Few-Cycle High-Harmonic Generation of Attosecond Vortex Pulses

We theoretically investigate high-harmonic generation (HHG) and the subsequent formation of attosecond pulses in gas-phase systems driven by few-cycle infrared (IR) optical vortex beams (OVBs) carrying orbital angular momentum (OAM). While low-intensity, linearly polarized OVB drivers generate harmonics whose OAM scales linearly with the harmonic order, we show that this simple scaling breaks down for intense few-cycle drivers. In this nonperturbative regime, higher-order strong-field processes lead to a more complex OAM composition of the harmonics, which in turn imprints a rich spatiotemporal structure on the emitted attosecond pulses. Using numerical simulations and a theoretical model, we analyze the harmonic OAM composition and characterize the resulting spatiotemporal structure of attosecond vortex light generated by intense few-cycle OVB drivers.