser cooling are identified.
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David Emin, David Emin, } "Localized electrons and phonon-induced laser cooling of solids", Proc. SPIE 6461, Laser Cooling of Solids, 64610C (7 March 2007); doi: 10.1117/12.707209; https://doi.org/10.1117/12.707209
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