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arXiv:2501.13559

Overcoming temperature limitations in laser cooling using dressed states and diamond vacancies

P. R. Eastham, Conor N. Murphy, Luísa Toledo Tude

Published Mar 19, 2025• 0 citations
arXivHTMLPDF

Abstract

The established approach to laser cooling of solids relies on anti-Stokes fluorescence, for example from rare earth impurities in glass. Although successful, there is a minimum temperature to which such a process can cool set by the electronic level spacing in the impurity. We propose an alternative method which does not suffer from this limitation. Our approach relies on the formation of dressed states under strong laser driving, which generates a spectrum in which the gaps can be tuned to optimize the heat absorption. This allows for a cooling cycle which operates at any temperature with a power comparable to the maximum dictated by thermodynamic principles. While this cooling cycle will compete with heating due to non-radiative decay and other mechanisms, it could in principle allow laser cooling to temperatures which are unachievable with anti-Stokes fluorescence.

DOI
10.1117/12.3041722

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The established approach to laser cooling of solids relies on anti-Stokes fluorescence, for example from rare earth impurities in glass. Although successful, there is a minimum temperature to which such a process can cool set by the electronic level spacing in the impurity. We propose an alternative method which does not suffer from this limitation. Our approach relies on the formation of dressed states under strong laser driving, which generates a spectrum in which the gaps can be tuned to optimize the heat absorption. This allows for a cooling cycle which operates at any temperature with a power comparable to the maximum dictated by thermodynamic principles. While this cooling cycle will compete with heating due to non-radiative decay and other mechanisms, it could in principle allow laser cooling to temperatures which are unachievable with anti-Stokes fluorescence.
Laser cooling of solids currently has a temperature floor of 50-100 K. We propose a method that could overcome this using defects, such as diamond color centers, with narrow electronic manifolds and bright optical transitions. It exploits the dressed states formed in strong fields which extend the set of phonon transitions and have tunable energies. This allows an enhancement of the cooling power and diminishes the effect of inhomogeneous broadening. We demonstrate these effects theoretically for the silicon vacancy and the germanium vacancy, and discuss the role of background absorption, phonon-assisted emission, and nonradiative decay.
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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.
In recent decades, laser cooling of solids based on anti-Stokes fluorescence, also known as Pringsheim ‘s optical cooling, has garnered a great deal of scientific interest due to its applications, including optical solid-state coolers, radiation-balanced lasers and rare-earth-doped particles for biological applications. This Reprint compiles cutting-edge research on the subject carried out by various scientific teams all over the world. It will be useful to anyone involved in solid-state physics, low-temperature physics or cryogenics.
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.