Symmetry Breaking in a Plasmonic Metamaterial at Optical Wavelength Andre ´ Christ,* ,† Olivier J. F. Martin, † Yasin Ekinci, ‡ Nikolai A. Gippius, §,⊥ and Sergei G. Tikhodeev ⊥ Nanophotonics and Metrology Laboratory, E ´ cole Polytechnique Fe ´de ´ral de Lausanne, Station 11, 1015 Lausanne, Switzerland, Laboratory of Metal Physics and Technology, Swiss Federal Institute of Technology Zu ¨rich, CH-8093 Zu ¨rich, Switzerland, LASMEA, UMR 6602 CNRS, UniVersite ´ Blaise Pascal, 24, aV des Landais, 63177 Aubie `re, France, and A. M. ProkhoroV General Physics Institute RAS, VaViloVa 38, Moscow 119991, Russia Received February 25, 2008; Revised Manuscript Received May 21, 2008 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. 3–6 The 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 sym- metry breaking. The influence of weakly asymmetric struc- tural elements has been discussed in the case of modulated metal films, 7 photonic crystal slabs, 8 and planar metamate- rials; 9 however, similar investigations for plasmonic struc- tures 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 re- ported, 11 the present work focuses on the excitation of so- called 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 * Corresponding author. E-mail: andre.christ@epfl. † E ´ cole Polytechique Fe ´de ´ral de Lausanne. ‡ Swiss Federal Institute of Technology Zu ¨rich. § Universite ´ Blaise Pascal. ⊥ A. M. Prokhorov General Physics Institute RAS. Figure 1. (a) Sketch of the metal nanowire based metamaterial. (b) Reflection (solid line) and absorption spectra (shaded area) for a plasmonic lattice with L sp ) 30 nm and d s ) 0 nm. The reflection spectrum of a single lattice plane (dotted line) is shown as reference. (c) Illustration of the particle plasmon hybridization scheme. NANO LETTERS 2008 Vol. 8, No. 8 2171-2175 10.1021/nl0805559 CCC: $40.75  2008 American Chemical Society Published on Web 06/26/2008