A. Christ

Symmetry breaking in a plasmonic metamaterial at optical wavelength

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.

Electric and magnetic resonances in arrays of coupled gold nanoparticle in-tandem pairs

We present an experimental and theoretical study on the optical properties of arrays of gold nanoparticle in-tandem pairs (nanosandwiches). The well-ordered Au pairs with diameters down to 35 nm and separation distances down to 10 nm were fabricated using extreme ultraviolet (EUV) interference lithography. The strong near-field coupling of the nanoparticles leads to electric and magnetic resonances, which can be well reproduced by Finite-Difference Time-Domain (FDTD) calculations. The influence of the structural parameters, such as nanoparticle diameter and separation distance, on the hybridized modes is investigated. The energy and lifetimes of these modes are studied, providing valuable physical insight for the design of novel plasmonic structures and metamaterials.

From Near-Field to Far-Field Coupling in the Third Dimension: Retarded Interaction of Particle Plasmons

We study the transition from the near-field to the far-field coupling regime of particle plasmons in a three-dimensional geometry. In the far-field regime, retardation plays the dominant role and the plasmonic resonances are radiatively coupled. When the spatial arrangement of the oscillators is matched to their resonance wavelength, superradiant-like effects are observed.

Octave-wide photonic band gap in three-dimensional plasmonic Bragg structures and limitations of radiative coupling

We demonstrate that a three-dimensional arrangement of particle plasmonic oscillators at Bragg distance leads to a superradiant plasmon mode. We observe the formation of a very broad photonic band gap that spans almost one octave.

Waveguide plasmon polaritons in metal-dielectric photonic crystal slabs

The optical properties of arrays of metallic (gold) nanowires deposited on dielectric substrates are studied both theoretically and experimentally. Depending on the substrate, Wood’s anomalies of two types are observed in the transmission spectra of such planar metal-dielectric photonic crystals. One of them is diffraction (Rayleigh) anomalies associated with the opening of diffraction channels to the substrate or air with an increase in the frequency of the incident light. The other type of Wood’s anomaly is resonance anomalies associated with excitation of surface quasi-guided modes in the substrate. Coupling of the quasi-guided modes with individual nanowire plasmons brings about the formation of waveguide plasmon polaritons. This effect is accompanied by a strong rearrangement of the optical spectrum and can be utilized to control the photonic bands of metal-dielectric photonic crystal slabs.

Linear and nonlinear optical properties of strongly coupled metal nanoparticles

Interaction of localized plasmons and propagating waveguide modes leads to strong coupling and polariton formation. These polaritons are the basis of nanoplasmonic effects in linear and nonlinear optics. Experiments, theory, and applications are presented.

Extremely slow coherent polarization decay of waveguide- plasmon-polaritons in metallic photonic crystal slabs

Autocorrelation measurements using sub-20 fs laser pulses show that dephasing times of quasiparticles consisting of waveguide modes and particle plasmons can be increased by more than a factor of two when tuning the photonic crystal slab into the strong coupling regime.

Phase-resolved pulse propagation through metallic photonic crystal slabs: plasmonic slow light

We characterized the electromagnetic field of ultra-short laser pulses after propagation through metallic photonic crystal structures featuring photonic and plasmonic resonances. The complete pulse information, i.e. the envelope and phase of the electromagnetic field, was measured using the technique of cross-correlation frequency resolved optical gating. In good agreement, measurements and scattering matrix simulations show a dispersive behaviour of the spectral phase at the position of the resonances. Asymmetric Fano-type resonances go along with asymmetric phase characteristics. Furthermore, the spectral phase is used to calculate the dispersion of the sample and possible applications in dispersion compensation are investigated. Group refractive indices of 700 and 70 and group delay dispersion values of 90 000 fs2 and 5000 fs2 are achieved in transverse electric and transverse magnetic polarization, respectively. The behaviour of extinction and spectral phase can be understood from an intuitive model using the complex transmission amplitude. An associated depiction in the complex plane is a useful approach in this context. This method promises to be valuable also in photonic crystal and filter design, for example, with regards to the symmetrization of the resonances. This article is part of the themed issue ‘New horizons for nanophotonics’.

From near-field to far-field: Radiative coupling of particle plasmon resonances in three-dimensional geometries

Coupling between individual particle plasmon resonances (PPRs) has been investigated quite thoroughly during the past years. Most of the work was dedicated to near-field coupling with a close spacing of the resonant particles. In this limit, retardation is negligible, but also the interesting properties of the radiating PPRs are neglected mostly. In contrast, when placing particles at distances of the order of their emission wavelength, coupling can be mediated by the radiation fields of the PPRs.

A waveguided parametric downconversion source for pure heralded single photons at telecommunication wavelength

Heralded pure single photons are an indispensable resource for linear optical quantum computing (LOQC), as the fidelity of single photon quantum gates depends critically on the state purity of the input ancilla photons. The heralded photons purity requires the absence of any kind of entanglement between itself and the heralding photon, i. e. their joint amplitude function factorizes: f (k⃗<inf>s</inf>,k⃗<inf>i</inf>) = f<inf>s</inf>(k⃗<inf>s</inf>) f<inf>i</inf>(k⃗<inf>i</inf>). Several schemes for spontaneous parametric downconversion (SPDC) based heralded single photon sources have been proposed and demonstrated. In order to provide spectrally separable pairs, they typically rely heavily on narrow spatial [2] and/or spectral filtering [3], thus trading effective source brightness for purity.