Optical Refrigeration of Crystals Doped with Yb3+ and Ti3+ Ions

Over the past two and a half decades, advancements in the field of laser cooling of solids have provided a way to reach cryogenic temperatures in rare-earth-doped glasses and crystals through the emission of up-converted fluorescence. However, significant factors still limit cooling, such as unavoidable heating due to unintended impurities and the slowness of cooling, which stems from the forbidden nature of transitions currently utilized to implement refrigeration. The primary contributions of this thesis are new experimental approaches that overcome impurity heating and speed up cooling by saturation of background absorption and the use of allowed transitions. Net cooling in various concentrations of Yb3+:KYW was demonstrated for the first time, and a maximum cooling of 10 K was achieved in air at room temperature. Using differential luminescence thermometry (DLT) and thermal lens spectroscopy (TLS), the efficiency of anti-Stokes fluorescence (ASF) cooling in a 10%Yb3+:LiYF4 crystal was shown to double at 1020 nm when background impurity absorption was partially saturated. The measured cooling efficiency was independent of pump intensity at a wavelength of 1000 nm in this crystal, an observation that was exploited to determine that the saturation intensity of impurities (thought to be iron-related) had the unexpectedly low value of 2.6x10^4 W/cm^2. Theoretically, the use of high intensity to reduce the background heat load lowers the minimum temperature achievable with laser cooling, whenever the saturation intensity of background absorption is lower than that of the coolant ions. Thus, in some materials, saturation of the background impurity absorption can address the main limitation to optical refrigeration, providing a post-growth method for improving ASF cooling performance. Laser cooling of a solid on an electric dipole (ED) allowed transition was also observed for the first time in this work. TLS was performed for pi- and sigma-polarized excitation in a 0.037%Ti3+:Al2O3 crystal characterized by low absorption in the Ti-pair band at 780 nm. Cooling occurred at 30 discrete resonances in the range 760-975 nm. These resonances were assigned to purely electronic or phonon-assisted transitions of perturbed Ti sites in the host lattice. Predicted and observed wavenumbers of the resonances differed by less than 0.08% for pi-polarized resonances and less than 0.05% for sigma-polarized resonances on average. Transitions that did not involve vibrational sidebands occurred at the same positions for both polarizations within experimental error. The cooling rate on the allowed transitions of Ti3+:Al2O3 was estimated to be more than 10^4 times higher than on the forbidden transitions of Yb3+ ions in LiYF4. The potential role of quantum effects on laser cooling was also investigated theoretically in this thesis. In a 3-level system modeled after Ce3+ ions, density matrix calculations on the long wavelength transition revealed Fano interference when excited state coupling was present. Induced transparency appeared on the short wavelength transition without requiring any coupling. This implies that self-cooled lasing without inversion is expected in such systems. These results open the door to future use of allowed transitions for rapid optical refrigeration, tunable radiation-balanced lasers based on transition metal ions like Ti3+, or Raman cooling for the purpose of reaching l-He temperatures. In addition, the demonstrated cooling of sapphire should enable vibration-free, cryogenic cooling of III-V nitride semiconductor sensors in space.PhDElectrical and Computer EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/175709/1/lbandre_1.pd