Ultrafast, All-Silicon Light Modulator

The ability to send high-speed messages between integrated circuit devices requires both high-frequency receivers and transmitters. The vast majority of integrated circuits are made from silicon-based semiconductors. Suitable receivers of silicon integrated circuits can be constructed from either metal-semiconductor-metal photodiodesl or P-type, intrinsic, N-type (PIN) photodiodes. These receivers are capable of acting as interconnection devices for frequencies up to tens of gigahertz. The transmitter half of this interconnection for silicon has proved to be more problematic. A semiconductor laser has yet to be made from silicon and may prove to be an impossible task. It may be possible to use an optical modulator in place of a semiconductor laser as the transmitter for siliconbased integrated circuits. The refractive index of silicon can be modified by the free-carrier density (the carrier-refraction e f f e ~ t ) ; ~ therefore, the refractive index can be changed by either the injection or depletion of free-carriers.