Characterization of Extreme Ultraviolet Vortex Beams with a Very High Topological Charge

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.

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