An all-optical refrigeration scheme has successfully cooled a photodetector to cryogenic temperatures for the first time. Mansoor Sheik-Bahae and co-workers from the University of New Mexico and Los Alamos National Laboratory in the US used anti-Stokes fluorescence from an optically pumped Yb3+-doped YLiF4 (YLF) crystal to cool a HgCdTe infrared sensor to 135 K (Light. Sci. Appl.; 2018). The sensor was part of a Fourier-transform infrared spectrometer and 47 W of 1,020 nm laser light was required for the cooling. Various demonstrations of all-optical refrigeration have been previously reported, indeed cooling temperatures to as low as 87 K have been achieved, but this is the first where an attached payload has successfully been cooled. Key to the success was the use of a specially designed transparent thermal link, made from a kinked undoped YLF waveguide, which connects the cooling crystal to the sensor (pictured). The cooling crystal was placed inside a Herriott cell (two mirror cavity) to allow for multiple passes of the pump laser light and the entire apparatus was placed within a vacuum chamber to minimize unwanted heat flow.

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Optical refrigeration works by using a material that can emit light at a shorter wavelength than it absorbs, with the energy difference between the absorption and emission being provided by phonons (heat) from the material. A variety of materials, such as doped optical fibres and semiconductors, have been tested, with Yb3+:YLF being the most effective to date.

The team are now exploring the opportunities for cooling with other crystal materials and potential applications. “We are also getting close on achieving cryogenic cooling in Tm- and Ho-doped crystals (at 2 μm wavelength) that show twice the efficiency over Yb-doped materials that use pumps at 1 μm,” Sheik-Bahae told Nature Photonics. “We have already cooled these materials, but have not attempted ‘power cooling’ or to use them for cooling a device yet.”

As for applications, optical cryocoolers could prove useful wherever there is a need for minimal or negligible vibration that precludes the use of traditional mechanical means, such as a Stirling or Gifford-McMahon refrigerator that feature moving parts. Potential examples that may benefit include germanium gamma-ray detectors, cryogenic microscopy and space-based sensors.

“The most exciting application that we are currently working on is in collaboration with Jun Ye’s group at NIST,” explained Sheik-Bahae. “We hope to soon cool their single-crystal Si cavity to 124 K using our Yb:YLF cooler in a totally vibration-free environment. This will be a game-changer as it will allow these ultra-stable lasers to be portable and accessible for myriad of high-precision meteorology and clock applications.”