Recent developments of high harmonic generation (HHG) have enabled the production of structured extreme-ultraviolet (EUV) ultrafast laser beams with orbital angular momentum (OAM). Precise manipulation and characterization of their spatial structure are paramount for their application in state-of-the-art ultrafast studies. In this work, we report the generation and characterization of EUV vortex beams bearing a topological charge as high as 100. Thanks to OAM conservation, HHG in noble gases offers a unique opportunity to generate ultrafast harmonic beams with a high topological charge from low charge infrared vortex beams. A high-resolution Hartmann wavefront sensor allows us to perform a complete spatial characterization of the amplitude and phase of the 25th harmonic beam (32.6 nm), revealing very high-topological charges in the EUV spectral regime. Our experimental results, supported by numerical HHG simulations, demonstrate the linear upscaling of the OAM of the high-order harmonics with that of low-charge driving vortex beams, showing the sensitiveness of the OAM content to the purity of the driving beam. The generation of structured EUV beams carrying large topological charges brings in the promising scenario of OAM transfer from light to matter at both macroscopic and microscopic scales.

Light beams carrying Orbital Angular Momentum (OAM), also known as optical vortices (OV), have led to fascinating new developments in fields ranging from quantum communication to novel light–matter interaction aspects. Even though several techniques have emerged to synthesize these structured-beams, their detection, in particular, single-shot amplitude, wavefront, and modal content characterization, remains a challenging task. Here, we report the single-shot amplitude, wavefront, and modal content characterization of ultrashort OV using a Shack-Hartmann wavefront sensor. These vortex beams are obtained using spiral phase plates (SPPs) that are frequently used for high-intensity applications. The reconstructed wavefronts display a helical structure compatible with the topological charge induced by the SPPs. We affirm the accuracy of the optical field reconstruction by the wavefront sensor through an excellent agreement between the numerically backpropagated and experimentally obtained intensity distribution at the waist. Consequently, through Laguerre–Gauss (LG) decomposition of the reconstructed fields, we reveal the radial and azimuthal mode composition of vortex beams under different conditions. The potential of our method is further illustrated by characterizing asymmetric Gaussian vortices carrying fractional average OAM, and a realtime topological charge measurement at a 10Hz repetition rate. These results can promote Shack-Hartmann wavefront sensing as a single-shot OV characterization tool.