Analysis of efficient generation and spatial intensity profiles of high-order harmonic beams produced at high repetition rate

We report in this paper an experimental study of the spatial profile of harmonic beams (up to the 23rd order) generated in argon with 100 fs titanium-sapphire laser pulses at high repetition rate. In particular, we have investigated high-order harmonic radiation features as a function of the gas target position with respect to the pump laser focus. We have, thus, obtained the experimental conditions that simultaneously optimize generation efficiency and divergence angles of the harmonic beams. As a general trend, both the laser intensity and the harmonic order affect the harmonic beam divergence and spatial profile, depending on whether the harmonics lie in the plateau or in the cutoff region. Finally, we discuss our results in the light of recent theoretical analyses of the process. (C) 1998 Elsevier Science B.V.