Attosecond vortex pulse trains

The landscape of ultrafast structured light pulses has significantly advanced thanks to the ability of high-order harmonicgeneration (HHG) to translate the spatial properties of infrared laser beams to the extreme-ultraviolet (EUV) spectralrange. In particular, the up-conversion of orbital angular momentum (OAM) has enabled the generation of high-orderharmonics whose OAM scales linearly with the harmonic order and the topological charge of the driving field. Havinga well-defined OAM, each harmonic is emitted as an EUV femtosecond vortex pulse. However, the order-dependentOAM across the harmonic comb precludes the synthesis of attosecond vortex pulses. Here we demonstrate a methodfor generating attosecond vortex pulse trains, i.e., a succession of attosecond pulses with a helical wavefront, resultingfrom the coherent superposition of a comb of EUV high-order harmonics with thesameOAM. By driving HHG with apolarization tilt-angle fork grating, two spatially separated circularly polarized high-order harmonic beams with order-independent OAM are created. Our work opens the route towards attosecond-resolved light-matter interactions withtwo extra degrees of freedom, spin and OAM, which are particularly interesting for probing chiral systems and magneticmaterials.