Diffractionless beams and their use for harmonic generation

Use of diffraction-free beams to generate low-order harmonies in xenon jets has been investigated. Nearly diffractionless beams have been generated from a Ti: sapphire, 100 fs, laser system by focussing ring-type radiation. which has been produced in two different ways: either, by inserting an annular slit into the incident Gaussian beam, or by placing a doublet of axicons in a vertex-to-vertex configuration before the focussing lens. The results obtained with the first scheme are discussed and interpreted in terms of a simple perturbative model of the harmonic generation (HG) process. The first measurements performed with the doublet of axicons are also reported and discussed.