When a brief pulse of waves is sent through a nonlinear material, the
peaks of the individual waves can travel at a different velocity than the
pulse of waves as a whole. I've seen a wonderful illustration of this in
a science museum's "Giant wave tank." When the water is still and the
wave generator suddenly turns on, the region of ripples moves forward...
but the peaks of the waves themselves move far faster! It's very strange
to watch the individual "waves" approaching the advancing edge of the
"wavy region", and then vanish as they touch it.
With light pulses, the modulation envelope carries the "information",
while the individual peaks of the EM field do not. It's OK if the peaks
of the EM field move at faster than c. Only the leading and trailing
edges of the light pulse as a whole must move at c or less. Another way
to say it: if light is AM modulated, the modulations must move at c or
less, while the individual peaks of EM intensity can move above or below
c.
(But then what about the recent announcement of a microwave tunneling
experiment where the group velocity supposedly exceeded c?)