"Most existing switches are still switching electrons on and off," team member Fengnian Xia told nanotechweb.org. "Given the present demand for high storage densities and fast data-processing rates, future nanoscale photonic devices will need to switch photons on and off."

The new device is made of nanoscale silicon photonic wires (450 nm by 220 nm) and is based on a coupled resonator waveguide (CROW), a periodic photonic structure consisting of five coupled resonators. Such a structure has a photonic bandgap through which light of certain wavelength ranges can pass through completely while light in other ranges is reflected.

"The device is switched by perturbing the optical properties of one of the resonators," explained Xia. "The pass band is eliminated in the 'on' state and light is reflected (or switched) thanks to this perturbation."

The switch is also insensitive to temperature variations of plus or minus 15 °C because the CROW structure has a large and flat pass band (of around 350 GHz). This means that temperature variations will only shift the pass band slightly and the optical signal can be kept intact when the switch is in the "off" state. Such temperature insensitivity will be important in realistic on-chip environments, where temperatures can vary dramatically near "hot spots" that move around on the chip depending on the way the computer processor is functioning at any given moment.

The device switches in times of less than 2 ns and has a bit error rate of lower than 10–12.

According to the team, potential applications include on-chip optical networks for multicore computer processors. "The switch could be used to route high-speed optical signals between different cores," explained Xia. "Using photons instead of electrons to connect different cores of a computer processor provides enormous opportunities for performance improvement, but also lots of challenges."

The IBM team is now also developing other silicon photonic devices needed for on-chip optical networks, and integrating these devices with CMOS circuits.

The work was reported in Nature Photonics.