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)×${10}^{14}$ W/${\mathrm{cm}}^2$] 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.