Abstract

We have measured the far-field angular distributions of high harmonics (orders 11-39) generated by a 1μm, 1ps laser pulse (0.3-3×10¹⁴ W/cm²) in low density Ar, Kr, and Xe targets (≤ 1 Torr, 1mm thick). Low-density targets were used to minimize phase mismatch in the case of strong ionization. The 1.2-times-diffraction- limited laser beam was weakly focused to minimize geometrical phase mismatch. The far-field harmonic distributions show a narrow central structure similar to that expected from lowest-order perturbation theory. They also display broad shoulders which appear to indicate an intensity-dependent phase of the atomic harmonic emission.

© 1993 Optical Society of America