  We study the effect of laser photon merging, or equivalently high harmonic
generation, in the quantum vacuum subject to inhomogeneous electromagnetic
fields. Such a process is facilitated by the effective nonlinear couplings
arising from charged particle-antiparticle fluctuations in the quantum vacuum
subject to strong electromagnetic fields. We derive explicit results for
general kinematic and polarization configurations involving optical photons.
Concentrating on merged photons in reflected channels which are preferable in
experiments for reasons of noise suppression, we demonstrate that photon
merging is typically dominated by the competing nonlinear process of quantum
reflection, though appropriate polarization and signal filtering could
specifically search for the merging process. As a byproduct, we devise a novel
systematic expansion of the photon polarization tensor in plane wave fields.
