  The advent of ultrahigh-power femtosecond lasers creates a need for optical
components suitable to handle ultrahigh light intensities. Due to the
unavoidable laser-induced ionization of matter, these components will have to
be based on a plasma medium. An archetype of such optical elements is a plasma
mirror, created when an intense femtosecond laser pulse impinges on a solid
target. It consists of a dense plasma, formed by the laser field itself, which
specularly reflects the main part of the pulse. Plasma mirrors have major
potential applications as active optical elements to manipulate the temporal
and spatial properties of intense laser beams, in particular for the generation
of intense attosecond pulses of light. We investigate the basic physics
involved in the deformation of a plasma mirror resulting from the light
pressure exerted by the ultraintense laser during reflection, by deriving a
simple model of this fundamental process, which we validate both numerically
and experimentally. The understanding of this deformation is essential for all
future applications of plasma mirrors, especially for the generation of
collimated attosecond beams. We show how its effect on the attosecond beam
divergence can be mitigated by using the laser phase, thus providing crucial
control for future applications in attosecond science.
