Quantum Point Defects for Solid-State Laser Refrigeration

Investigating Li2Mg2(WO4)3: structure, morphology, and electrical properties with ultra-low dielectric loss for optimizing laser host materials

With technological advancements driving the demand for innovative materials, triple tungstate compounds, like Li2Mg2(WO4)3 (LMWO), offer exceptional properties for optoelectronic technologies. To investigate these potential outcomes, the LMWO compound was prepared via the solid-state reaction approach. The X-ray diffraction analysis revealed a single-phase material crystallizing in the orthorhombic structure, belonging to the Pnma space group. The crystallite size of the material was determined to be 58.32 nm, which played a significant role in enhancing its electrical performance. Scanning electron microscopy (SEM) revealed prismatic or rod-shaped particles with an average grain size of approximately 2.83 μm. Additionally, EDX confirmed the elemental composition, verifying the presence of Mg, W, and O, and ensuring the material's purity. Nyquist plots indicated non-Debye type relaxation, and further analysis of the relaxation frequency confirmed long-range motion of charge carriers. The temperature dependence of dielectric relaxation followed the Arrhenius law, yielding an activation energy of 0.84 eV. The frequency dependent behavior of M'' and Z'' at various temperatures indicated a shift from short-range to long-range mobility of charge carriers. The conductivity of the material increased with both temperature and frequency, demonstrating its semiconducting behavior. The temperature dependence of Jonscher's exponent suggests that conduction follows the non-overlapping small polaron tunneling (NSPT) model. This compound exhibited a high dielectric constant (ε ∼ 105) and low dielectric loss at high frequencies, making it promising for applications in laser host materials and energy storage.

Mediation of Colloidal Encounter Dynamics by Surface Roughness

Rigorous understanding of assembly in colloidal systems is crucial to the development of tailored nanostructured materials. Despite extensive studies, a mechanistic understanding of the dynamics governing encounters of colloidal particles remains an ongoing challenge. We study colloidal encounter dynamics by inducing assembly through optical tweezers that impose an external attractive field for cubic-phase sodium yttrium fluoride nanocrystals. We show that surface roughness of the nanocrystals is a decisive factor for contact leading to assembly between the nanocrystals, manifested by the roughness-dependent hydrodynamic resistivity. This provides direct evidence that dynamics are equally important to energetics in understanding assembly.

Effect of the stability of 1,3-bis(diphenylphosphoryl)-2-oxapropane complexes on the separation of lanthanide ions and their detection

Phosphoryl podands of neutral type with a flexible ethylene glycol chain and diphenylphosphorylmethyl end groups are known for their complexation properties towards various cations. In this work, the complexation process between 1,3-bis(diphenylphosphoryl)-2-oxapropane (L) and lanthanide ions was studied. Namely, the stability constants of lanthanide complexes with L in acetonitrile were estimated by the method of spectrophotometric titration. It was found that the stability constants of Ln3+ complexes with L increase in the lanthanide series, which is consistent with the extraction and ion-selective properties of L. The extraction ability of L and this ligand in the presence of ionic liquids (ILs) such as methyltrioctylammonium nitrate (MTOAN) and bis[(trifluoromethyl)sulfonyl]imide 1-butyl-3-methylimidazolium (C4mimTf2N) was studied. It was found that in the presence of ILs, L extracts the elements of the yttrium subgroup of lanthanides much better than those of the cerium subgroup: SFLu/Ce(L-MTOAN) = 2.29; SFLu/Ce(L-C4mimTf2N) = 110.38. The use of the L-C4mimTf2N mixture in the processes of lanthanide separation into subgroups is much more efficient than the use of L without the addition of ILs or the use of the L-MTOAN mixture. The ion-selective properties of L towards Ln3+ ions were studied. Podand L exhibits potentiometric selectivity to Lu3+ ions.

Doublet-emissive materials for organic light-emitting diodes: exciton formation and emission processes

Doublet-emission is mainly discovered in stable radicals, lanthanide-metal complexes with an f¹ electron configuration and transition-metal complexes with a low-spin d⁵ electron configuration, and has a distinct radiation mechanism from closed-shell luminescent molecules and thus technology opportunities. There exists an unpaired electron in the frontier molecular orbitals which enables efficient nanosecond-scale luminescence in these materials due to the spin-allowed transitions between doublet-spin states. In this review, we summarize recent advances in these materials and their application in organic light emitting diodes (OLEDs). The photoluminescence and electroluminescence mechanisms of different doublet-emissive molecular systems are discussed, in addition to the photophysical phenomena arising from doublet states. We also outline the current challenges faced by each molecular system, and the potential outlook on the future research trends in this field.

Accurate Characterization of the Properties of the Rare-Earth-Doped Crystal for Laser Cooling

We present a method for calibrating a commercial thermal camera adopted to accurately measure the temperature change of the sample in a laser-induced temperature modulation spectrum (LITMoS) test, which is adopted for measuring two crucial parameters of the external quantum efficiency ηext and the background absorption coefficient αb for assessing the laser cooling grade of the rare-earth-doped materials. After calibration, the temperature resolution of the calibrated thermal camera is better than 0.1 K. For the cooling grade Czochralski-grown 5% Yb3+:LuLiF4 crystal, the corresponding values of ηext and αb are LITMoS = measured to be ηext=99.4 (±0.1)% and αb=1.5 (±0.1)×10−4 cm−1, respectively.

Safe and Scalable Polyethylene Glycol-Assisted Hydrothermal Synthesis and Laser Cooling of 10%Yb3+:LiLuF4 Crystals

Rare earth doped lithium fluorides are a class of materials with a wide variety of optical applications, but the hazardous reagents used in their synthesis often restrict the amount of product that can be created at one time. In this work, 10%Yb3+:LiLuF4 (Yb:LLF) crystals have been synthesized through a safe and scalable polyethylene glycol (PEG)-assisted hydrothermal method. A combination of X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and photoluminescence (PL) measurements were used to characterize the obtained materials. The influence of reaction temperature, time, fluoride source, and precursor amount on the shape and size of the Yb:LLF crystals are also discussed. Calibrated PL spectra of Yb3+ ions show laser cooling to more than 15 K below room temperature in air and 5 K in deionized water under 1020 nm diode laser excitation measured at a laser power of 50 mW.

Laser Refrigeration by an Ytterbium-Doped NaYF4 Microspinner

Thermal control of liquids with high (micrometric) spatial resolution is required for advanced research such as single molecule/cell studies (where temperature is a key factor) or for the development of advanced microfluidic devices (based on the creation of thermal gradients at the microscale). Local and remote heating of liquids is easily achieved by focusing a laser beam with wavelength adjusted to absorption bands of the liquid medium or of the embedded colloidal absorbers. The opposite effect, that is highly localized cooling, is much more difficult to achieve. It requires the use of a refrigerating micro-/nanoparticle which should overcome the intrinsic liquid heating. Remote monitoring of such localized cooling, typically of a few degrees, is even more challenging. In this work, a solution to both problems is provided. Remote cooling in D₂ O is achieved via anti-Stokes emission by using an optically driven ytterbium-doped NaYF₄ microparticle. Simultaneously, the magnitude of cooling is determined by mechanical thermometry based on the analysis of the spinning dynamics of the same NaYF₄ microparticle. The angular deceleration of the NaYF₄ particle, caused by the cooling-induced increase of medium viscosity, reveals liquid refrigeration by over -6 K below ambient conditions.

Laser refrigeration of optically levitated sodium yttrium fluoride nanocrystals

Solid state laser refrigeration can cool optically levitated nanocrystals in an optical dipole trap, allowing for internal temperature control by mitigating photothermal heating. This work demonstrates cooling of ytterbium-doped cubic sodium yttrium fluoride nanocrystals to 252 K on average with the most effective crystal cooling to 241 K. The amount of cooling increases linearly with the intensity of the cooling laser and is dependent on the pressure of the gas surrounding the nanocrystal. Cooling optically levitated nanocrystals allows for crystals prone to heating to be studied at lower pressures than currently achievable and for temperature control and stabilization of trapped nanocrystals.

Implementation of Laser-Induced Anti-Stokes Fluorescence Power Cooling of Ytterbium-Doped Silica Glass

Laser cooling of a solid is achieved when a coherent laser illuminates the material, and the heat is extracted by annihilation of phonons resulting in anti-Stokes fluorescence. Over the past year, net solid-state laser cooling was successfully demonstrated for the first time in Yb-doped silica glass in both bulk samples and fibers. Here, we report more than 6 K of cooling below the ambient temperature, which is the lowest temperature achieved in solid-state laser cooling of silica glass to date to the best of our knowledge. We present details on the experiment performed using a 20 W laser operating at a 1035 nm wavelength and temperature measurements using both a thermal camera and the differential luminescence thermometry technique.