ANGULARLY-RESOLVED HIGH-ORDER HARMONIC-GENERATION IN HELIUM

Harmonics up to 119th have been generated in helium using a 1.053 mum, 1 ps chirped pulse amplification laser at intensities up to 3x10(14) W cm-2 and their far-field spatial distributions have been measured. I n the presence of considerable ionization, complex angular distributio ns are found for harmonics in the plateau region, while near the cut-o ff, their distribution narrows to approximately that predicted by lowe st-order perturbation theory. The experimental results are found to be in good agreement with the predictions of a numerical propagation mod el which incorporates photoelectron dispersion and strong-field atomic data for the ionization rates and dipole moments.