CU provides a key component for light-switching photonic circuits

By making a ring resonator either transparent or opaque to a particular wavelength of light, a photonic circuit can control it as a tiny optical switch. Courtesy of Nanophotonics Group

By Bill Steele

Cornell researchers have demonstrated for the first time a device that allows one low-powered beam of light to switch another on and off. Such switching is a key component for future "photonic" microcircuits on silicon, in which light replaces electrons.

Photonics on silicon has been suggested since the 1970s and previous light-beam switching devices on silicon have been demonstrated, but they were either excessively large (by microchip standards) or have required that the beam of light that does the switching be very high-powered. The approach developed by Michal Lipson, Cornell assistant professor of electrical and computer engineering, confines the beam to be switched in a circular resonator, greatly reducing the space required and allowing a very small change in refractive index to shift the material from transparent to opaque.

The advancement of nanoscale fabrication techniques in just the last few years has made it possible to overcome some of the traditional limitations of silicon photonics, Lipson said.

Photonic circuits will find their first application in routing devices for fiber-optic communications, she suggests. At present, information that travels at the speed of light through optical fiber must be converted at the end into electrical signals that are processed on conventional electronic chips, then in many cases converted back into optical signals for retransmission, an extremely slow process. The all-optical switch makes it possible to route these signals without conversion.

The all-optical switch is described in the Oct. 28 issue of the journal Nature by Lipson and members of the Cornell Nanophotonics Research Group, which she directs. The researchers used the facilities of the Cornell Nanoscale Facility to manufacture the devices on silicon chips. "It is highly desirable to use silicon -- the dominant material in the microelectronic industry -- as the platform for these photonic chips," they report in their paper. The group has already developed other components for silicon photonic chips, including straight and curved waveguides. One of the key components needed, however, is a way for one optical signal to switch another on or off.

Lipson's optical switch is based on a ring resonator, a device already familiar to photonics researchers. When a ring-shaped waveguide is placed tangent to a straight one, photons traveling along the straight waveguide are diverted into the ring and travel around it many times, but only if they match the resonant frequency of the ring, which is determined by its circumference. For the reported experiments the researchers created a ring ten micrometers in diameter with a resonance wavelength of 1555.5 nanometers, in the near infrared.

To turn the switch "off," they pumped a second beam of light in the same wavelength range through the system. This light is absorbed by the silicon through a process known as two-photon absorption, creating many free electrons and "holes" (positively charged regions) in the material. This changes the refractive index and shifts the resonant frequency of the ring far enough that it will no longer resonate with the 1555.5-nanometer signal. The process can theoretically take place in a few tens of picoseconds, the researchers said.

A similar effect can be used in a straight waveguide, but requires a fairly long distance. Because light travels many times around the ring, the scattering effect is enhanced and the signal can be controlled in a very small space.

For routing applications, Lipson said, a ring resonator coupled to two waveguides could be used. The second waveguide would receive a signal only when the resonator is switched on. She notes that there is very little loss of light in the ring, meaning that light coming into a routing device could be "recycled" and sent on its way with no additional amplification needed.

Ring resonators could also be used as tunable filters, the researchers suggest, for example to separate the many wavelengths of light in multiplexed optical fiber communications systems.

The Nature paper is titled "All-optical switch on silicon: Controlling light with light on chip." Co-authors are Vilson Almeida, a former Cornell graduate student now in the Institute for Advanced Studies in the Technical Center of the Brazilian Air Force; Carlos Barrios, former Cornell postdoctoral researcher and now a scientist in the Nanophotonics Technology Centre, Universidad Politècnica de Valencia, Spain and Roberto Panepucci, former Cornell research associate and now an assistant professor at Florida International University.

Previous work on nanoscale optical waveguides and photonic coupling is described in a paper, "Overcoming the limitations of microelectronics using Si nanophotonics: solving the coupling, modulation and switching challenges," published in the Institute of Physics journal Nanotechnology, Aug. 2, 2004.