This PDF File contains the front matter associated with SPIE Proceedings Volume 11702, including the Title Page, Copyright information and Table of Contents

We report on the design, fabrication and testing of the first laser cryocooler prototype made in Europe. The proposed architecture is based on a 7.5%-Yb:YLF cooling crystal located at the center of an astigmatic multipass cavity. A 1020 nm, 50 W laser is fiber coupled through the first cavity mirror, allowing the laser source to be located at any distance from the cold head. Encouraging preliminary results show good coupling efficiency of the laser source, stable temperature of the crystal and a minimum achieved temperature below 150 K with only 8.8 W of laser power, leaving margins for further improvements.

Cooling via efficient emission of anti-Stokes photoluminescence can be utilized to realize novel cooling devices. It has been shown that rare earth-doped materials are good candidates for solid-state laser cooling because of their narrow emission spectra and discrete energy levels. We systematically study laser cooling efficiency of rare earth-doped oxide crystals. The photoluminescence spectrum evidences that the phonon-assisted energy transfer from resonant states to inhomogeneously distributed energy states sensitively depends on temperature. We discuss detailed properties of the cooling efficiency of Yb-doped yttrium aluminum oxide crystals and, furthermore, touch emission properties of their crystal thin films.

The first demonstration of optical refrigeration in 1% Ho:BYF crystal with near unity external quantum efficiency (ɳ_{ext}~99.7%) is reported. Laser cooling efficiency, ɳ_{ext}, and background absorption (α_b) of 1% Ho:BYF crystal is obtained from laser induced temperature modulation spectroscopy (LITMoS) test, and its minimum achievable temperature (MAT) and optimum laser cooling wavelengths are extracted via temperature-dependent spectroscopic analysis. MAT of 120 K at the optimum laser cooling wavelengths of 2056 nm or 2063 nm is estimated for 1% Ho:BYF crystal with measured ɳ_{ext}=99.7% and α_b~1e-3 cm^-1. Additionally, MATs of 80 K and sub-80 K are estimated for this crystal considering improved background absorptions of α_b~2e-4 cm^-1 and α_b~5e-5 cm^-1 respectively.

Background absorption has been studied for an YLF:10%Yb3+ crystal at different intensities at room temperature. The cooling efficiency was measured by both the DLT (Differential Luminescence Thermometry) and TLS (Thermal Lens Spectroscopy) methods. Results show that background and coolant absorption saturate differently at room temperature. A cw Ti:sapphire pump beam was tuned from 920 nm to 1040 nm with an intensity range from 100 W/cm2 to 20,000 W/cm2. Changes of temperature and thermal strength were measured by DLT and TLS methods, respectively. The cooling efficiencies with these approaches at different wavelengths were then compared based on theoretical fits to the experimental results. The cooling efficiency at 1000 nm was found to be independent of pump intensity. There, the saturation of intensity of background absorption had the same value as that of Ytterbium. The cooling efficiency below 1000 nm dropped at elevated intensity. In this range, Ytterbium absorption saturated easily, reducing the cooling ion absorption, while the absorption of background impurities did not saturate as much as Ytterbium. Consequently the cooling efficiency was lowered. For wavelengths above 1000 nm, increases in the pump intensity led to improved cooling efficiency. In this range, background absorption saturated more easily than absorption of coolant ions and parasitic heating was reduced, leading to higher cooling efficiency. Thus we have devised a method of measuring differential absorption saturation and determined its effect on laser cooling at room temperature. Saturation effects of this kind have important consequences in the heat equation for radiation-balanced lasers.

Anti-Stokes fluorescence cooling of a Yb-doped silica glass optical fiber preform is achieved using a high-power laser in a double-pass configuration. The coherent laser beam illuminates the silica glass preform in the red tail of its absorption spectrum, and the heat is carried out by anti-Stokes fluorescence of the blue-shifted photons. The high-purity Yb-doped silica glass preform has low parasitic absorption and is codoped with modifiers to mitigate the quenching-induced non-radiative decay for sufficiently high concentrations of Yb ions in silica glass. Therefore, sufficiently large laser absorption could be achieved to observe cooling while maintaining a near-unity external quantum efficiency.

We will report the first demonstration of an optically cooled fiber amplifier. The fiber was made of silica, had a core doped with Yb, and was core-pumped to achieve both gain and cooling in the core via anti-Stokes fluorescence. Gains larger than 10 dB were measured while maintaining a negative average temperature change along the fiber.

This paper reports the first experimental observation of anti-Stokes cooling of fibers in which both the core and the cladding doped with Yb³⁺ to increase to number of Yb ions contributing to cooling and induce greater refrigeration. Two ZBLAN fibers were designed, fabricated by Le Verre Fluoré, and evaluated experimentally. Two cladding profiles were tested, both with asymmetric boundaries to induce greater mode mixing, and therefore better pump filling of the fiber and greater cooling. Temperature measurements showed that the fiber with a double-D cladding did not perform as well (it cooled to –78 mK for 240 mW of input pump power at 1025.5 nm) largely due to limited mode mixing. The octagonal cladding profile of the second fiber produced greater cooling, down to –1.3 K with 3 W. Fitting experimental results to a model showed good agreement with theory, and confirmed the high critical quenching concentration (N_c = 3.2x10²⁷ Yb/m³), low absorptive background loss (40 dB/km), and good filling ratio (~38%) achieved in this second fiber. This study establishes that with straightforward improvement in mode filling, a cladding-pumped ZBLAN fiber can readily be cooled to ~10 K below room temperature at atmospheric pressure with only ~15 W of pump power.

Thermal processes are key limiting factors to power scaling in high-power fiber lasers. Heat generation can arise from the quantum defect, partly manageable through judicious selection of pumping and lasing wavelengths. Heat can also result from nonradiative processes, presently believed to be the main obstacle to efficient anti-Stokes fluorescence cooling in silicates. Therefore, it is meaningful and necessary to evaluate the quantum conversion efficiency (QCE), which is defined to be the fraction of pump photons that undergo the desired radiative process. Here, an accurate and sensitive Brillouin-based method to characterize the QCE in Yb-doped optical fiber is proposed and evaluated.

The realization of optical refrigeration in semiconductors has been a long-standing challenge for optical science and semiconductor physics, and many studies have been conducted. Recently, halide perovskite semiconductors have been attracting much attention as a possible candidate for optical refrigerators, because of their extremely high photoluminescence (PL) quantum efficiencies. Here we report anti-Stokes PL properties of halide perovskite bulk crystals and quantum wells. Using PL excitation spectroscopy, we determined the lower limit of external quantum efficiency for laser cooling. The strong electron-phonon coupling and unique thermal properties of perovskites are key factors of efficient anti-Stokes PL processes.

One photon up-conversion photoluminescence is an optical phenomenon whereby the thermal energy of a fluorescent material is used to increase the energy of an emitted photon compared with the energy of the photon that was absorbed. When this occurs with near unity efficiency, the emitting material undergoes a net decrease in temperature—so called optical refrigeration. Because the up-conversion is thermally activated, the yield of up-converted photoluminescence is also a reporter of the temperature of the emitter. Taking advantage of this optical signature, we have shown that cesium lead trihalide nanocrystals are cooled by as much as 66 K during the up-conversion of 532 nm CW laser excitation. Our work is the first demonstration of optical cooling of colloidal semiconductor nanocrystals, as well as a new record for optical cooling of any semiconductor system, highlighting the intrinsic advantages of colloidal nanocrystals for this goal.

Following the emergence of metal halide perovskites for optical and electronic applications, search for alternative non-toxic lead-free halides that demonstrate high environmental stability and excellent optoelectronic properties has accelerated. In this talk, our recent discoveries of brand-new families of high efficiency light-emitting materials based on all-inorganic copper halides will be summarized. These all-inorganic copper(I) halides demonstrate low-dimensional non-perovskite structures, and consequently, very flat bands around the band gap, leading to very localized charges. Such charge localization and low-dimensional structures typically result in the presence of high stability self-trapped excitons at room temperature producing record high photoluminescence efficiencies approaching unity. Our preliminary studies of the potential practical applications of these luminescent copper halides will also be discussed.

Semiconductor nanocrystals (NCs) are potential materials for verifiable demonstrations of semiconductor-based laser cooling. The key feature that makes NCs appealing for laser cooling is their near unity emission quantum yields (QYs). An unresolved issue regarding NC QYs, however, centers on the existence of an excitation energy dependent (EED) QY. Here, we study EED QYs on three NC systems, aimed at demonstrating NC-based laser cooling (CsPbBr3, CsPbI3, and CdSe/CdS core/shell NCs). We evaluate the impact of EED QYs using two approaches. The first involves direct QY measurements using an integrating sphere. The second entails photoluminescence excitation spectroscopy where changes to NC QYs with excitation energy can be assessed qualitatively.

This talk will discuss the integration of phase-transition materials into thermal emitters to engineer thermal radiation. Topics will include zero-differential and negative-differential thermal emission, radiative runaway, infrared privacy shielding, and passive thermoregulation.

We give an overview of our recent theoretical studies of the thermodynamics of excitons, and other solid-state qubits, driven by time-dependent laser fields. We consider a single such emitter and describe how the formation of strong-field dressed states allows the emitter to absorb or emit acoustic phonons in a controlled way. We present results for the heat absorption, and show that the form of the driving field can be tailored to produce different thermodynamic processes, including both reversible and irreversible heat absorption. We discuss these effects from the perspective of quantum thermodynamics and outline the possibility of using them for optical cooling of solids to low temperatures.

Radiation balanced laser (RBL) can be realized by managing the cooling process via the anti-Stokes photoluminescence (PL), the small-signal gain, and the heating processes including the Stokes shifts and the multi-phonon relaxation. Yttrium aluminum perovskite (YAP) shows lower phonon energy than yttrium aluminum garnet (YAG) which the radiation balanced laser was demonstrated. According to the single-frequency phonon model, the low maximum phonon energy of YAP makes the multi-phonon relaxation probability of Yb-doped YAP [(Yb:Y)AP] smaller than Yb-doped YAG [(Yb:Y)AG]. The low multi-phonon relaxation probability of YAP suggests that (Yb:Y)AP is suitable material for RBL. In this work, we evaluated the PL characteristics and estimated the ideal laser cooling efficiency and the small-signal gain of the (Yb:Y)AP (Yb_0.1Y_0.9AlO₃) ceramics fabricated by a solid-state reaction method. We used McCumber’s relationship and referred to the literatures to derive the absorption and the small-signal gain spectra. The fluorescence re-absorption is observed in the PL spectra of (Yb:Y)AP ceramics with a thickness of ~2 mm, whereas the re-absorption is not observed in (Yb:Y)AG(Yb _0.3Y_2.7Al₅O₁₂). This result indicates the strong absorbance of (Yb:Y)AP. The obtained ideal laser cooling efficiencies of (Yb:Y)AP and (Yb:Y)AG at 300 K were 1.4 and 1.8%, respectively. On the other hand, the maximum small-signal gain of 0.27 cm−1 in (Yb:Y)AP is 3.5 times larger than that of 0.078 cm⁻¹ in (Yb:Y)AG. The large smallsignal gain of (Yb:Y)AP arises from its strong absorbance and intrinsic energy structure.

Rare earth doped oxyfluoride glass-ceramics (GCs) have recently attracted much attention as a novel material candidate for solid state laser cooling application. In this study, highly transparent (~ 90 % in the infrared region) ytterbium doped aluminosilicate oxyfluoride glasses and glass ceramics containing YF3 nanocrystals prepared by the conventional melt quenching process have been investigated to determine and compare their potential to obtain high photoluminescence quantum yield (PLQY). The highest ASF emission intensity was observed in the GC composition with near-infrared PL emission centered at ~1010 nm under a laser excitation at 1020 nm. The PL spectra at different temperatures (25 °C – 200 °C) were measured using different excitation wavelengths varying from 920 nm to 1030 nm in order to understand the nature of Stokes and anti-Stokes emission in glass ceramics. The glass-ceramic has a net heating near to zero with an excitation between 1020 nm and 1030 nm showing its potential for optically induced heat-management applications. The optical properties such as refractive index, quantum efficiency and lifetime of GC and the precursor glass were also studied in detail in order to explore these properties for laser cooling applications.

We propose to mitigate heat generation in Raman lasers and amplifiers by coupling the Raman active waveguide to a wavelengthselective dissipative waveguide. The DW supports (lossy) propagation only at the anti-Stokes wavelength, and hence evacuates the anti- Stokes photons out of the Raman waveguide without affecting the Stokes and pump photons. In this manner, it suppresses the reversed CARS cycles that would otherwise result into heat generation in the Raman waveguide. This mechanism is investigated for different phase mismatches. It is demonstrated represent a promising new avenue to enhance cooling in Raman lasers and amplifiers.

We present a design principle for photonically-active double-heterojunction (DHJ) diodes that utilizes spatial control over the distribution of dopant impurities in the growth direction to suppress non-radiative Shockley-Read-Hall (SRH) carrier generation and recombination processes. These non-radiative processes constitute major parasitics in numerous devices, including light-emitting diodes (LEDs), photovoltaics (PVs), and photo-diode detectors (PDs). The design principle is general to all diodes with carrier-confining heterojunctions and is agnostic to material system. As a result, devices based on semiconductors in Group IV (e.g. SiGe), III-V (e.g. GaN, AlGaAs, GaInAsP, GaInAsSb), II-VI (e.g. MgZnCdTe, ZnCdSeS), and others can theoretically benefit from it. To illustrate the principle, here we will model LEDs, PVs, and PDs in the InP-lattice-matched GaInAsP material system. We show that LEDs’ Internal Quantum Efficiency (IQE) is raised, with a major impact at low forward bias voltages. The modeling presented here shows that, because thermophotonic refrigerator LEDs operate at these voltages, the design principle could prove to be a major step toward realizing the Kelvinscale and larger temperature reductions that have remained experimentally elusive to date. Next, we show that redesigned PVs exhibit higher open-circuit voltage and efficiency, with significant improvement in cells with the high defect densities typical of lattice mismatched III-V cells grown on inexpensive Silicon substrates. Finally, we show that PDs exhibit improvements in dark current during reverse bias operation and shunt resistance during photovoltaic operation, quantities that can impact the noise floor of receivers in optical communication systems and thus the overall power consumption of photonic links.

High purity is a fundamental requirement to enable laser-cooling-grade materials. The vertical Bridgman method is well suited for crystal growth on the few-grams scale, which is compatible with purification techniques that aim to exceed the typical 99.999% to 99.9999% purity of commercial precursor materials. Here, we present advances in the Bridgman crystal growth of cooling-grade LLF:Yb single crystals in a radio-frequency heated furnace. Optical spectroscopy, cooling efficiency, and power cooling characterization are reported. COMSOL simulations were used to investigate the thermal gradient inside the crucible as the crystal growth proceeds.

Microlasers based on lanthanide-doped upconverting nanoparticles (UCNPs) have been demonstrated but radia- tion balanced lasing (RBL) remains a challenge. We present a novel design of radiation-balanced microlaser using microspheres coated with UCNPs. The model is tested using nitrogen vacancy doped nanodiamonds (NV:NDs) coated on silica microspheres. High quality factor enhancement of selective bands in the NV spectral features due to coupled whispering gallery modes (WGMs) was measured. The temperature of NDs on silica spheres was measured experimentally using Debye-Waller factor thermometry and analyzed using a novel analytical heat transfer model for spherical coordinates with localized sources.

We report a new radiation-balanced laser design of a cladding-pumped double-clad fiber laser based on Yb-doped silica. The single-mode glass core has a high Yb doping level for lasing, whereas the inner cladding is cooling- grade silica glass with a low Yb concentration. Multimode pumping propagates along the inner cladding, scaling up the signal and extracting heat generated inside the core due to quantum defects and concentration quenching. Both analytical and numerical methods are used to calculate the electric-field and temperature distribution in the fiber. The core temperature is reduced significantly due to anti-Stokes cooling of the inner cladding.

Radiation balancing has been suggested as a possible path for heat mitigation, especially because Yb-doped silica has been recently shown to be amenable to solid-state refrigeration. In this work, we characterize a Yb-doped silica fiber preform and extract its laser cooling related parameters. We show that a fiber drawn from this preform is suitable for designing radiation-balanced fiber lasers (RBFLs). Numerical simulations based on measured values support such an observation.

In radiation balanced lasers, anti-Stokes fluorescence is used to minimize the heat generated by the quantum defect and other non-radiative processes. Thermo-optics distortions can be minimized, enabling the scaling to high power. Here, experimental results of radiation balanced operation in various disk gain materials (i.e. YLF:Yb and LLF:Yb ) are presented. Different multipass pumping schemes are investigated for pump beam area scaling towards high power CW operation. Laser cavity design and thermal management issues are also discussed.

Quantum technologies containing key GaN laser components will enable a new generation of precision sensors, optical atomic clocks and secure communication systems for many applications such as next generation navigation, gravity mapping and timing since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the u.v. to the visible. We report our latest results on a range of AlGaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for quantum sensors such as optical clocks and cold-atom interferometry systems. This includes the [5s₂S1/2-5p²P1/2] cooling transition in strontium+ ion optical clocks at 422 nm, the [5s^{2 1}S0-5p¹P1] cooling transition in neutral strontium clocks at 461 nm and the [5s²s1/2 – 6p²P3/2] transition in rubidium at 420 nm. Several approaches are taken to achieve the required linewidth, wavelength and power, including an extended cavity laser diode (ECLD) system and an on-chip grating, distributed feedback (DFB) GaN laser diode.