Impedance for Wave Guiding Devices from the Microwave FrequencyRegime to Optics and Plasmonics

Abstract

We have derived expressions that generalized the impedance concept for wave guiding devices from the microwave frequency regime to optics and plasmonics. Our expressions were based on electromagnetic eigen modes that are excited at the interface of a structure. Impedance in electromagnetic wave theory is the ratio of the electric and magnetic field strength. The ratio between the electric and magnetic field is not constant over the cross section in most devices. This caused several suggestions using averaged or integrated fields which were heuristically proposed for a manifold of photonic structures not only waveguides. The area of photonic crystals were several such heuristic approaches excited until it was proven that the so called Bloch impedance, the ratio of the surface averaged fields was analytically correct solution, provided that the photonic crystal operates only in its fundamental modes. Electromagnetic response closely resembles the solution in the quasistatic limit. Plasmonic metal-insulator-metal waveguide and a waveguide at microwave frequencies facilitated the use of traditional impedance for this kind of structure in plasmonics. For this we required lumped circuit parameters to the radio frequency domain. Impedance definition must correctly describe the reflection that occurs at the boundary between two different structures. We analysed the reflection at an interface of impedance discontinuity i.e. between two different structures. A plasmonic insulator-metal-insulator waveguide characterized by referential impedance; illustrates another photonic structure. We have used Bragg reflector, i.e periodic corrugations in a metal film which intended to describe impedance. In case of circuit theory; we considered as part of a photonic network. Our approach was based on a decomposition of the electromagnetic fields into eigenmodes.