The multiphoton electron-positron pair production through nonlinear channels by superintense opposed laser pulses of the same frequencies in vacuum is considered. On the basis of the Dirac model the resonance approximation for vacuum induced transitions is developed. The analytic formulas for energetic and angular distributions and total number of created particles in the limit of a short interaction time are obtained.

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Nonlinear dynamics of establishment of electron-hole coherent superpositions states in graphene by multiphoton resonant excitation of interband transitions in laser fields is considered. The singleparticle time dependent density matrix for such a quantized system is calculated in the multiphoton resonant approximation. The dependence of Rabi oscillations of Fermi-Dirac sea in graphene on the time, momentum, and photon number at multiphoton laser-excitation is analyzed.

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In this work a scheme of x-ray coherent radiation generation in the nonlinear quantum regime by means of mildly relativistic high density electron beams and a strong pump laser field is investigated. The consideration is based on a self-consistent set of Maxwell and relativistic quantum kinetic equations. The coupled equations are solved in the slowly varying envelope approximation.