"OPTICS: Silicon circuit demonstrates optical switch"
The world's first silicon chip that switches optical wavelengths has been brought to light by Cornell University researchers using nanoscale techniques. "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.
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