Portland, Ore. - Cornell University says it has fabricated the components necessary for silicon optical computers that use light instead of electrons. "Photonic microchips are now a reality," said assistant professor Michal Lipson.
The announcement last week at the American Association for the Advancement of Science (AAAS) in Seattle opens the door to practical optical microchips, she said. Lipson and her associates replaced wires with beams of light routed on-chip, through the air, by silicon waveguides and routed off-chip by a "pinhole" lens connecting to normal optical fibers.
"We have used nanoscale fabrication techniques to make the silicon components for optical routers and repeaters, and the optical computer is just around the corner," Lipson said. "Our technology will help to make home use of fiber-optic cables practical."
At the AAAS, Lipson revealed Cornell's cookbook for building photonic chips in nanoscale silicon by routing light through "slot waveguides" filled with air, vacuum or new organic polymers. The waveguides are controlled by electro-optical switches.
Working at the Cornell Nanoscale Facility (http://nanophotonics.ece.cornell.edu), Lipson has verified experimentally the theoretical prediction that nearly all of a beam of coherent light can be confined to a "slot waveguide" if the center of the guide has a much lower index of refraction than its wall. She fabricated such waveguides in silicon with parallel strips placed 50 to 200 nanometers apart, with a slot for the light to travel through. She discovered that silicon dioxide could form walls for light beams traveling through silicon. "The index of refraction of the medium in the gap has to be much lower than that of the wall, up to a ratio of about four to one," she said.
Lipson has demonstrated slot waveguides in such chips as ring resonators, which bleed off light from a straight beam into a ring, thereby letting a demultiplexer separate different-colored channels in a light beam. So far her switches have been electro-optical, but she is promising an all optically switched photonic chip soon.
To route optical signals off-chip, the tiny on-chip lasers must drive the vastly larger fiber-optic cables connecting chips. Instead of using the typical long, loss-prone taper, Lipson reported success with a pinhole lens, whereby the tiny laser is forced to focus to a point at the edge of the chip. The emerging beam can be perfectly mated to conventional fiber-optic cable. "We can couple on-chip 200-nm waveguides to 5-micron off-chip optical fibers with 95 percent efficiency," said Lipson.
Some work was done in collaboration with a group led by Alexander Gaeta, an associate professor of applied and engineering physics at Cornell. The National Science Foundation provided funding.