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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