Jens Dorfmüller

3D optical Yagi-Uda nanoantenna array.

We fabricated three-dimensional arrays of optical Yagi-Uda nano-antennas. Due to the high directivity of the array structure the incoming light is received efficiently at the resonant wavelength in the near-infrared (around λ = 1.3 µm).

Fabry-Perot Resonances in One-Dimensional Plasmonic Nanostructures

We study the near-field optical behavior of Fabry-Pérot resonances in thin metal nanowires, also referred to as quasi one-dimensional plasmonic nanoantennas. From eigenmodes well beyond quadrupolar order we extract both, propagation constant and reflection phase of the guided surface plasmon polariton with superb accuracy. The combined symmetry breaking effects of oblique illumination and retardation allow the excitation of dipole forbidden, even order resonances. All measurements are supported by rigorous simulations of the experimental situation.

Plasmonic nanowire antennas: Experiment, simulation, and theory

Recent advances in nanolithography have allowed shifting of the resonance frequency of antennas into the optical and visible wavelength range with potential applications, for example, in single molecule spectroscopy by fluorescence and directionality enhancement of molecules. Despite such great promise, the analytical means to describe the properties of optical antennas is still lacking. As the phase velocity of currents at optical frequencies in metals is much below the speed of light, standard radio frequency (RF) antenna theory does not apply directly. For the fundamental linear wire antenna, we present an analytical description that overcomes this shortage and reveals profound differences between RF and plasmonic antennas. It is fully supported by apertureless scanning near-field optical microscope measurements and finite-difference time-domain simulations. This theory is a starting point for the development of analytical models of more complex antenna structures.

Direct Near-Field Optical Imaging of Higher Order Plasmonic Resonances

We map in real space and by purely optical means near-field optical information of localized surface plasmon polariton (LSPP) resonances excited in nanoscopic particles. We demonstrate that careful polarization control enables apertureless scanning near-field optical microscopy (aSNOM) to image dipolar and quadrupolar LSPPs of the bare sample with high fidelity in both amplitude and phase. This establishes a routine method for in situ optical microscopy of plasmonic and other resonant structures under ambient conditions.

Near-Field Dynamics of Optical Yagi-Uda Nanoantennas

We present near-field measurements of optical Yagi-Uda nanoantennas that are used in receiving mode. The eigenmode imaging of amplitude and phase by apertureless scanning near-field optical microscopy allows us to investigate the dynamics of the local out-of-plane electric field components and to visualize the temporal evolution of this time-harmonic reception process. The antenna directionality manifests itself by the dependence of the local field enhancement at the feed element on the illumination direction. Simulations taking into account the substrate confirm our observation of the directionality. Our work demonstrates the possibility to characterize multielement nanoantennas by electromagnetic antenna near-field scanners.

Amplitude- and phase-resolved optical near fields of split-ring-resonator-based metamaterials

We investigate the local optical response of split-ring resonator-(SRR)-based metamaterials with an apertureless scanning near-field optical microscope. By mapping the near fields of suitably resonant micrometer-sized SRRs in the near-infrared spectral region with an uncoated silicon tip, we obtain a spatial resolution of better than lambda/50. The experimental results confirm numerical predictions of the near-field excitations of SRRs. Combining experimental near-field optical studies with near- and far-field optical simulations provides a detailed understanding of resonance mechanisms in subwavelength structures and will facilitate an efficient approach to improved designs.

Spectral shifts in optical nanoantenna-enhanced hydrogen sensors

In this work, we numerically investigate the nature of spectral shifts in antenna-enhanced hydrogen sensing geometries consisting of a gold bowtie antenna next to a palladium nanodisk. We find through extensive finite element (FEM) simulations that the hydrogen-induced spectral behavior of the system is governed by two competing effects: a small blueshift caused by dielectric function changes in the palladium and a much stronger redshift due to an expansion of the palladium lattice. Our findings enable the accurate numerical characterization and especially the optimization of sensitive antenna-enhanced hydrogen sensors. As a first application, we calculate the performance improvement of gap antennas compared to single cut-wire antenna elements.

Long-distance indirect excitation of nanoplasmonic resonances.

In nanoscopic systems, size, geometry, and arrangement are the crucial determinants of the light-matter interaction and resulting nanoparticles excitation. At optical frequencies, one of the most prominent examples is the excitation of localized surface plasmon polaritons, where the electromagnetic radiation is coupled to the confined charge density oscillations. Here, we show that beyond direct near- and far-field excitation, a long-range, indirect mode of particle excitation is available in nanoplasmonic systems. In particular, in amorphous arrays of plasmonic nanodiscs we find strong collective and coherent influence on each particle from its entire active neighborhood. This dependency of the local field response on excitation conditions at distant areas brings exciting possibilities to engineer enhanced electromagnetic fields through controlled, spatially configured illumination.

Surface plasmon coupling to nanoscale Schottky-type electrical detectors

We have investigated the near-field coupling of surface plasmons to a titanium/CdS nanowire interface for two different device configurations. A bare aluminum grating on an underlying aluminum layer exhibited the expected stronger electrical signal for perpendicular versus parallel light polarization. An opposite intensity ratio was detected when the grating and the Schottky contact are connected via an aluminum–silica–aluminum sandwich structure. Based upon finite difference time domain device simulations, the enhanced coupling for parallel polarization is attributed to the emergence of a transversal electric wave within the metal–insulator–metal structure.

Plasmonic Activity of Large-Area Gold Nanodot Arrays on Arbitrary Substrates

Highly efficient fabrication of well-ordered, embedded gold nanodot matrices using diffraction mask projection laser ablation is demonstrated. These gold nanodot arrays are ideally generated onto sapphire substrates but do also form onto AlO(x) thin films, enabling the application to arbitrary bulk substrates. Well-ordered gold dots become embedded into the Al(2)O(3) substrate during the process, thus improving their mechanical stability, chemical inertness, and technological compliance. Such substrates may be useful, for example, to enhance solar-cell efficiency by surface plasmons or as convenient, biocompatible focusing elements in nearfield optical tweezers.