Abstract
Solid-state optical refrigeration uses anti-Stokes fluorescence to cool macroscopic objects to cryogenic temperatures without vibrations. Crystals such as Yb3+-doped YLiF4 (YLF:Yb) have previously been laser-cooled to 91 K. In this study, we show for the first time laser cooling of a payload connected to a cooling crystal. A YLF:Yb crystal was placed inside a Herriott cell and pumped with a 1020-nm laser (47 W) to cool a HgCdTe sensor that is part of a working Fourier Transform Infrared (FTIR) spectrometer to 135 K. This first demonstration of an all-solid-state optical cryocooler was enabled by careful control of the various desired and undesired heat flows. Fluorescence heating of the payload was minimized by using a single-kink YLF thermal link between the YLF:Yb cooling crystal and the copper coldfinger that held the HgCdTe sensor. The adhesive-free bond between YLF and YLF:Yb showed excellent thermal reliability. This laser-cooled assembly was then supported by silica aerogel cylinders inside a vacuum clamshell to minimize undesired conductive and radiative heat loads from the warm surroundings. Our structure can serve as a baseline for future optical cryocooler devices.
Highlights
All-optical cooling of a solid was first observed in 1995 by Epstein et al.[1], and extensive developments over the past two decades in materials, characterization techniques, and optical designs have laid the groundwork for practical applications
Cooling crystal and Herriott cell As shown in Eq (1), the cooling power of a solid-state optical refrigerator linearly scales with the laser power absorbed by the cooling crystal (YLF:Yb), Pabs = Pinηcpl, which in turn depends on Nrt, αr, and Lx
We used solid-state optical refrigeration to cool a payload to cryogenic temperatures for the first time, which represents a breakthrough in this field and opens the door to using this technology for a variety of applications that benefit from a reliable cryogenic refrigerator without moving parts and associated vibrations
Summary
All-optical cooling of a solid was first observed in 1995 by Epstein et al.[1], and extensive developments over the past two decades in materials, characterization techniques, and optical designs have laid the groundwork for practical applications. The much smaller inhomogeneous broadening in Yb3+-doped fluoride crystals (e.g., YLiF4:Yb3+) allowed for higher cooling efficiencies, which helped enable the breakthrough into the cryogenic regime in 20103. This breakthrough has fueled further research into solid-state optical refrigeration[4,5], which is currently the only technology that can provide truly vibration-free cooling to cryogenic temperatures[6]. Solid-state laser cooling is achieved using anti-Stokes fluorescence, a process in which the average wavelength of the fluorescence (λf ) emitted by a material is shorter than the wavelength (λ) of the laser used for excitation[7].
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