phtonic crystal
Image: RANDY MONTOYA, Sandia
GLOWING GRID. Photonic crystal created at Sandia National Laboratories can trap photons in its lattice and force them to follow intricate pathways.
Close up, it looks a lot like a stack of firewood waiting for a match, but it's actually a synthetic crystal made of slivers of silicon. The crystal has the unique ability to bend specific frequencies of light in almost any direction, with almost no loss. This achievement, by researchers at Sandia National Laboratories, is far more than a laboratory curiosity: These devices hold the key to rapid advances in communications and optical computing.

The lattice of interlocking bars, called a photonic crystal, acts like a mirror to prevent light of a particular frequency caught in the cavities from escaping. By selecting the proper width and distance between the bars, researchers can select the frequency of light that becomes trapped in this photonic "band gap." Then, by carefully introducing impurities or variations in the lattice, they can create pathways that take the trapped light on a roller coaster ride through the crystal. No matter how sharp the turns, light of a frequency roughly in the middle of the band gap cannot escape.

The nearly leakproof lattices guide approximately 95 percent of the light within them, compared to approximately 30 percent for conventional optical waveguides. And they can turn the light on a dime: they take up only one-tenth to one-fifth the space required by conventional waveguides to bend the light. The Sandia photonic lattice's turning radius is currently in the one-wavelength range, rather than the traditional waveguide bend of more than 10 wavelengths.

Researchers have attempted to build practical "photonic band gap structures" since the idea was first proposed in 1987 by Eli Yablonovitch, now a professor at the University of California at Los Angeles. The first photonic crystal, which he built in 1990, was the size of a baseball and could channel the microwaves useful in antenna applications. In the mid-1990s, scientists at Iowa State University and the nearby Department of Energy's Ames Laboratory built crystals the size of Ping-Pong balls, also for microwaves. They were assembled by hand from the common straight metal pins used by tailors. Another group headed by J. D. Joannopoulos at the Massachusetts Institute of Technology was also pursuing similar goals.

Sandia's advancement was in taking the crystals down into the nano-realm. The present device, which was constructed by Sandia researchers Shawn Lin and Jim Fleming, functions in the infrared range (wavelengths of approximately 10-micron) and can be used to enhance or better transmit infrared images. "We had built the same structure, but more than 100 times larger. It is quite remarkable that Shawn Lin's group could do it at this size," says Rama Biswas, a researcher at Ames Lab.

LIGHT TRAP. Using techniques drawn from semiconductor manufacturing, researchers created a synthetic crystal made of interlocking silicon bars, 1.2 microns wide and1.5 microns high. As predicted, light of infrared wavelengths (10 microns) cannot escape from the structure.

But Lin isn't stopping with the infrared. The next step, already under-way, is a 1.5-micron crystal--the region in which almost all the world's optically transmitted information is passed. Other photonics scientists are confident that the Sandia team will achieve its goal. "With the structure Lin is using now, he'll be able to hit the mark within the next 12 months," predicts Pierre Villeneuve, a member of MIT professor Joannopoulos's group who has theorized about uses for photonic crystals.

The reason for the confidence of Villeneuve and others is the fabrication technique employed by the Sandia group. Lin and Fleming were able to draw on technology that Sandia has perfected for making micromachines--tiny gears and wheels carved out of silicon using variations of the techniques that produce computer chips. A silicon wafer like those used in semiconductor manufacturing was coated with silicon dioxide. Then trenches were etched into the silicon dioxide and filled with polysilicon.

The chip was polished and another layer added on top, this time with the trenches at right angles to those on the layer below. After repeating the process a number of times, the silicon dioxide was etched away with hydrofluoric acid, leaving a lattice of polysilicon bars that were 1.2 microns wide and 1.5 microns high, with a pitch of 4.8 microns--identical to a structure predicted by Ames Laboratory researchers necessary to make the photonic equivalent of a band gap for electrons. Tens of thousands of these devices can be fabricated on a single, six-inch silicon wafer.

Shawn Lin
Image: RANDY MONTOYA, Sandia
SHAWN LIN, holding an early prototype, headed the Sandia effort to develop the photonic crystal. The next goal: even smaller structures that will be able to control the frequencies of light used in most optical communications systems (1.5 microns).

If the Sandia workers make their 1.5-micron-frequency mark, the development will pave the way to tinier, cheaper, more effective optical waveguides, sensors, lasers--and even make optical computers a reality. Because little light is lost in the three-dimensional mirroring that sends light back at itself, a new type of microlaser requiring little start-up energy is theoretically achievable.

In addition, photonic crystals will be a boon to researchers trying to develop computers that utilize photons instead of electrons. Photons are faster and cooler than electrons, but no one has been able to bend useful frequencies of light around the tight corners needed to navigate the million turns on a computer chip the size of a postage stamp. In communications the devices will make it easier to separate data carried on various frequencies on a combined stream of white light that passes through optical fibers.

The Sandia discovery was first revealed in the July 16, 1998, issue of Nature. Laboratory officials say they have applied for a patent on the new devices and have already been contacted by at least one venture capitalist eager to commercialize the technology.

Will this new way of manipulating light rival the transistor?