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Using GaN to shrink communication devices

Laser cooling achieved with gallium nitride could enable scientists to observe novel quantum effects and make the HEMTS used in satellites more resistant to damaging ultraviolet rays
Light might one day be used to cool the materials through which it passes, instead of heating them.

This could be due to a breakthrough by engineers at Lehigh and Johns Hopkins Universities.

The discovery could lead to smaller, lighter and cheaper communication devices and enable faster switching times, higher operating voltages and increased output.

Yujie Ding from Lehigh and Jacob B. Khurgin at Johns Hopkins, have achieved what they say is the most favourable ratio to date between opposing types of light-scattering phenomena that occur in semiconducting materials.

Photons, units of light energy, typically maintain the same kinetic energy and wavelength when they exit a material as they do when they strike it.

Raman scattering, named after the 1930 Nobel Prize winner in Physics, refers to the small fraction of scattered photons whose kinetic energy and wavelength, or frequency, differ from those of incident photons. When this frequency is lower, it is called Stokes scattering. When it is higher, it is called anti-Stokes scattering.

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.

Ideally, they would like to reduce it even further, and initiate more anti-Stokes than Stokes scattering. In this case, the material imparts its energy, and thus its heat, to the light passing through it.

Ding and Khurgin, working with GaN, have succeeded in reducing the ratio of Stokes to anti-Stokes to 2:1.

GaN, considered one of the most important semiconducting material since silicon, is used in LEDs and laser diodes. Other applications include high-frequency, high-power transistors that can operate at high temperatures and solar cell arrays for satellites. And due to its relative biocompatibility, GaN can also be used in electronic implants in humans.

Laser cooling achieved with GaN could also enable scientists to observe novel quantum effects and could make the high-electron mobility transistors used in satellites more resistant to damaging ultraviolet rays.

“We are the only group to minimise the Stokes-anti-Stokes ratio from 35:1 to 2:1 at room temperature,” says Ding. “We have accomplished this by exploiting the different resonance behaviours of Stokes and anti-Stokes scattering.”

Researchers now achieve laser cooling, says Ding, by adding a dopant to the lattices of certain crystalline materials. But the portion of the lattice that actually cools represents only a tiny 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.

Ding and Khurgin plan next 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,” says Ding. “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 research, led by  Ding and Khurgin, has been supported by the National Science Foundation and the Defence Advanced Research Projects Agency (DARPA).

Further details of this work have been published in an invited article titled, “From anti-Stokes photoluminescence to resonant Raman scattering in GaN single crystals and GaN-based heterostructures,” in Laser and Photonics Review,  Rev. 6, No. 5, 660–677 (2012).  DOI 10.1002/lpor.201000028

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