Abstract
We have measured the far-field angular distribution of high-order harmonic beams generated in low-density gas at the focus of an intense laser beam. The experimental setup created conditions where the harmonics could be thought of as emerging from a two-dimensional plane at the focus rather than from a three-dimensional interaction region. Under these conditions, harmonic light scattered into broad angles indicates that intrinsic phases develop at the atomic level between the laser field and the emitted individual harmonics and that this phenomenon has a strong dependence on the laser intensity. Because the laser intensity varies radially in the focus, intensity-dependent phases cause the emerging harmonic beams to have distorted wave fronts which result in broad scattering. These conclusions are in agreement with theoretical calculations and have interesting implications.
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