ANGULAR-DISTRIBUTION OF HIGH-ORDER HARMONICS EMITTED FROM RARE-GASES AT LOW-DENSITY

We have measured the far-field angular distributions of high harmonics (orders 11-41) generated by a 1-μm, 1-ps laser pulse [(0.3-3)×1014 W/cm2] in low-density Ar, Kr, and Xe targets (≤1 Torr, 1-mm thick). The far-field harmonic distributions show a narrow central structure surrounded by broad wings that contain approximately 75% of the harmonic energy. Low-density targets and f/70 focusing were used to minimize phase mismatches due to ionization and propagation effects, thereby isolating the single-atom contribution to the response. The wings indicate an intensity-dependent phase of the atomic harmonic emission. © 1995 The American Physical Society.