Pair production by Schwinger and Breit-Wheeler processes in bi-frequent   fields
  Counter-propagating and suitably polarized light (laser) beams can provide conditions for pair production. Here, we consider in more detail the following two situations: (i) In the homogeneity regions of anti-nodes of linearly polarized ultra-high intensity laser beams, the Schwinger process is dynamically assisted by a second high-frequency field, e.g. by a XFEL beam. (ii) A high-energy probe photon beam colliding with a superposition of co-propagating intense laser and XFEL beams gives rise to the laser assisted Breit-Wheeler process. Prospects of such bi-frequent field constellations with respect to the feasibility of conversion of light into matter are discussed. 
A. Otto, T. Nousch, D. Seipt B. K\"ampfer, D. Blaschke, A. D.   Panferov, S. A. Smolyansky, A. I. Titov
hep-ph physics.plasm-ph