This paper presents a quasi-three-dimensional post-array designed to propagate a terahertz spoof surface plasmon polariton (terahertz spoof SPP) with confinement. A transmission line making use of a terahertz spoof SPP is a promising device in the terahertz wave band, and there are many previous reports of two-dimensional structures. Three-dimensional structures provide sophisticated designs for transmission lines propagating a terahertz spoof SPP. Eigenmode analysis is used to derive a dispersion diagram for one post with boundary conditions extracted from the full model. The propagation frequency of the terahertz spoof SPP increases with lower heights or smaller diameters, and that remains virtually unchanged for post-spacing and thickness of a substrate. The analysis of the full model confirms the confinement of a terahertz spoof SPP vertically on the post-array. The magnitude of the electric field is strong around the top and bottom and weak at approximately one-third height. The terahertz spoof SPP is confined in the space around the post-array as well as a substrate, while it is confined only on substrates in conventional two-dimensional structures. The designed post-array can control the three-dimensional focusing of a terahertz spoof SPP in an arbitrary volume of space.
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