We demonstrate first cryogenic operation in a Ytterbium doped YLF crystal by means of an optical refrigeration. We have achieved cooling to 155 Kelvin absolute temperature with heat lift of 90 mW, exceeding performance of multi-stack thermo-electric coolers. This progress was possible by pumping the system near the Stark-manifold resonance of highly pure Yb:YLF crystal and careful thermal management in the cooling experiment. Detailed spectroscopic analysis demonstrated that cooling to 110 Kelvin is currently possible if pumped exactly on that resonance.

The total crystal-field splitting of the 2F7/2 ground-state multiplet of Yb3+ critically determines the cooling efficiency of an optical refrigerator. Crystals with a small 2F7/2 splitting maintain a sizeable thermal population of the initial state of the pumped crystal-field transition at low temperatures, leading to a workable laser-cooling efficiency in the application-relevant cryogenic regime below 120 K. A comprehensive review of the crystal-field splitting of (2S+1)L(J) multiplets in rare-earth-doped fluoride crystals is presented. The concept of crystal-field strength is used to predict the splitting of the 2F7/2 ground-state multiplet from other fluoride crystals doped with other rare earth ions. The analysis correctly predicts the typical 350-450 cm-1 total splitting of 2F7/2 in fluorides, but the accuracy of the method is found to be rather limited. LiKYF5, K2YF5, and Cs2KLnF6 are predicted to have large 2F7/2 splittings that are unfavorable for laser cooling. KY3F10, YLiF4, LuLiF4, and GdLiF4 are among the group of crystals expected to have small 2F7/2 splittings and currently appear to be the most promising hosts for laser cooling with Yb3+. LiBiF4:Yb3+ may have a <350 cm-1 2F7/2 splitting and warrants further study.

High-power lasers (Raman and traditional solid-state) suffer from heat generated by the quantum defect, which deteriorates their performance. The radiation-balanced technique was proposed to solve the problem in the case of solid-state lasers, and the intrinsic heat-mitigation technique, which relies on coherent anti-Stokes Raman scattering (CARS) was proposed to solve the problem of heat generation in the high power Raman laser. Unfortunately radiated energy increases only linearly with the length of the laser medium in both schemes. We propose a radically new approach to solve the problem of heat generation in lasers. We have considered athermal fiber lasers, where rare-earth (RE) ions based refrigeration sources are incorporated in the body of the devices. This new technique, which has been developed for Raman and traditional solid-state lasers, ensures the exponential growth of radiation along the laser medium and provides almost athermal operation.

Spectroscopic characterization of YLF crystal doped with Yb reveals the performance potential of this material in laser cooling applications. Temperature-dependent spectra allow us to estimate the minimum achievable temperature and the parasitic background absorption.

We demonstrate cooling of a 2 micron thick GaAs/InGaP double-heterostructure to 165 K from ambient using an all-solid-state optical refrigerator. Cooler is comprised of Yb³⁺-doped YLF crystal, pumped by 9 Watt near E4-E5 Stark manifold transition.

The first experimental demonstration of local internal and bulk optical cooling in samples of Nd-doped KPb₂Cl₅ crystals and Nd-doped KPb₂Cl₅ nanocrystalline powders upon laser excitation between the ⁴F_3/2 and ⁴F_5/2 manifolds is reported. The possibility of controlling the dynamics of the bulk optical cooling process by adequately tuning the excitation wavelength is also demonstrated.

The state of current research in laser cooling of semiconductors is reviewed. Emphasis is placed on the characterization of external quantum efficiency and absorption efficiency in GaAs/InGaP double heterostuctures. New experimental results will be presented that characterize device operation as a function of laser excitation power and temperature. Optimum carrier density is obtained independently and used as a screening tool for sample quality. The crucial importance of parasitic background absorption is discussed.

Recently, interest in optical refrigeration of semiconductors, which is based on photo-luminescence up-conversion, has drawn extensive attention both theoretically and experimentally. Theoretical descriptions often treat spatially homogeneous semiconductors, because of their conceptual simplicity. In typical experiments, however, semiconductors are usually heterostructures designed to reduce non-radiative recombination at the sample's surface. In particular, GaAs/GaInP structures have been used in experiments. In these structures, the GaAs layers are usually unintentionally p-doped, while the surface layers of GaInP are n-doped. Recent measurements of the non-radiative recombiation lifetime yielded values in the desirable inverse microsecond regime, and it is believed that the non-radiative recombination processes occur mainly at the heterostructure interfaces and its surfaces. For this reason, it is important to know the spatial density distribution of the excited carriers. Furthermore, photo-luminescence and carrier lifetime measurements are not spatially resolved, and therefore it is desirable to have a theory that can simulate lifetime measurements using the spatially varying density profile as an input. We have developed such a theory, using the simplifying assumption of quasi-thermal equilibrium (at each time during the photo-luminescence decay process). Using this theory, we are able to relate measurable (i.e. spatially averaged) lifetime measurements to the underlying non-radiative decay processes that, in our simulations, occur predominantly at the GaAs/GaInP interface. From this, we find that spatial inhomogeneities in the carrier density, which are most pronounced at low optical excitation powers, can have appreciable effects on the interpretation of the lifetime measurements.

We characterize high quantum efficiency double GaAs/InGaP heterostructures used in semiconductor laser cooling. To identify potential samples for laser cooling, measuring the nonradiative recombination rate coefficient is necessary. We describe a technique called power dependent photoluminescence measurement, which when combined with timeresolved photoluminescence lifetime determines the nonradiative recombination coefficient.

We have recently proposed a solid state heat pump based on photon mediated heat transfer between two large-area light emitting diodes (LEDs) coupled by the electromagnetic field and enclosed in a semiconductor structure with a nearly homogeneous refractive index. We have shown that ideally the thermophotonic heat pump (THP) allows heat transfer at Carnot efficiency and studied the factors that in practice limit the efficiency. In this paper we link the previously parametrized loss mechanisms to the observed nonradiative loss mechanisms in LEDs and study the requirements of observing electroluminescent cooling by using the THP structure. Our results show that a very simple structure that optically couples two LEDs and fabricated using current standard fabrication methods should enable electroluminescent cooling.