  Although Gigahertz-scale free-carrier modulators have been previously
demonstrated in silicon, intensity modulators operating at Terahertz speeds
have not been reported because of silicon's weak ultrafast optical
nonlinearity. We have demonstrated intensity modulation of light with light in
a silicon-polymer integrated waveguide device, based on the all-optical Kerr
effect - the same ultrafast effect used in four-wave mixing. Direct
measurements of time-domain intensity modulation are made at speeds of 10 GHz.
We showed experimentally that the ultrafast mechanism of this modulation
functions at the optical frequency through spectral measurements, and that
intensity modulation at frequencies in excess of 1 THz can be obtained in this
device. By integrating optical polymers through evanescent coupling to
high-mode-confinement silicon waveguides, we greatly increase the effective
nonlinearity of the waveguide for cross-phase modulation. The combination of
high mode confinement, multiple integrated optical components, and high
nonlinearities produces all-optical ultrafast devices operating at
continuous-wave power levels compatible with telecommunication systems.
Although far from commercial radio frequency optical modulator standards in
terms of extinction, these devices are a first step in development of
large-scale integrated ultrafast optical logic in silicon, and are two orders
of magnitude faster than previously reported silicon devices.
