Comparison of high-order harmonic generation of Ar using truncated Bessel and Gaussian beams

We study high-order harmonic generation (HHG) spectra by a focused truncated Bessel beam versus a Gaussian beam, including macroscopic propagation of the fundamental laser and harmonic fields in the gas medium, with the single-atom-induced dipole response calculated from the recently developed quantitative rescattering theory. We first simulate the HHG spectra of Ar by an 8-fs and 780-nm short laser pulse, reported by Wörner et al. [Phys. Rev. Lett. 102, 103901 (2009)], assuming the incident beam is a truncated Bessel beam or a Gaussian beam. Both simulations fail to reproduce the observed wide and deep Cooper minimum in the HHG spectra. However, we are able to reproduce the HHG spectra of Ar generated by few-cycle 1800-nm near-infrared lasers, reported recently by Shiner et al. [Nature Phys. 7, 464 (2011)]. We also provide a systematic phase-mismatch analysis in the gas jet to examine the spatial growth of harmonics for tight-focusing versus loose-focusing lasers. The dependence of phase mismatch on the gas-jet position and gas pressure is investigated. Finally, we check the divergence of the harmonic beam generated by a tight-focusing versus loose-focusing truncated Bessel beam. These studies show that for a typical gas-jet experiment, whether the spatial mode is a truncated Bessel beam or a Gaussian beam is important only when the laser beam is tightly focused.