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Titanium dioxide (TiO2) displays photocatalytic behavior under near-ultraviolet (UV) illumination. In another scientific field, it is well understood that the excitation of localized plasmon polaritons on the surface of silver (Ag) nanoparticles (NPs) causes a tremendous increase of the near-field amplitude at well-defined wavelengths in the near UV. The exact resonance wavelength depends on the shape and the dielectric environment of the NPs. We expected that the photocatalytic behavior of TiO2 would be greatly boosted if it gets assisted by the enhanced near-field amplitudes of localized surface plasmon (LSP). Here we show that this is true indeed. We named this new phenomenon "plasmonic photocatalysis". The key to enable plasmonic photocatalysis is to deposit TiO2 on a NP comprising an Ag core covered with a silica (SiO2) shell to prevent oxidation of Ag by direct contact with TiO2. The most appropriate diameter for Ag NPs and thickness for the SiO2 shell giving rise to LSP in the near UV were estimated from Mie scattering theory. Upon implementing a device that took these design considerations into account, the measured photocatalytic activity under near UV illumination of such a plasmonic photocatalyst, monitored by decomposition of methylene blue, was enhanced by a factor of 7. The enhancement of the photocatalytic activity increases with a decreased thickness of the SiO2 shell. The plasmonic photocatalysis will be of use as a high performance photocatalyst in nearly all current applications but will be of particular importance for applications in locations of minimal light exposure.

We theoretically analyze the properties of metamaterials which have been designed by taking advantage of Babinet's principle. It is shown that the complementary structure exhibits both a complementary spectral response and field distribution of the respective eigenmodes. For complementary split-ring resonators, we show that the spectral resonance features can be explained from two different perspectives. On one hand they can be explained as plasmon polariton resonances in dielectric nanostructures surrounded by metal, on the other hand they can be understood as guided mode resonances with vanishing propagation constant. The physical origin of these modes and differences to the conventional split-ring geometry are discussed.

Properties of evanescent-field-coupled waveguide-mode sensors consisting of a multi-layer structure made of a SiO(2) waveguide, a thin metal layer (Au, Cu, W or Ti), and a high refractive index glass substrate illuminated under the Kretschmann configuration have been theoretically and experimentally investigated. In all cases, reflectivity changes attributed to streptavidin combining to biotinyl groups were observed in close spectral vicinity of the waveguide resonances. The sensors with the Au and the Cu layers show superior sensitivity as compared to those with the W and the Ti layers, whereas the W and Ti layers show better thermal and chemical stability. The results indicate that the materials of thin metal layers should be chosen in accordance with the purpose of sensors and/or environment in which the sensors are used.

The properties of an ensemble of densely packed metallic nanoparticles and the effective material properties of a metamaterial based on them are studied in detail using rigorous diffraction theory. We show that in the spectral vicinity of the plasmon resonance, the metamaterial shows a well-defined Lorentzian resonance in its effective permittivity whereas the permeability is unaltered. The metamaterial shows a stop band in the spectral domain where the real part of the effective permittivity is negative. Modifying the effective properties of the metamaterial allows efficient tuning of the band-gap properties. The implementation of the proposed approach towards a band-gap metamaterial employing self-organization in liquid crystals is discussed.

We show that properties attributed to planar metamaterials that support resonances due to an appropriately shaped unit cell can also be identified in a medium that exhibits a resonance evoked by its period only. By choosing a subwavelength period, the effective material parameters of such a medium can be retrieved from the complex reflected and transmitted amplitude. The parameters exhibit Lorentzian line shapes in the spectral vicinity of the resonances associated with the period. If this material is stacked to form a three-dimensional medium, a stop gap is observed in transmission in the frequency range where the real part of one effective material parameter becomes negative. The resonance at the origin of the response is related to the excitation of a higher-order Bloch mode. Because their negligible excitation is a prerequisite for deriving effective material parameters, the analyzed structure mimics only the response of a metamaterial but cannot be regarded as a metamaterial.

We explain the origin of the electric and particular the magnetic polarizabiltiy of metamaterials employing a fully electromagnetic plasmonic picture. As example we study an U-shaped split-ring resonator based metamaterial at optical frequencies. The relevance of the split-ring resonator orientation relative to the illuminating field for obtaining a strong magnetic response is outlined. We reveal higher-order magnetic resonances and explain their origin on the basis of higher-order plasmonic eigenmodes caused by an appropriate current flow in the split-ring resonator. Finally, the conditions required for obtaining a negative index at optical frequencies in a metamaterial consisting of split-ring resonators and wires are investigated.

We propose an artificial three-dimensional material that exhibits a strong resonance in the effective permeability in the visible spectral domain. This material may be implemented in a two-step procedure. First, a metamaterial made of densely packed metallic nanoparticles is fabricated that shows a Lorentz-type resonance in the permittivity at the collective plasmon frequency. Second, spheres are formed out of this material and arranged in a cubic lattice. This meta-metamaterial exhibits a strong resonance in the permeability which is caused by a Mie resonance associated with the magnetic mode of a single metamaterial sphere. Realization of this material based on self-organization in liquid crystals and the limitations of the approach are discussed.

Sensors based on surface plasmons or waveguide modes are at the focus of interest for applications in biological or environmental chemistry. Waveguide-mode spectra of 1 mum-thick pure and perforated silica films comprising isolated nanometric holes with great aspect ratio were measured before and after adhesion of streptavidin at concentrations of 500 nM. The shift of the angular position for guided modes was nine times higher in perforated films than in bulk films. Capturing of streptavidin in the nanoholes is at the origin of that largely enhanced shift in the angular position as the amplitude of the guided mode in the waveguide perfectly overlaps with the perturbation caused by the molecules. Hence, the device allows for strongly confined modes and their strong perturbation to enable ultra-sensitive sensor applications.

We numerically study the spectral response of 'U'-shaped split-ring-resonators at normal incidence with respect to the resonator plane. Based on the evaluation of the near-field patterns of the resonances and their geometry-dependent spectral positions, we obtain a comprehensive and consistent picture of their origin. We conclude that all resonances can be understood as plasmonic resonances of increasing order of the entire structure. In particular, for an electrical field polarized parallel to the gap the so-called LC-resonance corresponds to the fundamental plasmonic mode and, contrary to earlier interpretations, the electrical resonance is a second order plasmon mode of the entire structure. The presence of further higher order modes is discussed.

The origin of frequency gaps in the dispersion relation of periodic, quasi-periodic, and random photonic structures consisting of different arrangements of dielectric cylinders has been investigated. For TM polarization it was found that the formation and properties of gaps are strongly affected by Mie resonances of a single cylinder. Both the spectral position and size depend on the properties of this single scatterer. In contrast, for TE polarization no correlation between the scattering properties and bandgap formation was found, as Mie resonances are spectrally not well separated. For the inverted structure consisting of air cylinders in a dielectric material, the frequency gaps depend on the spatial arrangement of the cylinders because no pronounced Mie resonances exist in this case.

We present our investigation of the torque exerted on dielectric elliptical cylinders by highly focused laser beams. The calculations are performed with rigorous diffraction theory, and the size-dependent torque is analyzed as a function of the axis ratio. It is found that highly elongated particles will experience a reversal of the torque for a radius that is approximately one third of the wavelength. This effect is attributed to interference effects inside the structure due to multiple reflections of the incoming wave. The evolution from a perfectly sinusoidal angular dependence of the torque to a more complicated pattern for increasing particle size is presented in detail.

We use rigorous diffraction theory to analyze the force on elliptical cylinders made from silver as a function of the elongation. We find that, when the wires are illuminated at the plasmon wavelength and placed in a highly focused Gaussian beam, they are attracted toward the optical axis if the waist of the laser is behind the wire and repelled if the waist is before the wire. Also, the force distributions of illumination wavelengths smaller and larger than the plasmon wavelength are analyzed.

We apply the boundary element method to the analysis of the plasmon response of systems that consist of coupled metallic nanoscatterers. For systems made of two or more objects, the response depends strongly on the individual particle behavior as well as on the separation distance and on the configuration of the particles relative to the illumination direction. By analyzing the behavior of these systems, we determine the smallest interaction distance at which the particles can be considered decoupled. We discriminate the two cases of particle systems consisting of scatterers with the same and different resonance wavelengths.

The boundary-element method is applied to the interaction of light with resonant metallic nanoparticles. At a certain wavelength, excitation of a surface plasmon takes place, which leads to a resonantly enhanced near-field amplitude and a large scattering cross section. The resonance wavelength for different scatterer geometries is determined. Alteration of the scattering properties in the presence of other metallic nanoparticles is discussed. To treat this problem, a novel formulation of the boundary-element method is presented that solves the interaction problem for all the coupled particles.