Open-aperture Z-scan study for absorption saturation: accurate measurement of saturation intensity in YLF:Yb for optical refrigeration.

Knowledge of saturation intensity of gain or absorption plays a fundamental role in a variety of applications ranging from lasers to many nonlinear optical processes. Here, we present an analytical expression for open-aperture Z-scan transmission for accurately measuring the saturation intensity in the low absorbance samples but at arbitrary pump intensities. We exploit this formalism to investigate the absorption saturation of : (YLF:Yb) in the anti-Stokes excitation region for optical refrigeration at high pump intensities.

Optical refrigeration: the role of parasitic absorption at cryogenic temperatures.

Optical cooling of a YLF:Yb single crystal to 87 K, well below the minimum achievable temperature predicted from existing theory, has been observed. This discrepancy between theory and data has motivated us to revisit the current model of optical refrigeration, in particular the critical role of parasitic background absorption. Challenging experiments that measured the cooling efficiency as a function of temperature reveal that the background absorption coefficient decreases with temperature, resulting in a significant enhancement of the cooling efficiency at cryogenic temperatures.

Tm-doped crystals for mid-IR optical cryocoolers and radiation balanced lasers.

We report the complete characterization of various cooling-grade Tm-doped crystals including, to the best of our knowledge, the first demonstration of optical refrigeration in Tm:YLF crystals. Room temperature laser cooling efficiencies of 1% and 2% (mol) Tm:YLF and 1% Tm:BYF crystals at different excitation polarizations are measured, and their external quantum efficiency and background absorption are extracted. By performing detailed low-temperature spectroscopic analysis of the samples, global minimum achievable temperatures of 160 to 110 K are estimated.

Solid-state optical refrigeration to sub-100 Kelvin regime.

Since the first demonstration of net cooling twenty years ago, optical refrigeration of solids has progressed to outperform all other solid-state cooling processes. It has become the first and only solid-state refrigerator capable of reaching cryogenic temperatures, and now the first solid-state cooling below 100 K. Such substantial progress required a multi-disciplinary approach of pump laser absorption enhancement, material characterization and purification, and thermal management.