This PDF file contains the front matter associated with SPIE Proceedings Volume 7228, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Laser cooling of a semiconductor has been an elusive but highly desirable goal for several years. Although it is theoretically possible, tedious and often time-consuming sample preparation, processing and testing has slowed the progress on the experimental end. The work presented here focuses on a new approach to the first step, the growth of high quality starting samples by molecular beam epitaxy (MBE). MBE is believed to have an inherent advantage over chemical vapor deposition techniques since typically material with higher purity can be grown by MBE, thereby reducing the chance for parasitic absorption and nonradiative recombinations to occur. Additionally, with MBE very precise control over interfaces is possible, where a significant portion of the non-radiative traps are usually located. The most promising material for laser cooling is the binary compound GaAs. The lattice-matched material Ga_0.515In_0.485P is chosen for passivating the surface as it has shown much longer radiative lifetimes in GaAs than, for example, Al_xGa_1-xAs. The present study focuses on growth optimization of Ga^0.515In_0.485P/GaAs/Ga_0.515In_0.485P heterostructures and the influence of growth conditions on sample suitability for laser cooling as measured by non-radiative lifetimes in GaAs. In particular, parameters such as growth temperature, group V:III overpressure, substrate orientation, doping, and interface composition on a monolayer length scale are varied and analyzed. The suitability of an optimized sample for semiconductor laser cooling is discussed.

We present a microscopic many-body theory of optical refrigeration of semiconductors with nite spatial beam prole extension. The theory is an extension of our previous theory of optical refrigeration of GaAs, which had been limited to spatially homogeneous systems. In it, optically excited electron-hole pairs can be an unbound pairs, or pairs bound by the attractive Coulomb interaction (excitons). Assuming the electron-hole pairs to be in quasi-thermal equilibrium, our theory calculates its absorption and luminescence spectra within a diagrammatic (real-time) Green's function approach at the self-consistent T-matrix level. The present extension to lateral spatial inhomogeneities due to nite beam spot size utilizes a photon transport equation which is based on a diagrammatic formulation of Maxwell's equations for photon correlation functions. Assuming only radial ux for simplicity, and analytical solution for the pair density and power density rate equations is obtained, and numerical self-consistent solutions are presented. The results show that for typical beam waist parameters, lateral (radial) photon transport does not signicantly impede the theoretically predicted cooling process.

We review some of our recent results following the investigation of anti-Stokes photoluminescence (PL). Indeed, we have observed anti-Stokes photoluminescence from n-type free-standing GaN at room temperature. Such a process is induced by phonon-assisted absorption. When the excitation photon energy is sufficiently below the donor-acceptor transition energy, however, two-photon absorption becomes the dominant mechanism for anti-Stokes photoluminescence. By measuring the dependences of the photoluminescence spectra on temperature, excitation power, and excitation photon energy, we have demonstrated that donor-acceptor pair transition plays an important role in the generation of anti-Stokes photoluminescence. Our study could result in efficient laser cooling of semiconductors.

We present a detailed experimental study of photoluminescence upconversion in GaAs quantum wells over a wide temperature range. The dependence of the upconversion on the well width is discussed and the conversion efficiency is determined as a function of laser detuning. The best results are achieved when the laser detuning is comparable to the thermal energy of the system, ▵E≈2k_BT.

Significant progress has been made in synthesizing and characterizing ultra-pure, rare-earth-doped ZIBLAN (ZrF₄-InF₃- BaF₂-LaF₃-AlF₃-NaF) glass capable of laser refrigeration. Yb³⁺-doped ZIBLAN glass was produced from fluoride precursors which were individually purified by solvent extraction and subsequently treated with hydrofluoric gas at elevated temperatures to remove oxygen impurities. We have developed two-band differential luminescence thermometry (TBDLT) as a new non-invasive, spectroscopic technique to evaluate the intrinsic quality of Yb³⁺ doped laser-cooling samples. TBDLT measures changes in the local temperature upon laser excitation via the small changes in the ²F_5/2→²F_7/2 fluorescence spectrum. Two commercial band pass filters in combination with a balanced dual InGaAs photodetector are used to select and integrate regions of interest in the fluorescence spectrum with sub-millisecond resolution. The TBDLT technique successfully finds the zero-crossing temperature (ZCT), which is the minimum temperature to which a Yb³⁺ doped sample can cool, independent of surface heating. ZCT is a useful measure for the presence of impurities and the overall quality of the laser-cooling material. Favorable laser cooling results were obtained for several 1% Yb³⁺-doped glasses with varying degrees of purity.

The status of optical refrigeration of rare-earth-doped solids is reviewed, and the various factors that limit the performance of current laser-cooling materials are discussed. Efficient optical refrigeration is possible in materials for which hω_max < E_p/8, where h&omega;_max is the maximum phonon energy of the host material and E_p is the pump energy for the rare-earth dopant. Transition-metal and OH- impurities at levels >100 ppb are believed to be the main reason for the limited laser-cooling performance in current materials. The many components of doped ZBLAN glass pose particular processing challenges. Binary fluoride glasses such as YF3-LiF are considered as alternatives to ZBLAN, and the crystalline system KPb₂Cl₅ :Dy³⁺ is identified as a prime candidate for high-efficiency laser cooling.

We discuss recent progress in the laser cooling experiments via resonant cavity. Following analysis of the cooling efficiency, we highlight importance of wavelength dependence of the minimum achievable temperature for a given cryocooler. Following the analysis, we utilize pump detuning along with reduction of thermal load on the sample to achieve absolute temperature of nearly 200K, a 98.5 degree drop, starting from room temperature. Wavelength dependent analysis suggests that further improvement is possible.

This presentation will review and compare different purification processes of starting materials and different synthesis processes developed so far to prepare high purity rare-earth doped and un-doped fluoride glasses. Discovered in 1975 at Rennes University and intensively developed for more than 25 years, fluoride glasses have experienced an extraordinary development, due to their broad transmission spectrum and their low optical loss. With a theoretical optical loss of 0.001 dB| km fluoride at 2.6 micron, fluoride glass have been considered as material of choice for repeater less communication link. Since this goal wasn't easy to achieve the intensive development ended in early nineties. However, the technology is quite mature to provide high purity bulk glass, lenses and fibers for short and medium-length application. Single and multimode fibers with optical loss lower than 10 dB|km are routinely prepared. Furthermore, compared to silica and chalcogenide glasses, fluoride glasses can be doped with high concentration of rare-earth element required for different applications such as, fiber laser, fiber amplifier. They can also be prepared with lower rare earth and transition metals ions required for laser cooling applications.

The potential of three erbium based solids hosts has been investigated for laser cooling. Absorption and emission spectra have been studied for the low lying IR transitions of erbium that are relevant to recent reports of cooling using the ⁴I_15/2-⁴I_9/2 and⁴I_15/2 -⁴I_13/2 transitions. Experimental studies have been performed for erbium in three hosts; ZBLAN glass and KPb₂Cl₅ and Cs₂NaYCl₆ crystals. In order to estimate the efficiencies of cooling, theoretical calculations have been performed for the cubic Elpasolite (Cs₂NaYCl₆ ) crystal. These calculations also provide a first principle insight into the cooling efficiency for non-cubic and glassy hosts where such calculations are not possible.

Optical cryocoolers made of luminescent solids are very promising for many applications in the fields of optical telecommunications, aerospace industry, bioimaging, and phototherapy. To the present day, researchers have employed a number of crystal and glass host materials doped with rare-earth ions (Yb³⁺, Tm³⁺, and Er³⁺) to yield anti-Stokes optical refrigeration. In these host materials, the attainable minimum temperature is limited by the average phonon energy of the lattice and the impurity concentration. However, recently Ruan and Kaviany have theoretically demonstrated that the cooling efficiency can be dramatically enhanced when the host material doped with rare-earth ions is ground into a powder made of sub-micron size grains. This is due to two facts: firstly, the phonon spectrum is modified due to finite size of the grains and, secondly, light localization effects increase the photon density, leading to an enhanced absorptivity. In the present work, we propose that using a photonic crystal doped with rare earth ions offers many advantages with regards to getting a larger cooling efficiency at room temperature when compared to standard bulk materials or nano-powders. Indeed, apart to analogous phenomena to the ones predicted in nano-crystalline powders, there is the possibility of directly controlling the spontaneous emission rate of the ions embedded in the structure and, also, the absorption rate in the Stokes side of the absorption band by adequately tuning the density of photonic states, thus obtaining a large improvement in the cooling efficiency.

The physical mechanism of radiation cooling by anti-Stokes fluorescence was originally proposed in 1929 and experimentally observed in solid materials in 1995 by Epstein's research team in ytterbium-doped ZBLANP glass. Some specific combinations of the ions, host materials, and the wavelength of the incident radiation can provide anti-Stokes interaction resulting in phonon absorption accompanied by the cooling of the host material. Although the optical cooling of the Yb³⁺-doped ZBLANP sample was already observed there are broad possibilities for its improvement to increase the temperature-drop of the sample by optimization of the geometrical parameters of the cooling sample. We propose a theoretical model for an optimized tapered fiber structure for use as a sample in anti-Stokes laser cooling of solids. This tapered fiber has a fluorozirconate glass ZBLANP with a core doped with Yb³⁺ or Tm³⁺ ions. As evident from the results of our work, the appropriate choice of the fiber core and the fiber cladding radii can significantly increase the temperature-drop of the sample for any fixed pump power. The value of the maximum of the temperature-drop of the sample increases with an increase in the pump power. The depletion of the pump power causes a temperature gradient along the length of the cooled sample.

Differential luminescence thermometry (DLT) allows non-contact method of measuring temperature by timedifferencing luminescence spectra emitted from the material in study. Here, we present a modification to the DLT technique, termed "two-pixel DLT" (2pixDLT), that combines high temperature and temporal resolutions at the expense of reduced spectral sampling of the luminescence signal. We showcase our technique by demonstrating millisecond/millidegree resolution in time and temperature in heating dynamics of GaAs heterostructure sample. We utilize tnis technique to determine minimum achievable temperature in rare-earth doped fluoride crystal Yb:YLF to be 170K, when excited at 1030nm.