the focal spot of two counter-propagating, linearly polarized high-intensity laser beams with their polarization vectors pointing in the same direction.

It would be interesting to extend our study to other electromagnetic field configurations attainable in the overlapping foci of two high-intensity laser pulses, e.g., to deviate from the counter-propagation geometry by letting the beams collide under an relative angle and to study other laser polarizations. Moreover, the electromagnetic field profiles to mimic the laser beams should eventually be improved to account for more features of real, experimentally attainable pulses. In particular the Gaussian profile mimicking the finite Rayleigh length in the present study should be replaced by a Lorentzian profile. Besides, in a latter step of this program also a dedicated detection set-up should be worked out and the precise numbers of the detectable photons originating from the stimulated emission process should be specified. Let us emphasize again that in the present study we rather intended to underpin our viewpoint of interpreting the vacuum subjected to macroscopic strong electromagnetic (laser) fields as source term for outgoing photons. To this end, we present first estimates of the photon numbers attainable from the effect of stimulated photon emission in an all-optical experimental 