Manuel R. Gonçalves

Laser Fabrication of Large-Scale Nanoparticle Arrays for Sensing Applications

A novel method for high-speed fabrication of large scale periodic arrays of nanoparticles (diameters 40-200 nm) is developed. This method is based on a combination of nanosphere lithography and laser-induced transfer. Fabricated spherical nanoparticles are partially embedded into a polymer substrate. They are arranged into a hexagonal array and can be used for sensing applications. An optical sensor with the sensitivity of 365 nm/RIU and the figure of merit of 21.5 in the visible spectral range is demonstrated.

Surface-Enhanced Raman Spectroscopy of Dye and Thiol Molecules Adsorbed on Triangular Silver Nanostructures: A Study of Near-Field Enhancement, Localization of Hot-Spots, and Passivation of Adsorbed Carbonaceous Species

Surface-enhanced Raman spectroscopy (SERS) of thiols and dye molecules adsorbed on triangular silver nanostructures was investigated. The SERS hot-spots are localized at the edges and corners of the silver triangular particles. AFM and SEM measurements permit to observe many small clusters formed at the edges of triangular particles fabricated by nanosphere lithography. Finite-element calculations show that near-field enhancements can reach values of more than 200 at visible wavelengths, in the gaps between small spherical particles and large triangular particles, although for the later no plasmon resonance was found at the wavelengths investigated. The regions near the particles showing strong near-field enhancement are well correlated with spatial localization of SERS hot-spots done by confocal microscopy. Silver nanostructures fabricated by thermal evaporation present strong and fast fluctuating SERS activity, due to amorphous carbon contamination. Thiols and dye molecules seem to be able to passivate the undesired SERS activity on fresh evaporated silver.

Plasmonic nanostructures fabricated using nanosphere-lithography, soft-lithography and plasma etching

Summary We present two routes for the fabrication of plasmonic structures based on nanosphere lithography templates. One route makes use of soft-lithography to obtain arrays of epoxy resin hemispheres, which, in a second step, can be coated by metal films. The second uses the hexagonal array of triangular structures, obtained by evaporation of a metal film on top of colloidal crystals, as a mask for reactive ion etching (RIE) of the substrate. In this way, the triangular patterns of the mask are transferred to the substrate through etched triangular pillars. Making an epoxy resin cast of the pillars, coated with metal films, allows us to invert the structure and obtain arrays of triangular holes within the metal. Both fabrication methods illustrate the preparation of large arrays of nanocavities within metal films at low cost. Gold films of different thicknesses were evaporated on top of hemispherical structures of epoxy resin with different radii, and the reflectance and transmittance were measured for optical wavelengths. Experimental results show that the reflectivity of coated hemispheres is lower than that of coated polystyrene spheres of the same size, for certain wavelength bands. The spectral position of these bands correlates with the size of the hemispheres. In contrast, etched structures on quartz coated with gold films exhibit low reflectance and transmittance values for all wavelengths measured. Low transmittance and reflectance indicate high absorbance, which can be utilized in experiments requiring light confinement.

Hybrids of carbon Nanotube Forests and Gold Nanoparticles for Improved Surface Plasmon Manipulation

We report the fabrication and characterization of hybrids of vertically-aligned carbon nanotube forests and gold nanoparticles for improved manipulation of their plasmonic properties. Raman spectroscopy of nanotube forests performed at the separation area of nanotube-nanoparticles shows a scattering enhancement factor of the order of 1 × 10(6). The enhancement is related to the plasmonic coupling of the nanoparticles and is potentially applicable in high-resolution scanning near-field optical microscopy, plasmonics, and photovoltaics.

Strong dipole-quadrupole coupling and Fano resonance in H-like metallic nanostructures

Under certain conditions of the incident light polarization direction a Fano resonance arises in small gold nanorods arranged in a H-like configuration. This stems from the coupling between a bright dipole plasmon mode excited in the horizontal rod and dark quadrupole plasmon modes in both vertical rods. We investigate these surface plasmon modes, and analyze the dependence of the Fano resonance on the geometry parameters such as rod size and interparticle separation, and refractive index of embedding medium. To describe the degree of this energy transfer, we introduce a new parameter: the Fano resonance efficiency. We calculate absorption cross-sections for visible and NIR spectrum in each element of the structure, and near-field distributions at different wavelengths. We show that Fano resonance in small H-like structures exhibits high sensitivity with respect to the refractive index of the host medium, outperforming the values for larger plasmonic structures based on nanorods already investigated.

Near-field effects and energy transfer in hybrid metal-oxide nanostructures

Summary One of the big challenges of the 21st century is the utilization of nanotechnology for energy technology. Nanoscale structures may provide novel functionality, which has been demonstrated most convincingly by successful applications such as dye-sensitized solar cells introduced by M. Grätzel. Applications in energy technology are based on the transfer and conversion of energy. Following the example of photosynthesis, this requires a combination of light harvesting, transfer of energy to a reaction center, and conversion to other forms of energy by charge separation and transfer. This may be achieved by utilizing hybrid nanostructures, which combine metallic and nonmetallic components. Metallic nanostructures can interact strongly with light. Plasmonic excitations of such structures can cause local enhancement of the electrical field, which has been utilized in spectroscopy for many years. On the other hand, the excited states in metallic structures decay over very short lifetimes. Longer lifetimes of excited states occur in nonmetallic nanostructures, which makes them attractive for further energy transfer before recombination or relaxation sets in. Therefore, the combination of metallic nanostructures with nonmetallic materials is of great interest. We report investigations of hybrid nanostructured model systems that consist of a combination of metallic nanoantennas (fabricated by nanosphere lithography, NSL) and oxide nanoparticles. The oxide particles were doped with rare-earth (RE) ions, which show a large shift between absorption and emission wavelengths, allowing us to investigate the energy-transfer processes in detail. The main focus is on TiO2 nanoparticles doped with Eu3+, since the material is interesting for applications such as the generation of hydrogen by photocatalytic splitting of water molecules. We use high-resolution techniques such as confocal fluorescence microscopy for the investigation of energy-transfer processes. The experiments are supported by simulations of the electromagnetic field enhancement in the vicinity of well-defined nanoantennas. The results show that the presence of the nanoparticle layer can modify the field enhancement significantly. In addition, we find that the fluorescent intensities observed in the experiments are affected by agglomeration of the nanoparticles. In order to further elucidate the possible influence of agglomeration and quenching effects in the vicinity of the nanoantennas, we have used a commercial organic pigment containing Eu, which exhibits an extremely narrow particle size distribution and no significant agglomeration. We demonstrate that quenching of the Eu fluorescence can be suppressed by covering the nanoantennas with a 10 nm thick SiOx layer.

Influence of the light‐scattering form factor on the Bragg diffraction patterns of arrays of metallic nanoparticles

Accurate models for the light‐scattering form factors of nanoparticles are of crucial importance to characterize the optical properties of the particles and to develop new photonic devices. Analytical or semi‐empirical models exist for particles of spherical and cylindrical shape. The angular spectrum of scattering for particles of more complex shape is very complex and can only be obtained by numerical simulations. Moreover, the light scattering of metallic particles depends on many parameters as size, shape, optical constants, substrate and polarization of light. Experimental verification of the differential scattering cross‐sections obtained from different calculation methods is always necessary. Measurements done on single nanoparticles are very sensitive to their local properties and the sinal‐to‐noise ratio may be very poor. Arrays of identical particles illuminated by a planes waves produce Bragg diffraction and the resultant patterns depend on the averaged values of the form factors of the particles.

Diffractive Focusing of Waves in Time and in Space

We study the general wave phenomenon of diffractive focusing from a single slit for two types of waves and demonstrate several properties of this effect. Whereas in the first situation, the envelope of a surface gravity water wave is modulated in time by a rectangular function, leading to temporal focusing, in the second example, surface plasmon polariton waves are focused in space by a thin metal slit to a transverse width narrower than the slit itself. The observed evolution of the phase carrier of the water waves is measured for the first time and reveals a nearly flat phase as well as an 80% increase in the intensity at the focal point. We then utilize this flat phase with plasmonic beams in the spatial domain, and study the case of two successive slits, creating a tighter focusing of the waves by putting the second slit at the focal point of the first slit.

Super-resolution in diffractive imaging from hemispherical elastic light scattering data

Angle-resolved hemispherical elastic light scattering techniques have been used to reconstruct the surface profile of two-dimensional photonic crystals with submicron resolution and metrological precision. Iterative algorithms permit subsequent calculation of a surface autocorrelation function with additional numerical approximation of the power spectrum and then yield final reconstruction of the surface shape. The proposed method enables filtering out unwanted scattering background, precluding the convergence of phase-retrieval algorithms. The estimation of higher harmonics in the power spectrum provides the reconstruction of a realistic surface achieving subwavelength resolution.

SERS observed in periodic metallo-dielectric nanostructures fabricated using coated colloidal crystals

Surface enhanced Raman scattering (SERS) has been investigated for different molecules adsorbed on metallic nanostructures. The local field enhancements due to the confinement and resonances of surface plasmons, can reach many orders of magnitude. These field enhancements allow molecules to produce strong Raman spectra, although they have tiny Raman scattering cross sections. The high sensitivity demonstrated was relevant for sensing applications of single molecules. We have investigated experimentally the SERS effect on rhodamine 6G molecules, adsorbed on triangular silver particles and photonic metallo-dielectric structures based on polymers. These structures were fabricated by evaporation of a thin metallic film on colloidal crystals followed by casting in PDMS and epoxy resin. In the later, the polystyrene spheres were removed by sonication in organic solvents. The remaining structure allows molecules to be adsorbed at its metallic surface, on top of the triangular particles or inside the spherical holes. The SERS spectra were measured by a scanning confocal Raman microscope. The location of the SERS active centers (hot spots) in arrays of triangular particles (corners and edges) is correlated with the optical near-field enhancements obtained by numerical simulations. In metallo-dielectric photonic structures made of PDMS the Raman images show regions of stable SERS spectra (several pixels wide) and many isolated bright pixels.The isolated pixels are instable in time, i.e. show spectral blinking. The photonic structures we propose can be fabricated in a reproducible way. The field enhancements depend mainly on the size and shape of the arrays, which is not the case for etched silver films and for clusters prepared by colloidal silver. Thus, they are more suitable to investigate the electromagnetic contribution to SERS.