Determining the Orbital Angular Momentum of a Vortex Beam Using Strong Field Ionization

Vortex beams are a special case of laser beam in which photons possess orbital angular momentum (OAM) in addition to their intrinsic spin angular momentum. These beams possess unique properties which make them useful in many fields such as astronomy, communication, and biology. While the OAM is the main feature of interest in these beams, measuring this value can be very difficult. Several different setups have been created to measure OAM, many of which are intrusive. A recent study used a two-color beam pump-probe experiment to classify OAM from the photoelectron momentum distribution (PMD) [Fang et. al, Light: Science Applications 11 (2022)]. Inspired by this, the authors propose a new unique setup involving an ionizing vortex beam and a perturbing terahertz beam which does not require beam sculpting, and can be used to determine the OAM of the vortex beam. This setup takes advantage of the unique spatial structure of the vortex beams which has a beam radius proportional to the square root of the OAM. By generating initial conditions with a Monte Carlo simulation and calculating trajectories with the strong field approximation, the authors demonstrate that this setup can be used to determine the OAM of a Laguerre-Gaussian vortex beam from the PMD.