High-order harmonic generation by a bichromatic elliptically polarized field: conservation of angular momentum

We present a theory of high-order harmonic generation by a bichromatic elliptically polarized laser field which consists of two coplanar components having the frequencies and (r and s are integers) and is defined in the xy plane. Laser and harmonic fields are decomposed in the components having opposite helicities. Using the conservation laws for energy and projection of the total angular momentum of atom and laser and harmonic photons on the z axis we have derived a general selection rule. This rule reduces to the known result in the case of bicircular field which consists of two counter-rotating circularly polarized fields. We apply our results to explain recent experiment by Fleischer et al 2014 (Nature Photonics 8 543).