Enhanced subthreshold electron-positron production in short laser pulses
  The emission of electron-positron pairs off a probe photon propagating through a polarized short-pulsed electromagnetic (e.g.\ laser) wave field is analyzed.   A significant increase of the total cross section of pair production in the subthreshold region is found for decreasing laser pulse duration even in case of moderate laser pulse intensities. 
A.I. Titov, H. Takabe, B.Kampfer, and A. Hosaka
hep-ph