For the first time, researchers have made high-quality three-dimensional photonic crystals and used them to make a highly efficient light-emitting diode (LED). Three-dimensional photonic crystals promise to boost the performance of just about any optical device, be it a display, a solar cell, or an efficient lightbulb—but until now, no one had been able to make them using commercially viable methods or workable materials. Researchers at the University of Illinois at Urbana-Champaign are now working on solar cells based on the structures.
Photonic crystals can control the absorption, emission, and movement of light in a very precise way based on their structure. They’ve been a hot area of research since the late 1980s. So far, it’s only been practical to make flat, two-dimensional photonic crystals. These control the movement of light very well in two dimensions, but not perfectly in the third. Still, they’ve been very successful. A company called Luxtera, for example, has developed ways of building photonic-crystal-based optical interconnects directly onto computer chips. Bringing optical signals closer to computer processors helps speed data transmission, and using photonic crystals helps keep the size of these links compact. Luminus has focused on LEDs, for which the crystals help improve light output, making these devices brighter and more power-efficient.
However, three-dimensional photonic crystals would make even better optical devices. “The key advantage is, you can really control the propagation of light in all dimensions,” says Paul Braun, professor of materials science and engineering at the University of Illinois. Braun is leading the work on three-dimensional photonic crystals, and his group is also working on making solar cells from the crystals.
Making these structures is tricky. Photonic crystal structures vary, but they’re often made by drilling nanoscale holes, rods, and other features into a material. Patterning a flat slab of material with the necessary nanoscale structures to make a two-dimensional photonic crystal is a relatively simple process. It’s far more difficult to get that kind of patterning into a thick chunk of material to make a three-dimensional structure without degrading the material. And the kinds of photonic crystals that are most useful—those that can actively convert between electrical signals and optical ones, in addition to precisely manipulating the flow of light—are the hardest to make because material flaws are introduced during the process. This light-to-electricity and back conversion is critical in LEDs, solar cells, and optical data interconnects for computing.