Ralf Vogelgesang

Transition from isolated to collective modes in plasmonic oligomers.

We demonstrate the transition from isolated to collective optical modes in plasmonic oligomers. Specifically, we investigate the resonant behavior of planar plasmonic hexamers and heptamers with gradually decreasing the interparticle gap separation. A pronounced Fano resonance is observed in the plasmonic heptamer for separations smaller than 60 nm. The spectral characteristics change drastically upon removal of the central nanoparticle. Our work paves the road toward complex hierarchical plasmonic oligomers with tailored optical properties.

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.

Toward Plasmonics with Nanometer Precision: Nonlinear Optics of Helium-Ion Milled Gold Nanoantennas

Plasmonic nanoantennas are versatile tools for coherently controlling and directing light on the nanoscale. For these antennas, current fabrication techniques such as electron beam lithography (EBL) or focused ion beam (FIB) milling with Ga(+)-ions routinely achieve feature sizes in the 10 nm range. However, they suffer increasingly from inherent limitations when a precision of single nanometers down to atomic length scales is required, where exciting quantum mechanical effects are expected to affect the nanoantenna optics. Here, we demonstrate that a combined approach of Ga(+)-FIB and milling-based He(+)-ion lithography (HIL) for the fabrication of nanoantennas offers to readily overcome some of these limitations. Gold bowtie antennas with 6 nm gap size were fabricated with single-nanometer accuracy and high reproducibility. Using third harmonic (TH) spectroscopy, we find a substantial enhancement of the nonlinear emission intensity of single HIL-antennas compared to those produced by state-of-the-art gallium-based milling. Moreover, HIL-antennas show a vastly improved polarization contrast. This superior nonlinear performance of HIL-derived plasmonic structures is an excellent testimonial to the application of He(+)-ion beam milling for ultrahigh precision nanofabrication, which in turn can be viewed as a stepping stone to mastering quantum optical investigations in the near-field.

Toroidal Plasmonic Eigenmodes in Oligomer Nanocavities for the Visible

Plasmonics has become one of the most vibrant areas in research with technological innovations impacting fields from telecommunications to medicine. Many fascinating applications of plasmonic nanostructures employ electric dipole and higher-order multipole resonances. Also magnetic multipole resonances are recognized for their unique properties. Besides these multipolar modes that easily radiate into free space, other types of electromagnetic resonances exist, so-called toroidal eigenmodes, which have been largely overlooked historically. They are strongly bound to material structures and their peculiar spatial structure renders them practically invisible to conventional optical microscopy techniques. In this Letter, we demonstrate toroidal modes in a metal ring formed by an oligomer of holes. Combined energy-filtering transmission electron microscopy and three-dimensional finite difference time domain analysis reveal their distinct features. For the study of these modes that cannot be excited by optical far-field spectroscopy, energy-filtering transmission electron microscopy emerges as the method of choice. Toroidal moments bear great potential for novel applications, for example, in the engineering of Purcell factors of quantum-optical emitters inside toroidal cavities.

Electrospray Ion Beam Deposition: Soft-Landing and Fragmentation of Functional Molecules at Solid Surfaces

The ion beam deposition (IBD) of rhodamine dye molecules on solid surfaces in high vacuum is explored in order to characterize the possibility of fabricating molecular coatings or nanostructures from nonvolatile molecules. Molecular ion beams with a well-defined composition are deposited on silicon oxide surfaces with a controlled kinetic energy. Photoluminescence spectroscopy and time-of-flight secondary ion mass spectrometry (TOF-SIMS) are employed in order to characterize the sample with respect to coverage, homogeneity, and the fraction of intact landed ions (soft-landing ratio). We find that homogeneous rhodamine films of defined composition can be produced at energies of 2-100 eV. The coverage is found to be proportional to the ion dose. Soft-landing is observed for energies up to 35 eV.

Apertureless scanning near field optical microscope with sub-10nm resolution

We report on the implementation of a versatile dynamic mode apertureless scanning near field optical microscope (aSNOM) for nanoscopic investigations of optical properties at surfaces and interfaces. The newly developed modular aSNOM optomechanical unit is essentially integrable with a multitude of laser sources, homemade scanning probe microscopes (SPMs) as well as commercially available SPMs as demonstrated here. The instrument is especially designed to image opaque surfaces without a restriction to transparent substrates. In the description of the instrument we draw frequent attention to various possible artifact mechanisms, how to overcome them, and we present effective checks to ensure true near field optical contrast. Lateral optical contrast in optical amplitude and phase images below 10nm is demonstrated.