The multiphoton resonant excitation of three-level atoms by the two laser fields of different frequencies is investigated. The time evolution of the system and analytical solutions expressing Rabi oscillations of the probability amplitudes at the two-color multiphoton resonant excitation are found using a nonperturbative resonant approach. The specific examples for experimental implementation of two-color multiphoton resonant excitation of hydrogen atoms are considered.
The microscopic theory of the nonlinear interaction of coherent radiation with the quantum electrodynamic vacuum is developed. The evolution equation for a single-particle density matrix is solved in the multiphoton resonant approximation. The obtained solutions disclose Rabi oscillations and resonant multiphoton excitations of the Dirac sea. On the basis of the numerical simulations, we examine the rates of harmonics radiation at the electron–positron annihilation in the field of a strong pump wave.
Microscopic quantum theory of nonlinear stimulated scattering of 2D massless Dirac particles in doped graphene on Coulomb field of impurity ions at the presence of an external strong coherent electromagnetic radiation is developed. We consider high Fermi energies and low frequencies (actually terahertz radiation) to exclude the valence electrons excitations. The Liouville-von Neumann equation for the density matrix is solved analytically, taking into account the interaction of electrons with the scattering potential in the Born approximation. With the help of this solution, the nonlinear inverse-bremsstrahlung absorption rate for a grand canonical ensemble of 2D Dirac fermions is calculated. It is shown that one can achieve the efficient absorption coefficient by this mechanism.
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