Alexandre Vial

Description of dispersion properties of metals by means of the critical points model and application to the study of resonant structures using the FDTD method

Using a combination of Drude and critical points models, we show that the permittivity of several metals can be more efficiently described than using the well-known Drude–Lorentz model. The numerical implementation in a finite-difference time-domain code together with a non-uniform grid enables the study of thin metallic intermediate layers often neglected in simulations but found in realistic resonant structures. (Some figures in this article are in colour only in the electronic version)

Enhancement and Quenching Regimes in Metal−Semiconductor Hybrid Optical Nanosources

We report on the emission of hybrid nanosources composed of gold nanoparticles coupled with quantum dots. The emission relies on energy transfer from the quantum dots to gold nanoparticles which could be de-excited through radiative plasmon relaxation. The dependence of the emission efficiency is studied systematically as a function of the size of gold nanoparticles and interdistance between gold nanoparticles and quantum dots. We demonstrate a size-dependent transition between quenching and enhancement and a nonradiative energy transfer from the quantum dots to the gold nanoparticles.

Implementation of the critical points model in the recursive convolution method for modelling dispersive media with the finite-difference time domain method

We describe the implementation of the critical points model in a finite-difference time domain code dealing with dispersive media through the recursive convolution method. We show that this requires only minor modifications to existing codes able to take the Drude and Lorentz models into account. The use of this model of dispersion over the typical Drude–Lorentz one may be advantageous in specific cases.

Modeling recent experiments of apertureless near-field optical microscopy using 2D finite element method

Abstract Recently promising experiments of apertureless scanning near-field optical microscopy (ASNOM) have been reported [Appl. Phys. Lett. 79 (24) (2001) 4019]. They deal with the study of the confinement of the light in the vicinity of a nanometric tip which produces a nanosource of great interest to study local physical effects or to elaborate nanostructures. These experiments consist of “imaging” the electromagnetic field in the vicinity of a vibrating tip by using a photopolymer placed in the near-field region of the tip under laser illumination. Nanometer-size polymer dots, produced by local-field enhancement at the tip extremity, are characterized by atomic force microscopy. The dot height is related to the local-optical intensity at the tip end. In this paper, in order to model these experiments, we introduce for the first time in the context of SNOM a 2D harmonic finite element simulation enabling the calculation of the electromagnetic field in the vicinity of a tip for apertureless scanning near-field optical microscopy. Both tip vibration and realistic tip geometry have been taken into account and the dot height has been calculated as a function of both the vibration amplitude during exposure and the polarization of the incident light. Results of the finite element calculations are found to be in good agreement with the experimental data and are compared to simple analytical calculations.

Application of evolution strategies for the solution of an inverse problem in near-field optics

We introduce an inversion procedure for the characterization of a nanostructure from near-field intensity data. The method proposed is based on heuristic arguments and makes use of evolution strategies for the solution of the inverse problem as a nonlinear constrained-optimization problem. By means of some examples we illustrate the performance of our inversion method. We also discuss its possibilities and potential applications.

Crystalline structure’s influence on the near-field optical properties of single plasmonic nanowires

The finite difference time domain method is employed to study the crystalline structure’s influence on the propagation of a local excitation along metallic nanowires of subwavelength cross section. The metallic nanowires are elongated cylinders deposited on a transparent substrate. A tightly focused gaussian beam illuminates one end of the nanowires. According to recent experimental studies, the authors show that the propagation length of the localized surface plasmon excitations depends on the crystalline structure of the nanowire. Thus, they are able to determine the effective permittivity of metals in such a nanostructure versus its crystalline properties. The authors also demonstrate that the field of optical information transport could greatly benefit from the care of the subwavelength optical waveguide’s crystallinity.

Photoresponsive polymers for topographic simulation of the optical near-field of a nanometer sized gold tip in a highly focused laser beam

The local perturbation of a diffraction-limited spot by a nanometer sized gold tip in a popular apertureless scanning near-field optical microscopy (ASNOM) configuration is reproduced through topography changes in a photoresponsive polymer. Our method relies on the observation of the photochemical migration of azobenzene molecules grafted to a polymer placed beneath the tip. A local molecular displacement has been shown to be activated by a gold tip as a consequence of the lateral surface charge density present at the edges of the tips end, resulting from a strong near-field depolarization predicted by theory.

Modeling of metallic nanostructures embedded in liquid crystals: application to the tuning of their plasmon resonance

We numerically investigate arrays of metallic nanoparticles deposited on a glass substrate and covered by a liquid-crystal material. Extinction spectra at normal incidence are computed using the finite-difference time-domain method, and we show that by rotating the optical axis around an axis orthogonal to the main direction of illumination, it is possible to tune the resonance of the system according to a simple law. The spectral width of the tunability is studied as a function of different parameters.

Plasmonic electromagnetic hot spots temporally addressed by photoinduced molecular displacement.

We report the observation of temporally varying electromagnetic hot spots in plasmonic nanostructures. Changes in the field amplitude, position, and spatial features are induced by embedding plasmonic silver nanorods in the photoresponsive azo-polymer. This polymer undergoes cis-trans isomerization and wormlike transport within resonant optical fields, producing a time-varying local dielectric environment that alters the locations where electromagnetic hot spots are produced. Finite-difference time-domain and Monte Carlo simulations that model the induced field and corresponding material response are presented to aid in the interpretation of the experimental results. Evidence for propagating plasmons induced at the ends of the rods is also presented.

FDTD modelling of gold nanoparticle pairs in a nematic liquid crystal cell

In this paper, we numerically investigate a grating of gold dimer in a nematic liquid crystal (LC) media. We show that the plasmon resonance exhibits a high sensitivity to the distance between nanoparticles for all orientations of molecules of LCs. The behaviour of plasmon resonance can be described by a simple function called compressed hyperbola that overcomes the limitation of describing this behaviour by the well-known exponential function. Also we show that the orientation of the optical axis leads to an important spectral tunability. We demonstrate then that for certain orientations of the optical axis, we can induce a diffraction coupling featuring an additional narrow resonance peak. Finally near-field properties of the structure are investigated, and we demonstrate that by rotating the director we can control the local field enhancement.