Gallium nitride has emerged as one of the most widely used materials in the optoelectronics industry and the most important semiconducting material after silicon. However, Yujie Ding, professor of electrical and computer engineering, sees another, potentially more revolutionary role for GaN.
The compound, he says, can be engineered so that light passing through GaN actually cools it instead of heating it. This phenomenon, called laser cooling, or laser refrigeration, would eliminate the need for costly heat-dispersion methods that are employed to prevent electronic devices from overheating.
"GaN can be used to make lasers, optoelectronic and electronic devices," says Ding, who is a fellow of both the Institute of Electrical and Electronics Engineers and the Optical Society of America. "What if we could also use GaN for cooling? This would be one-stop shopping. We could monolithically integrate everything -- the laser, the laser-cooling device and the electronic devices -- on the same substrate."
Ding's group has reached the threshold for achieving laser refrigeration by utilizing a phenomenon called anti-Stokes photoluminescence (APSL), which refers to the tiny fraction of photons, or units of light energy, whose frequency increases after striking a material.
The ratio of the occurrence of Stokes to anti-Stokes scattering, says Ding, is typically 35:1. Scientists would like to reduce this to 1:1, at which point a 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. Recently, the group improved upon their results and recorded a ratio of 1:4.
The project is funded by DARPA. Ding's other collaborators include Guan Sun, who received his Ph.D. from Lehigh in 2013, and Ruolin Chen, a Ph.D. candidate. Sun now works for JDS Uniphase Corp., a company in San Jose, Calif., that designs and manufactures products for optical communications networks.
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