Theoretical Analysis of Plasmonic Modes in a Symmetric Conductor-Gap-Dielectric Structure for Nanoscale Confinement

A hybrid plasmonic waveguide is considered as one of the most promising architectures for long-range subwavelength guiding. The objective of this paper is to present a theoretical analysis of plasmonic guided modes in a symmetric conductor-gap-dielectric (SCGD) system. It consists of a thin metal conductor symmetrically sandwiched by two-layer dielectrics with low-index nanoscale gaps inside. The SCGD waveguide can support ultra-long range surface plasmon-polariton mode when the thickness of a low-index gap is smaller than a cutoff gap thickness. For relatively high index contrast ratios of the cladding to gap layers, the cutoff gap thickness is only a few nanometers, within which the electric field of the guided SCGD mode is tightly confined. The dispersion equations and approximate analytical expressions of the cutoff gap thickness are derived in order to characterize the properties of the guided mode. Our simulation results show that the cutoff gap thickness can be tailored by the metal film thickness and the indices of the cladding and gap materials. The geometrical scheme for lateral confinement is also presented. Such a structure with unique features of low-loss and strong confinement has applications in the fabrication of active and passive plasmonic devices.