INTENSITY-DEPENDENT PHASE-MATCHING EFFECTS IN HARMONIC-GENERATION

In high-order harmonic generation by an intense laser, intrinsic phase s can develop at the atomic level between the laser field and the indi vidual emitted harmonics. Because intrinsic phases can vary rapidly wi th the laser intensity, they can strongly influence phase matching to the extent that the laser intensity varies within the generating mediu m. Previously reported measurements of broad far-field harmonic emissi on patterns as well as measured asymmetries in the emission with respe ct to the axial positioning of the medium in the focus can be explaine d by intrinsic phases. An experimental method for further study of int rinsic phases is proposed that involves harmonic generation in two cou nterpropagating laser beams. The periodic intensity modulation created by the to beams coupled with the intensity-dependent intrinsic phases allows harmonic Light to propagate in directions with otherwise extre mely poor phase-matching conditions.