Andreas M. Kern

Excitation and Reemission of Molecules near Realistic Plasmonic Nanostructures

The enhancement of excitation and reemission of molecules in close proximity to plasmonic nanostructures is studied with special focus on the comparison between idealized and realistically shaped nanostructures. Numerical experiments show that for certain applications choosing a realistic geometry closely resembling the actual nanostructure is imperative, an idealized simulation geometry yielding significantly different results. Finally, a link between excitation and reemission processes is formed via the theory of optical reciprocity, allowing a transparent view of the electromagnetic processes involved in plasmon-enhanced fluorescence and Raman-scattering.

Molecule-Dependent Plasmonic Enhancement of Fluorescence and Raman Scattering near Realistic Nanostructures

The enhancement of fluorescence and Raman scattering by plasmonic nanostructures is studied theoretically with special focus on the effects of the observed molecules properties and the realistic geometry of the plasmonic nanostructure. Numerical experiments show that the enhancement factor may vary by many orders of magnitude depending on a fluorophores transition rates or intrinsic quantum yield. For different molecules, boosting fluorescence enhancement means optimizing different factors, leading to a different ideal geometric and spectral configuration. This framework, coupled with powerful new simulation tools, will facilitate the design and characterization of fluorescence-enhancing plasmonic nanostructures as well as yield experimental access to the intrinsic properties of the molecules under study.

Pitfalls in the Determination of Optical Cross Sections From Surface Integral Equation Simulations

Calculation of electromagnetic cross sections from surface integral equation simulations, a popular approach in microwave studies and recently also in optics and plasmonics, requires only a single post-processing step, which can, however, be very sensitive to the precision of the simulation result. We investigate the accuracy and robustness of two methods for cross section calculation, displaying when and why errors may occur, in certain cases even unphysical behavior. A calculation recipe which avoids unphysical results is given, ensuring convergence of all obtained cross sections. This study will help judge the accuracy of performed simulations and can prevent misinterpretation of modeling results.

Projective Noise Cleaning With Dynamic Neighborhood Selection

In recent years, several methods of noise cleaning have been devised, of which projective methods have been particularly effective. In our paper, we explain in detail why orthogonal projections are nonoptimal and how the nonorthogonal projections suggested by Grassberger et al., naturally emerge from the SVD method. We show that this approach when combined with a dynamic neighborhood selection yields optimal results of noise cleaning.

Au Nanotip as Luminescent Near-Field Probe

We introduce a new optical near-field mapping method, namely utilizing the plasmon-mediated luminescence from the apex of a sharp gold nanotip. The tip acts as a quasi-point light source which does not suffer from bleaching and gives a spatial resolution of ≤25 nm. We demonstrate our method by imaging the near field of azimuthally and radially polarized plasmonic modes of nonluminescent aluminum oligomers.

Assessing the plasmonics of gold nano-triangles with higher order laser modes

Summary Regular arrays of metallic nano-triangles – so called Fischer patterns – are fabricated by nano-sphere lithography. We studied such gold nano-triangle arrays on silicon or glass substrates. A series of different samples was investigated with a parabolic mirror based confocal microscope where the sample is scanned through the laser focus. By employing higher order laser modes (azimuthally and radially polarised laser beams), we can excite the Fischer patterns using either a pure in-plane (x,y) electric field or a strongly z-directional (optical axis of the optical microscope) electric field. We collected and evaluated the emitted luminescence and thereby investigated the respectively excited plasmonic modes. These varied considerably: firstly with the light polarisation in the focus, secondly with the aspect ratio of the triangles and thirdly with the employed substrate. Moreover, we obtained strongly enhanced Raman spectra of an adenine (sub-)monolayer on gold Fischer patterns on glass. We thus showed that gold Fischer patterns are promising surface-enhanced Raman scattering (SERS) substrates.

Strong and Coherent Coupling of a Plasmonic Nanoparticle to a Subwavelength Fabry-Perot Resonator

A major aim in experimental nano- and quantum optics is observing and controlling the interaction between light and matter on a microscopic scale. Coupling molecules or atoms to optical microresonators is a prominent method to alter their optical properties such as luminescence spectra or lifetimes. Until today strong coupling of optical resonators to such objects has only been observed with atom-like systems in high quality resonators. We demonstrate first experiments revealing strong coupling between individual plasmonic gold nanorods (GNR) and a tunable low quality resonator by observing cavity-length-dependent nonlinear dephasing and spectral shifts indicating spectral anticrossing of the luminescent coupled system. These phenomena and experimental results can be described by a model of two coupled oscillators representing the plasmon resonance of the GNR and the optical fields of the resonator. The presented reproducible and accurately tunable resonator allows us to precisely control the optical properties of individual particles.

Superluminescence from an optically pumped molecular tunneling junction by injection of plasmon induced hot electrons

Summary Here, we demonstrate a bias-driven superluminescent point light-source based on an optically pumped molecular junction (gold substrate/self-assembled molecular monolayer/gold tip) of a scanning tunneling microscope, operating at ambient conditions and providing almost three orders of magnitude higher electron-to-photon conversion efficiency than electroluminescence induced by inelastic tunneling without optical pumping. A positive, steadily increasing bias voltage induces a step-like rise of the Stokes shifted optical signal emitted from the junction. This emission is strongly attenuated by reversing the applied bias voltage. At high bias voltage, the emission intensity depends non-linearly on the optical pump power. The enhanced emission can be modelled by rate equations taking into account hole injection from the tip (anode) into the highest occupied orbital of the closest substrate-bound molecule (lower level) and radiative recombination with an electron from above the Fermi level (upper level), hence feeding photons back by stimulated emission resonant with the gap mode. The system reflects many essential features of a superluminescent light emitting diode.

Plasmonic oligomers in cylindrical vector light beams

Summary We investigate the excitation as well as propagation of magnetic modes in plasmonic nanostructures. Such structures are particularly suited for excitation with cylindrical vector beams. We study magneto-inductive coupling between adjacent nanostructures. We utilize high-resolution lithographic techniques for the preparation of complex nanostructures consisting of gold as well as aluminium. These structures are subsequently characterized by linear optical spectroscopy. The well characterized and designed structures are afterwards studied in depth by exciting them with radial and azimuthally polarized light and simultaneously measuring their plasmonic near-field behavior. Additionally, we attempt to model and simulate our results, a project which has, to the best of our knowledge, not been attempted so far.

Controlling the interaction of photons and single quantum systems in an optical microresonator

Small optical microresonators are structures which confine light to volumes with dimensions on the order of one wavelength and provide an important means for controlling light-matter interaction in integrated optics. In this Paper, we would like to present our work on the study of the interaction of single quantum emitters or nanoparticles located in the confined optical field of a single-mode microresonator. The interaction possibilities between a general photonic system and a quantum system are discussed, with special focus on the effect of resonant microcavities. For the case of the optical microresonator used in our experiments, we present a model based on the transfer matrix method which can analytically describe the radiative enhancement of even complex resonator geometries. This allows not only the optimization of resonator geometries, but gives accurate information on the radiative processes occurring in the cavity, allowing the extraction of information normally inaccessible in optical measurements.