A. Leitner

Optical properties of two interacting gold nanoparticles

We study surface plasmon excitation in pairs of identical Au nanoparticles by optical transmission spectroscopy. The samples produced by electron beam lithography consist of 2D particle arrangements with varying interparticle distance. With decreasing interparticle distance the surface plasmon resonance shifts to longer wavelengths for a polarization direction parallel to the long particle pair axis whereas a blueshift is found for the orthogonal polarization. These experimental findings can be explained by a dipolar interaction mechanism.

Electromagnetic energy transport via linear chains of silver nanoparticles

We propose the idea of a subwavelength-sized light guide represented by a linear chain of spherical metal nanoparticles in which light is transmitted by electrodynamic interparticle coupling. The light-transport properties of this system are investigated by use of model calculations based on generalized Mie theory. Considering Ag particles of 50-nm diameter, we find optimum guiding conditions for an interparticle spacing of 25 nm, and a corresponding 1/e signal-damping length of 900 nm is evaluated. The proposed principle of optical energy transport may be useful for subwavelength transmission lines within integrated optics circuits and for near-field optical microscopy.

Two-dimensional optics with surface plasmon polaritons

We report the experimental realization of highly efficient optical elements built up from metal nanostructures to manipulate surface plasmon polaritons propagating along a silver/polymer interface. Mirrors, beamsplitters, and interferometers produced by electron-beam lithography are investigated. The plasmon fields are imaged by detecting the fluorescence of molecules dispersed in the polymer.

Optimized surface-enhanced Raman scattering on gold nanoparticle arrays

In this letter, we show that tuning the maximum of the surface plasmon resonance of elongated gold nanoparticles to a wavelength, the position of which is precisely midway between the exciting laser line and the Raman line, results in an optimization of the surface-enhanced Raman-scattering effect.

Surface plasmon propagation in microscale metal stripes

Addressing the fundamental question of miniaturization of light guiding and routing towards nanoscale optics, we study experimentally surface plasmon propagation in silver and gold thin films of finite widths in the micrometer range. Spatially confined excitation of surface plasmons is realized by a prism coupling arrangement involving an opaque aluminum screen for a distinct separation of excitation and propagation (measurement) region. The surface plasmon propagation length as a function of film widths is measured by detecting stray light due to surface plasmon scattering with a conventional optical microscope.

Enhanced dye fluorescence over silver island films: analysis of the distance dependence

Fluorescence intensity enhancement has been studied for several coverages (10-3 to 1 monolayer) of Rhodamine 6G separated from a silver island film by quartz spacer layers of varying thickness. When the dye film is brought into the proximity of the island film support, a first enhancement maximum is observed at ≈ 60 nm spacer thickness. For smaller distances, decrease in the enhancement is followed by a second maximum at 5–10 nm interlayer thickness. The magnitude of the latter maximum exhibits a pronounced dependence on dye coverage. The far-field enhancement maximum is due to an increase in fluorescence quantum yield induced by the presence of the island film substrate, combined with a resonant excitation phenomenon due to collective action of the islands. This effect is analysed in terms of stratified medium theory, in which the island film is represented by a suitable model for the effective dielectric constant. The short distance enhancement results from competition between the strongly amplified loc...

Dielectric stripes on gold as surface plasmon waveguides

We report on surface plasmon polariton (SPP) waveguiding by SiO2 stripes on gold thin films. Compared to other SPP waveguide schemes, these systems provide relatively large effective refractive indices which can be described by the effective index method. By leakage radiation and near-field optical microscopy, we observe directly multimode and monomode behavior in straight SPP waveguides of different widths. Furthermore, we demonstrate waveguide bends and cross-talk free propagation across waveguide crossings.

Spectrally coded optical data storage by metal nanoparticles.

In metal nanoparticles the resonance wavelength of light-driven collective electron oscillations is determined by the particle shape. This shape dependence can be used for optical data storage by spectral coding. In this way the storage density can be increased by at least a factor of 5 compared with that for conventional optical storage principles.

Optical properties of Ag and Au nanowire gratings

The optical response of regularly arranged noble metal wires with nanoscopic cross sections (nanowire gratings) strongly depends on the polarization direction of the incident light. We use silver and gold nanowire gratings produced by electron beam lithography to study this effect by optical extinction spectroscopy. For a polarization direction perpendicular to the wire axis, the excitation of a dipolar plasmon mode dominates the extinction spectrum. The spectral position of the plasmon resonance can be tuned by an appropriate choice of nanowire geometry and material. For a polarization direction parallel to the wire axis, the profile of the extinction spectrum varies mainly as a function of the grating constant. In particular, a transmission maximum for small grating constants is found. By combining the surface plasmon excitation and grating effect for orthogonal polarization directions, a spectrally selective polarizer with an extinction ratio of 26 is demonstrated.

Fluorescence imaging of surface plasmon fields

We demonstrate that surface plasmon fields can be imaged in real time by detecting the fluorescence of a molecular film close to the plasmon carrying metal surface. We use this method to image the field profile of surface plasmons launched at lithographically designed nanoscopic defects.