  Electrons at the surface of a plasma that is irradiated by a laser with
intensity in excess of $10^{23}~\mathrm{W}\mathrm{cm}^{-2}$ are accelerated so
strongly that they emit bursts of synchrotron radiation. Although the
combination of high photon and electron density and electromagnetic field
strength at the plasma surface makes particle-particle interactions possible,
these interactions are usually neglected in simulations of the high-intensity
regime. Here we demonstrate an implementation of two such processes: photon
absorption and stimulated emission. We show that, for plasmas that are opaque
to the laser light, photon absorption would cause complete depletion of the
multi-keV region of the synchrotron photon spectrum, unless compensated by
stimulated emission. Our results motivate further study of the density
dependence of QED phenomena in strong electromagnetic fields.
