Theoretical Analysis of the Characteristic Impedance in Metal-Insulator-Metal Plasmonic Transmission Lines

TL;DR

This paper derives a closed-form expression for the impedance of MIM plasmonic transmission lines, analyzing the effects of insulator thickness and validating the model through practical scenarios involving reflections and junctions.

Contribution

It introduces a new analytical formulation for MIM plasmonic impedance, including approximations for different insulator thickness regimes, validated against existing results.

Findings

Impedance varies linearly with insulator thickness for very thin layers.

Impedance becomes independent of insulator thickness for very thick layers.

The formulation accurately predicts reflection behaviors in practical plasmonic structures.

Abstract

We propose a closed form formulation for the impedance of the Metal-Insulator-Metal (MIM) plasmonic transmission lines by solving the Maxwell's equations. We provide approximations for thin and thick insulator layers sandwiched between metallic layers. In the case of very thin dielectric layer, the surface waves on both interfaces are strongly coupled resulting in an almost linear dependence of the impedance of the plasmonic transmission line on the thickness of the insulator layer. On the other hand, for very thick insulator layer, the impedance does not vary with the insulator layer thickness due to the weak-coupling/decoupling of the surface waves on each metal-insulator interface. We demonstrate the effectiveness of our proposed formulation using two test scenarios, namely, almost zero reflection in Tee-junction and reflection from line discontinuity in the design of Bragg reflectors, where we compare our formulation against previously published results.