This PDF file contains the front matter associated with SPIE Proceedings Volume 9765, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Gadolinium oxysulfide crystal is a wide-gap semiconductor material known as an excellent host for trivalent rare-earth ions. The present investigation explores the upconversion and thermal properties of Er³⁺-doped Gd₂O₂S crystal powders as well as their potentiality for anti-Stokes cooling. A detailed study of the wavelength and pumping power dependence of the spectroscopic properties and temperature field for samples of various erbium concentrations is presented.

When pumped with a 10 W 1020 nm fiber laser, a temperature drop of 60.4 K from room temperature of a crystal in the vacuum is observed by utilizing the differential luminescence thermometry measurement method. The crystal is doped with 5wt.%Yb ³⁺ : LuLiF₄ and has a size of 3×3×5 mm³. The cooling power and the cooling efficiency of sample is estimated to be ~50.6 mW and ~2.53%, respectively, at the 232.6 K temperature.

Solid-state coolers have no moving parts, and are therefore compact, vibration-free, inherently durable, and scalable to low power levels. Unfortunately, they have a low coefficient of performance. The materials characteristic that sets this efficiency in solid-state coolers is the thermoelectric figure of merit, zT. This article reviews the factors limiting zT in conventional thermoelectric devices, then outlines modern approaches to increasing zT. It describes a new solid-state energy conversion technology, the spin-Seebeck effect (SSE). New concepts are outlined that combine classical thermoelectric and SSE physics. By exploiting the spin degree of freedom, we hope to increase the performance of solid-state coolers.

Transverse thermoelectrics promise entirely new strategies for integrated cooling elements for optoelectronics. The recently introduced p × n-type transverse thermoelectric paradigm indicates that the most important step to engineering artificial transverse thermoelectrics is to create alternate p- and n-doped layers with orthogonally oriented anisotropic conductivity. This paper studies an approach to creating extreme anisotropic conductivity in bulk-doped semiconductor thin films via ion implantation. This approach defines an array of parallel conduction channels with photolithographic patterning of an SiO₂ mask layer, followed by proton implantation. With a 10 μm channel width and 20 μm pitch, both n-type and p-type Al_0.42 Ga_0.58As thin films demonstrate a conductivity anisotropy ratio σ /σ⊥ > 10⁴ at room temperature, while the longitudinal resistivity along the channel direction after implantation only increased by a factor of 3.3 ∼ 3.6. This approach can be readily adapted to other semiconductor materials for artificial p × n-type transverse thermoelectrics as other applications.

Near resonant pumping of solid-state lasers offers the potential for high efficiency and minimal thermal loading. These lasers inherently operate in the regime where fluorescent cooling plays an important role. Here a model is developed to optimize efficiency and minimize heating for these laser systems. The model incorporates realistic background absorption and excitation quenching. Beyond the conventional laser modeling, this s includes both radiative cooling and fluorescence trapping. The model is illustrated with simulations of Yb:YAG lasers.

The laser refrigeration of solid-state materials with nanoscale dimensions has been demonstrated for both semi- conducting (cadmium sulfide, CdS) and insulating dielectrics (Yb:YLiF4, YLF) in recent years. During laser refrigeration it is possible to observe morphology dependent resonances (MDRs), analogous to what is well- known in classical (Mie) light scattering theory, when the characteristic dimensions of the nanostructure are comparable to the wavelength of light used to initiate the laser cooling process. Mie resonances can create substantial increases for internal optical fields within a given nanostructure with the potential to enhance the absorption efficiency at the beginning of the cooling cycle. Recent breakthroughs in the laser refrigeration of semiconductor nanostructures have relied on materials that exhibit rectangular symmetry (nanoribbons). Here, we will present recent analytical, closed-form solutions to the energy partial differential equation that can be used to calculate the internal spatial temperature profile with a given semiconductor nanoribbon during irradiation by a continuous-wave laser. First, the energy equation is made dimensionless through the substitution of variables before being solved using the classical separation-of-variables approach. In particular, calculations will be presented for chalcogenide (CdS) nanoribbons using a pump wavelength of 1064 nm. For nanostructures with lower symmetry (such as YLF truncated tetragonal bipyramids) it is also possible to observe MDRs through numerical simulations using either the discrete dipole approximation or finite-difference time-domain simulations, and the resulting temperature profile can be calculated using the finite element method. Theoretical predictions are presented using parameters that will allow comparison with experimental data in the near future.

Laser cooling via collisional redistribution of fluorescence utilizes dense mixtures of alkali metals with noble buffer gases. Typical pressure values of the buffer gas are of the order of a few hundred bar, ensuring a frequent number of collisions between the two atomic species. The energy levels of the alkali atoms are thus perturbed so that excitation using far red-detuned laser light becomes feasible. Energy is then extracted via spontaneous emission occuring close to the unperturbed atomic resonances. Optimization of the cooling effect strongly depends on excitation conditions, namely the choice of detuning of the cooling beam, which in turn is dependent on the used pressure. We here report on spectroscopy measurements of atomic rubidium under high pressure buffer gas conditions. While thermal deflection spectroscopy has been previously used to measure the temperature change of the laser-cooled gas, an alternative approach for temperature measurements utilizing the Kennard-Stepanov relation is also investigated. We here exhibit that the collisionally thermalized atomic resonances well follow this thermodynamic scaling, allowing for temperature extraction of the dense gas mixture.

Excitons, bound electron-hole pairs, possess distinct physical properties from free electrons and holes that can be employed to improve the performance of optoelectronic devices. In particular, the signatures of excitons are enhanced optical absorption and radiative emission. These characteristics could be of major benefit for the laser cooling of semiconductors, a process which has stringent requirements on the parasitic absorption of incident radiation and the internal quantum efficiency. Here we experimentally demonstrate the dominant ultrafast excitonic super-radiance of our quantum well structure from 78 K up to room temperature. The experimental results are followed by our detailed discussions about the advantages and limitations of this method.

Laser cooling with anti-Stokes fluorescencewas predicted by Pringsheim in 1929, but for solids was only demonstrated in 1995. There are many difficulties which have hindered laser assisted cooling, principally the chemical purity of a sample and the availability of suitable hosts. Recent progress has seen the cooled temperature plummet to 93K in Yb:YLF. One of the challenges for laser cooling to become ubiquitous, is incorporating the rare-earthcooling ion in a more easily engineered material, rather than a pure crystalline host. Rare-earth-doped nanocrystalline glass-ceramics were first developed by Wang and Ohwaki for enhanced luminescence and mechanical properties compared to their parent glasses. Our work has focused on creating a nanocrystalline environment for the cooling ion, in an easy to engineer glass. The glasses with composition 30SiO₂-15Al²O₃-27CdF₂-22PbF₂-4YF₃-2YbF₃ (mol%), have been prepared by the conventional melt-quenching technique. By a simple post fabrication thermal treatment, the rare-earth ions are embedded in the crystalline phase within the glass matrix. Nanocrystals with various sizes and rare-earth concentrations have been fabricated and their photoluminescence properties assessed in detail. These materials show close to unity photoluminescence quantum yield (PLQY) when pumped above the band. However, they exhibit strong up-conversion into the blue, characteristic of Tm trace impurity whose presence was confirmed. The purification of the starting materials is underway to reduce the background loss to demonstrate laser cooling. Progress in the development of these nano-glass-ceramics and their experimental characterization will be discussed.

We show theoretically that a bottle resonator with a nanoscale altitude made on the surface of an optical fiber can be used as a temperature sensor for laser cooling of solids. The operation of such sensors is based on whispering gallery modes (WGMs). Bottle resonators can be made at different positions along the length of the fiber, which undergoes laser cooling. A smooth perturbation with a small nanoscale altitude on the surface of the fiber does not couple fiber modes propagating along the fiber axis and does not influence the laser cooling process. The temperature of the sample at each of these positions can be monitored as a shift in the dips seen in the transmission spectrum of a biconically tapered fiber placed perpendicular to the fiber axis on the top of the resonator to excite WGMs. Temperature sensitivities ~12pm/K and ~16pm/K are obtained for Yb³⁺:ZBLAN and Yb³⁺:YAG samples, respectively. The possibility of using bottle resonators for other applications is also discussed.

We present our recent work in developing a robust and versatile optical refrigerator. This work focuses on minimizing parasitic energy losses through efficient design and material optimization. The cooler’s thermal linkage system and housing are studied using thermal analysis software to minimize thermal gradients through the device. Due to the extreme temperature differences within the device, material selection and characterization are key to constructing an efficient device. We describe the design constraints and material selections necessary for thermally efficient and durable optical refrigeration.

Laser cooling of solids can be achieved through various photon up-conversion processes including anti-Stokes photoluminescence and anti-Stokes light scattering. While it has been shown that cooling using photoluminescence-based methods can achieve efficiency comparable to that of thermoelectric cooling, the reliance on specific transitions of the rare-earth dopants limits material choice. Light scattering, on the other hand, occurs in all materials, and has the potential to enable cooling in most materials. We show that by engineering the photonic density of states of a material, one can suppress the Stokes process, and enhance the anti-Stokes radiation. We employ the well-known diamond-structured photonic crystal patterned in crystalline silicon to demonstrate theoretically that when operating within a high transparency regime, the net energy removal rate from phonon annihilation can overcome the optical absorption. The engineered photonic density of states can thus enable simultaneous cooling of all Raman-active phonon modes and the net cooling of the solid.

We report the first observation of laser cooling in 1% doped Tm:YLF by 0.5 K and in 0.8% doped Ho:YLF crystals by 0.1 K starting from room temperature in air. To achieve this, we designed and constructed a high power, broadly tunable (1735 nm-2086 nm) continuous wave singly-resonant optical parametric oscillator. (OPO). The cooling experiments were performed at ambient pressure, and temperature changes were measured using a thermal camera.

We present a study of non-resonant optical cavities for optical refrigerators. Designs have been studied to maximize pump light-trapping to improve absorption and thereby increase the efficiency of optical refrigeration. The approaches of non-resonant optical cavities by Herriott-cell and total-internal-reflection were studied. Ray-tracing simulations and experiments were performed to analyze and optimize the different light-trapping configurations. We present a trade-off analysis between performance, reliability, and manufacturability.