Electromagnetic coupling in chiral and nonlinear metamaterials

Abstract

This thesis is a study of the mechanisms and applications of the near-field interaction in metamaterial systems, investigated using pairs of resonators. Metamaterials are arrays of sub-wavelength resonant elements, and can be engineered to create exotic properties such as artificial magnetism and negative refraction. The response of a bulk metamaterial is strongly dependent on the lattice parameters, and the relative orientation of neighboring resonators has a particularly strong influence, due to the near-field interaction. To start with I study the coupling between two split ring resonators rotated through their common axis, in order to further understand the {near-field interaction}. The interplay between the electric and magnetic interactions in the system is analyzed, along with the resulting crossing of the resonant modes. A Lagrangian model is applied to the twisted rings to determine the mechanisms behind the crossing of the modes, which is dependent on the symmetry and losses in the system. The ability to use the near-field interaction to {control the nonlinear tuning} is then investigated. By introducing nonlinear inclusions in resonant elements, meta-atoms with a dynamic nonlinear response can be created. By modifying the spacing between such resonators, I can control this response via the near-field interaction. The resulting nonlinear response can be explained using the linear properties of the system such as the absorption in the resonators, and the voltage induced across the nonlinear inclusions. The possibility of manipulating {chiral properties} of twisted meta-atoms is also studied, in order to address the issue of resonant optical activity over the transmission band, along with the accompanying ellipticity in the output polarization. In particular, I propose a {u0300}{u0300}mixed pair" - a structure consisting of a meta-atom and its complement. Combining these elements together couples equivalent parallel electric and magnetic dipoles. This structure has a lower order symmetry than a pair of twisted identical resonators. The optical activity in the structure is optimized through manipulating the coupling in the structure. I also develop a method for retrieving the effective parameters, and present the results retrieved from a periodic array. The resulting retrieved parameters are verified by recalculating the scattering parameters theoretically.