Spin-to-Orbital Conversion of Angular Momentum in Harmonic Generation Driven by Intense Circularly Polarized Beam.

A scheme to achieve spin-to-orbital conversion of angular momentum based on an intense circularly polarized (CP) beam interacting with a plane foil is proposed for the first time. It has been verified by both simulation and theoretical analysis that vortex harmonic photons carrying orbital angular momentum (OAM) are generated after a relativistic CP beam impinges normally on a plane foil. The generation of this vortex harmonics is attributed to the vortex oscillation of the plasma surface driven harmonically by the vortex longitudinal electric field of the CP beam. During the HHG process, the OAM is converted from spin angular momentum because of the conservation of total angular momentum. In addition, if an initially vortex beam or a spiral phase plate is used, the OAM of harmonic photons can be more tunable and controllable.