Measurement and reduction of damping in plasmonic nanowires

We report on a spectroscopic study of surface plasmon damping and group velocity in polycrystalline silver and gold nanowires. By comparing to single-crystalline wires and by using different substrates, we quantitatively deduce the relative damping contributions due to metal crystallinity and absorption in the substrate. Compared to absorbing substrates, we find strongly reduced plasmonic damping for polycrystalline nanowires on quartz substrates, enabling the application of such wires for plasmonic waveguide networks.

High-resolution biosensor based on localized surface plasmons

We report on a new biosensor with localized surface plasmons (LSP) based on an array of gold nanorods and the total internal reflection imaging in polarization contrast. The sensitivity of the new biosensor is characterized and a model detection of DNA hybridization is carried out. The results are compared with a reference experiment using a conventional high-resolution surface plasmon resonance (SPR) biosensor. We show that the LSP-based biosensor delivers the same performance as the SPR system while involving significantly lower surface densities of interacting molecules. We demonstrate a limit of detection of 100 pM and a surface density resolution of only 35 fg×mm-2 that corresponds to less than one DNA molecule per nanoparticle on average.

Surface plasmon leakage radiation microscopy at the diffraction limit

This paper describes the image formation process in optical leakage radiation microscopy of surface plasmon-polaritons with diffraction limited spatial resolution. The comparison of experimentally recorded images with simulations of point-like surface plasmon-polariton emitters allows for an assignment of the observed fringe patterns. A simple formula for the prediction of the fringe periodicity is presented and practically relevant effects of abberations in the imaging system are discussed.

Local refractive index sensitivity of plasmonic nanoparticles

We report on an experimental characterization of the sensitivity of localized surface plasmons (LSP) to local changes in the refractive index at a nanometer scale. The method is based on forming a polymer mask covering different well defined areas of metallic nanoparticles and measuring the extinction peak shifts associated with the local refractive index changes. Arrays of nanoparticles (nanorod chains) are prepared using electron beam lithography and the dielectric mask is aligned with respect to the nanoparticle array in a second lithographic step. Extinction peak shifts corresponding to different positions of the mask are measured and values for the local refractive index sensitivity are deduced. A deconvolution procedure is established and used to map the local sensitivity across the surface of nanoparticle based on measured data. The experimental results are shown to correspond well with theoretical simulations obtained using the finite-difference time-domain method. The results indicate that the sensitivity is strongly correlated with the profile of the LSP electric field.

Design and Optical Properties of Active Polymer-Coated Plasmonic Nanostructures

The grafting of stimuli-responsive polymer brushes on plasmonic structures provides a perfectly controlled two-dimensional active device with optical properties that can be modified through external stimuli. Herein, we demonstrate thermally induced modifications of the plasmonic response of lithographic gold nanoparticles functionalized by thermosensitive polymer brushes of (poly(N-isopropylacrylamide), PNIPAM). Optical modifications result from refractive local index changes due to a phase transition from a hydrophilic state (swollen regime) to a hydrophobic state (collapsed regime) of the polymer chains occurring in a very small range of temperatures. The refractive index of the polymer in aqueous solution is estimated in both states, deduced from the discrete dipole approximation (DDA) method. The combination of lithographic gold NPs and thermoresponsive polymer chains leads to a new generation of perfectly calibrated and dynamically controlled hybrid gold/polymer system for real-time nanosensors.

Superresolution Moiré mapping of particle plasmon modes

The spontaneous emission rate of a fluorophore provides a direct probe of the photonic local density of states at the fluorophore position. Here we exploit this capability to map the plasmonic modes of gold nanoparticles by imaging the fluorescence intensity in combined regular arrays of identical gold and fluorophore-doped polymer nanoparticles. By varying the distance between gold and polymer particles across the array, the fluorophore emission generates an optical Moiré pattern corresponding to a magnified spatial map of the plasmonic mode, which can be directly imaged with an optical microscope. Our results are corroborated by supplementary theoretical model calculations.

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.

Active plasmonic devices with anisotropic optical response: a step toward active polarizer

Control of the optical properties of metallic nanoparticles (NP) is realized using an electrochemical switch consisting of a thin layer of conducting polymer (CP). It is shown that the quenching of localized surface plasmon (LSP) sustained by oblate particles depends of the frequency of the LSP resonance. This effect is attributed to the variation of the CP dielectric function with wavelength. As a consequence, prolate arrays show total quenching of the LSP resonance along the major axis of the particles whereas modulation and moderate damping are observed along the minor axis. Combining electroactive conducting polymer and prolate NP makes it possible to design active plasmonic devices with anisotropic optical response upon CP switching. In the present case, such devices can be used as active filters or polarizers.

Förster-type resonant energy transfer influenced by metal nanoparticles

We show experimentally and numerically that Forster-type resonant energy transfer between molecules strongly depends on the interaction with plasmonic resonances in a nearby metallic nanoparticle (MNP). Acceptor luminescence emerges at the expense of donor luminescence when an acceptor molecule harvests the donor's near-field energy. By tuning the resonance of a close-by MNP across the transition energy bands of the molecules, we show how the molecular luminescence is affected and in part even strongly increased.

Coupling dielectric waveguide modes to surface plasmon polaritons

We study dielectric/metal thin film multilayers designed for the coupling of dielectric waveguide modes and surface plasmons. The coupling as identified in calculated dispersion relations for extended multilayers is confirmed by measured angle-resolved reflectance data. By lateral structuring of the multilayers the mutual coupling of dielectric and plasmonic modes is directly observed by fluorescence based microscopy. For a light wavelength of 514nm we find a coupling length of 15microm. Our results highlight the potential of hybrid dielectric/metal waveguides for integrating surface plasmon based photonic circuitry or sensing elements into conventional optical devices.

Tunable electrochemical switch of the optical properties of metallic nanoparticles

Control of the optical properties of oblate metallic nanoparticles (NP) is realized using an electrochemical switch consisting of a thin layer of conducting polymer (CP). Reversible modulation, moderate damping, and almost total quenching of the localized surface plasmon (LSP) resonance is achieved as a function of the thickness of the CP layer and the potential applied to the electrochemical systems, that is, the charge carrier density injected into the CP layer. These experimental results can be qualitatively reproduced using the single-particle model in the electrostatic approximation. We believe that combining an electroactive conducting polymer and NP will prove to be a general strategy for controlling the properties of various types of NP (fluorescent, magnetic, semiconducting) in many fields.

Probing surface plasmon fields by far-field Raman imaging

This article reports on the imaging of the surface plasmon fields of lithographically designed micrometre-sized gold structures. We investigate rings made of disk-shaped particles and individual crescent-shaped particles. These structures are imaged with two techniques, dark field imaging of elastically scattered light and imaging the surface-enhanced Raman scattering signal of methylene blue dye adsorbed onto the structures. Although elastically scattered light images result from the coherently summed contributions from all elementary scattering volumes, surface-enhanced Raman scattering images reflect the optical near-field intensity incoherently averaged over a surface area corresponding to the spatial resolution of the microscope objective. The combination of both imaging methods enables us to emphasize the role of plasmon coupling and antenna effect in the surface-enhanced Raman scattering enhancement.

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%.

Analysis of the angular acceptance of surface plasmon Bragg mirrors

We analyze an important aspect of the behavior of surface plasmon polariton (SPP) Bragg mirrors: the dependence of the angular acceptance for reflection on the incidence angle. By means of leakage radiation microscopy, both in direct and Fourier space, we observe that the angular acceptance diminishes for increasing incidence angles. This effect, which can considerably affect the design of devices based on these elements, is shown to be the consequence of the decrease of the bandgap width with increasing incidence angle.

Surface plasmon polariton microscope with parabolic reflectors

We report the realization of a two-dimensional optical microscope for surface plasmons polaritons (SPPs) based on parabolic Bragg mirrors. These mirrors are built from lithographically fabricated gold nanostructures on gold thin films. We show by direct imaging by leakage radiation microscopy that the magnification power of the SPP microscope follows basic predictions of geometrical optics. Spatial resolution down to the value set by the diffraction limit is demonstrated.

Surface plasmon mediated near-field imaging and optical addressing in nanoscience

We present an overview of recent progress in "plasmonics". We focus our study on the observation and excitation of surface plasmon polaritons (SPPs) with optical near-field microscopy. We discuss in particular recent applications of photon scanning tunnelling microscope (PSTM) for imaging of SPP propagating in metal and dielectric wave guides. We show how near-field scanning optical microscopy (NSOM) can be used to optically and actively address remote nano objects such as quantum dots. Additionally we compare results obtained with near-field microcopy to those obtained with other optical far-field methods of analysis such as leakage radiation microscopy (LRM).

Plasmonic crystal demultiplexer and multiports

We report the realization of two-dimensional optical wavelength demultiplexers and multiports for surface plasmons polaritons (SPPs) based on plasmonic crystals, i.e., photonic crystals for SPPs. These SPP elements are built up of lithographically fabricated gold nanostructures on gold thin films. We show by direct imaging of laterally confined SPP beams in the visible spectral range by leakage radiation microscopy that SPPs of different wavelengths are efficiently rerouted into different directions. In addition we demonstrate the generation of three output SPP beams from one input beam.

Rapid prototyping of optical components for surface plasmon polaritons

Advanced femtosecond laser technology allows the fabrication of arbitrary 2D and 3D dielectric micro- and nanoscale structures by two-photon polymerization (2PP). In this paper, we present first investigations on excitation of surface plasmon polaritons (SPPs) on dielectric 2D structures fabricated on metal surfaces with this technology. Straight and curved line- and dot- structures built of the negative-tone photoresist ORMOCER (organically modified ceramic) are investigated by plasmon leakage radiation microscopy. Polarization dependent excitation efficiencies and focusing of SPPs are investigated.

Heteroepitaxy of organic-organic nanostructures

Highly crystalline organic heteroepitaxial layers with controlled molecular orientations and morphologies are one of the keys for optimum organic device performance. With studies of molecular orientation, structure, and morphology, we have investigated the ability of oriented organic films to act as substrate templates for the growth of a second organic layer. Depending on the molecular orientation in the sexiphenyl substrate, crystalline sexithiophene nanostructures of either pyramidal or needlelike morphology, with either near vertical or parallel molecular orientations, respectively, grow.

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.

Grating-induced plasmon mode in gold nanoparticle arrays

We study the dipolar coupling of gold nanoparticles arranged in regular two-dimensional arrays by extinction micro-spectroscopy. When the interparticle spacing approaches the plasmon resonance wavelength of the individual particles, an additional band of very narrow width emerges in the extinction spectrum. By systematically changing the particles dielectric environment, the particles shape, the grating constant and angle of incidence, we show how this band associated to a grating induced-resonance can be influenced in strength and spectral position. The spectral position can be qualitatively understood by considering the conditions for grazing grating orders whereas the strength can be related to the strength of dipolar scattering from the individual particles.