Optical refrigeration: perspectives for the 2 micron region and new architectures for 1 micron applications

This dissertation presents optical refrigeration experiments conducted on fluoride crystal samples of LiYF4 (YLF) doped with rare earth ions, with the aim to improve the overall achievable cooling power. Crystal boules of Ho:YLF were grown in the laboratory at different doping percentages (0.8% and 1%), from which multiple samples were obtained. The optical properties of these samples were evaluated: absorption and emission spectra were measured at room temperature, and also at low temperatures, down to 100 K. A tunable laser source in the 2 μm range was developed in collaboration with Politecnico di Milano, to be used for LITMoS experiments on Ho:YLF. After testing 0.8% Ho:YLF and 1% Ho:YLF samples at 2070-2080 nm, no cooling was observed. Cooling by 0.4 K was observed in a 1% Tm:YLF sample. Finally, a new architecture for optical refrigeration was proposed for the first time: by placing two active Yb:YLF crystal samples in contact through an undoped YLF sample, it was demonstrated that the overall cooling power could be increased by pumping both samples simultaneously with a 1 μm laser.