Nonlinear Propagation of Crossing Electromagnetic Waves in Vacuum due to   Photon-Photon Scattering
  We review the theory for photon-photon scattering in vacuum, and some of the proposals for its experimental search, including the results of our recent works on the subject. We then describe a very simple and sensitive proposal of an experiment and discuss how it can be used at the present (HERCULES) and the future (ELI) ultrahigh power laser facilities either to find the first evidence of photon-photon scattering in vacuum, or to significantly improve the current experimental limits. 
Daniele Tommasini, Albert Ferrando, Humberto Michinel, Marcos Seco