Absorption as an indication of vacuum $e^{+}-e^{-}$ pair creation in a strong nonstationary electric field

We discuss the high frequency conductivity and absorption coefficient of an electron - positron plasma (EPP) created from the vacuum in a strong nonstationary electric field (nonstationary Schwinger mechanism). It is shown that the basic contribution here is due to vacuum polarization effects. For subcritical linearly polarised fields, we obtain the general expression for the induced conductivity and the absorption coefficient, which is investigated in a wide range of frequencies from the optical to the $\gamma$-ray region.