Abstract:

According to the four-energy level theoretical model, the background absorption of a cooled sample is a key parameter affecting laser cooling performance. To investigate the relationship between background absorption coefficient and the temperature of the cooled sample, such as Yb³⁺:LuLiF₄(LLF), a low-temperature laser-induced temperature modulation spectrum(LITMoS) experiment was designed to obtain the background absorption coefficient of the crystal at low temperatures. Based on the heat load theory, the experimental formula for the cooling efficiency under low-temperature conditions was derived, and the source of heat load was theoretically analyzed, demonstrating that contact conduction is the main source of heat load for low-temperature LITMoS experimental samples. In the experiments, a liquid nitrogen cryostat and a specially designed cold finger structure were used to control the temperature of the crystal, thereby obtaining the temperature drop of the crystal by non-contact temperature measurement using time-valve-differential fluorescence spectroscopy thermometry. A feasibility analysis was conducted on the experimental design scheme, and the sample cooling efficiency at the corresponding wavelength was calculated. The results show that the experimental design scheme can measure the functional relationship between the background absorption coefficient and temperature under low-temperature conditions. The test results of refrigeration efficiency were in accordance with the prediction of the theoretical model of optical refrigeration.

Key words: four-level theory, background absorption coefficient, cooling efficiency, low-temperature LITMoS(laser-induced temperature modulation spectrum) experiment, thermal load