On the emission of high-order harmonics in the field of two coherent waves

We consider spontaneous generation of a high-order harmonic by an atomic beam passing through a focus produced by two coherent pump waves propagating at a small angle with respect to each other. It is demonstrated that, due to phase matching, the use of two coherent waves in a gas dispersive medium allows the efficiency of high-order harmonic generation to be considerably increased as compared with the case of a single wave with a doubled amplitude. Conditions permitting the optimization of harmonic generation are analyzed.