It has been recognized that plasma represents a medium that, having
already been ionized, is governed by nonlinearities of higher
strength than those associated with electrons bound to a nucleus. The
relativistic nonlinearity transcends octave spanning frequency
modulation and can produce compression of the time-span associated
with a fraction of an optical cycle. Simulations indicate that this
form of compression, which we label phase compression, is capable of
efficiently producing pulses of attosecond duration. Measurements of
the spatial modulation of the fundamental radiation are consistent
with simulations for non-normal incidence and for plasma having a
scale length near one wavelength. One may rightly be concerned with
questions of whether the ionization processes may yet play a role in
partially ionized plasma.