Numerical study on high-order harmonic generation by a Bessel-Gauss laser beam

We present a theoretical study of high-order harmonic generation by a Bessel-Gauss laser beam focused in a low-density gas medium. We consider in detail the generation of the 21st-order harmonic of a short-pulse Ti:sapphire laser in an argon gas jet. Within the strong-field approximation for the atomic dipole, and a complete account of the macroscopic propagation, we compute the number of photons produced and the brightness as a function of the medium parameters and the focusing conditions. We show that emission of the 21st-order harmonic can be maximized by adjusting the half-angle of the Bessel-Gauss beam. We find that, at given intensity and confocal parameter, the conversion efficiency is significantly enhanced with a Bessel-Gauss beam, as compared with a Gaussian one. Of particular interest for many applications, we also observe a spectacular increase of the brightness, by two decades, at maximum emission.