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Waveguide-resonator systems are particularly useful for the development of several integrated photonic devices, such as
tunable filters, optical switches, channel drop filters, reflectors, and impedance matching elements. In this paper, we
introduce nanoscale devices based on plasmonic coaxial waveguide resonators. In particular, we investigate threedimensional
nanostructures consisting of plasmonic coaxial stub resonators side-coupled to a plasmonic coaxial
waveguide. We use coaxial waveguides with square cross sections, which can be fabricated using lithography-based
techniques. The waveguides are placed on top of a silicon substrate, and the space between inner and outer coaxial
metals is filled with silica. We use silver as the metal. We investigate structures consisting of a single plasmonic coaxial
resonator, which is terminated either in a short or an open circuit, side-coupled to a coaxial waveguide. We show that the
incident waveguide mode is almost completely reflected on resonance, while far from the resonance the waveguide mode
is almost completely transmitted. We also show that the properties of the waveguide systems can be accurately described
using a single-mode scattering matrix theory. The transmission and reflection coefficients at waveguide junctions are
either calculated using the concept of the characteristic impedance or are directly numerically extracted using full-wave
three-dimensional finite-difference frequency-domain simulations.
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