Nonperturbative multiphoton electron-positron pair creation in strong laser fields

The generation of electron-positron pairs from vacuum in counterpropagating laser beams is investigated. Our particular interest lies in the nonperturbative multiphoton regime and the resonance structure of the process. By combining analytical and numerical methods we obtain rich information on momentum spectra, resonance widths, and the time dependence of the creation probability, both on and off the resonance. The impact of the laser magnetic field is also addressed. Moreover, we discuss tunneling pair production in a sub-critical laser field and a nuclear field, and demonstrate the possibility of controlling the tunneling barrier by the assistance of a high-energy photon.