Polarization tensor of a photon in an electric field

The polarization operator is investigated at arbitrary photon energy in a constant and homogeneous electric field of the strength E. When the photon energy is less than the vacuum energy =$\mathrm{eE}\hslash /\mathrm{mc}$, the found probability describes the absorption of a soft photon by virtual electron and positron in an electric field. At this energy, the main contribution to the probability gives the process of the absorption of a soft photon by real electron and positron produced by an electric field.