Solid-state laser cooling approaches liquid-nitrogen temperatures

April 27, 2015
A team of researchers has identified a way to approach the cooling temperature of liquid nitrogen by reducing parasitic heating losses and maximizing the absorbed laser power.
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1505lfw Nb F2 Web
1505lfw Nb F2 Web
1505lfw Nb F2 Web
1505lfw Nb F2 Web

When laser light tuned to energies below the mean fluorescence energy of a material is absorbed, followed by efficient fluorescence generation and escape, outgoing photons remove energy via phonon annihilation and cool the material being illuminated. Having been the first team to achieve laser cooling to a cryogenic (<123 K) temperature of 119 K, researchers at the University of New Mexico and Air Force Research Labs (both in Albuquerque, NM) have now identified a way to approach the cooling temperature of liquid nitrogen (77 K) by reducing parasitic heating losses and maximizing the absorbed laser power.

First, the team sourced higher-purity crystal growth materials, resulting in a crystal grown with a background (parasitic) absorption coefficient that is half the value previously achieved. Second, a nonresonant cavity in the shape of a ring increases the absorption path length through the cooling crystal. Here, increased pump light passes through the visible cross section of the gain crystal (approximately 3 mm2) thanks to a smaller mirror separation that allows 22 passes through the crystal (an increase of 8 passes), resulting in pump power absorption nearly 42% higher than the previous geometry. These improvements allowed the researchers to achieve a record cooling temperature of 91 K in a 10% ytterbium-doped yttrium lithium fluoride (Yb:YLF) crystal. To reach the liquid-nitrogen temperature, the team is testing new active rare-earth ions in purified crystalline hosts, as well as developing new laser sources. Reference: S. Melgaard et al., SPIE Newsroom online, doi:10.1117/2.1201503.005790 (Mar. 13, 2015).

About the Author

Gail Overton | Senior Editor (2004-2020)

Gail has more than 30 years of engineering, marketing, product management, and editorial experience in the photonics and optical communications industry. Before joining the staff at Laser Focus World in 2004, she held many product management and product marketing roles in the fiber-optics industry, most notably at Hughes (El Segundo, CA), GTE Labs (Waltham, MA), Corning (Corning, NY), Photon Kinetics (Beaverton, OR), and Newport Corporation (Irvine, CA). During her marketing career, Gail published articles in WDM Solutions and Sensors magazine and traveled internationally to conduct product and sales training. Gail received her BS degree in physics, with an emphasis in optics, from San Diego State University in San Diego, CA in May 1986.

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