Negative refraction in split-ring-resonator stack at normal incidence

Abstract

B. Kante, R. Ghasemi, A. de Lustrac, J.- M. Lourtioz IEF, Univ. Paris-Sud, UMR 8622, Or say, F91405; CNRS, Orsay, F-91405 boubacar.kante@ief.u-psud.fr Abstract: Metamaterials building blocks, from microwave to optical ra nge are mainly based on metal-dielectric composite. In almost all structures with true negative index (not coming from losses) two kind of meta-atoms (electric and magnetic) are mixed in order to drive simultaneously the effective permittivity and permeability to negative values and thus to obtain a negative index of refraction. In this pa per we show that two co upled structures with localized plasmons modes (e.g. Cut wires or Split-Ring-Resonators) can exhibit negative refractive index by their own, by appropriately c ontrolling the hybridization scheme of the so called plasmons modes. As a result, the metallic filling factor is drastically reduced and low losses especially at optical frequencies may allow realistic applications of metamaterials. Metal dielectric composite have been deeply studied in the recent year for their remarkable properties. Most of the properties of the so called metal-dielectric composite are based on collective oscillation of free carrier in metals, i.e. plasmons [1]. More recently the focus has turned to artificially structured materials i.e. metamaterials exhibiting on demand electromagnetic properties [2-3]. Their have been used to construct the counterintuitive negative index metamaterials [4]. The first negative index metamaterials have been realized by Smith et al. by combining two fundamental building bl ocks in the design of metamaterials [5]. The first building block is an array of continuous wires for negative permittivity and the second an array of SRRs for negative permeability. After this first demonstration in the microwave focus has turned to constructing simila r structures in optics. However, due to the saturation of SRRs, alternative solutions need to be analyzed. For this purpose, coupled dipoles have been realized as artificial magnetic atom at optical frequencies [6-7]. Artificial magnetism in these structures is based on the anti-symmetric current mode excited in the coupled dipoles forming a virtual current loop. The coupled dipoles also support a symmetric current mode oscillation that can be used to alte r the effective permittivity. It has been tried to overlap the symmetric and anti-symmetric eigenmodes so as to get negative index with only one type of meta–atom. However, most of the attempts were unsuccessful and negative index observed in such structures mainly comes from losses. We have recently shown that breaking the symmetry of paired cut-wires can lead to an unambiguous negative index [8]. The principle was based on the control localized plasmons hybridization in cut-wires. The negative index has also been verified using a prism experiment based on the only cut-wire structure [9]. In this paper we show that the prin ciple applied to cut-wires in [8] can in fact be generalized to structures supporting localized plasmons. Ne gative refraction is demonstrated in split ring resonator (SRR) stack at normal in cidence by breaking the symmetry of the SRR stack.