This PDF file contains the front matter associated with SPIE Proceedings Volume 9380, including the Title Page, Copyright information, Table of Contents, Authors, Introduction (if any), and Conference Committee listing.

Laser cooling of solids has great potential to achieve an all-solid-state optical cryo-cooler. The advantages of compactness, no vibrations, no moving parts or fluids, and high reliability have motivated intensive research. Increasing the pump power absorption is essential to reach lower temperatures. Here, using a high power broadly tunable InGaAs/GaAs vertical external-cavity surface-emitting laser (VECSEL) we demonstrate how we have increased the pump power absorption in an intra-cavity geometry cooling a 10% Yb:YLF crystal. We also discuss the progress, advantages, and challenges of laser cooling inside a VECSEL cavity, including the VECSEL active region design, cavity design, and cooling sample choice for optimal cooling. A novel method to increase the absorption of the pump power in the crystal has also been proposed.

Optical refrigeration by laser irradiation of YLiF₄:Yb³⁺ (YLF:Yb) crystals has been shown to be strongly deteriorated by impurities, which absorb energy at the laser wavelength, and relax non-radiatively, negating cooling produced from anti-Stokes fluorescence. We aim to increase the efficiency of optical refrigeration through materials purification. We start with the purest sources commercially available and process them in a cleanroom environment. Our method proceeds through electrochemical purification, separating out the transition metal impurities by their redox potentials, and can be scaled up to produce the amounts of material needed for crystal growth.

Single-beam laser-tweezers have been demonstrated over the past several decades to confine nanometer-scale particles in three dimensions with sufficient sensitivity to measure the spring constants of individual biological macromolecules including DNA. Large laser-irradiance values (on the order of MW/cm2) commonly are used to generate laser traps which can lead to significant laser-heating within the 3D optical potential well. To date, laser-refrigeration of particles within an aqueous medium has not been reported stemming primarily from the large near-infrared (NIR) optical absorption coefficient of liquid water (0.2 cm-1 at lambda = 1020nm). In this paper we will detail the methods on how single-beam laser-traps can be used to induce and quantify the refrigeration of optically trapped nanocrystals in an aqueous medium. Analysis of the Brownian dynamics of individual nanocrystals via forward light scattering provides a way to determine both a relative and absolute measurement of particle’s temperature. Signal analysis considerations to interpreting Brownian motion data of trapped particles in nonisothermal aqueous environments, or so-called hot Brownian motion, are detailed. Applications of these methods to determining local laser-refrigeration of laser trapped nanoparticles in water show promise at realizing the first observation of particles undergoing cold Brownian motion.

We have theoretically investigated the laser cooling process in Yb³+:YAG nanocrystals. We have developed an approach, which permits not only estimate the cooling process in Yb³+:YAG nanocrystals but compare this process with the laser cooling of the Yb³+:YAG bulk samples. The temperature dependences of all parameters of the system are taken into account. The cooperative effects such as re-absorption, the energy migration and cooperative luminescence have been considered.

The present investigation explores the upconversion properties of Er³⁺- doped La₂O₂S crystal powder as well as its potentiality for anti-Stokes cooling. A detailed study of the wavelength and pumping power dependence of the spectroscopic properties and of the temperature field of samples with various erbium concentrations is presented. The analysis of both spectroscopic and thermal measurements shows that after a transient heating induced by the background absorption, cooling can be attained by means of anti-Stokes processes.

Suspended semiconductor structures with high thermal isolation provide high temperature sensitivity of the dissipated thermal power. Therefore they can be used to obtain essential information about the underlying mechanisms of anti-Stokes laser cooling. Here, we experimentally investigate the electron-hole pair recombination processes in suspended and non-suspended MQW structures from 77 K up to room temperature. Excitation dependent and time-resolved micro-photoluminescence measurements have been used to conduct this study. To include the effects of lateral carrier diffusion, we preformed finite-element time-resolved analysis of carrier recombination and diffusion and thermal transients. The potential of these structures for laser cooling purposes is discussed.

Laser cooling in InGaP|GaAs double heterostructures (DHS) has been a sought after goal. Even though very high external quantum efficiency (EQE) has been achieved, background absorption has remained a bottleneck in achieving net cooling. The purpose of this study is to gain more insight into the source of the background absorption for InGaP|GaAs DHS as well as GaAs|AlGaAs DBRs by employing an excite-probe thermal Z-scan measurement.

The performance of a solid-state optical refrigerator is the result of a complex interplay of numerous optical and thermal parameters. We present a first preliminary study of an optical cryocooler using ray-tracing techniques. A numerical optimization identified a non-resonant cavity with astigmatism. This geometry offered more efficient pump absorption by the YLF:10%Yb laser-cooling crystal compared to non-resonant cavities without astigmatism that have been pursued experimentally so far. Ray tracing simulations indicate that ~80% of the incident pump light can absorbed for temperatures down to ~100 K. Calculations of heat loads, cooling power, and net payload heat lift are presented. They show that it is possible to cool a payload to a range of 90–100 K while producing a net payload heat lift of 80 mW and 300 mW when pumping a YLF:10%Yb crystal with 20 W and 50 W at 1020 nm, respectively. This performance is suited to cool HgCdTe infrared detectors that are used for sensing in the 8–12 μm atmospheric window. While the detector noise would be ~6× greater at 100 K than at 77 K, the laser refrigerator would introduce no vibrations and thus eliminate sources of microphonic noise that are limiting the performance of current systems.

Optical refrigeration is currently the only completely solid state cooling method capable of reaching cryogenic temperatures from room temperature. Optical cooling utilizing Yb:YLF as the refrigerant crystal has resulted in temperatures lower than 123K measured via a fluorescence thermometry technique. However, to be useful as a refrigerator this cooling crystal must be attached to a sensor or other payload. The phenomenology behind laser cooling, known as anti-Stokes fluorescence, has a relatively low efficiency which makes the system level optimization and limitation of parasitic losses imperative. We propose a variety of potential designs for a final optical refrigerator, enclosure and thermal link; calculate conductive and radiative losses, and estimate direct fluorescence reabsorption. Our simulated designs show losses between 60 and 255 mW, depending on geometry and enclosure choice, with a lower bound as low as 23 mW.

The Shockley-Queisser (SQ) efficiency limit for single-junction photovoltaic cell (PV) is to a great extent due to inherent heat dissipation accompanying the quantum process of electro-chemical potential generation. Concepts such as solar thermophotovoltaics^1,2,3 (STPV) and thermo-photonics⁴ aim to harness this dissipated heat, claiming very high theoretical limit. In practice, none of these concepts have been experimentally proven to overcome the SQ limit, mainly due to the very high operating temperatures, which significantly challenge electro-optical devices. In contrast to the above concepts for harnessing thermal emission at thermal equilibrium, Photoluminescence (PL) is a fundamental light-matter interaction under non-thermal equilibrium, which conventionally involves the absorption of energetic photon, thermalization and the emission of a red-shifted photon. Conversely, in optical-refrigeration the absorption of low energy photon is followed by endothermic-PL of energetic photon^5,6. Both aspects were mainly studied where thermal population is far weaker than photonic excitation, obscuring the generalization of PL and thermal emissions. Here we experimentally study endothermic-PL at high temperatures⁷. In accordance with theory, we show how PL photon rate is conserved with temperature increase, while each photon is blue shifted. Further rise in temperature leads to an abrupt transition to thermal emission where the photon rate increases sharply. We also show how endothermic-PL generates orders of magnitude more energetic photons than thermal emission at similar temperatures. Relying on these observations, we propose and study thermally enhanced PL (TEPL) for highly efficient solar-energy conversion. Here, solar radiation is absorbed by a low-bandgap PL material. The dissipated heat is emitted by endothermic PL, and harvested by a higher-bandgap photovoltaic cell. While such device operates at much lower temperatures than STPV, the theoretical efficiencies approach 70%, bringing its realization into reach.

We report on experiments investigating laser cooling of atomic gases by collisional redistribution of radiation, a technique applicable to dense mixtures of alkali metals with noble gases. Thermal deflection spectroscopy is one of the methods used to measure the temperature change of the laser-cooled gas. In this work we describe experiments focusing on a different technique for precise determination of the local temperature achieved by the cooling within the gas cell. We investigate the Kennard-Stepanov relation, a thermodynamic, Boltzmann-type scaling between the absorption and emission spectral profiles of an absorber, which applies in many liquid state dye solutions as well as in semiconductor systems. To this end, absorption and emission spectra of rubidium atoms and dimers in dense argon buffer gas environment have been recorded. We demonstrate experimentally that the Kennard-Stepanov relation between absorption and emission spectra is well fulfilled for the collisionally broadened atomic and molecular transitions of the system, which allows for the extraction of the thermodynamic temperature.

We propose an alternative excitation scheme based on paired-π-pulses input for laser cooling of solids in superradiance regime. The dynamics of the activated atoms being pumped is analyzed, and used to deduce the temperature dependent cooling power. Compared with the continue-wave plus pulse pumping scheme, our scheme works better for laser cooling of solids via superradiance, and can improve the cooling power by 4 and 50 times at room and cryogenic temperatures, respectively. Moreover, the propagation effects of the input pulses are investigated to determine the proper length of the cooling sample.

The infrared (IR) absorption and emission properties of Ho:KPC, Ho:KPB, and Ho:YAG were compared for possible applications in 2 µm laser cooling. Ho:KPC and Ho:KPB crystals were grown by vertical Bridgman technique using purified starting materials. A commercial Ho:YAG crystal was included in this study for comparison. Under resonant pumping at ~1.907 µm, the Ho-doped KPC/KPB crystals exhibited broad IR emission centered at ~2 µm based on the Ho³⁺ intra-4f transition ⁵I₇ → ⁵I₈. Under similar experimental conditions, Ho:YAG showed a narrow-structured emission band reflective of individual Stark levels. The average emission wavelength for Ho:YAG was determined to be ~2.03 µm. Initial heat loading/cooling experiments under ambient air were performed using a fiber laser operating at ~2.036 µm with an output power of 2 W. The Ho:KPC/KPB crystals exhibited small temperature increases of ~1.0 ºC. A significantly larger temperature increase of ~5 ºC was observed for Ho:YAG. IR transmission studies revealed the existence of OH impurities in the Ho-doped halides, which possibly lead to non-radiative decay channels.

Glass-ceramics are composite materials consisting of crystals which are controllably grown within a glass matrix usually by applying an appropriate heat treatment. They possess outstanding optical properties with applications in solid state lasers, optical amplifiers, and now, laser induced cooling. For laser cooling, the material should exhibit specific properties like low phonon energy environment around the lanthanide ions, low background losses, high transparency and high photoluminescence quantum yield. In the present study, oxyfluoride glasses and ultra-transparent nano glassceramics doped with different concentrations (2, 5, 8, 12, 16 and 20 mol %) of Yb ³⁺ ions have been prepared by conventional melt-quenching and subsequent thermal treatments at different temperatures, respectively. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) measurements have been performed to characterize the thermal properties of the glass and the structural changes in the glass-ceramics, respectively. The XRD patterns confirm the growth of β-PbF₂ nanocrystals as well as progressive incorporation of Yb ³⁺ ions. This enhances the Yb ³⁺ ion emission intensity which depends on the doping concentration and ceramization temperatures. The size (20 nm) of the nanocrystallites was estimated from the Sherrer’s formula and found to increase with increasing ceramization temperature, small enough to avoid scattering losses and ensure an excellent transparency of the glass-ceramics comparable with that of the parent glass. An enhancement of the luminescence properties of Yb ³⁺ ions surrounded by a crystalline low phonon environment is observed. Finally, the utilization of these heavily Yb ^3+-doped ultra-transparent materials for laser cooling and solid state laser applications is discussed.

This paper reports on the characterization of nanocrystalline powders of ytterbium doped YLiF4 for applications in optical refrigeration. Here we used powders with nanocrystals of Yb ³⁺ concentrations of (10, 15, 20) mol % and lengths (70, 66, 96) nm. Our preliminary spectroscopic measurements did not show an enhancement in the absorption at the long-wavelength tail of the spectra of the nanocrystalline powder when compared with bulk Yb:YLiF4, indicating that the increase of the phonon-assisted excitation is not large enough to play a significant role in cooling in the present conditions. One advantage of nanocrystalline powders over bulk crystals is the possibility of enhancing the absorption by the realization of cavity-less pump recycling through photon localization [1]. While photon localization also increases the reabsorption of the fluorescence depending on the quantum efficiency of the material and can mitigate cooling, it allows the use of crystals of low enough concentrations to avoid deleterious effects such as ion-ion energy transfer followed by quenching. The pump intensity enhancement favors upconversion luminescence to visible wavelengths, which can be used for optical refrigeration and extends the scope of the application for the material. We observed both green and blue emission from the samples and investigate the processes which lead to it. We present the experimental investigation of the nanocrystals’ absorption and emission spectra and the first excited state lifetime measurements, which are used to estimate the nanocrystal’s photoluminescence quantum efficiency.

We report on the characterization of oxyfluoride glasses and glass ceramics for their application in optical refrigeration. Oxide glasses are chemically and mechanically stable and relatively ease to handle and fabricate, but their high maximum phonon energy leads to a nonradiative decay rate which is unacceptable for optical refrigeration. On the other hand, low-maximum phonon energy hosts such as fluorides lack the desirable mechanical and chemical stabilities to make them widely used. The combination of the high chemical and mechanical stability of oxides and the low maximum phonon energy of fluorides make oxyfluorides strong potential candidates for wide-spread use in optical refrigeration. Glasses and ultra-transparent glass-ceramics of molar composition 30SiO₂-15Al₂O₃-(27-x)CdF₂-22PbF₂-4YF₃-xYbF₃, with x = (2, 5, 8, 12, 16 and 20) mol % are investigated. The absorption and photoluminescence spectra, as well as the lifetime and the external quantum efficiency of the photoluminescence for these samples using an integrating sphere are reported. The effects of reabsorption on the measured mean fluorescence wavelength are also reported. The cooling efficiencies of the samples were measured as a function of the pump wavelength using a calorimetric method with a Ti:Sapphire laser pump source and a fiber Bragg grating sensor for a direct temperature measurement. Impurities and background absorption are also investigated using different pump sources and the calorimetric method. From a comparison of the cooling/heating performance of the oxyfluoride glasses and glass-ceramics containing various Yb³⁺ amounts, we developed a strategy to realize and enhance optical refrigeration in this class of material.

Methods of coherent pumping through dipole-allowed 5d levels of RE ion are proposed for laser cooling. The coherent and complete population transfer between the ground and the first excited levels of 4f multiplet is achieved by using the different Raman techniques, namely two-photon scattering, adiabatic passage method, and π-pulse pumping. It is shown that the multiplication of the number of electrons that participate in cooling cycle leads to increasing of the cooling power and to acceleration of the cooling process. The increasing of cooling efficiency of 0.5% compared to the direct pumping between 4f levels is attained through the use of dipole-allowed optical transitions. Performed estimates show that the sample temperature can achieve 94 K for current purity materials. The calculations are obtained for Yb³⁺:CaF₂ system.