"We have demonstrated for the first time a silicon structure that enables one low-power optical beam to switch another one on and off," said Cornell University EE Michal Lipson.

Silicon circuits traditionally don't do optics. Because silicon is an indirect-bandgap material — meaning that the bottom of its conduction band is shifted with respect to the top of its valence band — the energy released during electron recombination with a hole is converted primarily into phonons (lattice vibration) instead of the photons you get from a "direct-bandgap" material like gallium arsenide.

The key is a ring-shaped nanoscale cavity whose resonant frequency depends on its refractive index, which can be optically switched by virtue of a second light beam controlling free-carrier dispersion. According to Lipson, the technique should eventually enable terahertz switching of signals on silicon chips with integrated ultralow-power, high-modulation-depth picosecond optical switches fabricated alongside conventional silicon circuitry.

"Now our photonic circuits are for carrying information, not for logic," said Lipson, principal investigator on the project and an assistant professor in Cornell's Electrical and Computer Engineering Department. "The first applications will likely be all-optical routers, not photonic circuitry, but that will come later."

Switches what?
Since the 1970s, physicists have demonstrated that silicon can switch optical signals, but only with bulky, high-powered laboratory setups. In recent years, several passive silicon components for optical circuits have been demonstrated, such as waveguides and filters, but active devices have lagged behind.

Very simple active optical silicon components have been demonstrated, such as a silica fiber doped with erbium that acts as an optical amplifier. Others have coaxed silicon into optical mode using nanoscale quantum confinement, such as porous silicon etched into nanometer-diameter pillars. STMicroelectronics has even demonstrated a silicon LED using quantum confinement that has the same brightness and efficiency as the gallium arsenide LED.

The simple task of switching an optical communications input line among various output lines has required costly optical-to-electronic-to-optical switches. And although micromirror-based all-optical switches have appeared recently, this is the first time that optical signals have been switched on an otherwise ordinary silicon chip.

If the Cornell University researchers fulfill their promises, eventually silicon chip makers will have a whole catalog of such silicon nanostructures in their libraries — enabling almost any optical task to be integrated into silicon systems rather than requiring additional gallium arsenide chips.

On to photons
"Our technique is appropriate for communications switches, not for logic, but with future nanoscale components derived from this one and others like it, we believe that photonic silicon will eventually be able to compute with photons instead of electrons," said Lipson.

In February, Lipson's group at Cornell University demonstrated a linear slot waveguide (www.eetimes.com/article/showArticle.jhtml?articleId=18310890) whose index of refraction could switch over a range of four to one. But it did not control a communications flow and required a long (20-micron) cavity on the chip.

To switch an optical communications line and to shrink the device, Lipson's group formed the cavity into a compact ring of 1 to 10 microns in diameter. The ring resonated at an optical frequency corresponding to the common communications wavelength of 1.55 microns (near infrared) and introduced no appreciable losses, thus requiring no amplification.

In operation, the linear optical communications waveguide was placed on a tangent to the circular ring resonator, enabling only photons of wavelength 1.55 microns to "jump" from the linear line to the ring. The photons take an indirect route as they travel around the ring partway, and come out switched to a different output communications line.

To turn the switch off, Lipson's group pumped in a second control beam, which caused two-photon absorption in the material. Consequently, the material's index of refraction was shifted because of free-carrier dispersion — where free electrons and holes dominate the cavity. When the index of refraction is changed by the second beam, the communication stops jumping to the ring, thereby switching off the signal.

"We think that all-optical silicon switches based on this technology will be able to switch in just tens of picoseconds," said Lipson. Today, switching is measured in nanoseconds.

For a picosecond, all-optical silicon router chip, each "input" communications line would travel down a waveguide past a line of ring resonators, each one of which would be on a tangent to a second "output" waveguide. The control beams would then switch the correct ring resonators on and off to connect the correct input lines to the correct output lines. By tuning ring resonators to different frequencies, the same setup could also demultiplex fibers carrying more than one channel.

Lipson directs the Cornell Nanophotonics Research Group (http://nanophotonics.ece.cornell.edu) at the Cornell NanoScale Facility.

Lipson was assisted in the work by Vilson Almeida, a former Cornell graduate student now at the Institute for Advanced Studies in the Technical Center of the Brazilian Air Force.

Additional contributions were made by Carlos Barrios, a former Cornell postdoctoral researcher who is now a scientist in the Nanophotonics Technology Centre, Universidad Politnica de Valencia, Spain; and Roberto Panepucci, a former Cornell research associate who is now an assistant professor at Florida International University.

The National Science Foundation provided funding for Lipson's work.