copyright (2006) Nature Materials

A key operation in any integrated, all-optical logic scheme is likely to be the controlled modulation of the optical signal intensity using a second optical beam — modulating light with light. An important phenomenon in achieving this is the Kerr effect, by which an intense modulation beam can induce optical nonlinearities in the waveguide material. Although the maturity of silicon-device fabrication and the low losses in silicon waveguides make it a desirable material system for integrated optical circuits, up to now, modulation speeds have been limited by the weak strength of the necessary optical nonlinearities in silicon.

Hochberg and colleagues at Caltech and the University of Washington have now demonstrated a method by which silicon all-optical modulators can operate at terahertz frequencies1. Their technique uses four-wave mixing. The source laser signal is divided at a beam splitter. One part is mixed with a modulation, or gate, signal, which induces a phase shift via the Kerr effect. Interference when this beam and the other part of the source are recombined on a second beam splitter then causes the desired intensity modulation. The key new development is the addition of polymer cladding to the silicon waveguide. By integrating polymer layers, the team can take advantage of not only the efficient light confinement, but also the stronger nonlinear properties of the polymer. Although the equipment used in the current experiment limited the demonstrable modulation to 10 GHz, indirect evidence indicates that the same technique can be used to extend into the terahertz regime. The possibility of all-optical computers operating at this speed is likely to spur on further interest in integrated silicon optical circuits.