Generation Dynamics of Broadband Extreme Ultraviolet Vortex Beams.

Light beams carrying orbital angular momentum (OAM) can be generated in the extreme ultraviolet and soft X-ray spectra by means of high harmonic generation (HHG). In HHG, phase properties of the drive laser, such as curvature, aberrations, and topological charge, are upconverted to the harmonic beams and coherently added to the inherent dipole phase. The strong nonlinearity of the HHG process, combined with the rapid phase variations corresponding to large OAM values in these vortex beams, leads to a high sensitivity to small variations in the driving field. However, a study of the generation dynamics via an accurate reconstruction of multiwavelength OAM beams is challenging. Here we show full complex field measurements of multiple individual harmonics of the HHG vortex beams. By using spectrally resolved ptychographic wavefront sensing, we retrieve the high-resolution amplitude and phase profiles for harmonics 23 to 29 in parallel, enabling detailed multiwavelength beam reconstructions. We study the influence of generation conditions and drive laser aberrations on the resulting vortex fields by comparing measured fields to numerical simulations and retrieving the propagation conditions around the focus and the OAM content of the beams. Specifically, we find that the multimodal content of such vortex beams can significantly influence the propagation and field distributions in the focal region. Such a beam propagation analysis allows a prediction of the resulting attosecond pulse trains and associated attosecond light springs that can be generated under realistic driving conditions.