Vacuum self-focussing of very intense laser beams
  We argue that long-range photon-photon attraction induced by the dipole interaction of two electron-positron loops can lead to "vacuum self-focussing" of very intense laser beams. The focussing angle theta(F) is found to increase with the beam intensity I as theta(F) I^(4/3); for the laser beams available at present or in the near future, theta(F) 10^(-10)--10^(-7). 
Dmitri Kharzeev and Kirill Tuchin