Minimizing the angular divergence of high-order harmonics by truncating the truncated Bessel beam

摘要

We have experimentally investigated high-order-harmonic generation driven by a few-cycle truncated Bessel (TB) laser beam which propagates through optical elements of finite aperture sizes. The TB beam was first investigated by Nisoli et al. [Phys. Rev. Lett. 88, 033902 (2002)], who assumed an infinite size for the optical elements so they concluded that the phase and intensity of the laser field oscillate dramatically around the laser focus in space. However, in all real experiments, the optical elements are always finite in size and would further truncate the TB beam, and so the oscillations would dwindle substantially. In this paper we take the finite size of the optical elements into account. We find that the further truncated TB beam has two intensity peaks around the focus. In front of the second peak position the curvatures of the laser phase front and the atomic-dipole phase front have the same absolute values but opposite signs, so the generated harmonic has a flat wavefront and hence a minimized angular divergence. In addition, at this position the pump intensity is not much less than its maximal value. This result is of significant importance in practical applications due to the finite aperture size of all real optical elements.