Harald Ditlbacher

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.

Spectroscopy of single metallic nanoparticles using total internal reflection microscopy

We have developed a simple, fast, and flexible technique to measure optical scattering spectra of individual metallic nanoparticles. The particles are placed in an evanescent field produced by total internal reflection of light from a halogen lamp in a glass prism. The light scattered by individual particles is collected using a conventional microscope and is spectrally analyzed by a nitrogen-cooled charge-coupled-device array coupled to a spectrometer. This technique is employed to measure the effect of particle diameter on the dephasing time of the particle plasmon resonance in gold nanoparticles. We also demonstrate the use of this technique for measurements in liquids, which is important for the potential application of particle plasmons in chemical or biological nanosensors.

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.

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.

Dielectric optical elements for surface plasmons

Basic optical elements for surface plasmons are fabricated and their functionality (focusing, refraction, and total internal reflection) is demonstrated experimentally. The optical elements consist of dielectric structures of defined geometry on top of a gold film. The working principle of these structures is discussed on the basis of calculated surface plasmon dispersion relations.

Dark plasmonic breathing modes in silver nanodisks.

We map the complete plasmonic spectrum of silver nanodisks by electron energy loss spectroscopy and show that the mode which couples strongest to the electron beam has radial symmetry with no net dipole moment. Therefore, this mode does not couple to light and has escaped from observation in optical experiments. This radial breathing mode has the character of an extended two-dimensional surface plasmon with a wavenumber determined by the circular disk confinement. Its strong near fields can impact the hybridization in coupled plasmonic nanoparticles as well as couplings with nearby quantum emitters.

Quantitative analysis of surface plasmon interaction with silver nanoparticles

The present insight into plasmon effects on the nanoscale seems sufficiently advanced to allow the development of surface-plasmon-polariton- (SPP-) based optical devices. Therefore quantitative information describing SPP phenomena is required. We investigate a SPP beam splitter constituted by silver nanoparticles on a silver thin film, fabricated by electron-beam lithography. We acquire quantitative information on the beam splitter performance by monitoring SPP leakage radiation, yielding SPP reflection, transmission, and scattering efficiencies.