INTENSITY-DEPENDENT ATOMIC-PHASE EFFECTS IN HIGH-ORDER HARMONIC-GENERATION

The fat-field angular distributions of high-order harmonics of a 1054- nm laser, with orders ranging from the lower teens to the upper thirti es, have been measured in thin, low-density Ar, Kr, and Xe targets. Th e 1.25-times-diffraction-limited, 1.4-ps-duration, Gaussian laser puls es were focused to intensities ranging from 3 X 10(13) to 3 X 10(14) W /cm(2), using f/70 optics. A gas target localized the gas distribution near the laser focus to a thickness of about 1 mm at pressures as low as 0.3 Torr. The weak focusing geometry and the low gas pressures cre ated experimental conditions for which the harmonics could be thought of as emerging from a plane at the laser focus rather than a three-dim ensional volume. The far-field distributions of nearly all of the harm onics exhibit narrow central peaks surrounded by broad wings of about the same angular divergence as the emerging laser beam. The spatial wi ngs are due to an intensity-dependent phase variation among the dipole moments of the individual target atoms. This phase variation gives ri se to broad spatial interferences in the scattered light due to the ra dial and temporal variation of the laser intensity.