We determine the
plasmon dispersion relation in coaxial waveguides composed of a circular channel separating a
metallic core and cladding. Optical transmission measurements are performed on isolated coaxial nanoapertures fabricated on a Ag film using focused ion-beam
lithography. The dispersion depends strongly on the dielectric material and layer thickness. Our experimental results agree well with an analytical model for
plasmon dispersion in coaxial waveguides. We observe large phase shifts at reflection from the end
facets of the coaxial cavity, which strongly affect the waveguide
resonances and can be tuned by changing the
coax geometry, composition, and surrounding dielectric index, enabling coaxial cavities with ultrasmall mode volumes.
