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.
Highlights
The harmonic vortices is complex, arising from the superposition of the contribution from the short and long quantum paths responsible of high-order harmonic generation (HHG)
In the previous section we have shown that the main features of the angular HHG emission profile can be understood in terms of a simple quantum-path orbital angular momentum (OAM) model
In the phase distribution we observe that the high-order topological charge of the 19th harmonic is l = 19, as expected from the conservation of OAM in HHG [15, 26]
Summary
The physical scenario under study is sketched in figure 1. Our quantum-path OAM model assumes that all the radiation has been emitted from an infinitely thin gas layer, placed perpendicularly to the propagation axis of the fundamental beam. This approximation neglects the longitudinal phase-matching effects and allows us to focus the discussion on the transversal phase-matching [40], that becomes relevant due to the unique transversal structure of beams carrying OAM. By considering the phase part of the fundamental Laguerre–Gaussian beam, equation (1), and performing the integral over the angular coordinate, we obtain a compact formula that includes the dependence both on the fundamental OAM (l) and on the quantum path contribution (j), ò ( ) Aq(j)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
