Researchers have discovered a new method of engineering gallium nitride (GaN) that could help eliminate the need for current heat dissipation methods.
Considered one of the most important semiconducting materials after silicon, GaN’s hardness, crystalline structure and wide bandgap make it ideal for a variety of applications in the optoelectronics industry, including LEDs, laser diodes that read blu-ray discs, transistors that operate at high temperatures, solar cell arrays for satellites and biochemical sensors.
Now, researchers at Lehigh University believe GaN can be engineered so that light passing through the compound cools it instead of heating it, a phenomenon known as laser cooling or laser refrigeration that would eliminate the need for expensive cooling methods currently in use.
“GaN can be used to make lasers, optoelectronic and electronic devices,” Yujie Ding, professor of electrical and computer engineering at Lehigh University, said. “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.”
The technique relies on a phenomenon known as anti-Stokes photoluminescence (APSL), which refers to the small fraction of photons whose frequency increases after hitting a material. Stokes scattering occurs when the frequency of scattered photons is lower than the frequency of incident photons; typically the ratio of the occurrence of Stokes to anti-Stokes scattering is 35:1. Scientists hope to reduce this to 1:1, at which point a material neither heats nor cools when struck by light, and even further, at which point when, with more anti-Stokes than Stokes scattering, a material imparts its energy, and thus its heat, to the light passing through it. In 2012, Ding and his students succeeded in reducing the ratio of Stokes to anti-Stokes to 2:1 in GaN; they recently improved their results to a ratio of 1:4 and are continuing efforts to reduce the ratio.
“We have not yet demonstrated cooling,” Ding said. “That will require further work. But we have demonstrated that we are above the threshold for laser cooling.”
Laser cooling was first achieved 20 years ago using glass doped with a rare earth element. However, this method is ineffective, says Ding, because only the small portion of the material that is doped contributes to the cooling. In contrast, GaN’s crystalline structure enables a much larger portion of the compound to assist with cooling. The phonons—collective vibrations at a uniform frequency—of the GaN molecules in the compound’s structure play a particularly important role to its success.
“Because of the nonlinear properties of the lattices, phonons vibrating at very high frequency break down to lower-frequency vibrations. At this lower acoustical vibrational frequency, the phonons become heat,” Ding said. To prevent this breakdown of phononic vibrations, the research group combined the higher-frequency-vibration phonons with incoming photons, effectively removing the high-frequency phononic vibrations before they break down. With this technique, the vibrations, instead of generating heat, are emitted as high-frequency photons.
“The advantage of GaN is that the collective vibration of all the GaN molecules in the lattice makes it possible for the entire lattice to potentially contribute to cooling by promoting the upconversion of high-frequency phonons,” Ding said. “We have learned how to use ASPL to convert input photons with low energy to outgoing photons with higher energy. To do this, we remove phonons by using resonance enhancement of outgoing photons’ energy with energy states of GaN. Thus, we enhance ASPL.”
“This is the best way to achieve laser cooling, because once the breakdown of high-frequency phonons occurs and produces heat, the process is not reversible. You have to work to remove heat and this is never effective.”
“Ours can be considered as a fundamental breakthrough in laser refrigeration because it shows that laser refrigeration can be obtained with a III-V semiconductor, that is, with the very materials from which the optoelectronic devices that require cooling are themselves made.”
