Subwavelength metallic coaxial waveguides in the optical range: Role of the plasmonic modes

A theoretical investigation is presented of the mode propagation and attenuation in a nanometric coaxial waveguide in real metal. By a rapid comparison with other structures, it is established that a coaxial waveguide has propagative modes with very interesting properties: the cutoff wavelengths are very large, they become larger when a perfectly electric conductor is replaced by gold or silver (real metal), and they can be increased when the outer and inner radii are very close one to other. By studying dispersion curves and field structures, it is shown that surface plasmon modes are responsible for these properties. By simply changing the geometrical parameters of the structure, a very large effective index and very low group velocities could be obtained. We also establish that, in spite of the metal losses, a reasonable large propagation length could be obtained (50 mu m) which should allow applications for guiding light in nano-optics.