This PDF file contains the front matter associated with SPIE Proceedings Volume 9000, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

With the coldest solid-state temperatures (ΔT <185K from 300K) achievable by optical refrigeration, it is now timely to apply this technology to cryogenic devices. Along with thermal management and pump absorption, this work addresses the most key engineering challenge of transferring cooling power to the payload while efficiently rejecting optical waste-heat fluorescence. We discuss our optimized design of such a thermal link, which shows excellent performance in optical rejection and thermal properties.

We present a characterization of the optical refrigeration properties of two 5% Ytterbium-doped YLF crystals. Measurements showed different cooling efficiencies for the samples, despite the same concentration of dopant. We carried out a spectroscopic study on the crystals devoted to the identification of foreign contaminants inside them. These searches determined the presence of Erbium and Holmium impurities in both the cooling samples. We attributed the reduction of the cooling efficiency in one of the crystals to an increased amount of these contaminants.

The role of transition-metal impurities in Yb³⁺-doped YLiF4 (YLF) laser-cooling crystals is studied. Divalent 3d transition-metal ions, in particular Fe²⁺, are found to have strong absorptions at the laser cooling pump wavelength and degrade the cooling efficiency by introducing background absorption. A set of eight substitutional and chargecompensated defects that form upon introduction of 1+, 2+, and 3+ transition-metal ions into the YLF crystal lattice is proposed. A calculation of solution energies for each defect type and for a range of 3d ions is carried out. It indicates that divalent 3d ions preferentially substitute for Y³⁺ accompanied by a fluoride vacancy for charge compensation. An electron paramagnetic resonance (EPR) study of a YLF crystal identifies Fe²⁺ in the crystal lattice, in agreement with the elemental analysis and the computational results. A strategy for purifying the YF₃, LiF, and YbF₃ starting materials for the YLF:Yb crystal growth is discussed. Chelate-assisted solvent extraction purification with pyrrolidine dithiocarbamate (APDC) for Y, Li, and Yb as well as ethylenediaminetetraacetic acid (EDTA) for Li was carried out.

Laser cooling of Yb:YLF crystal to 131 K from room temperature has been demonstrated in an active intracavity arrangement for enhanced pump absorption. The laser is a high-power, broadly-tunable InGaAs/GaAs MQW VECSEL capable of producing 20 Watts at 1020 nm, directly at the E4-E5 transition of the Yb-ion. This is the coldest temperature achieved to date in an intracavity geometry and without sophisticated heat load management of the crystal. This progress presents a significant advancement towards an all-solid-state compact cryocooler.

This work describes a band-engineered transverse thermoelectric with p-type Seebeck in one direction and ntype orthogonal, with off-diagonal terms that drive heat flow transverse to electrical current. Such materials are named p × n type transverse thermoelectrics. Whereas thermoelectric performance is normally limited by the figure of merit ZT, p × n type materials can be more easily geometrically shaped and integrated for devices, leading to more compact, longer lifetime, enhanced efficiency coolers for infrared detectors or photovoltaic generators.

Recent work on electro-luminescent cooling has focused either on diodes at forward bias voltages just below the bandgap energy, where high cooling power density is possible, or voltages below the thermal voltage, where the effect is more tolerant to parasitic non-radiative recombination. Here we consider the possibilities for diodes designed to operate at intermediate voltages. Numerical calculations suggest that design for this regime may enable near- and mid-infrared devices capable of solid-state refrigeration with sufficient power density for some applications.

We study laser cooling of atomic gases by collisional redistribution of fluorescence. In a high pressure buffer gas regime, frequent collisions perturb the energy levels of the alkali atoms; which enables the absorption of a far red detuned irradiated laser beam. Subsequent spontaneous decay occurs close to the unperturbed resonance frequency, leading to a cooling of the dense gas mixture by redistribution of fluorescence. Thermal defection spectroscopy indicates large relative temperature changes down to and even below room temperature starting from an initial cell temperature near 700K. We are currently performing a detailed analysis of the temperature distribution in the cell. As we expect this cooling technique to work also for molecular-noble gas mixtures, we also present initial spectroscopic experiments on alkali-dimers in a dense buffer gas surrounding.

A theoretical model for optical cooling is developed, which yields the overall efficiency of a single endpumped cooling system. This model includes the effects of background absorption and pump saturation, while in multi-level systems, the model accounts for the important energy transfer processes. Two-level efficiency is evaluated for the case of Yb:YAG and compared with a hypothetical three-level material with identical spectral properties. This model is readily modified to include more levels and different materials.

We consider a theoretical model for laser cooling with anti-Stokes fluorescence of a Yb³⁺:YAG sample. We have estimated the fluorescence power density removed from the system with spontaneous emission, the power density radiated with stimulated emission as well as the heat power density generated in the system by non-radiative decays from the impurities in the host material. We have investigated the influence of each of these power densities on the cooling process in Yb³⁺:YAG system. It has been shown how the temperature dependences of the different parameters of the system as well as the concentration of the impurities in the host influence the final temperature of the cooled sample.

Although Yb:YAG has been cooled in a vacuum environment1, we report for the first time an experimental demonstration of optical cooling at atmospheric pressure. A Yb:YAG crystal is supported on thin silica fibers, inside a matt-black chamber with air at atmospheric pressure, and pumped at 1029 nm in the pulsed and CW regimes. Direct measurement of the crystal surface temperature during pumping was made possible by using a low thermal-mass, transparent fiber Bragg grating (FBG) sensor. The FBG interrogation system has sufficient sensitivity to measure the background absorption of the sample to below 10^-4 cm^-1, and bulk cooling at a pump power as low as 17 mW. The dynamical measurement of the temperature allows the determination of the overall heat transfer coefficient of the sample in the air, of 22 W.m^-2K^-1. A temperature drop of 8.8 K from the chamber temperature is observed in the Yb:YAG crystal in air when pumped with 4.2 W at 1029 nm, close to 8.9 K observed in vacuum1. A background absorption α_b = 2.9×10-4 cm^-1 is estimated with a pump wavelength at 1550 nm. Simulations predict further cooling when the sample’s cross sectional area and the pump power are optimized, including absorption saturation effects. The choice of an efficient geometry, the use of a readily available temperature sensor in less controlled environments should simplify implementation of laser cooling systems and the development of commercial devices.

Realization of anti-Stokes cooling requires high enough photon extraction efficiency as well as quantum efficiency, making the implementation of this technique extremely difficult for semiconductors. Here, for the first time, we demonstrate that the Coulomb interaction between photogenerated electron-hole pairs in strong piezoelectric materials such as GaN/InGaN quantum wells could assist laser cooling. By comparing to the cavity back-action mechanism, we also explain how this process depends upon laser detuning with respect to bandgap. To demonstrate the advantage of this method even further, we present simulations by using experimentally reported parameters of GaN and In_0.15Ga_0.85N, in order to conclude that the net cooling is indeed possible even with current III-nitride growth technology.

A laser can convert pump energy to light in a coherent single mode field of an optical resonator. If the energy is taken from a thermodynamic reservoir the energy conversion process reaches Carnot efficiency. From a different viewpoint, a laser thus can also be seen as a refrigerator for efficient heat extraction from hot environment via stimulated emission. This process can be studied well at a toy model of a quantum well structure with suitably designed tunnel-coupled wells kept at different temperature. Lasing appears concurrent with amplified heat flow and points to a new form of stimulated solid state cooling. From a practical point of view, this mechanism could help to raise the operating temperature limit of quantum cascade lasers by substituting phonon emission driven injection, which generates intrinsic heat, by an extended model with phonon absorption steps. Other implementations in systems with phonon driven up conversion of photons could be equally envisaged.

Tm³⁺ doped solids have shown promising results for laser cooling applications at IR wavelengths of ~2 μm. The extended IR fluorescence of the involved Tm³⁺ transition (³H₆ → ³F₄), however, requires low-phonon energy hosts reducing the detrimental effect of non-radiative decay through multiphonon relaxation. In this work the temperature dependent absorption and emission properties of Tm doped KPC (hν_max<200 cm^-1) and KPB (hν_max<140 cm^-1) crystals were evaluated for applications in laser cooling. Under laser pumping both crystals exhibited broad IR fluorescence at room temperature with a mean fluorescence wavelength of 1.82 μm and bandwidth of 0.14 μm (FWHM). Initial experiments on laser-induced heating and cooling were performed using a combined IR imaging and fluorescence thermometry setup. Employing a continuous-wave laser operating at 1.907 μm, Tm: KPC and Tm: KPB crystals revealed a very small heat load resulting in a temperature increase of ~0.3 (±0.1) °C compared to undoped reference samples. Further work on material improvement will be necessary to identify possible non-radiative loss mechanisms and to improve the crystal quality.

Aimed to promote light conversion processes for radiative cooling of solids, we consider operation principles and fundamentals of two recently discovered approaches to make semiconductors cooled through photo excitation of free carriers. These are light up conversion that is due to above bandgap thermal-assisted luminescence converting heat into light and light down conversion occurring due to the enhancement of the thermal emission output when the overall energy of multiple below bandgap photons escaping a semiconductor exceeds the energy of a single pumped photon. These two processes come at the cost of the internal energy of an object by causing therefore it cooled. Figures of merit will be the microscopy of the processes, cooler band structure, entropy limitations, cooling power, cooling and power conversion efficiencies, and cooling temperature range.