  An electromagnetic surface wave carrying orbital angular momentum (AM) is
called plasmonic vortex that also carries spin AM originating from evanescent
field. In this study, spin and orbital AM of plasmonic vortex in a terahertz
cylindrical corrugated waveguide (CCW) are analyzed. We report the extrinsic
spin AM of the plasmonic vortex for the first time. The extrinsic spin AM is
produced by the rotation of the polarization of the plasmonic vortex whose
rotation axis is positioned at the vicinity of the CCW wall. The extrinsic spin
AM depends on the intrinsic orbital AM and radius of CCW. The plasmonic vortex
with sufficiently large orbital AM carries large spin AM per photon. This large
extrinsic spin AM expands degree of freedom of the spin AM and would be applied
to various terahertz technologies.

Optics