Stability and instability of dipole selection rules for atomic high-order-harmonic-generation spectra in two-beam setups

We show that the dipole selection rules for the high-order-harmonic-generation spectra of atoms interacting with two laser beams break down unless the polarization of each one of the beams is orthogonal to the wave vector of the other one. In a general two-beam setup, the interaction of the electrons with the magnetic components of the electromagnetic waves leads to sometimes very efficient generation of the harmonics that are symmetry forbidden within the dipole approximation, e.g., even harmonics. Our symmetry analysis and model calculations elucidate the essential differences between several schemes that have been recently proposed for the generation of circularly polarized high-order harmonics.