A Light Switch in More Ways than One

Hank Hogan

Researchers at Cornell University in Ithaca, N.Y., have fabricated a silicon-based, all-optical light switch. Built using standard semiconductor manufacturing methods, the device enables one beam of light to switch another on and off.

The technique could form the basis for photonic microchips, although that will require further research and development.

"The switch in its current form is not for logic," said team member and assistant professor of electrical and computer engineering Michal Lipson. "It is for routing signals on chip, not for computing."

A scanning electron micrograph reveals the ring resonator coupled to two straight waveguides. By making the resonator either transparent or opaque to a particular wavelength, a photonic circuit could control whether or not the light passes from one straight waveguide to the other.

Previous attempts at silicon optical switches have required large, nonplanar structures and high beam intensities. The new method requires neither because it takes advantage of the properties of silicon and light.

To construct the switch, the researchers created a 10-µm-diameter, ring-shaped resonator and a straight waveguide that came within 0.25 µm of the ring. Fabrication was performed in silicon using electron-beam lithography and reactive-ion etching. The waveguide and ring had rectangular cross sections measuring 450 nm wide and 250 nm high. Because of its diameter, the ring had a resonant wavelength of 1555.5 nm. Its circumference was an integral multiple of that wavelength.

Because silicon is transparent to light around 1500 nm, Lipson noted, a beam traveling down the waveguide would enter the ring and loop around without escaping. When the researchers sent a 1555.5-nm beam down the waveguide, less than 10 percent came out of the other end. At other wavelengths, transmission was greater than 80 percent.

The investigators then demonstrated that sending a 10-ps pump pulse from a tunable mode-locked optical parametric oscillator down the waveguide could turn on the beam. The pump also had a wavelength of approximately 1500 nm, and it was more intense than the 1555.5-nm probe.

It created free carriers in the resonator and changed its index of refraction, causing the percentage transmitted at the resonant wavelength to soar from less than 10 percent to more than 50 percent. The switching time was less than 500 ps.

The current device could form the basis of a photonic switch, although any commercial use is several years away. Further research is under way at Cornell, aimed at trimming transmission losses by smoothing out the edges of the waveguide and ring to prevent scattering.

The power of the device and its switching time also need to be reduced. Lipson said that both could be achieved with the use of smaller devices, which switch faster and require less power as a result of their smaller area.