Optical refrigeration of solids requires crystals with exceptional qualities. Crystals with external quantum efficiencies (EQE) larger than 99% and background absorptions of 4×10-4cm-1 have been cooled to cryogenic temperatures using non resonant cavities. Estimating the cooling efficiency requires accurate measurements of the above mentioned quantities. Here we discuss measurements of EQE and background absorption for two high quality Yb:YLF samples. For any given sample, to reach minimum achievable temperatures heat generated by fluorescence must be removed from the surrounding clamshell and more importantly, absorption of the laser light must be maximized. Since the absorption coefficient drops at lower temperatures the only option is to confine laser light in a cavity until almost 100% of the light is absorbed. This can be achieved by placing the crystal between a cylindrical and spherical mirror to form an astigmatic Herriott cell. In this geometry light enters through a hole in the middle of the spherical mirror and if the entrance angle is correct, it can make as many round trips as required to absorb all the light. At 120 K 60 passes and 150 passes at 100K ensures more than 95% absorption of the laser light. 5 and 10% Yb:YLF crystals placed in such a cell cool to sub 90K temperatures. Non-contact temperature measurements are more challenging for such a geometry. Reabsorption of fluorescence for each pass must be taken into account for accurate temperature measurements by differential luminescence thermometry (DLT). Alternatively, we used part of the spectrum that is not affected by reabsorption.

Hexagonal sodium yttrium fluoride (β-NaYF₄) crystals are currently being studied for a wide range of applications including color displays, solar cells, photocatalysis, and bio-imagβing. β-NaYF₄ has also been predicted to be a promising host material for laser refrigeration of solids. However, due to challenges with growing Czochralski β- NaYF₄ single-crystals, laser refrigeration of bulk β-NaYF₄ has not yet been achieved6. Recently hydrothermal processing has been reported to produce Yb-doped β-NaYF₄ nanowires (NWs) that undergo laser refrigeration during single-beam optical trapping experiments in heavy water. The local refrigeration of the individual nanowire is quantified through the analysis of its Brownian motion through the analysis of forward scattered light that is focused onto a quadrant photodiode. The individual β-NaYF₄ nanowires show maximum local cooling of 9°C below ambient conditions. Here we present the emission lifetime for the ⁴S_3/2 – ⁴I_15/2 transition for Er(III) ions in Yb/Er-codoped -NaYF₄ NW ensembles was measured to be (220 ± 6) μs using a an electron multiplying charge coupled device (EMCCD) as a detector with high spatial resolution. This lifetime is consistent with values reported in the literature.

We investigate high power laser cooling in Tm:YLF crystals both in ambient pressure and high vacuum. For this purpose, we have constructed a high power CW OPO broadly tunable from 1755 nm to 2000 nm. By using this tunable source, laser cooling for (3 mm3) 1% doped Tm:YLF crystal was observed from 1801 nm to 2000 nm. Cooling efficiency of the sample, external quantum efficiency (EQE), background absorption and optimum laser cooling wavelength are extracted from laser induced temperature modulation spectrum (LITMoS) test on the cooling sample. To improve cooling performance, we have designed multiple pass non-resonant cavities to maximize the absorption of the laser light inside the sample. We setup multiple pass cavities in a high vacuum chamber to reduce convective heat load and enhance laser cooling results.

The new breakthrough in photovoltaics, exemplified by the slogan “A great solar cell has to be a great light-emitting diode (LED)”, has led to all the major new solar cell records, while also leading to extraordinary LED efficiency. As an LED becomes very efficient in converting its electrical input into light, the device cools as it operates because the photons carry away entropy as well as energy. If these photons are absorbed in a photovoltaic (PV) cell, the generated electricity can be used to provide part of the electrical input that drives the LED. Indeed, the LED/PV cell combination forms a new type of heat engine with light as the working fluid. The electroluminescent refrigerator requires only a small amount of external electricity to provide cooling, leading to a high coefficient of performance. We present the theoretical performance of such a refrigerator, in which the cool side (LED) is radiatively coupled to the hot side (PV) across a vacuum gap. The coefficient of performance is maximized by using a highly luminescent material, such as GaAs, together with device structures that optimize extraction of the luminescence. We consider both a macroscopic vacuum gap and a sub-wavelength gap; the latter allows for evanescent coupling of photons between the devices, potentially providing a further enhancement to the efficiency of light extraction. Using device assumptions based on the current record-efficiency solar cells, we show that electroluminescent cooling can, in certain regimes of cooling power, achieve a higher coefficient of performance than thermoelectric cooling.

It is known that the wall-plug efficiency (WPE) of a light-emitting diode (LED) can exceed unity and that electroluminescence cooling (ELC) happens in this scenario. However, it is difficult to observe the associated temperature drop due to the relatively small cooling power and the overwhelming heat flux from the ambient. In this work, we design a photonic crystal (PhC) enhanced LED which has smaller surface area as well as thermal mass compared with an encapsulated LED. We also present thermal models to evaluate the temperature drop of the LED in air and vacuum.

Optical cooling of semiconductors has recently been demonstrated both for optically pumped CdS nanobelts and for electrically injected GaInAsSb LEDs at very low powers. To enable cooling at larger power and to understand and overcome the main obstacles in optical cooling of conventional semiconductor structures, we study thermophotonic (TPX) heat transport in cavity coupled light emitters. Our structures consist of a double heterojunction (DHJ) LED with a GaAs active layer and a corresponding DHJ or a p-n-homojunction photodiode, enclosed within a single semiconductor cavity to eliminate the light extraction challenges. Our presently studied double diode structures (DDS) use GaInP barriers around the GaAs active layer instead of the AlGaAs barriers used in our previous structures. We characterize our updated double diode structures by four point probe IV- measurements and measure how the material modifications affect the recombination parameters and coupling quantum efficiencies in the structures. The coupling quantum efficiency of the new devices with InGaP barrier layers is found to be approximately 10 % larger than for the structures with AlGaAs barriers at the point of maximum efficiency.

The influence of a solid-state sample placed in the vicinity of a laser cooled sample is theoretically investigated. The laser cooling process is based on anti-Stokes fluorescence. The laser cooled sample is a rare-earth doped low-phonon energy solid. In this system, all samples can support surface phonon polaritons (SPhPs) in the same or different wavelength regions. Two different cases are considered. In the first case a laser cooled ytterbium-doped yttrium aluminum garnet ( Yb³⁺:YAG ) sample is placed in a vacuum chamber near a YAG sample, which is at room temperature. In the second case a laser cooled Yb³⁺:YAG sample is placed near a silicon carbide (SiC) sample, which is at room temperature. It is shown that for short distances between samples, when there is coupling between SPhPs propagating in the sample undergoing laser cooling and SPhPs propagating in the next sample, the laser cooling process can deteriorate substantially. In the opposite case the SPhPs do not influence the laser cooling process significantly even if the distance between the samples is less than the dominant wavelength of thermal radiation.

The concept of optical cooling of solids has existed for nearly 90 years ever since Pringsheim proposed a way to cool solids through the annihilation of phonons via phonon-assisted photoluminescence (PL) up-conversion. In this process, energy is removed from the solid by the emission of photons with energies larger than those of incident photons. However, actually realizing optical cooling requires exacting parameters from the condensed phase medium such as near unity external quantum efficiencies as well as existence of a low background absorption. Until recently, laser cooling has only been successfully realized in rare earth doped solids. In semiconductors, optical cooling has very recently been demonstrated in cadmium sulfide (CdS) nanobelts as well as in hybrid lead halide perovskites. For the former, large internal quantum efficiencies, sub-wavelength thicknesses, which decrease light trapping, and low background absorption, all make near unity external quantum yields possible. Net cooling by as much as 40 K has therefore been possible with CdS nanobelts. In this study, we describe a detailed investigation of the nature of efficient anti-Stokes photoluminescence (ASPL) in CdS nanobelts. Temperature-dependent PL up-conversion and optical absorption studies on individual NBs together with frequency-dependent up-converted PL intensity spectroscopies suggest that ASPL in CdS nanobelts is defect-mediated through involvement of defect levels below the band gap.

Despite achievements of extremely high external quantum efficiency (EQE), 99.5%, the net cooling of GaInP|GaAs double heterostructures (DHS) has never been realized. This is due to an unknown source of parasitic absorption. Prior studies have ruled out the possibility of the bulk absorption from the GaAs layer. Thus it is thought to be either at the air- GaInP interface, through the presence of dangling bonds, or in bulk GaInP through impurities. Using two-color thermallens calorimetry (based on the Z-scan technique), this study indicates that that the parasitic absorption likely originates from the GaInP bulk layers.

Upon excitation of a material below its fundamental transition, cooling of the lattice results if the subsequent emission is predominantly radiative. Despite overwhelming experimental success, it remains a challenge to understand the microscopic nature of detrimental processes that can even prevent cooling. We apply ultrafast spectroscopy to resolve the laser refrigeration cycle in the time domain. Strong evidence for lattice cooling on picosecond timescales in bulk GaAs/InGaP double-heterostructures and GaAs/AlGaAs quantum wells establishes the non-local nature of the parasitic mechanisms. Further precision measurements investigating long-time dynamics are currently underway to resolve detrimental heating in bulk GaAs for the first time.

Radiation Balanced Lasers (RBL) use cooling from spontaneous emission to offset waste heat generation. This technique offers the potential for very high power operation without thermo-optic distortions or damage. Nevertheless establishing and maintaining radiation balance poses interesting problems for the laser designer. An analysis of RBL’s sensitivity to material losses, intensity variation, and temperature will be presented. This comparison of simulations and experiments is intended to assist in the design of future high power systems.

A new AFOSR MURI program, devoted to the pursuit of cooling solid state lasers internally, is underway and will be described. Comprising research teams from four universities, this program will focus on fiber and disc lasers and the demonstration of optical and/or phonon-based processes capable of maintaining beam quality as power loading of the medium rises. Emphasis will be placed on leveraging novel resonator designs to enhance a targeted optical field-material interaction such that localized cooling occurs within the gain medium. Examples will be given of two systems that are being pursued initially.

An overview of the diverse research activities under the newly funded MURI project by AFOSR will be presented. The main goal is to advance the science of radiation-balanced lasers, also known as athermal lasers, in order to mitigate the thermal degradation of the high-power laser beams. The MARBLE project involves researchers from four universities and spans research activities in rare-earth doped crystals and fibers to semiconductor disc lasers.

In this article, we review some of the ideas relevant to Peltier cooling at room temperatures and below. We identify challenges and directions for thermoelectric refrigeration. We also point out that measures other than ZT, should be used for active cooling cycle where heat is pumped from hot to cold. Finally, we review several strategies proposed in the past by our group and others to enhance the thermoelectric power factor.

Conventional thermoelectric coolers have been widely used for cooling of electronic devices. Utilizing bismuth telluride materials, these Peltier modules are typically categorized as high heat flux devices that can achieve modest temperature differences in a compact architecture. Breaking from convention of typical bismuth telluride thermoelectric devices, an alternative method of providing thermal-electric cooling will be discussed providing inspiration for new cooling directions and materials challenges. While this approach has application in electric cooling of solids, there are also wider applications including space cooling and heat pumping.

At conformal interfaces between dissimilar materials, a finite thermal resistance develops, governed by the transmission behavior of phonons. Understanding the engineering opportunities available for such interfaces thus requires an understanding of phonon transmission behavior. Due to its simplicity, the diffuse mismatch model (DMM) remains a popular description of phonon transmission across solid-solid boundaries. However, it remains unclear in which situations the DMM is good description of the underlying physics. In this talk we present theoretical and experimental observations of interfaces with tailored degrees of disorder. Using a 3-dimensional extension of the frequency domain, perfectly matched layer (FD-PML) method, we probe the validity of the diffuse mismatch model (DMM) on a mode-by-mode basis at the interface between solids with interdiffused atoms. It is found that small levels of disorder at an interface can increase the number of available modes for transmission, and subsequently reduce thermal interface resistance. These general observations are consistent with the DMM, and for submonolayer levels of interdiffusion, similar thermal interface conductance values as the DMM are seen. However, the mode-by-mode predictions of transmission coefficient vary drastically from the DMM. Particularly, (1) contrary to the fundamental assumption of the DMM, not all modes lose memory of their initial polarization and wavevector. (2) Interdiffusion in excess of a monolayer is generally found to make agreement between the DMM and the simulations worse, not better. On the other hand, experimental measurements between epitaxial and non-epitaxial versions of the same material interfaces indicate that the detailed structure of the interfaces are unimportant to the transport properties: a key result of the DMM.

Developing optical cooling technologies requires access to reliable efficiency measurement techniques and ability to detect spatial variations in the efficiency and light emission of the devices. We investigate the possibility to combine the calorimetric efficiency measurement principles with lock-in thermography (LIT) and conventional luminescence microscopy to enable spatially resolved measurement of the efficiency, current spreading and local device heating of double diode structures (DDS) serving as test vessels for developing thermophotonic cooling devices. Our approach enables spatially resolved characterization and localization of the losses of the double diode structures as well as other light emitting semiconductor devices. In particular, the approach may allow directly observing effects like current crowding and surface recombination on the light emission and heating of the DDS devices.

This PDF file contains the front matter associated with SPIE Proceedings Volume 10121, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.