Multiple multipole program analysis of metallic optical waveguides

Abstract

The focus of this paper is on the numerical analysis of ultra-small metallic waveguides at optical wavelengths that is very demanding 1) because the cross sections may be much smaller than the wavelength, 2) because strong plasmon-polariton effects must be accounted for, and 3) because of strong dispersion and material loss. After a short outline of available numerical methods with focus on eigenvalue solvers, the Multiple Multipole Program (MMP) - that is applied for obtaining the results shown in this paper - is outlined. Since the analysis of metallic waveguides leads to difficult complex eigenvalue problems, several techniques for solving such problems are introduced. Based on these procedures, simple plasmonic wires, metallic wires coupled with a dielectric fiber, partially coated optical fibers, and metallic waveguides with tiny V-grooves of only a few nanometers are analyzed. The impact of the material properties is demonstrated by comparing gold and silver wires with V-grooves. It is shown that such structures may exhibit Channel Plasmon-Polariton (CPP) modes with acceptable propagation lengths even when the grooves are only a few nm deep, but only within a narrow frequency range and only for metals with low loss in the desired frequency range. These modes show a strong field confinement within the groove that might be attractive for sensor applications. Furthermore, the partially coated optical fiber is attractive for optical nearfield microscopy and exhibits field enhancement due to wedge plasmon polariton and triple-point plasmon polariton effects.