Generation of vacuum ultraviolet vortex beams via near-threshold harmonics in argon gas driven by infrared Laguerre-Gaussian lasers

Vortex light beams in the vacuum ultraviolet (VUV) range are in high demand for many applications. Specifically, the energy range of 10-17 eV is of interest as it enables probing the ultrafast dynamics of interband transitions and excitonic resonances in solids. However, VUV vortex beams are usually produced using large-scale facilities or high-order harmonic generation. In this work, we present a compact, table-top method to generate such beams using near-threshold harmonics (NTHs) in rare gases, driven by intense infrared vortex lasers carrying orbital angular momentum. Our simulations reveal distinct spectral and spatial features that vary with the gas-jet position relative to the laser focus. Using time-frequency analysis at both single-atom and macroscopic levels, we show that the NTH generation mechanism depends on the harmonic order, governed by the interplay between multiphoton and recollision pathways. Coherence-length maps and field-propagation analysis further clarify the spatial behavior of NTHs. This work establishes a foundation for compact VUV vortex sources and their potential applications in the study of ultrafast electron dynamics. Vacuum ultraviolet light beams with orbital angular momentum are needed to study fast electron dynamics in solids. The authors demonstrate through simulations that these vortex beams can be generated in a compact, table-top setup using nearthreshold harmonics in rare gases.