This PDF file contains the front matter associated with SPIE Proceedings Volume 11298, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists

An optical fiber-based microheater is described. The fiber, a highly Yb-doped (23.4wt% Yb2O3) silicate glass, can produce thermal power densities in excess of 10 W/nL via optical pumping at 976nm. No evidence of luminescence is observed, indicating efficient conversion from optical to thermal energy. Demonstrated are two applications for this microheater. The first is an all-optical-fiber Pirani thermal vacuum gauge, which uses a dual-fiber configuration. The second is an all-optically-driven, all-optical-fiber, Mach-Zehnder-based modulator. The phase delay, introduced by inserting the microheater into one interferometer arm, is a function of its temperature and can be actively controlled by the pump power.

Establishing the optical refrigeration of semiconductors remains a longstanding goal due to potential applications in optoelectronics. Apart from stringent materials requirements, required to realize condensed phase laser cooling, namely the need to have near unity emission quantum yields, a practical challenge involves accurately measuring specimen temperatures in a non-contact fashion. Common all-optical approaches developed in response to this need include: pump– probe luminescence thermometry (PPLT) and differential luminescence thermometry (DLT). In this study, we compare and contrast PPLT and DLT to a newly developed up-conversion emission thermometry to establish the most robust approach for measuring semiconductor nanocrystal (NC) temperatures. Using high external quantum efficiency CdSe/CdS core/shell NCs, we reveal that up-conversion emission thermometry possesses higher accuracy than either PPLT or DLT. Up-conversion emission thermometry can also be used on specimens such as CsPbBr₃ NCs with temperature-insensitive band gaps.

Temperature measurement of an optical fiber core is a challenge as there is no way to reach the tiny core in order to probe its temperature using regular methods. By exploiting the birefringence properties of a polarization maintaining (PM) fiber, a highly sensitive temperature sensor can be realized to monitor the variations of temperature in the optical fibers. This temperature sensor is particularly useful to detect very small temperature changes of the gain fiber in radiation-balanced fiber lasers. The sensor performance is demonstrated by measuring the temperature changes of a PM fiber attached to a Peltier cooler with the sensitivity of 0.02 C for 6 cm of PM fiber. Also, the temperature rise in the core of a piece of PM fiber carrying a few mW of a cw laser is measured to be 0.18 C mW with a response time of 125 microseconds. PM fibers are designed in different ways to create different indices of refraction along two orthogonal slow and fast axes. The polarization of a linearly polarized light input to a PM fiber along one of these axes is maintained while propagating in the fiber. Any other input polarization will be periodically modified along the PM fiber. The transmitted polarization state depends on the initial polarization, fiber length, and the birefringence of the fiber which is varied by small temperature changes. In the case of using single-mode gain fiber in radiation-balanced fiber lasers, the temperature detection can be done by attaching the gain fiber to the PM fiber sensor. The possibility of direct fiber core temperature monitoring in laser cooled fibers will be achieved by using polarization maintaining fiber for laser cooling. The same fiber can then be used as temperature sensor of its core using circularly polarized light as probe.

This Conference Presentation, Thermally enhanced photoluminescence and fundamental upper limit of luminescence: theoretical study was recorded at Photonics West 2020 held in San Francisco, California United States.

Laser cooling allows vibration-free cryocooling down to 100K and appears as a promising technology for future satellite missions. We evaluate the impact of a laser cooler onboard a microsatellite on size, weight and power at platform levels and compare it to a mechanical cryocooler. Practically we intend to use a cooling head attached to the focal plane holding the instruments based on state-of-the-art cooling crystals 10 %Yb:YLF inside an astigmatic absorption cell. It will be linked by a fiber to a second system that includes the opto-electronics and laser. We will present initial results on a fiber-coupled cooling head.

Optical refrigeration of a Yb:YLF crystal can be used to cool an arbitrary payload, utilizing a novel MgF2 thermal link in a completely vibration free optical cryocooler. We discuss the latest advances in the design and implementation of an astigmatic Herriott cell to optimize pump laser absorption in the cooling crystal, and are studying the adverse effects of absorption saturation. We investigate novel spectrally selective coatings of the clamshell to reduce the parasitic heat load on the crystal. By overcoming these challenges, we work towards cooling a silicon reference cavity to a temperature of 124 K.

The role of pump saturation in high-power optical refrigeration is investigated. Employing both Z-scan and intensity-dependent PL techniques, we measure the pump saturation intensity in Yb:YLF versus the temperature. We find that the absorption efficiency, and consequently the cooling efficiency can be limited at cryogenic temperatures under power scaling (when substantial heat lift is desired) unless multi-pass pumping schemes are tailored to control the average pump intensity inside the crystal.

We report on the Bridgman crystal growth of Yb3+-doped LiLuF4 (LLF:Yb) for radiation balanced laser (RBL) applications. Crystals were grown on both radio-frequency (RF) heated and resistively heated furnaces starting from the binary fluorides LiF, LuF3, and YbF3. Graphite crucibles were found to introduce excessive amounts of carbon contamination, while glassy carbon crucibles offered significantly better results. In a preliminary experiment, a LLF:Yb crystal grown by this method showed laser-induced cooling when excited at 1055 nm with 810 mW in a setup with the crystal in air and measuring the temperature change by a thermal camera.

We demonstrated an enhancement of the ideal laser-cooling efficiency in Yb-doped yttrium aluminum garnet, (Yb:Y)AG, and Er-doped yttrium aluminum garnet, (Er:Y)AG, ceramic discs above 300 K. The temperature-dependent photoluminescence (PL) spectrum indicates that the phonon-assisted energy transfer from the narrowband, resonant state into the inhomogeneously distributed energy states is enhanced with increasing the temperature. The enhanced ideal cooling efficiency of the (Yb:Y)AG ceramic disc and the (Er:Y)AG ceramic disc at 470 K is 1.7 times higher than that at 300 K. The enhanced ideal cooling efficiency of (Er:Y)AG at 470 K marked 4.0% which is 1.8 times higher than that of (Yb:Y)AG.

The latest efforts in performing high-power mid-IR optical refrigeration in Tm-doped crystals are reported. A Thuliumdoped fiber amplifier (TDFA) seeded by a mid-IR Continuous Wave Optical Parametric Oscillator (CW-OPO) and diodepumped at 793 nm is developed to obtain a high-power source in the mid-IR for laser cooling and RBL experiments. Using the TDFA, experiments are underway for implementing a Herriott cell geometry for laser cooling to low temperatures, as well as demonstrating the first mid-IR RBL in Tm and Ho-doped crystals.

Oxyfluoride glass-ceramics are considered to be one of the promising materials for anti-Stokes laser cooling. Our previous results showed that glass-ceramics are more appropriate than glassy materials for laser cooling applications. In the present work, we describe the role of optimization of the oxide: fluoride ratio in oxyfluoride glass ceramics with the composition (SiO₂-Al₂O₃)_(100-x)(YLiF₄)_x: (YbF₃)₁ (x = 35 and 40; in mol.%) in order to obtain high photoluminescence quantum yield (PLQY) and low background absorption focusing on optical refrigeration applications. Glass-ceramics with high transparency (~ 90 % in the infrared region) were synthesized by the conventional melt-quenching process followed by heat-treatment. Near-infrared (NIR) photoluminescence (PL) emission due to the ²F_5/2 – ²F_7/2 Yb³⁺ transition, centered at ~1010 nm was observed. An improvement in the quantum efficiency is observed for all the samples, which varies between 64 % and 99 %, depending on the oxide: fluoride ratio. A decrease in the background absorption of the samples was investigated by calorimetry. The enhanced radiative (radiative emission) quantum efficiency is achieved due to the YLiF₄ crystals (low phonon energy ~450 cm^-1) which minimize non-radiative relaxations. At a laser wavelength of 1020 nm, the glass-ceramics show anti-Stokes PL emission, essential to achieve laser cooling. The proposed composition is an ideal candidate for laser cooling considering the low phonon energy and low background absorption compared to the other oxyfluoride glasses previously investigated.

Anti-Stokes fluorescence cooling in a silica-based fiber is reported for the first time. The fiber had a core with a 20-μm diameter doped with 2.06 wt.% Yb and co-doped with 0.86 wt.% Al and 0.88 wt.% F. Core-pumping the fiber with 1040- nm light, temperature changes as large at -50 mK were measured at atmospheric pressure. Temperature measurements were performed at 12 pump wavelengths, and the measured dependence of the temperature change as a function of pump wavelength was in excellent agreement with a previously reported model. With this model, the absorptive loss in the fiber was inferred to be less than 15 dB/km, and the critical quenching concentration to be ~15.6 wt.% Yb. This combination of low loss and high quenching concentration (a factor of 16 times higher than the highest reported values for Yb-doped silica) is what allowed the observation of cooling. The temperature measurements were performed at atmospheric pressure using a custom slow-light fiber Bragg grating sensor with an improved thermal contact between the test fiber and the FBG. The improved method involves isopropanol to establish a good thermal contact between the two fibers. This eliminated a source of heating and enabled more accurate measurements of the cooled-fiber temperature. This improved temperaturemeasurement set-up also led to a new cooling record in a multimode Yb-doped ZBLAN fiber at atmospheric pressure. When pumped at 1030 nm, the fiber cooled by -3.5 K, a factor of 5.4 times higher than the previous record.

We report the observation of anti-Stokes fluorescence cooling of Yb-doped silica glass by 0.7 degrees Celsius. We conduct a detailed investigation of the cooling parameters of this glass, including the wavelength dependence of the cooling efficiency as a function of the wavelength and also the parasitic absorption of the pump laser. The measurements are performed on three different glass samples with different compositions and cooling is observed in all samples to varying degrees. The results highlight the possibility of using Yb-doped silica glass for radiation-balancing in fibers. Radiation-Balancing is a viable technique for heat mitigation in lasers and amplifiers.

The modal instability due to heating has been a limiting factor in the ongoing progress towards power scaling in fiber lasers and amplifiers. Radiation balancing has been suggested as a feasible approach for heat mitigation, for which Yb-doped ZBLAN is a recommended gain medium. In this study, we introduce a non-destructive and non-contact method to characterize Yb-doped ZBLAN fibers and show that it is suitable for designing a radiation-balanced fiber laser. Numerical simulations based on measured values indicate that background absorption is the main hindrance to be overcome in such designs.

Thermophotovoltaic power conversion utilizes thermal radiation from a local heat source to generate electricity in a photovoltaic cell. It was shown in recent years that the addition of a highly reflective rear mirror to a solar cell maximizes the extraction of luminescence. This, in turn, boosts the voltage, enabling the creation of record-breaking devices. Now we report that the rear mirror can be used to create thermophotovoltaic systems with unprecedented high efficiency. This mirror reflects low-energy infrared photons back into the heat source, recovering their energy. This radically improves thermophotovoltaic efficiency. Therefore, the rear mirror serves a dual function; boosting the voltage and re-using infrared thermal photons. This allows the possibility of a practical >50% efficient thermophotovoltaic system. Based on this reflective rear mirror concept, we recently experimentally demonstrated a thermophotovoltaic efficiency of 29.1%, a new efficiency record. In this work,

Photon upconversion, a step toward laser cooling of solids, is an anti-Stokes process in which an absorption of a photon leads to a reemission of a photon at energy higher than the excitation energy. Here, we demonstrate room temperature upconversion photoluminescence process in a monolayer semiconductor WS2, with energy gain up to 150 meV. We attribute this process to transitions involving trions (T) and many phonons and free exciton complexes (X). We show that the energy gain significantly depends on the temperature. In order to gain insight into the temperature dependence of the mechanism of the upconverted emission, we combine the normal and upconverted photoluminescence of the monolayer WS2 at low, intermediate and high temperatures. At 7 K the energy gain of upconversion emission amounts about 40 meV, which is comparable with the energy difference between the X and T emission lines and also nearly resonates with the energy of one optical phonon (A’1 or E’). This suggests that at low temperature the upconversion process is related to the coupling between the T and X states mediated by one optical phonon. The higher energy gain of ~60 meV at 70 K suggests that more than one phonon is involved in the upconversion process.

Luminescent-solar-power spatially separates solar photon’s heat from free-energy for parallel harvesting of both. While the heat from a photoluminescent-absorber runs a turbine, its narrow emission matches the PV’s band-edge thus enabling cost-effective base-load solar energy generation.

CsPbBr3 nanocrystals possess optical properties amenable to achieving verifiable condensed phase laser cooling. This includes near unity emission quantum yields, a high tolerance to defects, and efficient up-conversion. We have previously demonstrated emission QYs that approach unity as well as up-conversion efficiencies that range from ~30% to 75%, depending on sub gap excitation energy. Emission up-conversion is also seen at the individual nanocrystal level. In this presentation, I will discuss our latest results on evaluating CsPbBr3 nanocrystals for the purpose of demonstrating condensed phase laser cooling.

The fundamental thermally-driven fluctuations of matter, present in all finite temperature systems and described by the fluctuation-dissipation theorem, are responsible for the precision-limiting noise processes of many measurement devices, including Johnson noise in resistors and thermo-mechanical noise in Fabry-Perot cavities. The LIGO interferometer’s noise floor was famously limited by thermo-elastic fluctuations on its mirror coatings. I will present measurements and analysis of thermo-refractive noise in photonic microresonators, examine how this noise sets a fundamental limit for the coherence of Kerr-microresonator optical frequency combs, and present a novel technique for beating this limit using laser cooling.

Anti-Stokes photoluminescence from colloidal CdSeS/ZnS quantum dots (QDs) is observed. The QDs were inserted into the core of wider-bandgap SiO2/Si3N4/SiO2 structure by thin film deposition and confirmed as promising nanoemitters for laser cooling due to efficient anti-Stokes emission. The nanoemitters were optically pumped by semiconductor lasers coupled to the waveguides using free-space optics. A direct evidence of local optical cooling in the waveguide structure has been demonstrated with a luminescence thermometry based on the detection of photoluminescence signal phase change versus power of the pumping laser, using a lock-in amplifier.

High Power Semiconductor Radiation Balanced Laser (RBL) is an attractive proposition, however, both theoretical and experimental research has shown that fully balanced semiconductor RBL remains an elusive goal. At the same time, rare-earth doped materials have been successfully optically cooled. In this talk we present the investigation of tandem RBL consisting of semiconductor vertically emitting laser (VECSEL) that is cooled by a rare earth doped material in thermal contact with it and show that substantial power can be obtain in the radiation balanced regime.

Abstract: The use of temperature-sensitive fluorescence up-conversion in Ho doped crystals for non-contact Differential Luminescence Thermometry (DLT) is investigated. The up-conversion fluorescence spectra of Ho³⁺-doped YLF crystals, subject to pumping within the ⁵I₈ - ⁵I₇ manifold (1899-2066 nm), are measured versus the temperature in a cryostat. Considerable changes in the measured photoluminescence spectra, and in the red, yellow and green upconversion spectra is observed as the temperature is reduced from 300 K to 80 K