Computational and theoretical investigation of micro- and nano-scale chiral electromagnetic metamaterials
Authors: Alkim Akyurtlu
Venue:  (2006)

Artificial chiral media composed of randomly oriented helices or other left-right asymmetrical (chiral) inclusions (with sizes typically less than a wavelength), embedded within an achiral background medium, belong to a new class of optical metamaterials (MTMs). Unlike other electromagnetic designs, which try to eliminate the cross-polarization, the sensitivity of rotation to the polarization state and the elliptization of visible light diffracted from the chiral structures, derived from the handedness form, appeals to optoelectronic technologies. It was shown that due to a strong resonant interaction between chiral particles and dipoles, a stop band is formed in the frequency area where the backward wave regime can be generated. Hence, some of the chiral metamaterials can exhibit negative refractive index (NRI) properties. Chiral materials are not invariant to inversion: there is a distinction between right- and left-handed materials. Chiral resonance can lead to negative refraction of one polarization, resulting in improved and simplified designs of negatively refracting materials. These NRI chiral metamaterials can be the key to the materialization of the negative-refraction effect, perfect lenses, invisible cloak, and other optical applications in the visible and optical frequency regimes. In this work, based on circuit analysis and group theory, several novel MTMs with different special properties are proposed. These MTMS are: (1) Novel broadband NRI metamaterials, a model of spilt-ring resonator/wire composite MTM which can create a negative index passband approximately two and a half times higher than those of the conventional SRR/wire structures, maintaining identical dimensions; (2) Novel isotropic planar and three-dimensional NRI metamaterials, consisting of "cross" structures. These proposed materials have the added advantage of not possessing any cross-coupling, and, due to the simple structure, are also easy to fabricate; (3) Novel NRI chiral metamaterials, which, besides providing magnetoelectric coupling, have a negative refractive index passband which can easily be tuned in both the frequency of operation and bandwidth. Moreover, the transmission bands can be generated over a broad frequency regime with a lower loss than other known chiral structures e.g. omega, gammadion, and the NRI metamaterial, e.g. the "S-shaped" resonator. The design methodology for the novel metamaterials development is presented. Proof of the negative refractive index of the proposed structure, through simulation, using a wedge model and a triangle-shaped structure of the design, is also provided. Furthermore, a couple of the metamaterial designs are fabricated and tested in the microwave regime for validation purposes. Circuit analysis is applied to design the negative index property. Group theory is used to derive material properties which are then used to analyze the magnetoelectric coupling of this design. Co- and cross-polarizations from the experimental and numerical results in the terahertz regime are discussed. Advantages and disadvantages of our novel chiral NRI structures are compared to those of the other recognized structures previously mentioned. The complimentary synthesis and modeling techniques of our novel chiral NRI metamaterials provide a guide to the optimization of metamaterial designs in the near-IR and visible frequency regime.