Strong coupling between localized particle plasmons and optical waveguide modes leads to drastic modifications of the transmission of metallic nanowire arrays on dielectric waveguide substrates. The coupling results in the formation of a new quasiparticle, a waveguide-plasmon polariton, with a surprisingly large Rabi splitting of 250 meV. Our experimental results agree well with scattering-matrix calculations and a polariton-type model. The effect provides an efficient tool for photonic band gap engineering in metallodielectric photonic crystal slabs. We show evidence of a full one-dimensional photonic band gap in resonant plasmon-waveguide structures.

We formulate a scattering-matrix-based numerical method to calculate the optical transmission properties and quasiguided eigenmodes in a two-dimensionally periodic photonic crystal slab ͑PCS͒ of finite thickness. The square symmetry ͑point group C 4v) is taken for the illustration of the method, but it is quite general and works for any point group symmetry for one-dimensional ͑1D͒ and 2D PCS's. We show that the appearance of well-pronounced dips in the transmission spectra of a PCS is due to the interaction with resonant waveguide eigenmodes in the slab. The energy position and width of the dips in transmission provide information on the frequency and inverse radiative lifetime of the quasiguided eigenmodes. We calculate the energies, linewidths, and electromagnetic fields of such quasiguided eigenmodes, and analyze their symmetry and optical activity. The electromagnetic field in such modes is resonantly enhanced, which opens possibilities for use in creating resonant enhancement of different nonlinear effects.

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New effects of polarization multistability and polarization hysteresis in a coherently driven polariton condensate in a semiconductor microcavity are predicted and theoretically analyzed. The multistability arises due to polarization-dependent polariton-polariton interactions and can be revealed in polarization resolved photoluminescence experiments. The pumping power required to observe this effect is of 4 orders of magnitude lower than the characteristic pumping power in conventional bistable optical systems.

We analyze the influence of near-field coupling on the formation of collective plasmon modes in a multilayer metallic nanowire array. It is shown that the spectral interference between super-and subradiant normal modes results in characteristic line shape modifications which are directly controlled by the spacing as well as the alignment of the stacked lattice planes. Moreover, a distinct near-field-induced reversal of particle plasmon hybridization is reported. Our numerical findings are in excellent agreement with experimental results.

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

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