Bernhard Lamprecht

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.

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.

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.

Design of multipolar plasmon excitations in silver nanoparticles

We report on the experimental observation of multipolar plasmon excitations in lithographically designed elongated silver particles. In contrast to spheres, where the extinction bands of the respective multipolar plasmons overlap considerably to form a broad spectrum, spectrally well-separated extinction bands corresponding to plasmons of multipolar order up to n=6 are found. The results agree well with numerical simulations based on the Green’s Dyadic method.

Organic diodes as monolithically integrated surface plasmon polariton detectors

The authors show that organic p∕n heterojunction diodes enable the direct electric detection of surface plasmon polaritons (SPPs). The organic diodes are built from two organic semiconductor thin film layers with an area of 150×500μm2 deposited on an extended silver thin film. Besides serving as the bottom electrode of the diode this silver thin film is as well a waveguide feeding SPPs to the diode area. The authors visualize the direct SPP detection by a spatially resolved induced current map.

Integrated organic electronic based optochemical sensors using polarization filters

A compact, integrated photoluminescence based oxygen and pH sensor, utilizing an organic light emitting device (OLED) as the light source and an organic photodiode (OPD) as the detection unit, is described. The main challenge in such an integrated sensor is the suppression of the excitation light at the detector, which is typically by many orders of magnitude higher in intensity than the emitted fluorescence. In our approach, we refrain from utilizing edge filters which require narrow band excitation sources and dyes with an adequate large Stokes shift. We rather developed an integrated sensor concept relying on two polarizers to separate the emission and excitation light. One polarizer is located right after the OLED, while the other one, oriented at 90° to the first, is placed in front of the OPD. The main advantage of this solution is that any combination of excitation and emission light is acceptable, even if the two signals overlap spectrally. This is especially important for the use of OLEDs as the ...

A planar waveguide optical sensor employing simple light coupling

The novel optical sensor concept utilizes the sensing layer as light propagating layer and employs a new method to couple light into a planar waveguide.