Increased Coherence Length in High-Order Harmonic Generation by a Self-Guided Beam

Abstract

The increase of the yield of high-order harmonic generation in gases is a topic of current interest in the search of high-brightness UVX sources. The yield depends on the pump intensity, the jet density, the nonlinear polarizability of the target and the coherence length. The first three parameters have been thoroughly investigated and optimized: the intensity is limited by the saturation of ionisation1, the jet density is limited by the defocusing effects of the free electrons2 and the rare gases offer, to the present knowledge, the most efficient targets3. The coherence length is limited by three different phase-mismatch factors: the medium dispersion, the intensity-dependent atomic phase and the phase-shift due to the focusing geometry (the Gouy phase) normally used to achieve the required intensity. While little can be gained from the weak dispersion of relatively low density gases, the suppression of the focusing geometry is potentially an important source of improvement of harmonic generation that would remove the Gouy phase itself, and also cancel the atomic phase longitudinal gradient. The ideal plane-wave configuration can be approached by different ways. In this contribution we investigate harmonic generation by self-guided beams which can propagate over distances of several tenth of meters 4–7.