Photonic Crystals Trap IR Light

Michael D. Wheeler

A team of scientists from Sandia National Laboratories and Ames Laboratory at Iowa State University revealed that they have constructed a three-dimensional IR photonic crystal on a silicon wafer that reflects light between 10 and 14 µm.
The recently published findings (Nature, July 16, page 251) could mark an important advance in the field and could lead to applications such as ultrafast optical switching, guiding light on a semiconductor chip and constructing efficient microlasers.
These photonic crystals, which measure less than half a micron, are different from other photonic crystals in several respects. Bragg gratings, for example, are one-dimensional and have a larger cavity. The size of the microcavity inside the three-dimensional crystals is much smaller, making them suitable for highly efficient nanolasers.
By introducing doping elements, physicists increase the density of electromagnetic states within the crystal at one specific frequency. At that frequency the atom inside the crystal will undergo a faster spontaneous emission (or faster rate of recombination). When atoms undergo that transition, photons will bounce back and forth against the crystal walls, which function like mirrors. A fraction of the light then escapes from the crystal.

Shorter wavelengths
The problem with early attempts to construct these crystals is that researchers had trouble fabricating three-dimensional devices that were small enough to reflect wavelengths in the IR. They used alumina rods or stycast material, but again it was difficult to create a bandgap lower than the millimeter range. An attempt to structure a three-dimensional crystal out of gallium arsenide proved a little more promising, although light attenuation was very faint in the IR range. This could be attributed to a different configuration, rather than the material used.
The Sandia/Ames researchers, led by Shawn Lin, decided to use silicon for its versatility and its ability to be micromachined. They constructed the crystal using a periodic layer-by-layer process pioneered at Ames. The structure of the crystal consists of layers of silicon rods with a stacking sequence that repeats itself every four layers. To stack the layers, the researchers used a micromachining process in which SiO₂ was deposited, patterned and etched to a desired depth. The resulting three-dimensional structure featured a high refractive index, corresponding to a large bandgap.
"It's no secret where we are trying to go with this," said Pierre Villeneuve, a research scientist at MIT who is a sometime collaborator with the Sandia/Ames team. "We're looking at telecommunications."
Villeneuve predicted Lin would soon be able to create crystals with bandgaps between 1.3 and 1.5 µm because the machining process used to construct the crystals can be scaled down easily. Besides nanolasers, the crystals could lead to developments in light-emitting diodes, filters and waveguides, he said.