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Features of laser cooling of Yb-doped fluorite nanocrystals using coherent population transfer techniques

Tatiana Vovk, Semyon S. Rudyi, Andrei V. Ivanov, E. Yu. Perlin, Yu. Rozhdestvensky

Published Apr 30, 2019• 1 citation
Publisher

Abstract

As of today, one of the milestones in quantum theory testing is obtaining of macroscopic quantum states, for which very low temperatures are necessary. Such low energies can be present during optical and optomechanical cooling of nanostructures. Here we investigate the deep cooling of ytterbium-doped fluorite nanocrystal via coherent population transfer techniques. We consider two main approaches towards coherent transfer, namely, Raman pulses and stimulated Raman adiabatic passage, and search for the most efficient cooling parameters. Optimization of the process of deep nanocrystal cooling opens up possibilities for various applications and technologies.

DOI
10.1117/12.2517320

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As of today, one of the milestones in quantum theory testing is obtaining of macroscopic quantum states, for which very low temperatures are necessary. Such low energies can be present during optical and optomechanical cooling of nanostructures. Here we investigate the deep cooling of ytterbium-doped fluorite nanocrystal via coherent population transfer techniques. We consider two main approaches towards coherent transfer, namely, Raman pulses and stimulated Raman adiabatic passage, and search for the most efficient cooling parameters. Optimization of the process of deep nanocrystal cooling opens up possibilities for various applications and technologies.
Methods of coherent pumping through dipole-allowed 5d levels of RE ion are proposed for laser cooling. The coherent and complete population transfer between the ground and the first excited levels of 4f multiplet is achieved by using the different Raman techniques, namely two-photon scattering, adiabatic passage method, and π-pulse pumping. It is shown that the multiplication of the number of electrons that participate in cooling cycle leads to increasing of the cooling power and to acceleration of the cooling process. The increasing of cooling efficiency of 0.5% compared to the direct pumping between 4f levels is attained through the use of dipole-allowed optical transitions. Performed estimates show that the sample temperature can achieve 94 K for current purity materials. The calculations are obtained for Yb3+:CaF2 system.
A vibronic model of laser cooling for crystals doped with Yb3+ ions is proposed. The model involves the stimulated Raman adiabatic passage pumping through dipole-allowed 4f–5d transitions, electron transitions between Stark-split sublevels due to vibronic interaction, and anti-Stokes fluorescence from the excited ion state. Dynamics of the cooling process are described by the density matrix formalism. It is shown that the cooling power in the case of vibronic interaction is higher at least by a factor of 3.4 compared to the case of electron–phonon interaction. In the proposed model, an increase in the cooling power yields deeper and faster refrigeration of a crystal doped with Yb3+ ions. Estimations show that, on cooling by the stimulated Raman adiabatic passage pumping, the temperature of a crystal with an irradiated volume of 2.12×10−4 cm3 can reach 64 K in a time of 2 s. In the proposed model, the cooling temperature is 30 K lower and the cooling process is 100 times faster compared to the traditional model of direct pumping. The estimates are applicable to present-day materials of technologically attainable purity.
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.
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.
Coherent population transfer in a laser-driven four-level system in a tripod configuration is experimentally investigated with a rare-earth-metal-ion-doped crystal (Pr3+:Y2SiO5)({\mathrm{Pr}}^{3+}:{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5})(Pr3+:Y2​SiO5​). The population transfers observed here indicate that a main process inducing them is not optical pumping, which is an incoherent process inducing population transfer. Moreover, numerical simulation, which well reproduces the experimental results, also shows that the process inducing the observed population transfers is similar to stimulated Raman adiabatic passage (STIRAP) in the sense that this process possesses characteristic features of STIRAP.