On electron-positron pair production using a two level on resonant   multiphoton approximation
  We present an indepth investigation of certain aspects of the two level on resonant multiphoton approximation to pair production from vacuum in the presence of strong electromagnetic fields. Numerical computations strongly suggest that a viable experimental verification of this approach using modern optical laser technology can be achieved. It is shown that use of higher harmonic within the presently available range of laser intensities can lead to multiphoton processes offering up to 10^12 pairs per laser shot. Finally the range of applicability of this approximation is examined from the point of view of admissible values of electric field strength and energy spectrum of the created pairs. 
I. Tsohantjis, S. Moustaizis, I. Ploumistakis