A Gem of an Idea

By copying a structure found in nature -- the gemstone opal -- a team of scientists from the NEC Research Institute and Princeton University has created a new optical material that could be used to make the ultimate in microscopic integrated optical circuits (see Scientists Grow Opals).

Opal, which consists of millions of miniature silica spheres stacked in a repeating pattern, owes its shimmering colors to multiple reflections of visible light from the surfaces of the spheres. In fact, the opal is a naturally occurring example of what's called a "photonic crystal."

An artificial photonic crystal can be designed to have unusual reflective properties at the telecom wavelengths around 1.3 and 1.55 microns. If such a structure can be fabricated in a nice, simple way, it will provide optics engineers with a new way of controlling light inside optical integrated circuits (see The Hole Thing).

So far, however, actually making photonic crystals has proved tricky. That's why the work reported by NEC's Yurii Vlasov and colleagues appears to be such a breakthrough.

In yesterday's edition of the journal Nature, Vlasov describes a method of fabricating a layer of artificial opal on top of a silicon wafer. This results in a photonic crystal structure that's compatible with microelectronics processing.

"This is important if silicon-based photonic components and microelectronics are to be integrated onto a single chip, leading to advanced optoelectronic devices that will speed up the next generation of telecommunications," writes John Joannopoulos, a professor at Massachusetts Institute of Technology (MIT) in a research review article published simultaneously in Nature. Incidentally, Joannopoulos sits on the advisory boards of at least two companies that aim to exploit photonic crystal ideas, namely Clarendon Photonics Inc., and OmniGuide Communications Inc. (see A Fiber Filled With Air).

In general, there are two approaches to the fabrication of photonic crystals. With a "top down" approach, the crystal is created by using lithography and etching to selectively remove material. This is very complex, although some good results have been achieved.

A "bottom up" technique involves creating millions of identical particles and then assembling them into the desired structure. Ideally, the particles can be persuaded to self-assemble -- to organize themselves into the crystal structure while floating around in a liquid, for example.

The NEC/Princeton team set out to overcome the drawbacks of the self-assembly technique, according to David Norris, a co-author and leader of NEC's research effort into photonic crystals. "Usually the chemists who make [photonic crystals] end up with a large chunk of material in a beaker, and its hard to see how an engineer can translate it into a device," he says.

Spanish researchers had previously tried to self-assemble glass spheres on top of silicon, but their structure was so full of defects that it didn't show the desired optical properties. In contrast, the artificial crystal structure produced by Vlasov and co-workers is near perfect. "A key finding of our work was we were able to show optical results that were consistent with a photonic bandgap," Norris claims.

The starting point for making the structure was a batch of identical glass marbles, approximately 1 micron in diameter, which are suspended in a colloidal solution. These are deposited onto a silicon substrate, held vertically, by evaporating the solution slowly from the top. The next step is to use commercial deposition apparatus to fill the spaces between the glass marbles with silicon. Finally, the original glass spheres are removed with a chemical etch, leaving a porous material that's called an inverted opal.

But this is just the beginning. "Now that we have a material that's good enough, we need to go back and intentionally introduce defects because that's what devices are based on," says Norris. To make a waveguide, a line of defects must be introduced. A single point defect that acts like a microcavity is required to make a laser. "All device ideas revolve around waveguides, or point defects or some combination thereof," he adds.

Defects can be added relatively easily, for example, by including some undersized marbles in the mix. But although the researchers can control the number of defects in the structure, they cannot determine where they end up. The self-assembly technique is not controllable enough to allow precise placement of the odd-size marbles. "We need to come up with some clever tricks so we can put defects where we want them," says Norris.

The other thing he hopes to achieve is to make a similar structure out of a compound semiconductor such as gallium arsenide or indium phosphide, which emits light easily. The problem here is process technology -- deposition of silicon, which is widely used in the microelectronic industry, is better understood than deposition of compound semiconductors.

— Pauline Rigby, Senior Editor, Light Reading
http://www.lightreading.com