Axicon center intensity and polarization in THz vector vortex beams

Determine whether the intensity at the center (r=0) of the silicon axicon–generated terahertz vector vortex beam is zero, as in conventional Laguerre–Gaussian vortex beams, and characterize the polarization state at r=0 for the axicon-based retarder used to produce the vector vortex beams in the h-Lu0.6Sc0.4FeO3 experiments.

Background

To probe magnetic excitations, the paper employed an undoped silicon axicon retarder to generate broadband terahertz vector vortex beams with orbital angular momentum l=±1 and spin angular momentum σ=0. The authors note that, in theory, the electric field distribution of the vector beam has a singular behavior at r=0, and that intensity and polarization at this point are problematic.

Because the central behavior of the axicon-generated beam was uncertain, the authors masked the center (r<2 mm) to avoid contributions from the ill-defined region. Clarifying the intensity and polarization at r=0 is relevant for interpreting vortex-beam coupling to magnons and for the broader development of VVB spectroscopy in antiferromagnetic materials.