In this work we present a comprehensive review of recent work carried out by our group in the field of optical refrigeration of Nd-doped solids. Several infrared thermography measurements in Nd-doped KPb₂Cl₅ crystals and micro-powders both above and below the barycentre of the ⁴F_3/2 are presented. These include some of our most recent ones obtained by employing a novel technique that allows one to perform differential temperature measurements. The role of both the direct anti-Stokes absorption processes and those assisted by either excited state absorption or energy transfer upconversion in the cooling process is discussed.

Since recent demonstration of cryogenic optical refrigeration, a need for reliable characterization tools of cooling performance of different materials is in high demand. We present our experimental apparatus that allows for temperature and wavelength dependent characterization of the materials' cooling efficiency and is based on highly sensitive spectral differencing technique or two-band differential spectral metrology (2B-DSM). First characterization of a 5% w.t. ytterbium-doped YLF crystal showed quantitative agreement with the current laser cooling model, as well as measured a minimum achievable temperature (MAT) at 110 K. Other materials and ion concentrations are also investigated and reported here.

We report on the first observation of intracavity laser cooling inside of a vertical external-cavity surface-emitting laser (VECSEL). A Yb:YLF crystal is placed under Brewster angle inside the cavity of an InGaAs quantum well VECSEL emitting around 1030 nm. With the crystal in air, we observed cooling by about 0.5 degrees. By placing the sample and cavity end mirror inside a vacuum chamber, with the window also at Brewster angle to the laser mode, cooling by 20 degrees has been realized. Furthermore, the development of a compact and efficient integrated cryocooler device is underway.

An all fiber approach to optical cooling is being investigated experimentally and theoretically using Tm-doped fiber laser and Tm-doped fiber cooler. A single mode, high efficiency and high power Tm-doped fiber laser is used to pump at the absorption edge of Tm-doped fiber coolers, one made by germanate and the other by tellurite glasses. The glass characterization shows that the quenching effect, which is negative for cooling processes in the fiber, in germanate glass is much stronger than that in tellurite glass. The preliminary results of experiments indicate cooling effects could occur in the fiber, but net cooling in the system has not been achieved. A theoretical framework aimed at understanding the nature of cooling in this laser cooling system has been developed which shows that the temperature in the sample could increase even if the fiber core is indeed cooling. The details of the temperature dynamics depend on many factors such as background loss and absorption of scattered light by the heat spreader.

We describe experiments on the laser cooling of both helium-rubidium and argon-rubidium gas mixtures by collisional redistribution of radiation. Frequent alkali-noble gas collisions in the ultradense gas, with typically 200 bar of noble buffer gas pressure, shift a highly red detuned optical beam into resonance with a rubidium Dline transition, while spontaneous decay occurs close to the unshifted atomic resonance frequency. The technique allows for the laser cooling of macroscopic ensembles of gas atoms. The use of helium as a buffer gas leads to smaller temperature changes within the gas volume due to the high thermal conductivity of this buffer gas, as compared to the heavier argon noble gas, while the heat transfer within the cell is improved.

A new method is analyzed for 3-D cooling of solids based on near-resonant, stimulated Raman scattering on a narrowband transition together with optical pumping on a broadband transition. Estimates of achievable cooling rates indicate that Raman cooling offers significant improvement over anti-Stkes fluorescence cooling at cryogenic temperatures and should enable the attainment of sub-Kelvin termperatures, starting from ambient conditions in bulk samples. Also the method is not restricted to ions with small Stokes shifts, and should therefore lead to diversification of the material platforms used for optical refrigeration.

We present a scheme, in which colloidal lead-salt PbSe QDs doped in a glass host are used for laser cooling with anti- Stokes fluorescence. The relatively short (microsecond range) lifetime of the excited level of the PbSe QD allows the cooling process to be accelerated, and new materials with higher phonon energy to be used as hosts, which are normally considered unsuitable for cooling with rare-earth ions. The considerable increase (by ~10⁴) in the absorption cross section of PbSe QD in comparison with the absorption cross section of the rare-earth ions doped in glasses or crystals increases the efficiency of the cooling process considerably, lowering the pump power requirements.

Fluoride glasses are the only material that transmit light from ultraviolet to mid-infrared and can be drawn into industrial optical fibers. The mechanical and optical properties of new indium fluoride glass fibers have been investigated. Multimode fiber 190 microns, has very high mechanical strength greater than 100 kpsi and optical loss as low as 45 dB/km between 2 and 4 microns. Unlike chalcogenide glass fibers, indium fluoride fiber has a wide transmission window from 0.3 to 5.5 microns without any absorption peak. Indium fluoride glass fibers are the technology of choice for all application requiring transmission up to 5 micron such as infrared contour measure (IRCM) and chemical sensing. Furthermore, Indium fluoride glasses have low phonon energy and can be heavily doped and co-doped whit rare-earth elements. Therefore they are very promising candidates for infrared fiber lasers.

Based on a highly sensitive dierential spectroscopy technique, we present a non-contact method of optical- scanning thermal imaging with a possibility of sub-thermal-wavelength spatial resolution. This technique is general and can also be applied to imaging of strain or impurity distributions at the surfaces of semiconductors. This procedure is particularly well suited for near-eld imaging and investigation of thermal transport on the nanoscale. Applications to optical refrigeration in semiconductors are discussed.

Optical refrigerator is based on an optical heat removal from a cooling element by satisfying conditions of anti-Stokes fluorescence. This imposes a challenge of optically shielding a payload from the anti-Stokes radiation. Therefore, a link between the cooling element and a payload (the thermal link) has to exhibit high thermal conductivity while blocking the fluorescence and scattered laser light in a non-absorbing manner. In this paper we examine photon blockade structures ranging from textured kinks to hybrid systems involving light-shedding geometries and semiconductor-based epitaxially-grown distributed Bragg mirrors.

Noncontact temperature measurements with large (thermal) dynamic range are desirable in many applications. Aside from interferometric techniques, fluorescence intensity and spectral shape have been exploited in the past for sensitive thermometry in luminescent materials. Here, we present a novel method that utilizes the polarization-sensitive reflection and/or transmission of light from (through) an optical material without relying on any fluorescence. A balanced photodetector will measure the difference signal corresponding to two orthogonal polarization states with high singal-tonoise ratio. Temperature resolution of 5 mK have been demonstrated.