The influence of the spatial inhomogeneity of the field on the nonlinear-optical response of an atom

The theory of the interaction of a centrosymmetric atom with a superstrong spatially inhomogeneous laser field is developed. This theory employs the two-level approximation to describe the dynamics of spatially nonlocal interactions related to variations in the populations of the levels. We propose a model that includes spatially nonlocal interactions, such as magnetic-dipole and quadrupole interactions and interactions due to the gradient of the ponderomotive potential of the field. We consider the interaction of a homogeneous medium of centrosymmetric atoms with a superstrong laser field, which is represented as a superposition of two plane-wave ultrashort pulses propagating at some angle with respect to each other. Perturbation theory, which is valid for the fields of moderate intensity, is developed for the atomic response. The results of numerical simulations are compared with the predictions of this perturbation theory. The specific features of the nonlinear-optical response of an atom in a superstrong field are investigated. The angular distribution of the second- and third-harmonic emission is calculated in the constant-field approximation for different polarisations of the incident field.