Generation of Attosecond Vortex Pulse Trains

High-order harmonic generation (HHG) has been key for developing extreme ultraviolet (EUV) structured light beams [1]–[4]. The up-conversion of orbital angular momentum (OAM) follows a linear scaling with the harmonic order and the OAM of the driver[5], [6]. This selection rule has precluded the emission of vortex beams with attosecond pulse duration. In this work, we demonstrate theoretically and experimentally a novel setup of HHG that produces harmonic vortices with the same topological charge as the driver[7]. The noncollinear superposition of two circularly polarized vortex beams with opposite spin and OAM helicity creates a forked polarization tilt-angle grating at the generation plane. Then, the simultaneous conservation of linear momentum, spin, and orbital angular momentum yields two spatially separated high-harmonic beams (Fig. 1a-c) with order-independent OAM (Fig. 1d). In the temporal domain, the synthesis of a comb of high-order harmonics with the same OAM results in an attosecond vortex pulse train, wherein the electric field describes spatiotemporal helicoids evolving over hundreds of attoseconds (Fig. 1e-g). This control of the OAM of light at the attosecond timescale provides an additional degree of freedom for attosecond spectroscopy.