Strong Field Ionization with a Twist: Determining Vortex Beam Orbital Angular Momentum from Photoelectrons

Vortex beams are exotic states of structured light that exhibit spiral phase profiles and carry nontrivial orbital angular momentum (OAM). While these beams display unique properties and introduce an additional control parameter in light-matter interactions, namely the orbital angular momentum index, it has been difficult to experimentally observe effects of the vortex beam OAM on resulting photoelectron momenta created via strong field ionization [Sen et al. PRA 106 (2022)]. Techniques for distinguishing vortex beams in the strong field regime include the application of beam sculpting [Fang et al. Light: Science & Applications 11 (2022)] and the implementation of a transverse terahertz pulse [Pasquinilli et al. Photonics 10 (2023)]. In this presentation, we propose a new method for determining the OAM index of an intense, low-frequency vortex beam that involves excitation of the target gas atoms by an extreme ultraviolet pulse into an excited (Rydberg) state at a particular location within the vortex beam profile. Excited electrons are then tunnel ionized by a composite beam consisting of a circularly polarized vortex field plus a mirror-image copy of itself, transforming the beam’s space-dependent phase into a space-dependent linear polarization. By performing classical trajectory Monte-Carlo simulations for helium and comparing momentum distributions corresponding to different ionization locations within the beam profile, we demonstrate that one can unambiguously determine the ionizing radiation’s OAM index.