The University of Illinois group made high-quality three-dimensional photonic crystals by growing them on a template, from the bottom up, rather than by trying to introduce the nanoscale patterns into hunks of material. The researchers start by making the template, stacks of packed nanoscale spheres. They then put the template inside a vapor-deposition chamber and flow in a series of gases containing gallium and arsenide. The materials get deposited onto the template and grow around it. It’s like filling up a box of ping-pong balls with water: the material that’s flowed in fills the spaces between the spheres. Then they chemically remove the spheres, leaving behind a three-dimensional photonic crystal—a hunk of crystalline gallium arsenide that’s riddled with nanoscale holes.
Gallium arsenide is used to make optical devices such as photodetectors, but making it into three-dimensional photonic crystals has not been possible before. Not only were the Illinois researchers able to make a three-dimensional photonic crystal out of the material, they were also able to use it to make an LED that’s driven by an electrical current.
“I’ve been waiting a long time for someone to accomplish what [the Illinois group] has accomplished,” says Eli Yablanovitch, a professor of electrical engineering and computer science at the University of California, Berkeley. In the late 1980s, Yablanovitch did some of the foundational work on photonic crystals, advancing the idea that certain designs can emit light in a very controlled way that’s useful for LEDs.
Yablanovitch says it’s difficult to predict what will result from this work, and when, because no one has made practical three-dimensional photonic crystals before. “Some of the most compelling applications are already being filled by two-dimensional photonic crystals,” he says. If it becomes as easy to make three-dimensional photonic crystals as it is to make their flat counterparts already in products, they would always be the first choice, says Yablanovitch.
The technology is probably still several years away from commercialization, says Braun.
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