Modelling beam-transport diffraction in near-infrared few-cycle strong-field experiments

Abstract

Few-cycle high peak power laser pulses produced in gas-filled hollow fibres have a EH11 Bessel mode. Control of the intensity and phase-matching conditions requiring no additional dispersion is introduced, and is often achieved by closing a hard aperture around the beam. We use the Huygens–Fresnel diffraction integral to propagate such a beam through a numerical model of a typical attosecond beamline, quantifying the distribution of spatial intensity and the shape of the wavefront through the focal volume as the beam is truncated by finite-diameter optics. State-of-the-art attosecond and single-harmonic experiments combining the fundamental NIR with high-harmonic XUV radiation also employ annular optics, again as material dispersion must be avoided. We simulate the additional changes to wavefront shape and spatial intensity distribution on reflection from a spherical annular mirror for a range of annulus and aperture sizes. Finally we predict the change in intensity and focal position as the aperture is reduced. The model's usefulness as a tool for optimisation of an experimental beamline is therefore highlighted.