Dark modes and Fano resonances in plasmonic clusters excited by cylindrical vector beams

Control of the polarization distribution of light allows tailoring the electromagnetic response of plasmonic particles. By rigorously extending the generalized multiparticle Mie theory, we show that focused cylindrical vector beams (CVB) can be used to efficiently excite dark plasmon modes in nanoparticle clusters. In addition to the small radiative damping and large field enhancement associated to dark modes, excitation with CVB can give place to unusual phenomenology like the formation of electromagnetic cold spots and the generation of Fano resonances in highly symmetric clusters. Overall, the results show the potential of CVB to tailor the plasmonic response of nanoparticle clusters in a unique way

Optical forces induced by metal nanoparticle clusters

The strong field localization generated between closely placed metal particles excited by electromagnetic radiation induces intense forces on small polarizable objects. In this study we investigate the optical forces that can be generated in the vicinity of metal nanoparticle clusters using fully electrodynamic numerical simulations. The influence of the cluster configuration as well as of the excitation parameters is analyzed

Procedures for the measurement of the extinction cross section of one particle using a Gaussian beam

Two procedures for the measurement of the extinction cross section (ECS) of one particle using a slightly focused Gaussian beam have been introduced and numerically tested. While the first one relies on previously introduced ideas and has close connection with the optical theorem, the second procedure is new and is mostly related with light measurements where the detector collects much of the energy of the incident beam. Both procedures prove to be valid and somehow complementary up to particle sizes of the order of the beam waist, thus enlarging the capability of simple measurement set-ups based on Gaussian beams for the estimation of the ECS of one particle

Light scattering by coupled oriented dipoles: decomposition of the scattering matrix

We study the optical response of two coupled oriented dipoles with the dimer axis perpendicular to the wave vector of light by analyzing how their scattering matrix can be decomposed. The scattering matrix can be written as a linear combination of three terms with a clear physical meaning: one for each particle and another that is responsible for the coupling and that vanishes for noninteracting or distant particles. We show that the interaction term may generate optical activity for certain scattering directions and that this effect manifests itself mostly in the near field. This simple and intuitive theory based on matrix and vector states of oriented dipoles also describes hybridization processes and Fano resonances. The decomposition method can be also formulated in terms of a hybrid basis that allows us to quantitatively determine the individual contribution of the in-phase and out-of-phase coupling modes to the overall intensity. Our method can help to understand the optical response of more complex nanostructures that can be decomposed into dipole terms. The results are illustrated in gold nanoantenna dimers which exhibit a strong dipolar resonanc

Metal island film-based structures for sensing using spectrophotometry and ellipsometry

Metal island films (MIF) are good candidates for sensors due to the strong sensitivity of the localised surface plasmon resonance to the environment refractive index. The strong near field enhancement in the vicinity of the island surface can be even higher if a metal layer (ML) is placed close to a MIF. Structures containing MIF with and without ML are prepared and sensitivities of spectrophotometric and ellipsometric features of the measurements are compared. It is shown that simple MIF is preferable for ellipsometry-based sensing and the one including ML in the case of spectrophotometric measurements

Relation between 2D/3D chirality and the appearence of chiroptical effects in real nanostructures.

The optical activity of fabricated metallic nanostructures is investigated by complete polarimetry. While lattices decorated with nanoscale gammadia etched in thin metallic films have been described as two dimensional, planar nanostructures, they are better described as quasi-planar structures with some three dimensional character. We find that the optical activity of these structures arises not only from the dissymmetric backing by a substrate but, more importantly, from the selective rounding of the nanostructure edges. A true chiroptical response in the far-field is only allowed when the gammadia contain these non-planar features. This is demonstrated by polarimetric measurements in conjunction with electrodynamical simulations based on the discrete dipole approximation that consider non-ideal gammadia. It is also shown that subtle planar dissymmetries in gammadia are sufficient to generate asymmetric transmission of circular polarized light

Detection and characterization of single nanoparticles by interferometric phase modulated ellipsometry

We introduce a new measurement system called Nanopolar interferometer devoted to monitor and characterize single nanoparticles which is based on the interferometric phase modulated ellipsometry technique. The system collects the backscattered light by the particles in the solid angle subtended by a microscope objective and then analyses its frequency components. The results for the detection of 2 μm and 50 nm particles are explained in terms of a cross polarization effect of the polarization vectors when the beam converts from divergent to parallel in the microscope objective. This explanation is supported with the results of the optical modelling using the exact Mie theory for the light scattered by the particles

Caracterització òptica de materials en estructures multicapa per a filtres interferencials

[cat] L'estudi de les propietats òptiques de materials és una de les àrees de recerca més habituals en l'àmbit de les capes primes. L'interès es troba sovint motivat per l'ample ventall d'aplicacions (com els filtres òptics interferencials) on aquestes propietats tenen un paper primordial. Com a conseqüència, els mètodes de caracterització òptica de materials han estat intensament desenvolupats, tant a nivell de les tècniques experimentals com de les modelitzacions teòriques i els tractaments numèrics. L'objectiu d'aquesta Tesi ha estat el desenvolupament i la utilització pràctica d'un programari de càlcul per a la caracterització òptica de materials en estructures multicapa. El programari pretén ser d'abast general, però ha estat dissenyat principalment per a l'estudi de materials usats en filtres interferencials. En termes generals, les mostres que s'analitzen són estructures formades per una o vàries làmines primes de materials isòtrops dipositades sobre un substrat. Les propietats òptiques de les mostres es descriuen amb un nombre reduït de paràmetres (en general els gruixos de les capes i els paràmetres que defineixen les relacions de dispersió de les constants òptiques dels materials). Per a la caracterització de les mostres s'utilitzen mesures espectroscòpiques fotomètriques i el·lipsomètriques. La present Memòria està estructurada en sis Capítols que es poden agrupar en dues parts. Així, els dos primers Capítols són de caire fonamentalment teòric i presenten els conceptes bàsics necessaris per al desenvolupament del programari de càlcul. D'aquesta manera, al Capítol 1 es recullen els models matemàtics per a la parametrització de les constants òptiques dels materials. S'hi estudien diferents formulacions que determinen la dependència de les constants òptiques amb la freqüència de la radiació electromagnètica (models dispersius). Es pretén donar una visió general dels models habitualment utilitzats en el domini espectral òptic, però amb especial incidència en els models que s'han emprat en la Tesi. El segon Capítol aborda els aspectes més rellevants del mètode de caracterització desenvolupat. La primera part del segon Capítol analitza diferents models matemàtics per al càlcul de les propietats òptiques de les estructures multicapa. Seguidament es fa una breu descripció de les tècniques experimentals utilitzades: l'espectrofotometria i l'el·lipsometria. Finalment, es discuteixen aspectes computacionals dels programes de càlcul: la modelització matemàtica de la mostra, la definició d'una funció de mèrit i la tria d'un algoritme d'ajust. La segona part de la Memòria consisteix en un recull d'estudis i aplicacions realitzats amb els programes de càlcul. A cada Capítol s'exposen diferents exemples que presenten com a denominador comú una determinada problemàtica en la caracterització. En lloc d'agrupar els exemples segons el tipus de material o aplicació, s'ha optat per aquesta organització perquè il·lustra de forma clara diversos conceptes clau d'acord amb la nostra recerca. Així, el Capítol 3 conté un seguit d'exemples que posen de relleu la importància del model dispersiu dels materials per a la correcta descripció de les mesures experimentals. Al Capítol 4 es posa de manifest la necessitat d'una modelització realista de l'estructura física de la mostra. El Capítol 5 descriu diverses estratègies d'ajust per a casos en què la inversió numèrica de les dades experimentals és especialment complexa. Finalment, el Capítol 6 introdueix l'anàlisi multimostra com a eina de caracterització que augmenta la fiabilitat dels resultats. Finalment cal esmentar que del programari de càlcul desenvolupat se n'ha realitzat una versió Graphic User Interface formada per un seguit de programes que poden ser utilitzats de forma senzilla per a usuaris treballant en laboratoris de dipòsit de capes primes. A l'Apèndix es presenta el NKD Software, que descriu breument les característiques i el funcionament d'aquests programes.[eng] The study of the optical properties of materials is one of the most common research areas in thin film technology. The interest is often motivated by the large number of applications where optical properties play an important role, like the interferential optical coatings. Consequently, the methods for the optical characterization of materials in thin film phase have been intensively developed, both the experimental techniques and the numerical approaches. The aim of this work has been the development and application of numerical methods for the optical characterization of materials in multilayer structures and its implementation in a software package. The software consists of a general purpose program, although it has been mainly designed for the study of materials used in interferential optical coatings. The analyzed samples are structures consisting of one or several layers of isotropic materials deposited on a substrate. The optical behaviour of the samples is described by a reduced number of parameters (in general, the thicknesses of the layers and the parameters defining the dispersion relations for the optical constants of the materials). Spectrophotometric and ellipsometric measurements are considered for the characterization of the samples. The dissertation is divided in two parts: the first one describes the theoretical aspects of the development of the characterization method and the software and the second part illustrates the application of the program to the study of several types of samples. Thus, the first chapter contains the theoretical background of the dispersion models that are used to describe the optical properties of the analyzed materials. In the second chapter, different numerical methods to calculate the optical properties of multilayer structures are compared. After that, a brief description of the used experimental techniques is given. Finally, the chapter describes the proposed characterization method and its implementation in a software package. The second part of the dissertation consists of a collection of studied cases. In each chapter, the different studies present a common problem or difficulty in the procedure of characterization. This organization has been preferred rather than a standard classification of the studies according the type of material or sample, because it illustrates in a better way the typical problems involved in the optical characterization of materials. Thus, Chapter 3 shows the relevance of an accurate choice of the dispersion model for the optical constants of the investigated materials. In Chapter 4, the importance of the physical structure of the sample is illustrated. Strategies for the cases where the numerical difficulties arise during the fitting procedure are shown in Chapter 5. Finally, in Chapter 6 is demonstrated the utility of the simultaneous analysis of different samples. An Appendix depicts the main features of the developed software package

Dark modes and Fano resonances in plasmonic clusters excited by cylindrical vector beams

Control of the polarization distribution of light allows tailoring the electromagnetic response of plasmonic particles. By rigorously extending the generalized multiparticle Mie theory, we show that focused cylindrical vector beams (CVB) can be used to efficiently excite dark plasmon modes in nanoparticle clusters. In addition to the small radiative damping and large field enhancement associated to dark modes, excitation with CVB can give place to unusual phenomenology like the formation of electromagnetic cold spots and the generation of Fano resonances in highly symmetric clusters. Overall, the results show the potential of CVB to tailor the plasmonic response of nanoparticle clusters in a unique way

Procedures for the measurement of the extinction cross section of one particle using a Gaussian beam

Two procedures for the measurement of the extinction cross section (ECS) of one particle using a slightly focused Gaussian beam have been introduced and numerically tested. While the first one relies on previously introduced ideas and has close connection with the optical theorem, the second procedure is new and is mostly related with light measurements where the detector collects much of the energy of the incident beam. Both procedures prove to be valid and somehow complementary up to particle sizes of the order of the beam waist, thus enlarging the capability of simple measurement set-ups based on Gaussian beams for the estimation of the ECS of one particle