Abstract
We investigate twisted electrons with a well-defined orbital angular momentum, which have been ionised via a strong laser field. By formulating a new variant of the well-known strong field approximation, we are able to derive conservation laws for the angular momenta of twisted electrons in the cases of linear and circularly polarised fields. In the case of linear fields, we demonstrate that the orbital angular momentum of the twisted electron is determined by the magnetic quantum number of the initial bound state. The condition for the circular field can be related to the famous ATI peaks, and provides a new interpretation for this fundamental feature of photoelectron spectra. We find the length of the circular pulse to be a vital factor in this selection rule and, employing an effective frequency, we show that the photoelectron OAM emission spectra are sensitive to the parity of the number of laser cycles. This work provides the basic theoretical framework with which to understand the OAM of a photoelectron undergoing strong field ionisation.Graphic
Highlights
Since being first recognised in the classical context of tides [1–3] vortex phenomena have held an iconic status across a diverse range of disciplines [4–7]
We find the length of the circular pulse to be a vital factor in this selection rule and, employing an effective frequency, we show that the photoelectron orbital angular momenta (OAM) emission spectra are sensitive to the parity of the number of laser cycles
In this study we have developed a new version of the strong field approximation (SFA) to explore the orbital angular momentum of photoelectrons undergoing strong field ionisation
Summary
Since being first recognised in the classical context of tides [1–3] vortex phenomena have held an iconic status across a diverse range of disciplines [4–7]. The OAM has been studied indirectly via the coherent combination of pairs of vortex states that lead to the interference vortices [41–51], which occur for two counter rotating circularly polarised fields separated by a time delay. D (2021) 75 :199 ever, ideas as advanced as knots or self-torque in electron vortices have not been explored This can partly be attributed to the difficulty in experimental implementation; for example, no measurement scheme has been devised to detect the OAM of photoelectrons emitted in strong-field experiments. The OAM computations were performed numerically without full derivation of the analytical expressions and the SFA calculations employed a monochromatic field, whereas in this work we extend the model to include a sin envelope.
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