Pair production and optical lasers
  Electron-positron pair creation in a standing wave is explored using a parameter-free quantum kinetic equation. Field strengths and frequencies corresponding to modern optical lasers induce a material polarisation of the QED vacuum, which may be characterised as a plasma of e^+ e^- quasiparticle pairs with a density of ~10^{20}cm^{-3}. The plasma vanishes almost completely when the laser field is zero, leaving a very small residual pair density, n_r, which is the true manifestation of vacuum decay. The average pair density per period is proportional to the laser intensity but independent of the frequency, \nu. The density of residual pairs also grows with laser intensity but n_r \propto \nu^2. With optical lasers at the forefront of the current generation, these dynamical QED vacuum effects may generate 5-10 observable two-photon annihilation events per laser pulse. 
D.B. Blaschke, A.V. Prozorkevich, C.D. Roberts, S.M. Schmidt, S.A.   Smolyansky