Using laser light to cool down glass

What is it about?

We used 2-µm wavelength lasers to cool thulium atoms mixed into indium fluoride glass. Thulium atoms in an optical glass can absorb a certain range of wavelengths and emit a range of wavelengths of light. Because the thulium atoms interact with heat and vibrations in their environment, the wavelengths they emit are different from the wavelengths they absorb. If the absorbed wavelength of light is chosen so that the total energy of the emitted light is greater than the absorbed energy, the glass material around the thulium atoms loses energy and cools down. We have demonstrated the most efficient cooling of thulium in a glass to date.

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Photo by Daniele Levis Pelusi on Unsplash

Photo by Daniele Levis Pelusi on Unsplash

Why is it important?

Optical refrigeration by using the energy difference between absorbed and emitted light was proposed by Pringsheim in 1929, and first demonstrated by Epstein with ytterbium in ZBLAN glass in 1995. Unlike traditional refrigeration systems with moving parts and liquid coolants, optical refrigeration is vibration free and does not require fluids for heat exchange which allows for highly compact, physically robust cooling systems. Optical refrigeration using ytterbium in glasses and crystals and near-infrared lasers in the 1-µm wavelength region has been extensively studied. This work studied the cooling of thulium in indium fluoride glass with mid-infrared lasers in the 2-µm region. Using 2-µm lasers is an important development, because they could allow for better energy efficiency by allowing use of lower energy photons to drive the optical refrigeration process.

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