Abstract
We investigate the discrete orbital angular momentum (OAM) of photoelectrons freed in strong-field ionization. We use these 'twisted' electrons to provide an alternative interpretation on existing experimental work of vortex interferences caused by strong field ionization mediated by two counter-rotating circularly polarized pulses separated by a delay. Using the strong field approximation, we derive an interference condition for the vortices. In computations for a neon target we find very good agreement of the vortex condition with photoelectron momentum distributions computed with the strong field approximation, as well as with the time-dependent methods Qprop and R-Matrix. For each of these approaches we examine the OAM of the photoelectrons, finding a small number of vortex states localized in separate energy regions. We demonstrate that the vortices arise from the interference of pairs of twisted electron states. The OAM of each twisted electron state can be directly related to the number of arms of the spiral in that region. We gain further understanding by recreating the vortices with pairs of twisted electrons and use this to determine a semiclassical relation for the OAM. A discussion is included on measuring the OAM in strong field ionization directly or by employing specific laser pulse schemes as well as utilizing the OAM in time-resolved imaging of photo-induced dynamics.
Highlights
Phase vortices in light and matter are the product of the orbital angular momentum (OAM) carried by free particles resulting in a rotating wavefront
From eqn (24) for any particular angle f there is a minimum value of n, below which there will be no real solutions for the radius p. This is reminiscent of similar interference conditions that may be derived in the SFA e.g. to describe above-threshold ionization (ATI) peaks.[41]
In the previous sections the main dynamics of the interference vortices in the strong eld regime were captured and new light was shed on their formation by analysing the orbital angular momenta (OAM)
Summary
OAM of a free particle is a quantum number that can be measured and manipulated, with many of the same properties as spin, yet it has only been observed in experiments fairly recently in photons (3 decades ago)[1] and even later for electrons (1 decade ago)[2] (see ref. 3 and 4 for reviews). A more explicit example of twisted electrons in multiphoton processes is the formation of interference vortices.[18,19,20,21,22,23,24,25,26,27] Computations of the momentum distribution of photoelectrons ionized via two time-delayed, counter-rotating circularly-polarized laser elds reveal a Fermat spiral interference pattern. Understanding the OAM in photoelectrons, which undergo strong- eld ionization via circularly-polarized light, imbues the seemingly plain momentum distributions with previously unseen structure revealed only by such interferometric schemes. This opens the question: is there a generalised way to measure, directly or indirectly, the OAM of photoelectrons in strong- eld experiments?
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
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 callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
