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DARPA kicks off green laser project

Non-polar GaN substrates and low-defect crystal growth will have an important part to play in powerful, practical green laser diodes.

Nine research groups have begun tackling the challenge of producing a high-power 500 nm semiconductor laser in a three-year US-based research program called VIGIL.

The teams met to initiate the program at the end of November, and they have until June 2009 to hit the first milestone and produce a workable green laser based on GaN.

VIGIL stands for Visible InGaN Injection Lasers, a name that reflects the need to include high proportions of indium to obtain green light from a GaN-based laser diode.

"There s a technical problem with getting green [light] out of nitride material," explained Henrik Temkyn, VIGIL s program manager at the US Defence Advanced Research Projects Agency, DARPA. "If you increase the amount of indium, the efficiency goes down."

Once room-temperature operational lasers have been demonstrated, official second and third phase goals include attaining 100 mW and 1 W continuous wave power output respectively. Temkyn, however, suggests that after initial lasing is shown he simply "would like power output at 700 mW from a single aperture".

The majority of the approaches being used in the program see reducing the level of defects in high-indium-content InGaN as central to producing efficient 500 nm lasers.

However, the University of California, Santa Barbara team that has exploited non-polar GaN to improve LED performance and make GaN lasers without AlGaN cladding layers will focus on using its expertise in this area for VIGIL, Temkin says.

A Georgia Institute of Technology group will also pursue non-polar GaN, with a combined focus on low-defect substrates and strain management - an approach that has recently proved to be successful for green LEDs, according to Temkin.

The other groups funded under VIGIL come from the University of South Carolina, TDI Incorporated, the Rensselaer Polytechnic Institute, the Palo Alto Research Center, Kansas State University, Arizona State University, and the High Pressure Research Center of the Polish Academy of Sciences.

Expertise in MOCVD fabrication of laser diodes is an attribute that is common to most of these groups, although not the Polish outfit. Led by Sylwester Porowski, this team has previously been able to produce "nearly defect-free" GaN substrates, onto which it will deposit InGaN by MBE.

• Arizona State has announced its award of $800,000 funding under the VIGIL program. The team will perform transmission electron microscopy to study the impact the InGaN materials structures make on their electrical and optical properties.

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