  We have performed ultrafast pump-probe experiments on a GaAs-AlAs microcavity
with a resonance near 1300 nm in the "original" telecom band. We exploit the
virtually instantaneous electronic Kerr effect to repeatedly and reproducibly
switch a GaAs-AlAs planar microcavity. We achieve repetition times as fast as
300 fs, thereby breaking the THz modulation barrier. The rate of the switching
in our experiments is only determined by optics and not by material related
relaxation. Our results offer novel opportunities for fundamental studies of
cavity-QED and optical information processing in sub-picosecond time scale.
