MIT researchers reported in the October 9 issue of Science that they have proven through experiments their long-standing theory on a new way to manipulate light waves. Their photonic crystals do what no other waveguide has managed to do: guide light around a 90-degree turn without losing even an iota of efficiency.
Although only marginally faster than electricity, light can send and process much more information than electrical signals through wires, said Pierre R. Villeneuve, a research scientist in the Research Laboratory of Electronics (RLE). Among the most promising applications for improved optical circuits are telecommunications and data processing, including CDs and other optical data storage devices incorporating microlasers.
After eight years of research, John D. Joannopoulos, the Francis Wright Davis professor of physics at RLE, Dr. Villeneuve and colleagues from Sandia National Laboratories in Albuquerque, NM, have come up with a way to control and guide light that takes a different approach from standard waveguides. Their photonic crystals -- symmetrical and geometrical arrangements of materials that reflect light -- give the word "tiny" a whole new meaning. At their smallest, they are about one-eightieth the diameter of human hair.
This is an exciting new class of materials, Professor Joannopoulos said. "This new ability to mold and guide light leads to many novel applications of these materials as optoelectronic components."
Because optical fibers use pulses of light to transmit information typically at the rate of 20 billion pulses, or bits, per second, they are used widely for transmitting data by telephone and the Internet. But because optical signals cannot yet be manipulated efficiently enough, tasks such as routing calls between locations is still done electronically. This causes bottlenecks and delays.
Light, like a speeding bullet, is good at zooming straight ahead but bad at turns. Current waveguides that navigate light around sharp turns tend to cause the light to escape and scatter. What's more, a standard waveguide for a 90-degree turn takes up a square millimeter, a veritable airport of space on a photonic integrated circuit.
The researchers' goal is to find a way to miniaturize photonic devices while improving their function. "If we can manipulate photons on the scale of 1.5 microns, the length of one wavelength of light, or even less, the advantages are low-energy, low-cost devices in which a lot of components can fit on a single chip so we can integrate as many components as possible," Dr. Villeneuve said. Real estate is at a premium on your average computer chip. The Pentium chip, for instance, holds half a million transistors in the space of the head of a thimble. The goal here is to duplicate this high-density integration of components onto optical circuits.
"We would like to be able to develop photonic integrated circuits, in which a single (optical) chip contains components such as multiple waveguides, switches and modulators," Dr. Villeneuve said. "We need to be able to send light down the maze of channels and cross-channels on a chip. So we need to come up with a way to bend the light more sharply than in one square millimeter."
With the goal of designing materials that affect light in the way that semiconductors affect electrons, the researchers turned to photonic crystals. When silicon atoms are arranged in a crystal, some electron waves get through the matrix and some get reflected. This interference creates a range of energies where electrons cannot exist. The introduction of impurities or defects allows one to control the flow of electrons, i.e., their conductivity.
To recreate the same principle using light, the researchers built a checkerboard of perfect rows of tiny posts made of alternating materials. If the system is just perfect, destructive interference -- the bouncing around and reflections of light waves as in a maze of mirrors -- will cause the waves to cancel each other out. The result is an energy gap -- a range of frequencies in which there are no photons. "The resulting multiple reflections and interference creates the effect we are seeking," Dr. Villeneuve said.
If you then insert a defect in the crystal, the photon can localize onto the defect. Light waves can move along the defect. If they move away from the defect, they are rebuffed by walls of perfect crystal.
The researchers then built the defect so it makes a sharp turn. The light either turns or gets reflected back. With a slight adjustment in the placement of the posts, the researchers got the light to travel down the defect and turn without any loss in efficiency. All the light that was sent down the defect turned and continued on its way without being reflected.
While photonic crystals have been tested with microwaves and seem to work in 1,000 times less space than traditional waveguides, Dr. Villeneuve warned that optical computers containing only chips that carry light waves and none that operate with electrons are still a long way off. "There are still staggering obstacles to overcome," he said.
This work at MIT is supported in part by the National Science Foundation.
A version of this article appeared in MIT Tech Talk on October 21, 1998.