Wideband Circularly Polarized Vortex Surface Modes on Helically Grooved Metal Wires

We report the characteristics of a right-handed helically grooved metal wire waveguide. For the helically grooved waveguide, the normal degenerate HE1 spoof surface plasmon polariton (SSPP) mode could be decomposed into two opposite circularly polarized modes, i.e., HE−1 and HE+1 SSPP modes, due to the chirality of helical grooves. The dispersion characteristics of the nondegenerate $\mathbf{HE}\pm\mathbf{1}$ SSPP modes were numerically obtained, and their spiral electromagnetic distributions were also presented. It was found that the $\mathbf{HE}\pm\mathbf{1}$ modes were circularly polarized; they were also vortex modes and possessed an orbit angular monument. In addition, the focusing characteristics of such waves near the waveguide end were investigated. The distinct advantages, such as a relatively long focusing length and the circular polarization of the chiral $\mathbf{HE}+\mathbf{1}$ SSPP modes, make the helically grooved metal wire waveguide a potential for near-field circular dichroism detection.