Evanescent-wave amplification studied using a bilayer periodic circuit structure and its effective medium model

Abstract

In this paper, we present both theoretical analysis and experimental verification of evanescent-wave amplification by using a bilayer periodic circuit structure and its effective medium model. We propose a series-shunt capacitor $(C\text{\ensuremath{-}}C)$ structure to simulate a magnetic plasma, whose permittivity is positive and permeability is negative, and a series-shunt inductor $(L\text{\ensuremath{-}}L)$ structure to simulate an electric plasma, whose permittivity is negative and permeability is positive, in which the structure cells are not required to be electrically small. In addition, we derive and define an effective permittivity and permeability for the $C\text{\ensuremath{-}}C$ and $L\text{\ensuremath{-}}L$ structures in closed forms, which are completely different from the published ones. When the two structures are cascaded together to form a bilayer structure, we show that evanescent waves which exist in two single layers independently can be amplified exponentially if a certain resonant condition is satisfied. Such a resonant condition is equivalent to the antimatching condition for the permittivity and permeability of the effectively electric and magnetic plasmas. To show the accuracy of this equivalent medium model, we compare both circuit-simulation results for the $C\text{\ensuremath{-}}C$ and $L\text{\ensuremath{-}}L$ structures and theoretical-prediction results for the effective magnetic and electric plasmas, which have excellent agreement. Finally, we design an experiment using lumped capacitors and inductors mounted on a printed circuit board to verify the amplification of evanescent waves sufficiently. The measurement results have good agreement with the simulation results.