This PDF file contains the front matter associated with SPIE Proceedings Volume 6461, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

Here we report the first experimental evidences of anti-Stokes laser-induced cooling in two different low phonon erbium-doped matrices: a KPb₂C_l5 crystal and a fluorochloride glass. The local cooling was detected by using a photothermal deflection technique whereas the bulk cooling was detected by means of a calibrated thermal sensitive camera. The Er³⁺ ion was excited in the ⁴I_9/2 manifold. It is worthwhile to mention that the cooling was observed in the spectral region where some upconversion processes that initiate at the pumped level occur. Together with the spectroscopic characterization, a short discussion on the experimental and theoretical background of the cooling process including the possible influence of upconversion processes is presented.

A quantitative description of optical refrigeration in Yb³⁺-doped ZBLAN glass in the presence of transition-metal and OH impurities is presented. The model includes the competition of radiative processes with energy migration, energy transfer to transition-metal ions, and multiphonon relaxation. The cooling efficiency is sensitive to the presence of both 3d metal ions with absorption in the near infrared and high-frequency vibrational impurities such as OH. The calculation establishes maximum impurity concentrations for different operating temperatures and finds Cu²⁺, Fe²⁺, Co²⁺, Ni²⁺, and OH to be the most problematic species. Cu²⁺ in particular has to be reduced to <2 ppb, and Fe²⁺, Co²⁺, Ni²⁺, and OH have to be reduced to 10-100 ppb for a practical ZBLAN:Yb³⁺ optical cryocooler to operate at 100-150 K.

We discuss a cavity enhanced resonant absorption (CERA) approach for increasing optical pump absorption. This is demonstrated in the context of laser cooling of solids, where the absorption in ytterbium (Yb⁺³) ion is increased by over an order of magnitude. This corresponds to more than 90% absorption efficiency. We demonstrate cooling with Yb:ZBLAN and obtain &Dgr;T = -3 K starting from room temperature.

We present a brief analysis of laser cooling in semiconductor quantum wells with emphasis on a previous experiment that gave evidence for local cooling. This work is re-examined in the context of light management, heat removal, and how increasing photo-carrier density affects luminescence. Our main conclusion is that using a single laser to both pump the sample and monitor temperature may lead to ambiguity in semiconductor cooling experiments.

Understanding and quantifying nonradiative recombination is a critical factor for the successful laser cooling of semiconductors. The usual approach to measuring the nonradiative lifetime employs pulsed photoexcitation and monitors the luminescence decay via time-resolved photon counting. We present an alternative approach that employs phase fluorometry with a lock-in amplifier. A sinusoidally modulated diode laser is used for excitation. Lifetime data are extracted from the frequency dependent phase shift and amplitude response of the photolumi-nescence signal, detected by a photomultiplier tube. Samples studied include high quality AlGaAs/GaAs/AlGaAs and GaInP/GaAs/GaInP double heterostructures, grown by MBE and MOCVD. Data over a temperature range from 10 to 300 K is compared with results obtained in time-domain measurements.

We investigate the role of surface defects on semiconductor fluorescence lifetime using near-field scanning optical microscopy (NSOM) and time correlated single photon counting (TCSPC). A conventional far-field microscope is used to excite a GaAs sample and subsequent fluorescence is collected with a fiber coupled near-field probe. With the application of custom fitting algorithms, we find fluorescence lifetimes in the vicinity of surface defects to be significantly reduced with respect to fluorescence lifetimes measured in defect free regions.

We have developed a non-contact spectroscopic technique to measure the temperature change of Thulium-doped glasses and crystals with high precision. The approach makes use of temperature-dependent broadening of various peaks in the luminescence spectra. A weak, cw probe laser excites the sample in vacuum. The luminescence spectrum is collected by a fiber and routed to a high-resolution spectrometer. We have demonstrated temperature resolution of 62 mK in Tm:BaY₂F₈.

Under appropriate conditions absorption of light by a solid can initiate a process by which it is cooled. Here, two schemes for laser cooling via localized electrons are addressed. The first scheme utilizes two states of a localized center. In this two-level scheme, the cooling process is initiated with photon absorption in the low-energy tail of a localized state's strain-broadened absorption spectrum. The subsequent atomic relaxation transfers energy of especially large vibratory atomic strains into electrical energy that is then extracted via photon emission. Cooling can occur at elevated temperatures but is suppressed as the temperature is lowered. The second scheme involves three energy levels of a localized center. Cooling is facilitated when i) the photo-excitation of an electron promotes it to the lower of the two upper levels followed by ii) its electron-phonon-induced promotion to the upper-most level and the subsequent iii) return of the electron to its initial state via emission of a photon of higher energy than that of the absorbed photon. However, competing relaxation processes contribute to heating. The net cooling power is calculated. Heating predominates at low temperatures. Significant cooling at elevated temperatures requires satisfying very restrictive conditions. Among these: i) the energy separation between two highest states must be very small; ii) the degeneracy of the highest state must exceed that of the state below it, and; iii) the effective electron-phonon interaction, responsible for energy levels' Stokes shifts, must be exceptionally weak. Different avenues to promising systems to achieve laser cooling are identified.

We present a novel approach to intrinsically mitigate the heat dissipated in Raman lasers due to the pump-Stokes quantum defect. We explain the principle of this so-called "Coherent Anti-Stokes Raman Scattering (CARS)-based heat mitigation", which is based on decreasing the amount of phonons created in the Raman medium by increasing the ratio of the number of anti-Stokes photons to the number of Stokes photons coupled out of a Raman laser. To illustrate the potentialities of this optical cooling technique, we numerically demonstrate that for mid-infrared silicon-based Raman lasers the heat dissipation can be reduced with at least 35% by the use of CARS-based heat mitigation.

We report the successful growth and the laser cooling results of Yb³⁺-doped single fluoride crystals. By investigating the mechanical and thermal properties of Yb-doped BaY₂F₈ and LiYF₄ crystals and using the spectroscopic data we collected from our samples, the theoretical and experimental cooling efficiency of fluoride crystals are evaluated and compared with respect to those of ZBLAN. Two different methods, a thermal camera and a fluorescence intensity ratio technique, have been used to monitor the temperature change of the samples. The temperature change is clearly exponential, as expected from theory, and the temperature drops are 6.3 K and 4 K for Yb:LiYF₄ and Yb:BaY₂F₈ respectively in single-pass configuration, corresponding to a cooling efficiency of about 2% and 3%. This last value is slightly larger than that observed in Yb-ZBLAN in similar experimental condition.

Laser refrigeration of solids is a promising technology capable of remotely removing heat from a small active device with minimum expended energy. The most critical factors limiting performance of laser cooler are low quantum efficiency and trapping of fluorescence inside the cooling volume due to total internal reflection. This issue becomes especially pressing in semiconductors due to their large refractive index. In this work we explore two new methods of improving the refrigeration efficiency - using type II quantum wells and using surface plasmon polaritons.

Doping of the clad layers in thin GaAs/GaInP heterostructures, displaces the band energy discontinuity, modifies the carrier concentration in the active GaAs region and changes the quality of the hetero-interfaces. As a result, internal and consequently external quantum efficiencies in the double heterostructure are affected. In this paper, the interfacial quality of GaAs/GaInP heterostructure is systematically investigated by adjusting the doping level and type (n or p) of the cladding layer. An optimum structure for laser cooling applications is proposed.

One of the challenges of laser cooling a semiconductor is the typically high index of refraction (greater than 3), which limits efficient light output of the upconverted photon. This challenge is proposed to be met with a novel concept of coupling the photon out via a thin, thermally insulating vacuum gap that allows light to pass efficiently by frustrated total internal reflection. This study has the goal of producing a test structure that allows investigation of heat transport across a 'nanogap' consisting of a thin film supported over a substrate by an array of nanometer-sized posts. The nanogap is fabricated monolithically by first creating a film of SiO₂ on a silicon substrate, lithographically defining holes in the SiO₂, and covering this structure including the holes with silicon. Selective lateral etching will then remove the SiO₂, leaving behind a thin gap between two Si layers spaced apart by nanometer-scale Si posts. Demonstration of this final step by successfully undercutting the a-Si upper layer due to the hydrophobic nature of silicon and the slow etch rate of buffered oxide etch in the small gap has proved to be problematic. Arriving at a feasible solution to this conundrum is the current objective of this project in order to begin investigating the thermal conductivity properties of the structure.

In efforts underway to achieve laser cooling of semiconductors, an electron-hole population is generated in the sample and maintained in a steady state. The analysis of light absorption by and luminescence from this population is basic to the understanding of feasibility and efficiency issues of the cooling process. It is commonly understood that, when this electron-hole plasma is in quasi-thermal equilibrium (equilibrium at a fixed density), the KMS (Kubo-Martin-Schwinger) relation holds between its luminescence and absorption spectra: their ratio is proportional to the Bose distribution function characterized by the temperature and chemical potential of the plasma. The proportionality factor, which affects the total luminescence rate, may generally depend on the dimensionality and geometry of the system. In this Contribution, as a preliminary step to extend our theoretical analysis of semiconductor cooling to quantum well systems, we discuss the application of the KMS relation to their spectra. In particular, we derive and discuss the geometrical proportionality factor in the KMS relation for quantum wells and compare it to its counterpart for bulk semiconductors.

It has been proposed recently that thermally assisted electroluminescence may in principle provide a means to convert solar or waste heat into electricity. The basic concept is to use an intermediate active emitter between a heat source and a photovoltaic (PV) cell. The active emitter would be a forward biased light emitting diode (LED) with a bias voltage, V_b, below bandgap, E_g (i.e., qV_b < E_g), such that the average emitted photon energy is larger than the average energy that is required to create charge carriers. The basic requirement for this conversion mechanism is that the emitter can act as an optical refrigerator. For this process to work and be efficient, however, several materials challenges will need to be addressed and overcome. Here, we outline a preliminary analysis of the efficiency and conversion power density as a function of temperature, bandgap energy and bias voltage, by considering realistic high temperature radiative and non-radiative rates as well as radiative heat loss in the absorber/emitter. From this analysis, it appears that both the overall efficiency and net generated power increase with increasing bandgap energy and increasing temperature, at least for temperatures up to 1000 K, despite the fact that the internal quantum yield for radiative recombination decreases with increasing temperature. On the other hand, the escape efficiency is a crucial design parameter which needs to be optimized.

We present the growth and characterization of high quality semiconductor laser cooling material. The structure consists of GaAs passivated by InGaP which has been reported to have a longest surface recombination lifetime. GaAs was grown on 10 degree miscut GaAs substrate and sandwiched between lattice matched In_0.49Ga_0.51P. This structure was grown by a low-pressure metal organic chemical vapor deposition (MOCVD) system. The material was grown in the temperature range of 550 to 700 °C at 60.8 Torr. The morphology of InGaP was improved by the growth on 10 degree miscut substrate along <110> direction, which is confirmed by X-ray diffraction (XRD). The uninterrupted growth technique and GaP separation layer are employed to prevent the indium segregation and P/As intermixing at the interface between InGaP and GaAs. The effects of V/III ratio, growth temperature and material precursors on material impurities were also studied. The carrier lifetimes were measured using the time resolved photoluminescence (TRPL) technique at cryogenic temperatures. The experimental results show that the carrier lifetime was increased by 5 times with the use of TBA as arsenic source in place of AsH₃. Recent results show a highest room temperature carrier lifetime of 2 &mgr;sec.