| Laser cooling of solids is an interesting, vagarious phenomenon of nonlinear photo-thermal transition dynamics in the field of photon-matter interaction. It is generally based on the principle of anti-Stokes fluorescence, where the emitted photons have a mean frequency higher than that of the absorbed photons. Laser cooling of solids offers a feasible approach to all-solid-state optical refrigeration technique. It has potential applications in the fields of integrated optoelectronic circuit and aerospace detecting devices due to the pollution-free, vibrationless, no electro-magnetic radiation. Recently, laser cooling has experienced rapid progress in various rare-earth (Yb,Tm and Er) doped solids,with the record of a cryogenic cooling temperature to 110K (net cooling drop ?190K from room temperature). In contrast, laser cooling of Tm doped solids has only achieved a net cooling drop of -24K, and Er doped silids exhibited the much weak cooling effects. As for Tm doped mediums,the net cooling ability has the limitations via the conventional anti-Stokes cooling scheme due to the multi-level structures of Tm ions. Following the successful progress in experiment, the fundamental theories with new cooling schemes or physical mechanisms need to be developed to provide guidance for enhancing laser cooling performance. In this paper,we present a physical scheme of upconversion-assisted anti-Stokes fluorescence cooling in Tm doped low phonon materials, in order to enhance the net cooling power and improve the cooling ability towards a lower temperature region.Based on the popular SBE theory of solid laser cooling, the up-conversion fluorescence cooling scheme is proposed for Tm doped solids, and the analytic descriptions are completed.The advantages of the up-conversion cooling scheme are discussed. The enhancement of cooling power induced by an up-conversion pumping is analyzed. The matching relationship between the ground and up-conversion pumping is given. The dominant cooling role transforming from the fundamental anti-Stokes cooling cycle to the up-conversion cooling cycle is discussed, as well as the critical problems. The enhancement effects of the up-conversion cooling scheme is remarkable to promote the net cooling power, and improve the cooling ability towards a lower temperature.By employing an ?-polarized Tm-doped YLF crystal, the up-conversion cooling characteristics and its refrigeration capacity are analyzed systematically. The behavior of light absorption and radiation in the cooling system, and the cooling spectra are simulated. The cooling temperature at the thermal equilibrium is calculated. The cooling potential of up-conversion fluorescent refrigeration is also evaluated numerically.Taking into account the optical anisotropy of YLF crystal with ?-polarized and?-polarized responses, there are different cooling performances for an YLF crystal due to the different polarization spectra. By comparing the a-polarized and ?-polarized spectra, the distinctions in the up-conversion cooling scheme,including the pump absorption and radiation dynamics, and the minimum achievable cooling temperature are simulated and discussed. The intrinsic and practical cooling limitations are also numerically evaluated to point out that which polarized crystal and the optimal pump parameters are preferable to perform the lower cooling refrigeration. The theoretical results provides a guidance and a new cooling scheme for improving the laser cooling of Tm doped solids towards a lower temperature. |
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