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Training Light to Cool Materials

By exploiting the resonance behaviors of opposing light-scattering phenomena, photons may one day be used to cool the materials through which they pass, rather than heating them. The breakthrough could lead to smaller, lighter and cheaper communication devices with faster switching times, increased output and higher operating voltages.

Photons typically maintain the same kinetic energy and wavelength when they exit a material as they do when they strike it. The small fraction of scattered photons whose kinetic energy and wavelength differ from those of incident photons is called Raman scattering. When this frequency is lower, it is called Stokes scattering, and when it is higher, anti-Stokes scattering.

The ratio of the occurrence of Stokes to anti-Stokes scattering is typically 35:1, but scientists would like to reduce this to 1:1. At this point, the material neither heats nor cools when struck by light, and even further, when, with more anti-Stokes than Stokes scattering, a material imparts its energy, and thus its heat, to the light passing through it.

Scientists at Lehigh University and at The Johns Hopkins University in Baltimore have achieved the most favorable ratio to date, reducing the ratio of Stokes to anti-Stokes in gallium-nitride (GaN) to 2:1.

Laser cooling could improve the utility of gallium-nitride, the most important semiconducting material after silicon, said Lehigh University electrical and computer engineering professor Yujie Ding. Courtesy of Lehigh University.

“We are the only group to minimize the Stokes-anti-Stokes ratio from 35:1 to 2:1 at room temperature,” said Lehigh electrical and computer engineering professor Yujie Ding. “We have accomplished this by exploiting the different resonance behaviors of Stokes and anti-Stokes scattering.”

GaN, considered the most important semiconducting material since silicon, is used in LEDs, laser diodes, solar cell arrays, biochemical sensors and transistors, and, because of its biocompatibility, electronic implants in humans. Laser cooling achieved with GaN could enable scientists to observe novel quantum effects and could make the high-electron mobility transistors used in satellites more resistant to damaging ultraviolet rays.

Laser cooling is currently achieved by adding a dopant to the lattices of certain crystalline materials, Ding said. But the portion of the lattice that actually cools represents only a small fraction of the entire lattice. If the right Stokes-anti-Stokes ratio can be achieved, every atom in the GaN lattice would cool and contribute to the cooling effect.

The scientists now plan to build an optical resonator.

“We are still puzzled by the fundamental limit to the Stokes-anti-Stokes ratio and by the feasibility of reaching a ratio of 1 or less,” Ding said. “We want to see, experimentally, how an optical resonator affects this ratio. We have already done the theoretical work for this. We want to conduct experiments inside a nanowire or other nanostructure to show how this ratio is affected by the structure.”

The study was published in Laser and Photonics Review (doi: 10.1002/lpor.201000028).  

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