Intensity-dependent phase-matching effects in harmonic generation

In high-order harmonic generation by an intense laser, intrinsic phases can develop at the atomic level between the laser field and the individual emitted harmonics. Because intrinsic phases can vary rapidly with the laser intensity, they can strongly influence phase matching to the extent that the laser intensity varies within the generating medium. Previously reported measurements of broad far-field harmonic emission patterns as well as measured asymmetries in the emission with respect to the axial positioning of the medium in the focus can be explained by intrinsic phases. An experimental method for further study of intrinsic phases is proposed that involves harmonic generation in two counterpropagating laser beams. The periodic intensity modulation created by the two beams coupled with the intensity-dependent intrinsic phases allows harmonic light to propagate in directions with otherwise extremely poor phase-matching conditions.