We have numerically investigated the influence of a nanoscale silicon tip in proximity to an illuminated gold nanoparticle. We describe how the position of the high-permittivity tip and the size of a nanoparticle impact the absorption, peak electric field and surface plasmon resonance wavelength under different illumination conditions. We detail the finite element method (FEM) approach we have used for this, whereby we specify a volume excitation field analytically and calculate the difference between this source field and the total field (i.e., scattered-field formulation). We show that a nanoscale tip can locally enhance the absorption of the particle as well as the peak electric field at length scales far smaller than the wavelength of the incident light.

The interaction of an evanescent wave and plasmonic nanostructures are simulated in Finite Element Method. Specifically, the optical absorption cross section (Cabs) of a silver nanoparticle (AgNP) and a gold nanoparticle (AuNP) in the presence of metallic (gold) and dielectric (silicon) atomic force microscope (AFM) probes are numerically calculated in COMSOL. The system was illuminated by a transverse magnetic polarized, total internally reflected (TIR) waves or propagating surface plasmon (SP) wave. Both material nanoscale probes localize and enhance the field between the apex of the tip and the particle. Based on the absorption cross section equation the author was able to demonstrate the increment of absorption cross section when the Si tip was brought closer to the AuNP, or when the Si tip apex was made larger. However, the equation was not enough to predict the absorption modification under metallic tips, especially for a AgNP's Cabs; neither it was possible to estimate the optical absorption based on the localized enhanced field caused by a gold tip. With the help of the driven damped harmonic oscillator equation, the Cabs of nanoparticles was explained. In addition, this model was applicable for both TIR and Surface Plasmon Polaritons illuminations. Fitting the numerical absorption data to a driven damped harmonic oscillator (HO) model revealed that the AFM tip modifies both the driving force (F0), consisting of the free carrier charge and the driving field, and the overall damping of the oscillator beta. An increased F0 or a decreased beta will result in an increased Cabs and vice versa. Moreover, these effects of F0 and beta can be complementary or competing, and they combine to either enhance or suppress absorption. Hence, a significantly higher beta with a small increment in F0 will result in an absorption suppression. Therefore, under a Si tip, Cabs of a AuNP is enhanced while Cabs of a AgNP is suppressed. In contrast, a Au tip suppresses the Cabs for both Au and Ag NPs. As an extension of this absorption model, further investigation of the guided mode and a close by nanostructure is proposed, where the scattered wave off the structure attenuates the guided mode with destructive interference.

Dans ce travail, les propriétés des réseaux diffractifs ordonnés de nanoparticules d'or sont étudiées numériquement et expérimentalement. Ces résonances sont beaucoup plus étroites que celles observées dans le cas d'une nanoparticule isolée. D'après les simulations numériques, deux régimes distincts de réponse sont identifiés, l'un correspond à l'anomalie de Rayleigh (RA) l'autre au mode plasmon de réseau 2D (LPM). Dans la partie expérimentale nous avons fabriqué des réseaux de nanoparticules d'or en utilisant la lithographie d'électronique. La transmission spectrale a été mesurée dans le domaine optique pour caractériser ces réseaux. Toutes les caractéristiques essentielles des spectres expérimentaux sont en bon accord avec les simulations numériques. Les distributions du champ électrique pour différents paramètres de réseau sont étudiées pour obtenir le maximum d'augmentation du champ à la surface de la nanoparticule. L'excitation des résonances plasmon dans les réseaux diffractifs de nanoparticules d'or en condition asymétrique de l'indice de réfraction est examinée expérimentalement. L'excitation des modes plasmon à profil spectral étroit dans l'environnement asymétrique a été expérimentalement vérifiée. La possibilité d'accorder la longueur d'onde de ces résonances dans le proche infrarouge en changeant les paramètres structurels des réseaux périodiques en combinant taille et forme des nanoparticules est discutée. Ces résultats sont importants pour les applications telles que les spectroscopies en champ électrique exalté et la détection en biologie ou en chimie. / The properties of ordered diffractive arrays of gold nanoparticles are studied numerically and experimentally. Using numerical simulations I identify, two distinct regimes of lattice response, associated with two-characteristic states of the spectra: Rayleigh anomaly and lattice plasmon mode. In experimental part gold nanoparticle arrays were fabricated using e-beam lithography. Spectroscopic transmission measurements then were carried out to optically characterize these arrays. All the essential features of the experimental spectra were reproduced well by numerical simulations. Electric field distributions for different lattice parameters are studied in order to maximize the enhancement of electric field at the nanoparticle surface. The excitation of plasmon resonances in diffractive arrays of gold nanoparticles placed in asymmetric refractive index environment is examined experimentally. The excitation of the plasmon modes with narrow spectral profile in asymmetric environment was experimentally verified. The ability to tune the wavelength of these resonances in the near infrared range by varying the structural parameters of the periodic arrays in combination with size and geometry of the constituent nanoparticles is discussed. The presented results are of importance for the field enhanced spectroscopy as well as for plasmonic bio and chemical sensing.