Conservation laws for Electron Vortices in Strong-Field Ionisation

TL;DR

This paper develops a theoretical framework to understand the conservation and behavior of orbital angular momentum in electrons ionized by strong laser fields, revealing new insights into photoelectron spectra and angular momentum transfer.

Contribution

It introduces a novel variant of the strong field approximation that derives conservation laws for electron angular momentum in strong-field ionization.

Findings

Orbital angular momentum is conserved and determined by initial states in linear fields.

Circularly polarized fields relate ATI peaks to electron OAM, influenced by pulse length.

Photoelectron OAM spectra depend on laser cycle parity and effective frequency.

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 is 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.