This PDF file contains the front matter associated with SPIE Proceedings Volume10450, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

We have investigated the effect of co-doping Yb –Tm on the cooling parameters :  and EQE. We have studied six different samples, three single doped with 5% and 10% Yb3+ and three co-doped with Tm3+: 16 ppm, 40 ppm and 80 ppm, respectively. The samples have been cut and optically polished for cooling investigation. We obtained for a three co-doped samples an interesting effect on cooling parameters: the decrease of the  , the increase of EQE and the red shift of the peak wavelength. To understand these effects it has been studied the absorption and emission spectroscopy. In particular we studied the emission of the manifolds of Tm3+ (1G4 and 3H4) and the energy transfer mechanisms between Yb-Tm to investigate the variation of cooling parameters. This study continued in-depth both with dynamic spectroscopic of Yb 3+ and Tm3+ levels in single and co-doped samples to put in evidence the cross-relaxation effect on the levels and possible effects caused by impurities transitions metals.

Laser cooling of solids, also known as optical refrigeration, is an area of optical science investigating the interaction of light with condensed matter to remove thermal energy of a solid through the interaction of the pump photons and phonons in a solid. Apart from being of fundamental scientific interest, this topic addresses a number of important practical issues such as the development of all solid state optical cryo-coolers, and biological applications. A short history of laser cooling as well as latest achievement of optical refrigeration in rare-earth (RE) doped macro-samples are presented and discussed in the paper. The main technique of laser cooling of RE doped solids based on anti-Stokes fluorescence is presented in this paper. The new approach to optical refrigeration based on the Raman cooling is also considered. It is shown that the future prospects of the research are connected with laser cooling of μm- and nm-sized samples, are in their applications in biophysics in the fundamental studies of low-temperature physics.

Optical refrigeration of rare-earth doped crystals has exceptional qualities that can be used for building a compact and vibration-free all-solid-state optical cooler. Estimating the lowest achievable temperature and cooling power of such a device requires accurate measurements of external quantum efficiency, mean fluorescence wavelength, and parasitic absorption. Here we discuss temperature dependent measurements of these parameters for a high quality Yb:YLF sample by performing a LITMoS test (Laser Induced Temperature Modulation Spectrum) combined with contact-free differential luminescence thermometry. These measurements are challenging at low temperatures, but by integrating these two methods, we can perform LITMoS test at any temperature.

Radiation-balanced lasers (RBL) combine solid-state optical refrigeration and lasing in one material to enable a net zero thermal load that allows for favorable scaling to high laser powers. A high-performance RBL material, therefore, has to first qualify as a high-performance laser-cooling material. This necessitates exquisite material purity in order to achieve the required near-unity external quantum efficiency and low background absorption. Solvent extraction, ion exchange, and electrochemical treatment of aqueous solutions or melts are some of the techniques available for the purification of starting materials used in the growth of RBL crystals. Scaling these methods to the 100s of gram scale needed for traditional Czochralski crystal growth while maintaining parts-per-billion level impurity concentrations however has proven challenging in several past efforts. In contrast, we have previously shown solvent extraction and electrochemical treatment to be effective on the several gram scale. This creates a need for exploring alternative methods for growing optical-cooling-grade fluoride crystals on the small scale. We will present results on growing Yb-doped YLiF4 (YLF) and LuLiF4 (LLF) single crystals using the vertical Bridgman method. The external quantum efficiency and background absorption of these samples will be reported and discussed in the context of RBL.

We will review strategies for using the radiation-balanced technique to address the problem of excessive heat generation in optical fiber lasers and amplifiers. Our approach is to study the design parameter space and explore the role of different parameters in making an effective athermal device. The fiber geometry (core and cladding dimensions), its optics (fiber materials and refractive indices), the choice of the rare-earth gain materials and their spatial distribution in the fiber, and device configuration (pumping schemes and laser-cavity or amplifier design) will be investigated. Other issues such as the thermometry of the optical fiber will also be discussed.

Recent experimental breakthroughs in the laser-refrigeration-of-solids (LRS) have demonstrated that cryogenic temperatures can now be achieved opening up a range of promising applications using compact, vibration-free optical cryocoolers. These results also have stimulated significant interest in the development of new material designs for applications in radiation balanced lasing (RBL). The development of practical host materials for RBLs requires the understanding of how both spontaneous emission rates and non-radiative decay rates change under a wide range of thermal conditions and dielectric host environments. In this work the photoluminescence lifetime of 4S3/2 transitions from Er(III) ions within co-doped Yb3+/Er3+-codoped hexagonal sodium-yttrium-fluoride (beta-NaYF4) nanostructures is presented as a rapid, low-cost, spatially resolved method of quantifying the temperature of within RBL materials. Lifetime measurements from single nanostructures are made using single-beam laser-traps, where the focal plane of the trapping laser is used to control the spacing between single nanowires and dielectric chamber surfaces that are supported by a temperature-controlled piezo-stage. The lifetime of Er(III) ions is observed to change significantly based on the distance between emitting dipoles and nearby dielectric interfaces and also as a function of chamber temperature. Lifetime measurements are also presented for measuring the temperature within polydimenthylsiloxane-polymer/nanocrystal composite materials that serve as a model system for future optical-fiber cladding materials. Lastly, ratiometric photoluminescence and lifetime measurements will be presented for Yb(III):YLiF4 microcrystals supported on cadmium sulfide nanoribbon cantilevers, indicating the potential for hybrid semiconductor/RE-fluoride composite structures for future RBL applications.

Radiation balanced lasing (RBL) is an attractive pathway towards development of high power and good beam quality lasers because heat removal via anti-Stokes luminescence (optical refrigeration) does not require additional connections and components and the heat is dissipated away from the active medium. Optical refrigeration had been demonstrated in rare-earth doped laser medium, but is far more difficult to achieve it in semiconductors laser medium. The main obstacle to RBL in semiconductors that the most efficient cooling occurs at relatively low carrier densities, while the gain required to sustain laser operation requires much higher densities. In this talk we explore the means of resolving this conundrum by separating the optical refrigeration and lasing in temporal, spatial, and/or spectral domains. Time multiplexing involves modulating the pump and operating the laser in pulse modes with lasing and cooling intervals. Space multiplexing involves having separate regions (quantum wells and dots) for lasing and cooling. The spectral multiplexing involves operating with two separate pumps – one for lasing and one for cooling. This methods will be compared in the talk with the goal of selecting the optimal path towards radiation balanced semiconductor lasers.

Employing large surface-area-to-volume ratio gain, thin-disk lasers have shown great potential in power scaling. But thermal management for these devices is still challenging. One possible approach is to balance the heat load generated by the lasing process with cooling power from the anti-Stokes cooling process, forming radiation balanced lasers (RBLs). Compared to bulk RBLs, thin-disk RBLs can be better thermally balanced with reduced thermal gradients, promising higher output power and better beam quality. In this paper, we analyze and investigate radiation balanced disk lasers with Yb:YAG and Yb:YLF crystals in different pumping configurations.

The concept of condensed phase optical cooling has existed for nearly 90 years ever since Pringsheim proposed a conceptual approach for cooling 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 low background absorption. Until recently, solid state 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. 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 via the involvement of donor-acceptor states.

Cadmium sulfide (CdS) has been studied for decades due to a variety of applications from photovoltaic cells to solid state lasers owing to its direct bandgap of 2.42 eV (512 nm) and a high radiative quantum efficiency. Recently it has been considered to be a potential candidate for radiation balanced lasing. In particular, nanoribbons (NRs) of CdS have been claimed to laser cool at 514 and 532 nm wavelengths, due to annihilation of phonons to produce antistokes fluorescence near the bandgap. In an effort to verify the claim, we demonstrate a novel optomechanical experimental technique for micro-thermometry of a single CdS-NR where the material’s Young’s modulus is the primary temperature-dependent observable. The eigenfrequency of individual cantilevers is measured as a function of the laser irradiance by processing the time-dependent photovoltage of an avalanche photodiode. We observe a red-shift in the cantilever’s eigenfrequency with increasing laser power, suggesting net heating at low laser irradiances. However, a heating effect combined with possible laser trapping forces has been hypothesized for higher powers based on a modified Euler-Bernoulli elastic-beam model. The experimental cantilever heating results are supported by a heat transfer analysis to obtain the temperature distribution in the cantilever and the time required to reach steady state (<1ms). This thermometry technique can be used to probe the effects of laser irradiation on CdS cantilevers fabricated from thin films grown by pulsed laser deposition.

Despite achievements of extremely high external quantum efficiency (EQE), 99.5%, the net cooling of GaAs|GaInP double heterostructures (DHS) has been elusive. This is primarily due to the parasitic absorption, which originates from the GaInP passivation layers at long wavelengths. In samples with thin GaInP passivation layers, we report an EQE of 99%, approaching theoretical requirement for being heat neutral. Additionally, we investigate the EQE of MBE-grown GaAs|AlGaAs DHS versus temperature; the results compare well with that of GaAs|GaInP at and below 150 K. Also, initial measurements of parasitic absorption at shorter wavelengths is presented.

As the best performing light emitting diodes (LEDs) are approaching the conventional limit of unity efficiency, a unique heat-pump operating mode of the devices has been proposed to address this problem, in which case lattice heat is pumped from the phonon field of the device into the incoherent photon field of emission at the expense of consuming zero-entropy electrical power. To better understand the potential of visible LEDs for further efficiency improvement in this mode, we present a thermodynamic framework that allows us to estimate the Carnot limit for their wall-plug efficiency (WPE) at different operating conditions. We find that the theoretical efficiency limit drops at higher light intensities but can still be well above 100% even at 10 W/cm^2. Ideally, realizing such high efficiency at useful output powers requires the device to possess an external quantum efficiency (EQE) close to unity. Here we are able to introduce dissipation into the thermodynamic model and thus determine a minimum EQE required for an LED to achieve unity WPE. In addition, the thermodynamic study for visible LEDs yields one surprising result. The first observation of above-unity WPE was on a heated mid-infrared (2.2 um) LED, and the subsequent demonstration at room temperature necessarily required a longer-wavelength 3.4 um device in order to realize sufficient carrier injection for measurable optical output. On the contrary, this thermodynamic analysis indicates that at useful optical powers – and hence useful cooling powers – visible LEDs of shorter wavelength are expected to show higher cooling at a lower current density.

Conversion efficiency of broad-band sunlight in single-junction photovoltaics (PV's) is limited due to heat dissipation to less than 32%. Overcoming this requires inventive techniques, where their viability is compared to cheap and abundant silicon photovoltaics on one hand; or the efficient and costly multi-junction cells on the other. The recently proposed Thermally Enhanced Photoluminescence (TEPL) conversion device may take the place in-between, and potentially present high conversion efficiencies – with a single-junction solar cell. A PV cell is placed adjacent to a thermally insulated photo-luminescent (PL) absorber. The absorber is excited and heated by concentrated sun-light, consequently radiating blue-shifted PL emission toward the PV cell, resulting in higher conversion efficiencies compared to direct illumination or Thermal-PV at similar temperature. Spectral measurements based calculations show that efficiencies over 46% may be reached using a GaAs PV, and absorber working temperatures below 1500°C. TEPL prototype faces two major design challenges: absorber material, and photon management. Broad absorption, together with high PL external quantum efficiency (EQE), must be maintained at high temperatures. Here we demonstrate our achievements toward a TEPL converter. Using Cr, Ce & Nd, co-doped in YAG, we reach over 85% EQE with full absorption of sunlight up to 1.1µm. For photon recycling, paramount to maintain high chemical potential of the solar radiation, highly reflective surfaced and dichroic mirrors surround the absorber; reflecting photons not used by the PV cell to be reabsorbed in the absorber. We demonstrate a TEPL conversion device predicted to support conversion efficiencies over 15% under these conditions.

The industrial sector consumes one third of its total incoming energy and the remaining energy is discharged as wasted heat. Here, we experimentally demonstrate Thermally Enhanced Photoluminescence device for harvesting industrial wasted heat using low band gap photovoltaics. Specifically, the emission of a low band gap photoluminescence material excited next to its band edge and is heated in parallel, is characterized by a conserved photon flux of a blue-shift emission that can be coupled to a higher bandgap PV generating enhanced electricity due to the high operating voltage. The efficiency of such concept is orders of magnitude higher than the efficiency of thermal emission at the same temperature. Using a hot plate at 50oC-100oC as a heat source and Er:Tm doped silica, we have observed the absorption at a spectral range between 1.6 um and 1.9 um that is followed by a blue-shifted photoluminescence at 1.6 um that can be efficiently coupled to GaInAs or Ge solar cell. Using efficient LED and solar cell can result in a pseudo “perpetual motion” where the excessive heat together with the LED pump generates electric power exceeding the LED power consumption. This device can be extended for harvesting solar radiation between 1µm and 2 µm, which is considered as wasted radiation. Theoretical result shows that the ideal system can reach up to 28% efficiency cells, overcoming thermo-electrics concepts.

We use a comprehensive model of cooling by anti-Stokes fluorescence in a single-mode fiber that includes the effects of fiber loss, concentration quenching, mode profiles, and amplified spontaneous emission to analyze the trends of cooling in single-mode Yb-doped ZBLANP fibers. Simulations demonstrate that heat extraction varies significantly along the fiber. There is an optimum pump power (58 mW at 1015 nm for the modeled fiber) for which the maximum heat extracted per unit length is at the start of the fiber. Launching more power moves the coolest point further down the fiber. At substantially higher powers, ASE has a significant heating effect, and coupled with the heating due to absorptive loss, the entire fiber warms up. For a given fiber length, the total extracted heat is maximized for a different pump power (430 mW for a 20-m length). The temperature change is then negative along the entire fiber, and the total extracted heat is 7.12 mW (1.65% cooling efficiency). When the fiber absorptive loss is negligible, this value increases to 30.5 mW for a 2-W pump, giving a 3.48% cooling efficiency, only slightly below the quantum limit (3.7%). The optimum dopant concentration has a similar trade-off: the total extracted heat is maximized for a Yb concentration of 2 wt.%, and the cooling efficiency for 0.5 wt.%. A model of ASF cooling in fiber lasers is also described and exploited to investigate how to select the fiber laser parameters to extract the most power output from a radiation-balanced fiber laser. It shows that increasing the cavity length increases cooling at the expense of laser efficiency, and that a low output coupler reflectivity enhances ASF cooling. Simulations predict that a large-mode-area fiber laser should produce 12.7 W of output power at 63% efficiency, a performance limited by the fiber’s absorptive loss, the core diameter (30 μm), and concentration quenching.

A numerical study of laser cooling in a large mode area Single Mode Photonic Crystal Yb3+:ZBLAN glass fiber is presented. In a recent analyses on conventional single mode fibers (SMFs), strategies to maximize the cooling efficiency were highlighted and it was shown that the cooling scales quadratically with the core and inversely with the cladding radius. For conventional SMFs, heat source density can hardly be increased due to limitations in the total Yb doping concentration and the fact that the system easily operates in the pump saturation regime due to very low saturation intensities in small core single mode fibers. Therefore, it is essential to use the largest possible core radius and smallest cladding radius to obtain detectable cooling. A trivial design approach to obtain large mode areas is to decrease the numerical aperture (NA). However, there are several difficulties in applying this concept to rare-earth-doped fibers. Here, we propose large mode area single mode Yb3+:ZBLAN photonic crystal fibers as a robust alternative design. The radial distribution of the pump and laser mode intensities are numerically calculated using Finite Element Method and the heat source density is directly calculated using the pump and laser intensity distributions. The heat source density is fed back to the heat transfer module of COMSOL and radial temperature distribution across the large core of the fiber and its surrounding photonic crystal structure is calculated. Our results show that a much higher laser cooling efficiency is achievable in large mode area single mode photonic crystal fibers.

The experimental study of cooling by anti-Stokes fluorescence in a fiber or a radiation-balanced fiber laser necessitates the development of a sensor that can measure the temperature of the fiber core with an excellent temperature and spatial resolution, a large dynamic range, a small drift, a fast response, and a low absorptive loss. We report an in-situ slow-light fiber sensor written directly in a Yb-doped silica fiber using a femtosecond laser. The sensor has a spatial resolution of 6.5 mm, an excellent measured temperature resolution of 0.9 m°C/√Hz, and a measured drift as low as 20 m°C/min. One of the grating’s slow-light resonances is interrogated with a tunable 1.55-μm laser to measure the temperature-induced shift in the resonance wavelength when the fiber is optically pumped. The laser frequency is also modulated at 30 kHz to greatly reduce the detection noise. The sensor was pumped with 0.58 mW from a 1020-nm laser and measured a positive temperature change of 0.33 °C. The dominant source of heating is shown to be likely the photodarkening loss induced in the Yb-doped fiber when the FBG was written. The total FBG loss is predicted to be ~24 m^-1 at 1020 nm and expected to reduce after annealing. Projections indicate that if the loss of the rare-earth doped FBG can be decreased to the level of the loss observed in slow-light FBGs written in SMF-28 fibers, these sensors can be used to measure ASF cooling.

Measurement of cooling efficiency and temperature of the doped optical fiber is critical for the development of optical refrigerators and radiation balanced lasers. Measuring the optical fiber temperature, especially for single mode fibers, is challenging. Non-contact thermometry is required because a temperature sensor which is in thermal contact with the fiber can potentially be a heat load when exposed to the scattered pump power and fiber luminescence and can lead to inaccuracies in thermometry. One of the best non-contact methods is using differential luminescence thermometry (DLT). DLT works based on the fact that the 4f electrons in rare-earths are shielded from the surroundings and host field transitions; therefore, the temperature-induced intensity changes in rare-earth material luminescence are mainly caused by changes in Boltzmann population of emitting states. We propose a variant of DLT for finding a relation between the spontaneous emission of the fiber and the temperature. Our method is based on the normalized correlation between the spontaneous emission spectrum at each temperature and the reference spontaneous emission. In this method, we chose a section of the spontaneous emission spectrum as the reference and calculate the normalized correlation factor of the spontaneous spectrum at each temperature with the reference spontaneous spectrum. We make a calibration curve, and based on the calibration curve we estimate the temperature difference from the reference. Comparisons with the conventional DLT will be presented.

We report on the structural and optical properties of 50GeO₂-30PbF₂-(20-x)PbO-xYbF₃, with x = 0.5, 1.5, 2.0, 2.5 mol% glass-ceramics for optical refrigeration. XRD measurements reveal the formation of nanocrystals embedded in glass samples after heat treatment at 360°C ⁄ 20h. Spectroscopic measurements show that samples have near infrared photoluminescence emission due to the ²F_5/2 − ²F_7/2 Yb³⁺ transition, centered at ∼1020 nm with excitation at 919.7 nm, or 1011.2 nm, and the highest PL emission efficiency occurs for samples with 2.0 mol% of Yb³⁺. The PL quantum yield varies between 95% and 75%, depending on the lanthanide concentration and excitation wavelength, for 1.5 and 2.5 mol% Yb³⁺doped samples being the most efficient under 1011.2 nm excitation. The UV-Vis-NIR spectroscopy shows a transparency as high as 80% in the infrared region, and the absorption between 900-1050 nm increases with Yb³⁺ concentration, in good agreement with the theoretical doping levels. Preliminary measurements monitoring the sample temperature dependence using a fiber Bragg grating sensor, as a function of pump laser wavelength and Yb³⁺ concentration shows that the heating process approaches zero for an excitation wavelength of around 1030 nm, which is an indication that phonons are annihilated in these glass-ceramic materials, and shows promise for applications in optical refrigeration.

Radiation-balanced lasers are lasers where the heat generation in the gain medium is compensated by optical refrigeration due to anti-Stokes fluorescence emission. To investigate the feasibility of RBL operation in an optical fiber, a diagnostic test and a comprehensive model are essential. The model presented here is based on a two-level system and includes the intensity saturation effect, which has been usually neglected in bulk materials. We will show that for a material with a very low dopant area such as a single mode fiber (SMF) in which the saturation power is easily attainable, there is an optimum power at which the best cooling efficiency is obtained. The effect of the dopant density on the cooling power is investigated to find the maximum cooling efficiency which can be extracted for the material. We also present data for the extraction efficiency and other parameters of commercial Yb:ZBLAN glass and Yb:Silicate SMFs to discuss their cooling feasibility. Due to the structural defects, a double exponential behavior is usually observed in the fluorescence decay of the fibers that includes an slow and a fast decay channels. Some of the ions usually reside in the fast decay side and cause a large decrease in the heat extraction efficiency. Using our model, we will first analytically show that there is a maximum limit for the fast decay lifetime below which the cooling can still be functional and secondly discuss the effect of the measured decay lifetimes on the cooling efficiency.

Laser cooling in Tm:YLF and Tm:BYF crystals has recently been reported. We investigate high power laser cooling of Tm doped crystals under high vacuum using multiple-pass Herriott cell configuration. We also model potential mid-IR Radiation Balanced Lasers (RBLs) in available Tm:YLF and Tm:BYF crystals. Our experiments and modelling shows that our 1% wt. Tm:BYF sample is a promising 2 µm RBL candidate, since it has high gain and high external quantum efficiency as well as good room temperature cooling efficiency. We will attempt to demonstrate the first mid-IR RBL experimentally in Tm:BYF crystal as well.

Vertical external cavity semiconductor lasers (VECSELs) have shown a promise of becoming efficient sources of high power and high beam quality coherent radiation. In order to live up to their true potential potential, the VECSELs must be thermally managed in order to avoid thermal damage as thermal lensing and filamentation causing preventing it from operating in a single mode regime. For optically pumped VECSELs optical cooling presents an elegant solution for thermal management as it does not require electrical or thermal conduction. The goal of optical refrigeration is to achieve radiation balance lasing (RBL) when the active medium is maintained at the steady uniform temperature. In this work we investigate the active medium characteritics and operating conditions that can lead to RBL in semiconductor medium and show that in order to achieve RBL the gain medium should be engineered to create the density of states that simultaneously allows gain and strong Anti-Stokes Luminescence. Such media may incorporate bandtail states, impurities or quantum dots. We provide the recepee for optimization of such bandstructure-engineered materials to achieve the lowest threshold and highest output power.