  We describe a compact modulator based on a planar photonic crystal nanocavity
whose resonance is electrically controlled. A forward bias applied across a
p-i-n diode shifts the cavity into and out of resonance with a continuous-wave
laser field in a waveguide. The sub-micron size of the nanocavity promises
extremely low capacitance, high bandwidth, and efficient on-chip integration in
optical interconnects.
