This PDF file contains the front matter associated with SPIE Proceedings Volume 6907, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

Simultaneous near-field fluorescence lifetime imaging and atomic force microscopy identify radiative, interface and subsurface defect recombination sites in GaAs/GaInP heterostructures. This instrumentation helps characterize samples for laser cooling.

We present a microscopic many-body theory of optical refrigeration of p-doped semiconductors. Conceptually, the refrigeration mechanism is the upconversion of pump photons through absorption and subsequent luminescence by electron-hole pairs. The electron-hole pair can be an unbound pair, a pair bound by the attractive Coulomb interation (exciton), or a pair in which the hole is located at an acceptor site. 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 strong on-site Coulomb repulsion of holes at acceptor sites is taken into account via a truncation of the acceptor Fock space, which excludes states with higher than single-hole occupation. The resulting absorption and luminescence spectra are used in a cooling threshold analysis for GaAs that also takes into account other losses into heat. We compare the present results for p-doped GaAs with previous ones obtained for undoped GaAs.

One of the challenges of laser cooling a semiconductor is its typically high index of refraction (greater than 3), which limits efficient light output of the upconverted photon. This issue is addressed with a novel concept of coupling the photon out via a thin, thermally insulating vacuum gap that allows light to pass efficiently by frustrated internal reflection. Although silicon technology is mature and inexpensive, the indirect nature of the bandgap of silicon makes it unsuitable for laser cooling. The material of choice is the binary compound semiconductor GaAs, which can be fabricated with high quality necessary for laser cooling experiments. Moreover, process technology exists that enables a relatively simple fabrication of a thin vacuum gap in this material system. This paper will present an investigation of heat transport and light transmission across a "nanogap" consisting of a thin epitaxial film supported over a substrate by an array of nanometer-sized posts. The structure is manufactured by crystal growth of a sacrificial Al_0.98Ga_0.02As layer on a single crystal GaAs substrate. After lithographically defining holes in the Al_0.98Ga_0.02As layer, the holes are filled with GaAs and a top GaAs layer is deposited. Lateral selective etching of the Al_0.98Ga_0.02As will create a nanogap between two GaAs layers separated by GaAs posts. We are demonstrating the successful fabrication of various size nanogaps in this material system, as well as their properties with respect to reduced heat transfer across the gap. We are also presenting data supporting that the interface quality is high enough to allow evanescent tunneling of light at angles otherwise forbidden by total internal reflection. The implications for semiconductor laser cooling will be discussed.

The fluorozirconate glass ZBLAN:1%Yb³⁺ was synthesized, for the first time, from fluoride precursors that were individually purified by solvent extraction and hydrofluoric (HF) gas treatment. The synthesis used aqueous solutions of high-purity commercial precursors that were subjected to ultra-filtration followed by solvent extraction using ammonium pyrrolidine dithiocarbamate (APDC) and methyl-isobutyl-ketone (MIBK). The purified metal fluorides were precipitated and treated in hot HF gas to remove water, hydroxyl (OH-), and oxide impurities. ZBLAN:1%Yb³⁺ was fabricated from these precursors by melting under inert atmosphere, yielding glasses with excellent mechanical properties and having a clear, bubble-free, and crystallite-free matrix. The effect of adding 0.5 mol% of In³⁺ as an oxidizer to suppress the reduction of Zr⁴⁺ and the accompanying formation of black precipitates was studied. We found evidence for an oxidizer concentration threshold of ~0.8 mol%. Glasses made from purified fluorides formed black precipitates even with the addition of 0.5 mol% In³⁺, while glasses made from commercial fluorides did not. In the latter, additional oxidizers were likely present in the form of transition-metal impurities. An In³⁺ oxidizer concentration of >0.8 mol% is expected to eliminate the black precipitates in purified glasses and to yield ZBLAN:Yb³⁺ glass for efficient laser cooling.

Using a cavity resonant absorption scheme we demonstrate record laser cooling for the rare-earth doped crystalline solid Yb:YLF. A temperature drop of nearly 70 degrees is obtained with respect to the ambient. Our preliminary results indicate that minimum achievable temperature in this material/sample is 170 K, as measured using a modified differential luminescence thermometry technique. This indicates outstanding potential for Yb:YLF as a cryogenic laser cooler material.

We have used the thermal modeling tool in COMSOL Multiphysics to investigate factors that affect the thermal performance of the optical refrigerator. Assuming an ideal cooling element and a non-absorptive dielectric trapping mirror, the three dominant heating factors are blackbody radiation from the surrounding environment, conductive heat transfer through mechanical supports, and the absorption of fluoresced photons transmitted through the thermal link. Laboratory experimentation coupled with computer modeling using Code V optical software have resulted in link designs capable of reducing the transmission to 0.04% of the fluoresced photons emitted toward the thermal link. The ideal thermal link will have minimal surface area, provide complete optical isolation for the load, and possess high thermal conductivity. Modeling results imply that a 1cm³ load can be chilled to 102 K with currently available cooling efficiencies using a 100 W pump laser on a YB:ZBLANP system, and using an ideal link that has minimal surface area and no optical transmission. We review the simulated steady-state cooling temperatures reached by the heat load for several link designs and system configurations as a comparative measure of how well particular configurations perform.

Dielectric mirror leakage at large angles of incidence limits the effectiveness of solid state optical refrigerators due to reheating caused by photon absorption in an attached load. In this paper, we present several thermally conductive link solutions to greatly reduce the net photon absorption. The Los Alamos Solid State Optical Refrigerator (LASSOR) has demonstrated cooling of a Yb³⁺ doped ZBLANP glass to 208 K. We have designed optically isolating thermal link geometries capable of extending cooling to a typical heat load with minimal absorptive reheating, and we have tested the optical performance of these designs. A surrogate source operating at 625 nm was used to mimic the angular distribution of light from the LASSOR cooling element. While total link performance is dependent on additional factors, we have found that the best thermal link reduced the net transmission of photons to 0.04%, which includes the trapping mirrors 8.1% transmission. Our measurements of the optical performance of the various link geometries are supported by computer simulations of the designs using Code V, a commercially available optical modeling software package.

We present measurements of an optomechanical system in which the mechanical element is inside the cavity, and couples dispersively to the intracavity field. This geometry makes it easier to simultaneously achieve high optical finesse and high mechanical quality factor in an optomechanical device. We measured the linear optical properties of a such a device in which the mechanical element is a 50 nm thick silicon nitride membrane. We find that the device's finesse, resonant transmission and resonant reflection can be explained with a simple model which allows us to extract the membrane's optical loss. Our results indicate that it should be possible to increase the finesse of these devices to 5 × 105 or higher.

One of the formulations of the third laws of thermodynamics is that a processes become more isentropic as one approaches the absolute zero temperatures. We examine this prediction by studying an operating model of a quantum refrigerator pumping heat from a cold to a hot reservoir. The working medium consists of a gas of noninteracting harmonic oscillators. The model can be solved in closed form in the quasi-static limit or numerically for general conditions. It is found that the isentropic limit for T_c → 0 is approached only on the expansion segment of the refrigeration cycle. The scaling of the cooling rate with temperature is shown to be consistent with the second law of thermodynamics. This scaling is also consistent with the unattainability principle which is an alternative formulation of the third law of thermodynamics.

Thermally assisted electro-luminescence may provide a means to convert heat into electricity. In this process, radiation from a hot light-emitting diode (LED) is converted to electricity by a photovoltaic (PV) cell, which is termed thermophotonics. Novel analytical solutions to the equations governing such a system show that this system combines physical characteristics of thermophotovoltaics (TPV) and the inverse process of laser cooling. The flexibility of having both adjustable bias and load parameters may allow an optimized power generation system based on this concept to exceed the power throughput and efficiency of TPV systems. Such devices could function as efficient solar thermal, waste heat, and fuel-based generators.