Harmonic generation in gases by use of Bessel–Gauss laser beams

We have experimentally investigated the use of diffraction-free beams to generate low-order harmonics in gas jets. Bessel–Gauss beams have been generated from a Ti:sapphire, 100-fs laser by use of annular apertures and then by focusing of the ring-type–produced radiation. The conversion efficiencies for these beams for the third- and fifth-order harmonic generation in xenon have been measured and compared with those of pure Gaussian beams of the same energy content. We have thus found that, by matching the gas medium length to the range of the Bessel–Gauss beams, conversion efficiencies considerably higher than those of Gaussian beams can be obtained. The experimental results have also been discussed and interpreted in terms of a simple numerical model that emphasizes the effects of phase matching in the two different geometries.