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.

Multi-functionalization of lithographically designed gold nanodisks by plasmon-mediated reduction of aryl diazonium salts.

Plasmon-driven surface functionalization of nanoparticles is receiving increasing attention as it allows locally tailored chemical reactivity to be generated on the nanoparticle surface. The extension to surface multi-functionalization still represents a major breakthrough in chemistry. We address this issue by triggering regiospecific surface double-functionalization under plasmon excitation, using diazonium salts as surface functionalization agents.

Photochromic-fluorescent-plasmonic nanomaterials: towards integrated three-component photoactive hybrid nanosystems.

Silica-coated gold nanorods functionalized with grafted fluorescent and photochromic derivatives were synthesized and characterized. Spectroscopic investigations demonstrated that cross-coupled interactions between plasmonic, photochromic, and fluorescence properties play a major role in such nanosystems, depending on the thickness of the silica spacer, leading to multi-signal photoswitchability.

Revisiting Surface-Enhanced Raman Scattering on Realistic Lithographic Gold Nanostripes.

In this article, we investigate the Surface-Enhanced Raman Scattering (SERS) efficiency of methylene blue (MB) molecules deposited on gold nanostripes which, due to their fabrication by electron beam lithography and thermal evaporation, present various degrees of crystallinity and nanoscale surface roughness (NSR). By comparing gold nanostructures with different degrees of roughness and crystallinity, we show that the NSR has a strong effect on the SERS intensity of MB probe molecules. In particular, the NSR features of the lithographic structures significantly enhance the Raman signal of MB molecules, even when the excitation wavelength lies far from the localized surface plasmon resonance (LSPR) of the stripes. These results are in very good agreement with numerical calculations of the SERS gain obtained using the discrete dipole approximation (DDA). The influence of NSR on the optical near-field response of lithographic structures thus appears crucial since they are widely used in the context of nano-optics or/and molecular sensing.

Multipolar surface plasmon peaks on gold nanotriangles.

In this paper, we report on the observation of multipolar surface plasmon excitation in lithographically designed gold nanotriangles, investigated by means of far-field extinction microspectroscopy in the wavelength range of 400-1000 nm. Several bands are observed in the visible and near infrared regions when increasing the side length of the triangles. The assignment of these peaks to successive in-plane multipolar plasmon modes is supported by calculations using the discrete dipole approximation method. We show that the lowest three multipolar excitations are clearly resolved in the visible and near infrared range. These new spectral features could be very promising in nanooptics or for chemosensing and biosensing applications.

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.

Surface enhanced Raman scattering arising from multipolar plasmon excitation.

Visible and near infrared extinction spectra of gold nanorod regular arrays exhibit several bands assigned to high multipolar order plasmon resonances. These up to ninth order multipolar resonances generate surface enhanced Raman scattering spectra with typically 5 x 10(4) enhancement which is of similar magnitude as those obtained for dipolar excitations.

Imaging surface plasmon of gold nanoparticle arrays by far-field Raman scattering.

We present Raman spectra and Raman images of the methylene blue molecule adsorbed as a single layer on gold nanoparticles regularly arranged in periodic arrays. Spectra and images are recorded in the same spatial and spectral regions using an excitation under total internal reflection. Images of the Raman scattering appear as spots of circular shape located at the particle positions with size defined by the diffraction limit. It appears that all excited particles contribute equally to the Raman signal if the Gaussian intensity distribution of the laser beam is taken into account. These results demonstrate that Raman scattering can be a useful technique to study plasmon properties.

Study of Langmuir-Blodgett phospholipidic films deposited on surface enhanced Raman scattering active gold nanoparticle monolayers.

Surface enhanced Raman scattering (SERS) was used to study phospholipid monolayers transferred by the Langmuir-Blodgett (LB) technique to SERS active substrates. These substrates, which were constituted of gold colloidal nanoparticles bound to polysilane films grafted onto glass plates, showed a uniform and homogeneous layer with strong interacting particles as revealed from UV-visible extinction spectra and atomic force microscopy images. Laser excitation at 632.8 nm within the red part of the localized surface plasmon resonance leads to intense and reproducible SERS spectra of trans-1,2-bis(4-pyridyl)ethylene (BPE). From SERS measurements at different pHs it was possible to determine the apparent pK(a) of BPE adsorbed on gold-coated silanized substrates in the absence and presence of one LB monomolecular layer of phospholipids. These SERS titrations allowed the estimation of the pH at the metal-LB film interface.

Surface enhanced Raman scattering of a lipid Langmuir monolayer at the air-water interface.

Surface enhanced Raman spectra were recorded from a phospholipid monolayer directly at the air-water interface. We used an organized monolayer of negatively charged tetramyristoyl cardiolipins as a template for the electrochemical generation of silver deposits. This two-dimensional electrodeposition of silver under potentiostatic control was the substrate for enhancement of Raman spectra. We report the optimized conditions for the Raman enhancement, the microscopic observations of the deposits, and their characterization by atomic force microscopy. Laser excitation at 514.5 nm leads to intense and reproducible surface enhanced Raman scattering spectra recorded in situ from one monolayer of cardiolipin, using 0.5 mol % of 10N nonyl acridine orange or 5 mol % of acridine in the film, and demonstrates the possibility of estimating the pH at the metal/phospholipidic film interface.

Gold particle interaction in regular arrays probed by surface enhanced Raman scattering.

Lithographically designed two-dimensional arrays consisting of gold nanoparticles deposited on a smooth gold film are used as substrate to examine the SERS effect of the trans-1,2-bis (4-pyridyl) ethylene molecule. These arrays display two plasmon bands instead of the single one observed for the same arrays of particles but deposited on indium tin oxide coated glass. Laser excitation within the short wavelength band does not bring about any SERS spectrum, while excitation within the long wavelength band yields SERS spectra with a gain per molecule rising up to 10(8). The simultaneous investigation of extinction and Raman spectra of arrays exhibiting various topography parameters enables us to suggest an interpretation for both the occurrence of the two plasmon resonances and for the high Raman enhancement. We suggest to assign the short wavelength band to a plasmon wave propagating at the gold glass interface and the long wavelength one to an air/gold surface plasmon mode modified by particle-particle interaction.

Light field propagation by metal micro- and nanostructures.

The ability to sustain plasmon oscillations gives rise to unique properties of metal nanostructures, which can be exploited for the controlled manipulation of light fields on the nanoscale. In this context we investigate electromagnetic coupling effects within lithographically produced ensembles of gold nanoparticles with a photon scanning tunnelling microscope. To provide an interface between these nano-optical devices and classical far-field optics, we investigate surface plasmon propagation on microstructured metal thin films.