Research On Novel Mechanisms For Laser Cooling Of Rare-earth Doped Fluoride Crystals

Laser cooling of solids,or optical refrigeration refers to the refrigeration technology based on rare-earth doped solids or semiconductors,via laser-induced anti-Stokes fluorescence.Due to its advantages of compactness,no vibrations,no electromagnetic interference and no contamination,optical refrigeration can be potentially applied to local cooling of space-based electronic devices.In the past two decades,this research had received intensive attention,and had systematically developed.To realize its application,the cooling performance of optical refrigeration waits to be improved,and the operating wavelength region still needs to be broadened.In this thesis,we carry on a series of theoretical studies on novel mechanisms of optical refrigeration,based on the rare-earth doped fluoride crystal.To this end,the traditional cooling model is first improved to describe the case of Ho³⁺ optical refrigeration,and then several modified cooling schemes are provided,such as efficient and enhanced co-pumping Ho³⁺ optical refrigeration,energy transfer enhanced laser cooling of rare-earth co-doped solids,and double-pulse excitation scheme for efficient laser cooling of solids via superradiance.First,according to the analysis on the emission spectrum of a Ho³⁺ doped fluoride crystal,we generalize the traditional cooling model,so it can be used to describe precisely the case of Ho³⁺ optical refrigeration.Based on the improved cooling model,the criterion f'or upconversion-assisted cooling is derived.With reference to the spectrum parameters of the Ho³⁺:YLiF4 crystal,the cooling performance of Ho³⁺ optical refrigeration is numerically simulated.The pumping intensity and doped concentration dependent cooling power density and cooling efficiency are provided with different background absorption coefficients;The maximum cooling power density with various of pumping wavelengths,and the corresponding optimal doped concentration are also presented.Second,the co-pumping enhanced Ho³⁺ optical refrigeration is studied.According to the energy level structure of Ho³⁺ ions,the heat-producing aspect of each transition channel is analyzed.We illustrate how the co-pumping scheme improves the cooling performance of Ho³⁺ optical refrigeration.Then,the co-pumping Ho³⁺ optical refrigeration is theoretically modeled,and the expressions of the cooling power density and cooling efficiency are deduced.The application range of the co-pumping scheme is also pointed out in consideration of background absorption.With reference to the spectrum parameters of the Ho³⁺::YLiF4 crystal,the pumping intensities dependent cooling power density and cooling efficiency with various of doped concentrations are numerically simulated;The maximum cooling power density and the corresponding optimal pumping intensities are also obtained through optimization procedure.Third,we study the energy transfer enhanced laser cooling of solids.By comparing the emission spectrum of Ho³⁺ doped and Ho³⁺,Tm³⁺ co-doped fluoride crystals,the enhancing ratio in the cooling efficiency induced by Ho³⁺?Tm³⁺ energy transfer is roughly estimated.The energy transfer enhanced laser cooling of Ho³⁺,Tm³⁺ co-doped solid is modeled according to the rate equation theory of energy transfer,and the analytical expressions of the cooling power density and cooling efficiency are deduced.Then,the dependencies of the cooling ability on the energy transfer parameters,doped concentration,pumping intensity and pumping wavelength are analyzed.With reference to the spectrum parameters of the Ho³⁺Tm³⁺:YLiF4 crystal,the pumping intensity dependent cooling power density and cooling efficiency of the Ho³⁺-Tm³⁺ optical refrigeration system are numerically simulated,andcomparison is made with the traditional Ho³⁺ optical refrigeration case.By using the equilibrium parameter of energy transfer,the temperature dependence of energy transfer induced cooling enhancement is also provided.Last,we study the double pulse excitation scheme for efficient laser cooling of solids via superradiance?LCSSR?.The principle of LCSSR is reviewed to reveal the advantages of the double-pulse excitation scheme over the CW-pulse excitation scheme.The analytically expressions of the atomic density matrices are derived from the dynamic equantion of a closed ? level system.With reference to the spectrum parameters of the Yb³⁺:YLiF4,Tm³⁺:YLiF4 and Ho³⁺:YLiF4 crystals,the temperature dependent cooling powers of LCSSR via the two excitation schemes are numerically simulated.By using the area theorem,the proper sample length for LCSSR is estimated.The influence of choosing different pumping levels on the cooling performance is discussed,and the optimal selection guidance of pumping levels for LCSSR is also deduced with rational approximations.