This PDF file contains the front matter associated with SPIE Proceedings Volume 10936, including the Title Page, Copyright information, Table of Contents, Introduction, and Author Conference Committee lists

We will summarize the progress made by us on the efficient conversion of phonon polariton waves from input optical waves at lower frequencies to output optical waves at higher frequencies. The efficient conversion is made possible by utilizing waveguide structures and giant second-order nonlinearities. Such giant nonlinearities originate from huge enhancements due to phonon polariton resonances. We have calculated the highest conversion efficiencies from the phonon polariton waves to the output optical waves for a variety of semiconductor materials. Our results show that such a novel scheme can be used to efficiently remove the energies carried by the phonons.

Laser cooling of gallium arsenide is of high industrial and academic interest and has been actively investigated using different experimental methods based on steady-state concepts. We demonstrate for the first time an onset of transient cooling in GaAs after an impulsive excitation, verified by two independent methods of time-resolved spectroscopy. We observe that transient cooling lasts for about 100 ps, becoming overwhelmed on longer timescales by parasitic heating associated with the nonlocal character of the background absorption processes. In addition, new insights into the physics of the Urbach tail excitation, being at the heart of the laser cooling in semiconductors, are revealed.

Much effort has gone into realizing laser cooling with solids over the last two decades. Multiple attempts have been made with systems that include rare-earth doped glasses, GaAs heterostructures, CdS nanobelts and hybrid perovskite nanoplatelets. Here we suggest that CsPbBr₃ perovskite nanocrystals may eventually lead to verifiable demonstrations of condensed phase laser cooling. The highest emission quantum yield we have realized in CsPbBr₃ nanocrystal ensembles is 99.9% at room temperature. This value lies above the critical quantum yield for CsPbBr₃, needed to realize laser cooling. We also find that associated CsPbBr₃ nanocrystal emission up-conversion efficiencies are large and are 75% and 32% for laser detuning energies of 24 meV and 105 meV.

Since optical refrigeration has been demonstrated in rare-earth doped media, its most logical application has been development of a radiation balanced laser (RBL). While efficient and versatile rare-earth solid state laser operates only on a few discrete wavelengths. At the same time semiconductor gain medium can be easily engineered to lase over the entire visible and near IR spectrum. But achieving cooling and RBL in semiconductor medium has proven elusive due to both fundamental (hard to simultaneously achieve gain and anti-Stokes luminescence) and practical (background absorption) reasons. Therefore, it would be worthwhile to consider an alternative, tandem structure in which rare earth doped medium would cool the optically pumped semiconductor laser. In this work we provide guidelines for development of such a laser and estimate its characteristics under realistic conditions.

Laser radiation has conventionally been used to cool the mechanical amplitude of oscillators with approaches based on electronic feedback and cavity-induced radiation pressure. However, the direct laser refrigeration of an optomechanical oscillator has remained a challenge. Optically refrigerating the lattice of an optical resonator promises to impact several fields including the development of radiation balanced lasers. In this work, we demonstrate laser refrigeration of a hydrothermally synthesized 10 % ytterbium (Yb³⁺) doped lithium yttrium fluoride (YLF) crystal placed at the free end of a cadmium sulfide nanoribbon (CdSNR). An incident 1020 nm laser is used to cool the crystal and the back-scattered up-converted Yb³⁺ emission is analyzed using two-band differential luminescence thermometry (DLT) to monitor the temperature of the YLF crystal. A temperature drop of 23.6 K below room temperature is recorded at a focused laser power of 40.1 mW. Lastly, a combination of finite element wave optics and heat transfer calculations were used to estimate the imaginary part of the refractive index of the YLF crystal.

Conventionally, the theory of radiative energy transport is based on the concept of blackbody, which is known to emit the largest amount of radiative energy between objects. However, when two objects are separated within a sub-wavelength distance, the radiative energy transfer can exceed the blackbody limit by orders of magnitude owing to near-field coupling of the electromagnetic waves radiated by the objects. This near-field enhancement of radiative energy transport can be beneficially used for emerging nanoscale heat engines. In addition, a nanoscale gap can also make a significant effect on the charge transport phenomena by suppressing the space charge effect, creating image charges in the electrodes, concentrating electric fields, and allowing electron tunneling across the gap. The coupling of such near-field charge transport with near-field radiative energy transfer will be discussed along with our experimental efforts to demonstrate its viability for energy applications.

The cooling of a light emitting diode (LED) by photons carrying out more energy than was used to electrically bias the device, has been predicted decades ago.^{1, 2} While this effect, known as electroluminescent cooling (ELC), may allow e.g. fabricating thermophotonic heat pumps (THP) providing higher efficiencies than the existing solid state coolers,³ ELC at powers sufficient for practical applications is still not demonstrated. To study high-power ELC we use double diode structures (DDSs), which consist of a double heterojunction (DHJ) LED and a photodiode (PD) grown within a single technological process and, thus, enclosed in a cavity with a homogeneous refractive index.^{4, 5} The presence of the PD in the structure allows to more directly probe the efficiency of the LED, without the need for light extraction from the system, reducing undesirable losses. Our analysis of experimentally measured I − V curves for both the LED and the PD suggests that the local efficiency of the high-performance LEDs we have fabricated is approximately 110%, exceeding unity over a wide range of injection current densities of up to about 100A/cm² . At present the efficiency of the full DDS, however, still falls short of unity, not allowing direct evidence of the extraction of thermal energy from the LED. Here we review our previous studies of DDS for high-power EL cooling and discuss in more detail the remaining bottlenecks for demonstrating high-power ELC in the DDS context: the LED surface states, resistive and photodetection losses. In particular we report our first surface passivation measurements. Further optimization therefore mainly involves reducing the influence of the surface states, e.g. using more efficient surface passivation techniques and optimizing the PD. This combined with the optimization of the DDS layer thicknesses and contact metallization schemes is expected to finally allow purely experimental observation of high-power ELC.

Refrigeration is an intrinsic feature of light-emitting diodes, a fact that was recognized decades ago but has so far eluded direct experimental observation at practical power densities. The problem is insufficient external luminescence efficiency; for net cooling to occur, the losses in the device must be close to zero, and a sufficiently efficient LED has yet to materialize. We propose a possible structure for such an LED, and predict that with existing optoelectronic material quality and device processing, electroluminescent refrigeration is not only possible but is potentially more efficient than its solid-state alternatives, particularly at low temperature.

Optical refrigeration of Yb:YLF crystals is used to cool an arbitrary payload (HgCdTe IR sensor) for the first time to below 135K in a completely vibration free environment. This milestone is made possible by the design and fabrication of a specialty thermal link that efficiently shields the payload from the intense fluorescence while sustaining frequent thermal cycling. We also show the latest advances in the design and implementation of novel thermal links that promise sub-100K payload temperatures. This investigation considers thermal link materials that are CTE-matched to YLF and includes rigorous optical and thermal modeling under various geometries.

We report the first observation of laser cooling in 1%Yb³⁺:KYW and discuss factors that limit the cooling efficiency. Cooling by 10 K from room temperature at atmospheric pressure was achieved in this crystal at a wavelength of 1025 nm using 8 W from a seeded fiber amplifier. The temperature of the sample was measured using a calibrated differential luminescence thermometry method and was verified with a thermal camera. Infrared imagery and 3-D modeling were used to analyze the impact of thermal conduction, thermal convection, black-body radiation, and background impurities. The simulated results agree with experimental measurements confirming that the chief limitation of laser cooling at room temperature and atmospheric pressure is thermal conduction from sample supports. Best results were obtained for samples mounted on silica aerogel. Theoretical improvement of cooling efficiency in vacuum by exploiting impurity absorption saturation is also discussed.

We report for the first time cooling by anti-Stokes fluorescence (ASF) of a single-mode fiber, and cooling of a fiber at atmospheric pressure. This demonstration and the ability of our model to accurately predict cooling are crucial steps towards the development of radiation-balanced fiber lasers and were the primary focus of this work. We also experimentally investigated the effects of pump power and wavelength, the core size, and the dopant concentration on ASF cooling, in order to maximize this process. Experiments were performed on two Yb-doped ZBLAN fiber from Le Verre Fluoré: a single-mode fiber doped with 1 mol% Yb and a multimode fiber doped with 3 mol% Yb. The maximum temperature change achieved in the two fibers was -5.2 mK and -0.64 K, respectively, confirming that cooling scales with doped area. However, we also discuss limitations to this scaling, namely the absorptive loss, concentration quenching, and the mode profile of the pump. We use our previously reported model to quantify these scarcely reported parameters. For the multimode fiber, comparison between the experimental data and the model gave an inferred absorptive loss of 45 dB/km and a critical quenching concentration of 3.57x10²⁷ m^-3. In addition to these parameters, accurate modeling also requires precise knowledge of the absorption and emission cross-sections. To this end, we propose a method to obtain spectra that obey the McCumber relation and accurately represent the material under investigation. Finally, we report on the cooling efficiencies achieved in the single-mode (2.0%) and multimode (0.85%) fibers and show that the efficiency decreases with increasing pump power due to absorptive loss.

Several silicate fibers doped with 0.5-6 wt.% Yb were evaluated experimentally for optical cooling, namely three nanoparticle-doped fibers (LaF₃, BaF₂, and YbF₃) and three fibers with network modifiers (borophosphosilicate, fluorosilicate, and aluminosilicate). Their performance was compared to that of a commercial Yb-doped silica fiber. Simulations were first carried out to investigate the influence of absorptive loss and concentration quenching on the relationship between temperature change and pump power. This analysis provided a method for inferring the values of these two quantities from the measured dependence of the temperature change on pump power. For fair comparison of the cooling performance of different fibers, we show that the temperature change exhibited by the fibers must be compared at the same pump power absorbed per unit length. Although none of the fibers exhibited negative temperature changes, this metric was used to show that nanoparticles and network modifiers effectively reduce heating and increase the cooling efficiency. The borophosphosilicate and BaF2 nanoparticle fibers performed best, exhibiting 92.7% and 93.9% less heating than the silica fiber. Based on this result, we propose a borophosphosilicate fiber design with a lower Yb concentration and a larger core that is predicted to produce cooling at atmospheric pressure by -12 mK for 100 mW of 1020-nm pump.

Detailed characterization of Tm- and Ho-doped crystals is performed to identify optimum operation parameters for reaching cryogenic temperatures. The energy-gap scaling law states that these materials can deliver double the cooling efficiency compared with the Yb-doped systems. Here, we show our recent measurements of external quantum efficiency 𝜂ext and background absorption 𝛼b in Ho- and Tm-doped YLF and BYF crystals. Together with temperature-dependent spectroscopy, these data are then used to determine the minimum achievable temperature and the optimum cooling wavelengths for each crystal. Finally, the potential of these crystals for implementing mid-IR radiation balanced lasers is discussed.

Although the output power of current commercial fiber lasers has been reported to exceed 500kW, the heat generated within fiber gain-media has limited the generation of higher laser powers due to thermal lensing and melting of the gain -media at high temperatures. Radiation balanced fiber lasers promise to mitigate detrimental thermal effects within fiber gain- media based on using upconverted, anti-Stokes photoluminescence to extract heat from the optical fiber core. In this manuscript, we demonstrate that Yb(III) ions within YLiF₄ (YLF) crystals are capable of cooling both the core and cladding of optical fibers. We also demonstrate a novel radiation balanced fiber laser design using a composite fiber cladding material that incorporates YLF nanocrystals as the active photonic heat engine. YLF crystals are mixed with the cladding material to mitigate thermal gradients within the core and cladding. Analytical models of heat transfer within the fiber are presented where the electric- field amplitude within the fiber core is responsible for both the heating of the core, and also the excitation of Yb(III) ions for anti-Stokes laser refrigeration in the cladding.

A comprehensive study was performed to quantify anti-Stokes-fluorescence (ASF) cooling in fibers of various host compositions (telluride, fluorozirconates, fluorophosphates, phosphates, and chalcogenides) doped with Yb³⁺ or Er³⁺. Published expressions were used to calculate the maximum heat that can be extracted per unit length and time from a single-mode fiber in the limit of negligible absorptive loss, and the associated cooling efficiency. These expressions consider host- and ion-dependent parameters, namely the absorption and emission cross-section spectra, the radiative and nonradiative lifetimes, and the critical concentration for quenching. Using these expressions with published values for these parameters, the maximum extractable heat was calculated for a large-mode-area fiber (NA = 0.05) doped with either Yb³⁺ or Er³⁺ in a variety of hosts. The results show that for a given ion, the maximum heat that can be extracted depends strongly on the host due to the strong dependence of quenching on host composition. In contrast, the cooling efficiency (ratio of extracted heat to pump power absorbed) depends very weakly on the host. The cooling efficiency is also almost twice as high for Er³⁺ (average of 3.8%) than for Yb³⁺ (average of 2.2%) due to the larger gap between the pump and mean fluorescence energy in Er³⁺. Of the limited number of materials for which a full set of data was found in the literature, the highest extractable heat for Yb³⁺ is in phosphate (-51.5 mW/m), and for Er³⁺ is in chalcogenide (-10.3 mW/m). This work provides a simple methodology to evaluate the quantitative cooling performance of these and other rare-earth ions in any amorphous host, a procedure that should guide researchers in the selection of optimum materials for ASF cooling of fibers.

To study the cooling efficiency and radiation-balanced condition in optical fiber lasers, it is essential to measure the absorption coefficient over the appropriate spectral range accurately. The most common technique to measure the absorption coefficient in rare-earth doped optical fibers is the cut-back method. Unfortunately, the cut-back method is destructive and requires mechanical movement and optical realignment, which are troublesome for fragile fibers like ZBLAN fibers or tapered optical fibers. Moreover, the presence of the cladding modes is a source of inaccuracy in the final result, and it is challenging to properly remove the cladding modes in highly doped fibers to achieve high accuracy in the measurement of the absorption coefficient. We introduce a non-destructive method based on analyzing the side-light along the doped fiber. The method is presented and explained in detail, and it is successfully used for measuring the resonance absorption of the Yb-doped silica and ZBLAN fiber in single-mode and multi-mode rare-earth doped fibers.

Laser cooling and trapping of Yb³⁺:YAG nano-crystals has been theoretically investigated. It has been shown that in the system of Yb³⁺:YAG nano-crystals pumped at 1030 nm in the long wavelength tail of the Yb³⁺ absorption spectrum both internal and translational laser cooling processes take place. The internal laser cooling process based on anti-Stokes fluorescence serves as a supplementary process to translational cooling of rare-earth (RE) doped nano-crystals. The internal laser cooling process can help to reach lower equilibrium translational temperatures of RE-doped nano-crystals.

Focal cooling is a promising alternative therapy for intractable focal epilepsies, avoiding the irreversible neuronal damages induced by resection surgery. However, due to thermal conduction losses, local cooling of a deep brain region remains a challenging objective for thermoelectric or fluidic technologies. Here, we investigated the viability of an optical micro-cooler based on anti-Stokes refrigeration of ytterbium doped YLF crystals, taking into account the medical constraints for implantable device. We realized significant cooling under atmospheric pressure and developed a solution drastically reducing the harmful fluorescence heating of brain-like liquids below 2 K, thus demonstrating the relevance of this technology for biomedical applications.

We present the experimental progress in the investigation of radiation-balanced lasers in Yb:YAG and Yb:YLF thin discs. Due to low absorption of the pump beam, we explored pump absorption schemes including VECSEL intracavity pumping as well as multipass approach. The results are compared with theory and predictions are made for prospects of these lasers for high power (multi kW) operation.

Single crystals of LiYF₄ (YLF) and LiLuF₄ (LLF) doped with Yb³⁺, Tm³⁺, or Ho³⁺ are attractive materials for enabling radiation balanced lasers (RBLs) operating in the 1–2 μm wavelength range. High material purity and short growth times are critical for exploring and optimizing the performance of these materials. We show that the Bridgman growth method can yield single crystals within ~1 week and requiring only a few grams of the respective starting materials. We report on progress growing YLF and LLF crystals by the vertical Bridgman method and discuss the challenges posed by the incongruent melting properties of YLF. The congruently-melting LLF is found to be an attractive candidate material for RBL applications.

Using vibronic transitions in rare-earth doped crystals as a case-study, we present detailed analysis for the optimum operation of radiation-balanced lasers. In particular, conditions for achieving highest output power and highest optical-to-optical efficiency in Yb:YAG and Yb:YLF thin disc RBLs are given. Finally, we extend our analysis to two-tone RBL systems involving Ho-Tm doped crystals.

A 3-D thermal model applicable to arbitrary sample geometries was developed in COMSOL to analyze laser cooling of 1% Yb3+:KYW crystals. The model includes the effects of thermal conduction, black-body radiation, and background impurities. The simulation results showed that the largest limitation to cooling was the thermal conduction between the crystal and its glass capillary tube supports. Although glass has a low thermal conductivity, it absorbs a significant amount of heat through black body radiation because of its high emissivity (~0.9). The absorbed heat is transferred to the crystal sample through the thermal contact, causing an observable dip in the curve of temperature versus time thereby reducing the net, steady-state cooling power. This limitation was overcome using silicon aerogel, whose conductivity and emissivity are 3 orders and 1 order smaller than glass, respectively. The aerogel maintains the temperature gradient and the heat transported to the crystal is negligible, resulting in a much lower minimum achievable temperature and removing the dip in the temperature evolution curve. By changing the sample support from glass to aerogel, the minimum achievable temperature under ambient conditions was lowered from 0.2 K to 1.5 K in a 1% Yb:KYW crystal with 1W pump at 1023 nm. These results, together with analysis of radiation balance in a 10 mm long crystal of 1% Yb:KYW, were used for a preliminary investigation of self-cooled lasing in this tungstate host.

Recent advances in power scaling of fiber lasers and amplifiers are hampered by the transverse mode instability, which deteriorates the output laser beam quality: the main cause is the overheating of the optical fibers. Radiative cooling has been suggested as a potentially viable heat removal scheme: the rare-earth-doped optical fiber is pumped at the pump wavelength, which is higher than the mean fluorescence wavelength of the active ions; therefore, the anti-Stokes fluorescence removes some and ideally most of the excess heat. In practice, the pump absorption cross section is considerably lower than its peak value, because the pump wavelength must be sufficiently longer than the mean fluorescence wavelength for efficient radiative cooling. Therefore, the design of such lasers and amplifiers must naturally depart from the conventional designs. I will focus on general scaling laws that govern the design of radiation-balanced fiber amplifiers and lasers. In particular, I will show that the undesirable parasitic absorption can make conventional designs unrealistic or at best irrelevant. In other words, a sufficiently large (and realistic) value of parasitic absorption can totally dominate the thermal balance, because its contribution scales linearly with the power, while the radiative cooling saturates at high power values. I will show that in conventional designs, radiation balancing can only be achieved at power values sufficiently low that may not even warrant any sophisticated cooling effort. However, there exist unconventional design strategies that make radiation balancing relevant even at high powers and I will explore such designs and their consequences.

Currently heat management is a big hurdle for power-scaling of high-power fiber lasers and amplifiers. Different methods have been developed over the years to mitigate the heat generation in high-power lasers and amplifiers; radiation balancing is a new method that leverages the radiative cooling for heat mitigation. In this study, the effects of different design parameters on the operation of a doped-double-cladding (DC) fiber laser and amplifier for radiation balancing are investigated. The results show that the value of the internal quantum efficiency for an effective heat mitigation by radiation balancing is not required to be very close to unity. Conversely, it is shown that in high-power operation, even small values of background absorption can be detrimental to radiation balancing. It is argued that both the background absorption and the small dopant area of the doped-DC fibers are big hurdles in achieving an effective heat mitigation by radiation balancing in high power operation. In order to address this issue, we suggest to dope the inner cladding of the DC fiber such that at the pump wavelength, the inner cladding cools down by optical refrigeration. Due to the geometry of the fiber, the temperature is uniformly homogenized across the fiber and the effective fiber temperature decreases, allowing Yb-doped DC fiber lasers and amplifiers to operate at Kilo-Watt levels in radiation balanced mode.

Electroluminescent cooling (ELC) of light–emitting diodes (LEDs) at high powers is yet to be demonstrated. Earlier studies of photoluminescent cooling (PLC) suggested that temperature strongly affects the light emission efficiency and therefore it is useful to explore the temperature range below room temperature (RT) where ELC might be easier to observe. With that purpose in mind, we electrically characterised three different sized (0.2, 0.5 and 1 mm diameter) double–diode structure (DDS) devices, consisting of a coupled LED and photodiode (PD), at temperatures ranging from 100 K to 325 K to investigate how the temperature affects the efficiency of the structures in practice. We found that, for the studied devices, the coupling quantum efficiency (CQE) as well as the overall efficiency indeed increase when temperature decreases and reach their highest values at temperatures below room temperature.

We explore candidate materials for solid-state laser cooling on electric-dipole allowed transitions that could accelerate refrigeration. We analyze the required external quantum efficiencies and tolerable impurity absorption required to achieve net cooling, and examine limitations imposed by charge transfer and excited state absorption in Yb²⁺:SrF₂, Ce³⁺:LiCAF and Ti³⁺:Al₂O₃.