Few-Cycle Surface Plasmon Polaritons.

Surface plasmon polaritons (SPPs) can confine and guide light in nanometer volumes and are ideal tools for achieving electric field enhancement and the construction of nanophotonic circuitry. The realization of the highest field strengths and fastest switching requires confinement also in the temporal domain. Here, we demonstrate a tapered plasmonic waveguide with an optimized grating structure that supports few-cycle surface plasmon polaritons with >70 THz bandwidth while achieving >50% light-field-to-plasmon coupling efficiency. This enables us to observe the─to our knowledge─shortest reported SPP wavepackets. Using time-resolved photoelectron microscopy with suboptical-wavelength spatial and sub-10 fs temporal resolution, we provide full spatiotemporal imaging of co- and counter-propagating few-cycle SPP wavepackets along tapered plasmonic waveguides. By comparing their propagation, we track the evolution of the laser-plasmon phase, which can be controlled via the coupling conditions.

Photoconductivity of PbS/perovskite quantum dots in gold nanogaps.

We demonstrate that the photoconductance of colloidal PbS/MAPbI3 quantum dots in nanoscale gold electrode gaps shows a consistent power law dependence of the photocurrent on the light intensity with an exponent slightly below 0.7. The gap sizes are between 25 and 800 nm and by scanning photocurrent microscopy we evidence the strong localization and high reproducibility of photocurrent generation. We probe different flat-faced and pointed electrodes for excitation light in the red and near infrared spectral range and laser irradiances from 10-2 to 102 W cm-2. Our material combination provides practically identical photocurrent response for a wide range of gap sizes and geometries, highlighting its generic potential for nanoscale light coupling and detection.

Plasmonic Dispersion Relations and Intensity Enhancement of Metal-Insulator-Metal Nanodisks.

We show that the plasmon modes of vertically stacked Ag-SiO2-Ag nanodisks can be understood and classified as hybridized surface and edge modes. We describe their universal dispersion relations and demonstrate that coupling-induced spectral shifts are significantly stronger for surface modes than for edge modes. The experimental data correspond well to numerical simulations. In addition, we estimate optical intensity enhancements of the stacked nanodisks in the range of 1000.

Edge Mode Coupling within a Plasmonic Nanoparticle.

The coupling of plasmonic nanoparticles can strongly modify their optical properties. Here, we show that the coupling of the edges within a single rectangular particle leads to mode splitting and the formation of bonding and antibonding edge modes. We are able to unambiguously designate the modes due to the high spatial resolution of electron microscopy-based electron energy loss spectroscopy and the comparison with numerical simulations. Our results provide simple guidelines for the interpretation and the design of plasmonic mode spectra.

Plasmon modes of a silver thin film taper probed with STEM-EELS.

By focusing propagating surface plasmons, electromagnetic energy can be delivered to nanoscale volumes. In this context, we employ electron energy loss spectroscopy in a scanning transmission electron microscope to characterize the full plasmonic mode spectrum of a silver thin film tapered to a sharp tip. We show that the plasmon modes can be ordered in film and edge modes and corroborate our assignment through supplementary numerical simulations. In particular, we find that the focused plasmon field at the taper tip is fueled by edge modes.

Morphing a plasmonic nanodisk into a nanotriangle.

We morph a silver nanodisk into a nanotriangle by producing a series of nanoparticles with electron beam lithography. Using electron energy loss spectroscopy (EELS), we map out the plasmonic eigenmodes and trace the evolution of edge and film modes during morphing. Our results suggest that disk modes, characterized by angular order, can serve as a suitable basis for other nanoparticle geometries and are subject to resonance energy shifts and splittings, as well as to hybridization upon morphing. Similar to the linear combination of atomic orbitals (LCAO) in quantum chemistry, we introduce a linear combination of plasmonic eigenmodes to describe plasmon modes in different geometries, hereby extending the successful hybridization model of plasmonics.

Universal dispersion of surface plasmons in flat nanostructures.

Dimensionality has a significant impact on the optical properties of solid-state nanostructures. For example, dimensionality-dependent carrier confinement in semiconductors leads to the formation of quantum wells, quantum wires and quantum dots. While semiconductor properties are governed by excitonic effects, the optical response of metal nanostructures is dominated by surface plasmons. Here we find that, in contrast to excitonic systems, the mode dispersions in plasmonic structures of different dimensionality are related by simple scaling rules. Employing electron energy loss spectroscopy, we show that the modes of silver nanodisks can be scaled to the surface and edge modes of extended silver thin films. We thereby introduce a general and intuitive ordering scheme for plasmonic excitations with edge and surface modes as the elementary building blocks.

2D crystalline protein layers as immobilization matrices for the development of DNA microarrays.

There is a growing demand for functional layers for the immobilization of (bio)molecules on different kinds of substrates in the field of biosensors, microarrays, and lab-on-a-chip development. These functional coatings should have the ability to specifically bind (bio)molecules with a high binding efficiency, while showing low unspecific binding during the following assay. In this paper we present rSbpA surface layer proteins (S-layer proteins) as a versatile immobilization layer for the development of DNA microarrays. S-layer proteins show the ability to reassemble into two-dimensional arrays on solid surfaces and their functional groups, such as carboxylic groups, are repeated with the periodicity of the lattice, allowing for immobilization of other (bio)molecules. Different fluorescently labeled amino functionalized DNA oligomers were covalently linked to the S-layer matrices to allow the characterization of DNA binding on S-layers. Hybridization and dissociation of DNA-oligomers were studied on S-layer coated slides, revealing low levels of unspecific adsorption of DNA on S-layer based immobilization matrices. In the following the principle was transferred to a DNA microarray design showing successful spotting and hybridization on whole microarray slides. Besides common laser scanning for fluorescence detection, S-layer based microarrays were evaluated with a compact, low cost platform for direct fluorescence imaging based on surface plasmon enhanced fluorescence excitation. It could be shown that S-layer protein layers are promising as immobilization matrices for the development of biosensors and microarrays.

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.

Electron-energy-loss spectra of plasmonic nanoparticles.

We investigate electron-energy-loss spectroscopy (EELS) on metallic nanoparticles, through simulations, and provide a comprehensive comparison between EELS and the photonic local density of states (LDOS). Most importantly, we show that there is no direct link between EELS and LDOS maps, and that EELS can even be blind to hot spots in the gap between coupled nanoparticles. Although intimately related, the two quantities provide complementary information. This finding is in marked contrast to recently reported results.

Coupling efficiency of light to surface plasmon polariton for single subwavelength holes in a gold film.

The excitation of surface plasmon polaritons (SPP) by focusing a laser beam on single subwavelength holes opened in a thin gold film is studied both experimentally and theoretically. By means of leakage radiation microscopy, quantitative measurements of the light-SPP coupling efficiency are performed for holes with different sizes and shapes. The system is studied theoretically by using a modal expansion method to calculate the fraction of the incident energy which is scattered by the hole into a surface plasmon. We demonstrate that a single subwavelength hole can be used to generate SPP with an efficiency up to 28%.

Silver nanowires as surface plasmon resonators.

We report on chemically prepared silver nanowires (diameters around 100 nm) sustaining surface plasmon modes with wavelengths shortened to about half the value of the exciting light. As we find by scattered light spectroscopy and near-field optical microscopy, the nonradiating character of these modes together with minimized damping due to the well developed wire crystal structure gives rise to large values of surface plasmon propagation length and nanowire end face reflectivity of about 10 microm and 25%, respectively. We demonstrate that these properties allow us to apply the nanowires as efficient surface plasmon Fabry-Perot resonators.

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.

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.