The role of beam profile in high-order harmonic generation by few-optical-cycle pulses

Few-optical-cycle light pulses, produced by the hollow-fiber compression technique, are used to generate high-order harmonics with excellent spatial characteristics at wavelengths as short as 10 nm. We show that the spatial features of the harmonic radiation are strongly affected by the spatial profile of the excitation beam. The role of the fundamental beam shape has been carefully investigated by means of numerical simulations. The use of a fundamental beam with truncated Bessel radial intensity profile, produced upon propagation along a hollow fiber, allows us to obtain low-divergence, high-brightness harmonic radiation.