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Photonic Heat Engines: Science and Applications

Laser cooling of optically trapped nanocrystals

Galina A. Nemova

Published Mar 1, 2019• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.2510420

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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.
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.
Laser cooling of solids with anti-Stokes fluorescence is currently attracting widespread attention because of the wide range of its applications, including all-optical cryocooling for airborne and space-based applications, heat suppression in high-power lasers, and cooling of nanoparticles for biological and mesoscopic physics. This laser cooling process has typically very low (only a few%) efficiency and its enhancement is therefore highly desirable. In this work, we propose to leverage Mie resonance to enhance anti-Stokes fluorescence cooling in RE-doped nanoparticles. As an example, we consider an Yb3+:YAG nanosphere pumped at the long wavelength tail of the Yb3+ absorption spectrum, at 1030 nm. We show that if the radius of the nanosphere is adjusted to the pump wavelength in such a manner that the pump excites some of the Mie resonant modes of the sample, the cooling power density generated in the sample is considerably enhanced and the temperature of the sample is consequently appreciably decreased.
We study hybrid optical cooling of rare-earth-doped mesoscopic crystals via the anti-Stokes fluorescence and collective photon recoil effect inside them. The proposed cooling scheme, composed of a radio-frequency trap and a three-dimensional optical potential, makes it possible to cool doped nanocrystals down to an external temperature of 10−8 K. By achieving the mechanical action values of ∼10ℏ, we come close to the macroscopic quantum states of a solid matter. Such deep translational cooling of nanocrystals paves the way to fundamental studies on the solid state and to applications in low-temperature chemistry and biophysics.
Abstract YAlO 3 (YAP) shows lower phonon energy than Y 3 Al 5 O 12 (YAG), which implies a lower temperature can be obtained in Yb 3+ -doped YAP according to the single-frequency phonon model. Here, a 2% Yb 3+ : YAP crystal was refrigerated by anti-Stokes fluorescence and compared to YAG crystal with the same doping concentration in experiments. Yb 3+ exhibited a broad spectral cooling range in YAP from 1020nm to 1070nm, which was broader than in YAG from 1020 nm to 1055 nm. A minimum achievable temperature (MAT) dropped to ~ 249.7 K at 1040 nm for Yb 3+ : YAP, which was lower than that of ~ 259.3 K obtained in 2% Yb 3+ : YAG. Further analysis predicted that the cooling limit of two crystals can reach ~167 K for Yb 3+ : YAP and ~171 K for Yb 3+ : YAG from room temperature. This work exhibited the excellent cooling properties of Yb 3+ : YAP compared with Yb 3+ : YAG in experiment and provided a unique perspective for the practical application of high-power Radiation balance lasers (RBLs).
A strongly focused laser beam with high power density employed in optical tweezers typically results in photothermal damage and instability to the trapped nanoparticles or bio-targets. Here, we report an effective laser cooling of trapped Yb3+-Er3+ co-doped rare-earth nanoparticle by employing a new cooling channel provided by Er3+ and utilizing an enhancement of the anti-Stokes emission through energy transfer between Yb3+ and Er3+. Especially in high temperature conditions, the co-doped system demonstrates a high laser cooling performance, that enable the temperature to decrease from 500 K to around room temperature. But for the initial temperature of around room temperature, the cooling temperature does not drop below freezing, which can avoid not only thermal but also cold damage to bio-targets in optical tweezers. This work provides experimental evidence of the Yb3+-Er3+ co-doping rare-earth nanoparticle for laser cooling, which can hold a potential to address the challenge of precise measurements in optical tweezers. The high-power trapping lasers in optical tweezers generate damaging heat. Here, the authors demonstrate efficient laser cooling using an erbium-ytterbium co-doped nanoparticle, which cools targets from high temperatures to near room temperature while avoiding sub-zero cooling from ambient conditions, thus enabling safe trapping of biological specimens.