This PDF file contains the front matter associated with SPIE Proceedings Volume 8638, including the Title Page, Copyright Information, Table of Contents, Introduction and Conference Committee listing.

Anti-Stokes fluorescence cooling has been demonstrated in a number rare earth doped materials. Ytterbium doped oxides and fluorides, such as ZBLAN, YLF, and YAG, were the first materials to exhibit cooling.^1,2,3 These materials were originally developed as laser gain media and fluorescence cooling was eventually incorporated into the 1μm lasers to reduce detrimental thermal loading.⁴ Anti-Stokes cooling can offset quantum defect heating allowing laser power to be scaled to very high average powers. Since the early work in ytterbium, fluorescence cooling has been demonstrated in both erbium and thulium doped materials.^5,6 These materials were also initially developed as lasing media and their fluorescence cooling could be used to increase laser powers at 1.5μm and 2.0μm. In this study we examine the radiative efficiency of holmium and ask the question, “Can anti-Stokes fluorescence cooling be extended beyond 2μm?”

We have achieved cryogenic optical refrigeration with a record low temperature in optical refrigeration by cooling 5% wt.Yb:YLF crystal to 119K ±1K (~-154 C) at 1=1020 nm corresponding to its E4-E5 Stark manifold resonance with an estimated cooling power of 18 mW. This demonstration confirms the predicted minimum achievable temperature (MAT). Further cooling is achievable as shown by measurements of a doping study where a 10% wt. Yb:YLF crystal with reduced parasitic heating has predicted cooling below 100K (~-173K).

We report on the use of a high power InGaAs quantum well vertical external-cavity surface-emitting laser (VECSEL) emitting at a wavelength of 1020 nm for intra-cavity cooling of a 5% Yb-doped YLF crystal to 148 K from room temperature. Similar crystals have now reached temperatures below the NIST-defined cryogenic temperature of 123 K when pumped outside a laser cavity. We 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.

We present a theoretical scheme for laser cooling with ytterbium doped indium phosphide (Yb³⁺:InP). Yb³⁺:InP is a rareearth doped direct band-gap semiconductor. The cooling process in our system is based on thermal quenching of excited ytterbium ions. The mechanism of cooling in our system consists of laser excitation of ytterbium ions in the long wavelength tail of the Yb³⁺absorption spectrum followed by thermal quenching of excited ions accompanied by phonon absorption providing cooling. The band-to-band radiative recombination completing the cooling cycle removes energy from the system. This new approach to laser cooling of solids permits an increase in the efficiency of the cooling cycle approximately by the order, to accelerate the cooling process considerably, and allows cooling with pump wavelengths shorter than the mean fluorescence wavelength.

Experimental demonstration of net electro-luminescent cooling in a diode, or equivalently electroluminescence with wall-plug efficiency greater than unity, had eluded direct observation for more than five decades. We review experiments demonstrating light emission from a light-emitting diode in which the electron population is pumped by a combination of electrical work and heat.

We have demonstrated the first net laser cooling of semiconductors using CdS nanoribbons (or nanobelts) in this work. This net cooling effect is found to be facilitated by resonant high order annihilation of longitudinal optical (LO) phonons due to a strong exciton-LO phonon Fröhlich interactions. Using a pumpprobe luminescence thermometry technique to measure the local temperature change, we have achieved as large as 40 K cooling temperature from room temperature pumped by a 514 nm laser while a 532 nm laser pumping led to a cooling of 20 K. At 100 K, only the 532 nm laser pumping can lead to a net cooling of around 15 K. Our work opens new directions to search laser cooling semiconductors and makes it feasible to achieve all solid-state cryocoolers based on semiconductors.

Laser cooling by collisional redistribution of radiation has been successfully applied in the past for cooling dense atomic gases. Here we report on progress of work aiming at the demonstration of redistribution laser cooling in a molecular gas. The candidate molecule strontium monohydride is produced by laser ablation of strontium dihydride in a pressurized noble gas atmosphere. The composition of the ablation plasma plume is analyzed by measuring its emission spectrum. The dynamics of SrH molecular density following the ablation laser pulse is studied as a function of the buffer gas pressure and the laser intensity.

Heat switches are a key enabling element of efficient refrigerators that are based on the electrocaloric effect. We demonstrate a new concept for a heat switch that is based on micro-scale electrohydrodynamic (EHD) flows in thin layers of dielectric fluids. In this device, convective flow of the fluid is controlled by applying an electric field across the fluid layer. This creates a heat switch that can be cycled between a “closed” state with efficient convective heat transport and an “open” state with less efficient conductive heat transport. Substantial switching of the thermal transport coefficient was achieved in 500 μm thick layers of commercial hydrofluoroethers and bias voltages of typically 390 V. The efficacy of the heat switch varied by almost four orders of magnitude for different biasing schemes. The highest efficacy was achieved by biasing a patterned strip electrode and using a planar ground electrode. A preliminary experiment found a thermal conductivity contrast of 4.7±1.1 for the switch in the closed vs. open state. We also characterize the electrocaloric response of commercial multilayer ceramic chip capacitors and show that they can serve as serve as a useful surrogate material for first-generation electrocaloric refrigerators until higher performing multilayer structures of ferroelectric polymers are available.

We present our experimental and theoretical results of optical cooling in Tm-doped glass fibers as optical cooler pumped by single-mode, high efficiency and high power Tm-doped glass fiber lasers. The effects of impurities including OHabsorption and transition metals have been investigated systematically using different purified glasses for fiber fabrication. Our experimental results of spectroscopic measurements show temperature drops of more than 30 degrees from room temperature with pump powers of less than 3W. The results are in good agreement with theoretical simulation.

The state of current research in laser cooling of semiconductors is reviewed. Record external quantum efficiency (99.5%) is obtained for a GaAs/InGaP heterostructure bonded to a dome lens at 100 K by All-optical Scanning Laser Calorimetry (ASLC). Pulsed-Power-dependent photoluminescence measurement (Pulsed-PDPL) is proved to be an efficient way to determine the quantum efficiency and screen the sample quality before processing and fabrication. Second harmonic generation (767nm) from a 5ns Er:YAG laser is used as the pump source for the pulsed-PDPL experiment.

We show for the first time to our knowledge, measurements of anti-Stokes fluorescence and lifetime measurements in quantum dot (QD) doped glass, which has been proposed as a potential material for optical refrigeration recently. The glass host studied here is known as SNAB (SiO₂, Na2CO₃, Al2O_3, B2O₃) and is doped with PbS QDs. We show that when excited at a proposed pump wavelength (1550 nm) for cooling, anti-Stokes fluorescence is emitted, required for laser cooling. We also show fluorescence lifetime measurements in this glass, which is in the order of 500 ns. This lifetime is 3-4 orders of magnitude shorter than the typical lifetime of rare earth dopants in glass. From additional fluorescence spectrum measurements at a higher pumpenergy (1.064 μm), we estimate the quantum efficiency of such a system. The observation of anti-Stokes fluorescence and the short lifetime is evidence that QDs could be developed as potential candidates for laser cooling in the solid state, however improvements would have to be made in the quantum efficiency as well as in the background absorption of the host glass for successful applications.