Symmetry Breaking in a Plasmonic Metamaterial at Optical Wavelength

Abstract

We numerically study the effect of structural asymmetry in a plasmonic metamaterial made from gold nanowires. It is reported that optically inactive (i.e., optically dark) particle plasmon modes of the symmetric wire lattice are immediately coupled to the radiation field, when a broken structural symmetry is introduced. Such higher order plasmon resonances are characterized by their subradiant nature. They generally reveal long lifetimes and distinct absorption losses. It is shown that the near-field interaction strongly determines these modes.Resonant metallic nanostructures supporting localized surface plasmon polariton modes (i.e., so-called particle plasmons) play a remarkable role in current nanoscience, where their optical properties are the subject of considerable research efforts.1 When illuminated at their resonance frequency, extremely strong and confined optical fields can be generated to alter light-matter interactions on the nanoscale.2 A very important recent example is the use of periodically arranged metallic wire pairs to mimic so-called magnetic atoms. 3-6The observation of magnetic activity generally relies on the fact that wire-wire coupling results in the formation of two energetically separated plasmonic eigenstates with opposite electric dipole moments. The antisymmetric nature of the low-energy mode is thereby essential to provide negative permeability, that is, strong magnetic components which are opposing the external magnetic field. So far, most metamaterial studies have focused on geometries with a symmetrical translation cell as elementary building block, not addressing fundamental effects caused by spatial symmetry breaking. The influence of weakly asymmetric structural elements has been discussed in the case of modulated metal films, 7 photonic crystal slabs, 8 and planar metamaterials; 9 however, similar investigations for plasmonic structures are rarely found. A recent example is the investigation of plasmon hybridization in nanoshells with a nonconcentric core. 10 It was shown that the presence of a core offset, i.e., a structural asymmetry, induces optical activity of higher multipolar plasmons. In this communication, we numerically analyze the optical response of a plasmonic metamaterial in the presence of structural asymmetry. While the related near-field induced inversion of plasmon hybridization has already been reported, 11 the present work focuses on the excitation of socalled dark plasmon modes (also known as trapped modes). 9 It is shown that these spectrally narrow additional modes possess a unique electromagnetic field distribution and only become optically active when a broken spatial symmetry is introduced. The described fundamental phenomena might pave the way for novel metamaterial applications.As shown in Figure 1, the studied model system consists of two identical nanowire gratings separated by a finite

Negative index and mode coupling in all-dielectric metamaterials at terahertz frequencies

“…The mode coupling effect we report on is different from hybridization [46] which is observed with plasmonic metameterial [12,47], split-ring resonators metametarials [48], inductor-capacitor resonators [49], cut wires [50], nanowires [51], nano-rings [52], nanoparticle dimers [53] or silicon nanoparticles [54]. In the latter cases, the coupling between the two identical meta-atoms which constitute the dimer leads from a trapped mode to the formation of new hybridized modes because it lifts the degeneracy of the mode of the individual meta-atoms.…”

Broadband wide-angle dispersion measurements: Instrumental setup, alignment, and pitfalls

“…Finally, the significance of deviations in the reflected/transmitted beam resulting from the finite thickness of the sample substrate were highlighted and accounted for. Besides allowing for extremely accurate measurements of SPP dispersions, the simplicity and robustness of this device make it suitable and desirable for multi-angle spectroscopic studies of any nanostructure or microstructure array, such as multilayers supporting Bloch surface waves, 38 nanoscale grating/particle structures, 39,40 or composite metallic nanostructures.…”

Design and Characterization of Metamaterials for Optical and Radio Communications

“…The inversion process resulted from a radical change in near field Coulomb interactions between cut-wires in each pair [Kante, 2009, Christ, 2008. Let us consider for instance a periodic array of paired SRRs, which are fundamental building blocks in the design of metamaterials.…”