Quantum-path signatures in attosecond helical beams driven by optical vortices

Abstract

High-order harmonic generation (HHG) driven by beams carrying orbital angular momentum has been recently demonstrated as a unique process to generate spatio-temporal coherent extreme ultraviolet (XUV)/x-ray radiation with attosecond helical structure. We explore the details of the mapping of the driving vortex to its harmonic spectrum. In particular we show that the geometry of the harmonic vortices is complex, arising from the superposition of the contribution from the short and long quantum paths responsible of HHG. Transversal phase-matching and quantum path interferences provide an explanation of the dramatic changes in the XUV vortex structure generated at different relative positions of the target respect to the laser beam focus. Finally, we show how to take advantage of transversal phase-matching to select helical attosecond beams generated from short or long quantum paths, exhibiting positive or negative temporal chirp respectively.

Generation and Applications of Extreme-Ultraviolet Vortices

“…The harmonics detected in the far field are computed using the Fraunhofer diffraction formula. Despite its simplified approach, the TSM provides a very useful insight of the transverse phase matching effects for a vortex driving field, where it has shown excellent agreement with the 3D SFA calculations including propagation [21,24]. A review of both theoretical approaches can be found in Ref.…”

“…Two theoretical approaches were described in Refs. [18,24] in order to describe the nonlinear process of HHG with a vortex-structure driving beam. The first method includes a complete quantum description both at the microscopic (quantum single-atom) and 3D macroscopic (phase-matching) levels [25].…”

“…This is specially important for the case of a vortex driver, where the singularity at the center creates a transverse annular intensity profile. The transverse phase matching leads to a different divergence angle of the harmonics generated by long and short trajectories [24]. It is possible, therefore, to select the radiation emitted from short and/or long paths and, consequently, to control the positive or negative temporal chirp of the associated attosecond pulses.…”

“…Phase-matching control of the temporal and spatial properties of attosecond helical beams is explored in Refs. [24,42].…”

Attosecond twisted beams from high-order harmonic generation driven by optical vortices

“…One can define longitudinal -or transverse -phase-matching as the interference of the harmonic emission emitted from different atoms in the target placed along the longitudinal or transverse direction [34] . Transverse phase-matching is especially relevant in HHG driven by vortex beams due to their involved transverse field structure [35] . If driven by pure vortex beams, the harmonic vortices are generated with a topological charge q = q (q being the harmonic order and the topological charge of the driver) [13,20] , and with similar divergence [13,20,36] .…”