  We propose a new catalysis mechanism for non-perturbative vacuum
electron-positron pair production, by superimposing a plane-wave X-ray probe
beam with a strongly focused optical laser pulse, such as is planned at the
Extreme Light Infrastructure (ELI) facility. We compute the absorption
coefficient arising from vacuum polarization effects for photons below
threshold in a strong electric field. This set-up should facilitate the (first)
observation of this non-perturbative QED effect with planned light sources such
as ELI yielding an envisioned intensity of order 10^{26}W/cm^2.
