High-order harmonic generation in colliding beams

A method for the generation of high-order harmonics in two interfering waves is proposed. It is shown that phase matching is possible in such a method and its consequence is a considerable increase in the efficiency of conversion of radiation to a harmonic with a given number. The increase is particularly large when harmonics are generated in a low-density (subcritical) plasma. The conditions are found under which the increase is about four orders of magnitude. The use of two interfering beams makes it possible to separate angularly the generated harmonics without recourse to spectroscopic devices.