Abstract
We summarize the development of high harmonic generation (HHG) with linearly polarized Laguerre–Gaussian (LG) beams and their superpositions to explain the non-perturbative aspects of HHG. Furthermore, we show that circularly polarized extreme ultraviolet vortices with well-defined orbital angular momentum (OAM) can be generated by HHG with bicircular LG beams. We introduce photon diagrams in order to explain how to calculate the OAM and the polarization of the generated harmonics by means of simultaneous conservation of spin angular momentum and OAM. Moreover, we show how the intensity ratio of the driving fields in HHG with bicircular LG beams further enhances the generation of circularly polarized twisted attosecond pulse trains.
Highlights
Light beams can carry spin (SAM) and orbital angular momentum (OAM) [1]
We summarize the development of high harmonic generation (HHG) with linearly polarized Laguerre–Gaussian (LG) beams and their superpositions to explain the non-perturbative aspects of HHG
We show that circularly polarized extreme ultraviolet vortices with welldefined orbital angular momentum (OAM) can be generated by HHG with bicircular LG beams
Summary
While the SAM is known to refer to the polarization of the (light) beam, the OAM characterizes the spatial field distribution and the formation of helical wave fronts. Because of their helical phase fronts these beams are often referred to as twisted beams. If HHG is driven by beams with helical phase fronts, we need to consider the twofold nature of HHG, namely the microscopic interaction of the individual atoms with the intense laser field and as well the (macroscopic) superposition of the radiation emitted from different atoms at the detector, to determine the OAM and the divergence of the emitted harmonics.
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