  Optical modulators can be made nowadays with high modulation speed, broad
bandwidth, while being compact, owing to the recent advance in material science
and microfabrication technology. However, these optical modulators usually work
for low intensity light beams. Here, we present an ultrafast, plasma-based
optical modulator, which can directly modulate high power lasers with intensity
up to 10^16 W/cm^2 level to produce an extremely broad spectrum with a
fractional bandwidth over 100%, extending to the mid-infrared regime in the
low-frequency side. This concept relies on two co-propagating laser beams in a
sub-mm-scale underdense plasma, where a drive laser pulse first excites an
electron plasma wave in its wake while a following carrier laser beam is
modulated by the plasma wave. The laser and plasma parameters suitable for the
modulator to work are presented. Such optical modulators may enable new
applications in the high field physics.
