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Optical Manipulation and Structured Materials Conference (OMC)

Laser refrigeration and anti-Stokes luminescence of Ytterbium-doped YVO4 and CaF2 microcrystals

Paweł Karpinski, Kamil Bruchal, Magdalena Dudek, Vitor Paschoal

Published Aug 5, 2025• 0 citations
Publisher

Abstract

We study laser refrigeration and anti-Stokes luminescence of Ytterbium-doped YVO4 and CaF2 microcrystals. The studies are performed on a single particle level using optical tweezers. The temperature of the microcrystals is measured using Raman scattering and analysis of the motion of the particles inside the optical trap. We additionally performed spectroscopic studies of anti-Stokes luminescence versus excitation wavelength and laser power for particles with different dopant concentrations. The combination of various spectroscopic techniques allowed us to identify different competitive luminescence pathways, which determine the cooling and heating of microcrystals.

DOI
10.1117/12.3072294

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We study laser refrigeration and anti-Stokes luminescence of Ytterbium-doped YVO4 and CaF2 microcrystals. The studies are performed on a single particle level using optical tweezers. The temperature of the microcrystals is measured using Raman scattering and analysis of the motion of the particles inside the optical trap. We additionally performed spectroscopic studies of anti-Stokes luminescence versus excitation wavelength and laser power for particles with different dopant concentrations. The combination of various spectroscopic techniques allowed us to identify different competitive luminescence pathways, which determine the cooling and heating of microcrystals.
Abstract Emission and visible photoluminescence excitation spectra of CaF2:Er (0.5 at %) crystals have been studied under different optical pumping conditions. The results demonstrate that anti-Stokes luminescence of the erbium-doped crystals has higher intensity in the case of pumping of the 4I13/2 metastable state of the dopant. Resonance IR photon wavelengths have been determined for two-photon excitation of visible emission in CaF2:Er. A method has been developed for the optical modulation of anti-Stokes luminescence, which allows the nature of the corresponding electron transitions in emission centers to be identified.
This review focuses on optical refrigeration with the anti-Stokes fluorescence of rare-earth (RE)-doped low-phonon micro- and nanocrystals. Contrary to bulk samples, where the thermal energy is contained in internal vibrational modes (phonons), the thermal energy of nanoparticles is contained in both the translational motion and internal vibrational (phonons) modes of the sample. Much theoretical and experimental research is currently devoted to the laser cooling of nanoparticles. In the majority of the related work, only the translational energy of the particles has been suppressed. In this review, the latest achievements in hybrid optical refrigeration of RE-doped low-phonon micro- and nanoparticles are presented. Hybrid cooling permits the suppression of not only the translational energy of the RE-doped particles, but also their internal vibrational phonon thermal energy. Laser cooling of nanoparticles is not a simple task. Mie resonances can be used to enhance laser cooling with the anti-Stokes fluorescence of nanoparticles made of low-phonon RE-doped solids. Laser-cooled nanoparticles is a promising tool for fundamental quantum-mechanical studies, nonequilibrium thermodynamics, and precision measurements of forces.
This paper reports on the characterization of nanocrystalline powders of ytterbium doped YLiF4 for applications in optical refrigeration. Here we used powders with nanocrystals of Yb 3+ concentrations of (10, 15, 20) mol % and lengths (70, 66, 96) nm. Our preliminary spectroscopic measurements did not show an enhancement in the absorption at the long-wavelength tail of the spectra of the nanocrystalline powder when compared with bulk Yb:YLiF4, indicating that the increase of the phonon-assisted excitation is not large enough to play a significant role in cooling in the present conditions. One advantage of nanocrystalline powders over bulk crystals is the possibility of enhancing the absorption by the realization of cavity-less pump recycling through photon localization [1]. While photon localization also increases the reabsorption of the fluorescence depending on the quantum efficiency of the material and can mitigate cooling, it allows the use of crystals of low enough concentrations to avoid deleterious effects such as ion-ion energy transfer followed by quenching. The pump intensity enhancement favors upconversion luminescence to visible wavelengths, which can be used for optical refrigeration and extends the scope of the application for the material. We observed both green and blue emission from the samples and investigate the processes which lead to it. We present the experimental investigation of the nanocrystals’ absorption and emission spectra and the first excited state lifetime measurements, which are used to estimate the nanocrystal’s photoluminescence quantum efficiency.
This review discusses the progress and ongoing efforts in optical refrigeration. Optical refrigeration is a process in which phonons are removed from a solid by anti-Stokes fluorescence. The review first summarizes the history of optical refrigeration, noting the success in cooling rare-earth-doped solids to cryogenic temperatures. It then examines in detail a four-level model of rare-earth-based optical refrigeration. This model elucidates the essential roles that the various material parameters, such as the spacing of the energy levels and the radiative quantum efficiency, play in the process of optical refrigeration. The review then describes the experimental techniques for cryogenic optical refrigeration of rare-earth-doped solids employing non-resonant and resonant optical cavities. It then examines the work on laser cooling of semiconductors, emphasizing the differences between optical refrigeration of semiconductors and rare-earth-doped solids and the new challenges and advantages of semiconductors. It then describes the significant experimental results including the observed optical refrigeration of CdS nanostructures. The review concludes by discussing the engineering challenges to the development of practical optical refrigerators, and the potential advantages and uses of these refrigerators.
Anti-Stokes cooling has been observed in an Yb3+-doped BaY2F8 single crystal. Single crystals have been grown by the Czochralski technique. The absorption spectra and the emission properties have been measured at room temperature and at 10K. The energy positions of the Stark sublevels of the ground and the excited state manifolds have been determined and separated from the vibronic substructure. The intrinsic decay time of the F5∕22 level has been measured taking care of avoiding the effect of multiple reabsorption processes. The theoretical and experimental cooling efficiencies of Yb:BaY2F8 are evaluated and compared with respect to those of the most frequently investigated materials for laser cooling. A temperature drop of almost 4K was measured by pumping the crystal with 3W of laser radiation at ∼1025nm in single pass configuration with a cooling efficiency of ∼3%.
Yttrium aluminum perovskite (YAP) is a host material that can provide a strong emission from a rare-earth dopant and it has a lower phonon energy than yttrium aluminum garnet (YAG). Therefore, YAP is a promising material for optical refrigeration and radiation balanced laser. We measured the photoluminescence (PL) spectra of ytterbium (Yb)-doped YAP and compared them with those of Yb-doped YAG. The estimated ideal laser cooling efficiencies of Yb-doped YAP and Yb-doped YAG were comparable at 300 K. Based on the temperature-dependent anti-Stokes PL, we found that the laser cooling power of Yb-doped YAP at 470 K is 14.3 times higher than that at 200 K. This enhancement at higher temperatures is 3.2 times larger than that observed for Yb-doped YAG. We attributed the higher laser cooling power of Yb-doped YAP to a lower multi-phonon relaxation rate (and/or a higher energy transfer rate) and an antenna effect caused by the energy transfer from Yb ions that are located at the Y-site to Yb ions that are surrounded by an inhomogeneous alloy structure. The calculated small signal gain of (Yb:Y)AP is 3.5 times larger than that of (Yb:Y)AG. The larger small signal gain of (Yb:Y)AP arises from its strong absorbance and small Stark splitting width.