Nonlinear atomic response in superstrong spatially inhomogeneous field

Summary form only given. The study of nonlinear optical processes in an isotropic medium that are forbidden according to symmetry properties of medium attracts considerable interest. One of these processes is second harmonic generation in isotropic medium that is forbidden in the electric-dipole approximation. Several symmetry-breaking mechanisms have been proposed to explain the experimental results. We show that second harmonic generation could arise from the spatially nonlocal interactions, because in spatially inhomogeneous field the atomic wave function could be significantly distorted. We consider the harmonics generation in the case when an isotropic atomic medium interacts with the interference field of two noncollinear propagating pulses. We have developed a theoretical model, which describes atomic response in two-level approximation. This model takes into account spatial-nonlocal interaction with precision up to the first derivative of the laser field vector potential. The advantage of our approach over traditional perturbation theory is account of population inversion dynamics, that influences strongly on the frequency spectrum of atomic response.