Vacuum pair-production in a classical electric field and an   electromagnetic wave
  Using semiclassical WKB-methods, we calculate the rate of electron-positron pair-production from the vacuum in the presence of two external fields, a strong (space- or time-dependent) classical field and a monochromatic electromagnetic wave. We discuss the possible medium effects on the rate in the presence of thermal electrons, bosons, and neutral plasma of electrons and protons at a given temperature and chemical potential. Using our rate formula, we calculate the rate enhancement due to a laser beam, and discuss the possibility that a significant enhancement may appear in a plasma of electrons and protons with self-focusing properties. 
Hagen Kleinert and She-Sheng Xue
physics.plasm-ph hep-th