High-order harmonic generation (HHG) has been recently proven to produce harmonic vortices carrying orbital angular momentum (OAM) in the extreme-ultraviolet (XUV) region from the nonlinear up-conversion of infrared vortex beams. In this work we present two methods to control and extend the OAM content of the harmonic vortices. First, we show that when a driver combination of different vortex modes is used, HHG leads to the production of harmonic vortices with a broad OAM content due to its nonperturbative nature. Second, we show that harmonic vortices with two discrete OAM contributions –so called fractional OAM modes– are generated when HHG is driven by conical refraction beams. Our work offers the possibility of generating tunable OAM beams in the XUV regime, potentially extensible to the soft x rays, overcoming the state of the art limitations for the generation of OAM beams far from the visible domain.
Extreme-ultraviolet (EUV) attosecond vortices carrying orbital angular momentum (OAM) are produced through high-order harmonic generation (HHG) from the nonlinear conversion of infrared twisted beams. While previous works demonstrated a linear scaling law of the vortex OAM content with the harmonic order, an unexpectedly rich scenario for the OAM buildup appears when HHG is driven by a vortex combination. The non-perturbative nature of HHG increases the OAM content of the attosecond vortices when the driving field presents an azimuthally varying intensity profile. We theoretically explore the underlying mechanisms for this diversity and disentangle the perturbative and non-perturbative nature in the generation of EUV attosecond twisted through numerical simulations.