Saïd Bakhti

Photo-directed organization of silver nanoparticles in mesostructured silica and titania films

This paper quantifies the migration of silver contained within mesostructured hybrid silica films and mesoporous titania films under exposure to modulated light. This quantization allows to demonstrate that all of the initial silver salt can be concentrated and reduced in domains accumulating the higher photonic energy. Entirely reduced in the form of nanoparticles of few nanometers size embedded in the silica matrix, silver is then quite stable even under subsequent homogeneous exposures. It is also shown that thanks to the relatively slow nanoparticle growth, successive multiple exposures can be used to create complex 3D microstructures within silica films using a simple dual beam interferometer. In mesoporous titania films, the UV photo-growth of silver nanoparticles remains limited to the vicinity of the film interface because of the matrix absorption and cannot provide deep 3D patterns of silver nanoparticles. However, 2D refractive index patterns can be obtained under UV exposure, erased with visible light and updated thanks to a reversible photochromic behavior. In such films, opposite migration flows of silver species are proven under UV intensity gradient and homogeneous visible exposure.

Fano-like resonance emerging from magnetic and electric plasmon mode coupling in small arrays of gold particles

In this work we theoretically and experimentally analyze the resonant behavior of individual 3 × 3 gold particle oligomers illuminated under normal and oblique incidence. While this structure hosts both dipolar and quadrupolar electric and magnetic delocalized modes, only dipolar electric and quadrupolar magnetic modes remain at normal incidence. These modes couple into a strongly asymmetric spectral response typical of a Fano-like resonance. In the basis of the coupled mode theory, an analytical representation of the optical extinction in terms of singular functions is used to identify the hybrid modes emerging from the electric and magnetic mode coupling and to interpret the asymmetric line profiles. Especially, we demonstrate that the characteristic Fano line shape results from the spectral interference of a broad hybrid mode with a sharp one. This structure presents a special feature in which the electric field intensity is confined on different lines of the oligomer depending on the illumination wavelength relative to the Fano dip. This Fano-type resonance is experimentally observed performing extinction cross section measurements on arrays of gold nano-disks. The vanishing of the Fano dip when increasing the incidence angle is also experimentally observed in accordance with numerical simulations.

Singular Representation of Plasmon Resonance Modes to Optimize the Near- and Far-Field Properties of Metal Nanoparticles

A singular representation of the complex-valued extinction coefficient of metal nanoparticles is developed to characterize the resonant behavior of plasmonic systems exhibiting an arbitrary number of resonances. This complex coefficient is analytically represented in form of a meromorphic function of the pulsation containing a singular (resonant) and a regular part, and an original algorithm based on a numerical derivation is proposed to find all resonant parameters of each excited mode. This approach, applied to silver nanoparticles, allows a characterization of the resonance red-shift and broadening when increasing the particle size or the local refractive index, as well as a particular sphere radius presenting a minimal bandwidth corresponding to minimal losses in the system. The optical cross sections of individual modes present an optimal particle size that maximizes the absorption cross section and from which the scattering process becomes predominant with respect to the absorption. Optical efficiencies can also be optimized regarding the particle size, and their variations are correlated to those of the maximum near-field intensity. The hybrid modes in silver dimers are also analyzed, and the hot spot intensity resulting from the longitudinal mode excitation can also be maximized by optimizing the particle size and the local refractive index.

Polarization-driven self-organization of silver nanoparticles in 1D and 2D subwavelength gratings for plasmonic photocatalysis.

One of the main challenges in plasmonics is to conceive large-scale, low-cost techniques suitable for the fabrication of metal nanoparticle patterns showing precise spatial organization. Here, we introduce a simple method based on continuous-wave laser illumination to induce the self-organization of silver nanoparticles within high-index thin films. We show that highly regular and homogeneous nanoparticle gratings can be produced on large areas using laser-controlled self-organization processes. This very versatile technique can provide 1D and 2D patterns at a subwavelength scale with tunable features. It does not need any stabilization or expensive devices, such as those required by optical or electron lithography, and is rapid to implement. Accurate in-plane and in-depth characterizations provide valuable information to explain the mechanisms that lead to pattern formation and especially how 2D self-organization can fall into place with successive laser scans. The regular and homogeneous 2D self-organization of metallic NPs with a single laser scan is also reported for the first time in this article. As the reported nanostructures are embedded in porous TiO2, we also theoretically explore the interesting potential of organization on the photocatalytic activity of Ag-NP-containing TiO2 porous films, which is one of the most promising materials for self-cleaning or remediation applications. Realistic electromagnetic simulations demonstrate that the periodic organization of silver nanoparticles can increase the light intensity within the film more than ten times that produced with randomly distributed nanoparticles, leading as expected to enhanced photocatalytic efficiency.

Modeling and Interpretation of Hybridization in Coupled Plasmonic Systems

The coupled mode formalism is introduced to provide a phenomenological understanding of the coupling effects in finite systems of particles. Within this approach, a metal nanoparticle can be viewed as an optical resonator and the formation of hybrid modes, resulting from the coupling between particles, can be anticipated. An efficient numerical algorithm is proposed to extract the characteristics (complex poles and amplitudes) of each resonance of the system.