  We study the effect of stimulated photon emission from the vacuum in strong
space-time-dependent electromagnetic fields. We emphasize the viewpoint that
the vacuum subjected to macroscopic electromagnetic fields with at least one
nonzero electromagnetic field invariant, as, e.g., attainable by superimposing
two laser beams, can represent a source term for outgoing photons. We believe
that this view is particularly intuitive and allows for a straightforward and
intuitive study of optical signatures of quantum vacuum nonlinearity in
realistic experiments involving the collision of high-intensity laser pulses,
and exemplify this view for the vacuum subjected to a strong standing
electromagnetic wave as generated in the focal spot of two counterpropagating,
linearly polarized, high-intensity laser pulses. Focusing on a comparably
simple electromagnetic field profile, which should nevertheless capture the
essential features of the electromagnetic fields generated in the focal spots
of real high-intensity laser beams, we provide estimates for emission
characteristics and the numbers of emitted photons attainable with present and
near future high-intensity laser facilities.
