The terahertz frequency range is a relatively unstudied region of the electromagnetic spectrum. However with the emergence of numerous applications for terahertz light in diverse areas such as security scanning, biological imaging, gas spectroscopy and astrophysics there has been considerable recent growth in the volume of research activity in this area. The studies presented in this thesis aim to introduce the physics of surface plasmons to the terahertz frequency range, and on the way to use some of the unique capabilities of terahertz spectroscopy to try and find new information about fundamental surface-plasmon based electromagnetic structures. Four distinct experiments are described in this work, all of them underpinned by the technique of terahertz time-domain spectroscopy (Chapter 2). This is a very powerful and adaptable spectroscopic method which allows us to measure the electric field of pulsed terahertz radiation as a function of time. This in turn allows us to directly extract both phase and amplitude of the terahertz light as a function of frequency, over a broad frequency range. Furthermore, this method of terahertz spectroscopy can be combined with photoexcitation pulses of visible/NIR light which can be used to make dynamic changes to the properties of materials in the terahertz beam. The first experiment reported (Chapter 3) measures the propagation of coupled surface plasmons in a resonant slit cavity. We use terahertz time-domain spec- troscopy to determine the characteristics of the cavity resonances in a semiconductor slit near the surface plasma frequency of the material, where we are able to mea- sure very large red-shifts in the frequency of the cavity resonance. By considering the phase information which can be extracted directly from time-resolved terahertz measurements we are able to link the behaviour of the resonances to the propagation characteristics of the surface plasmon modes inside the slits. The second experiment (Chapter 4) is a more direct measurement of surface plasmons, propagated over the surface of a semiconductor wafer. We show that the electric field of the surface plasmon is confined to a subwavelength region around the surface, and that the confined field is useful for spectroscopy of very thin layers above the surface. We are able to measure films with thickness less than 1/600th of the wavelength of the terahertz light. After these two experiments with confined semiconductor surface plasmons we move on to a pair of experiments looking at terahertz surface modes mediating the transmission of light through holes in metal films. In the initial experiment (Chapter 5) we use the time-domain data from terahertz spectroscopy to determine the role that surface mode lifetime plays in modifying the amplitude and width of Extraor- dinary Optical Transmission (EOT) resonances, which arise from the periodicity of a hole-array lattice. By changing the temperature of the lossy dielectric semicon- ductor substrate we are able to modify the surface mode lifetime, and link this to the resonant transmission characteristics. In Chapter 6 we extend the hole array EOT experiment by making dynamic changes to the propagation of the surface mode which mediates the transmission. This is achieved by photo-exciting the semiconductor substrate inside the holes and forming a thin layer of material with high charge carrier density on the surface. Interaction of the surface mode with the photoexcited region quenches the resonant transmission. We show that by changing the hole size so that the surface-mode mediated transmission pathway predominates in the spectrum it is possible to use optical pulses to modulate the transmission of terahertz radiation with very high efficiency. In the conclusions (Chapter 7) we link together some of the insights and infer- ences which can be drawn from the above results, as well as evaluating the efficacy of the experimental and simulation methodology.

This research was mainly focused on the design and optimization of aperture-based structures to achieve the greatest third harmonic conversion efficiency. It was discovered that by tuning the localized surface plasmon resonance to the fundamental beam wavelength, and by tuning the propagating surface plasmons resonance to the Bragg resonance of the aperture arrays, both the directivity and conversion efficiency of the third harmonic signal were enhanced. The influence of the gap plasmon resonance on the third harmonic conversion efficiency of the aperture arrays was also investigated. The resulted third harmonic generation (THG) from an array of annular ring apertures as a closed loop structure were compared to arrays of H-shaped, double nanohole and rectangular apertures as open-loop structures. The H-shaped structure had the greatest conversion efficiency at approximately 0.5 %. Moreover, it was discovered that the maximum THG did not result from the smallest gap; instead, the gap sizes where the scattering and absorption cross sections were equal, led to the greatest THG. The finite difference time domain (FDTD) simulations based on the nonlinear scattering theory were also performed. The simulation results were in good agreement with the experimental data. Moreover, a modified quantum-corrected model was developed to study the electron tunneling effect as a limiting factor of the THG from plasmonic structures in the sub-nanometer regime. / Graduate / 0544 / 0794 / 0752 / 0756 / mrnezami@gmail.com

Les antennes optiques sont des structures qui permettent de convertir, dans les deux sens, l'énergie électromagnétique entre un faisceau lumineux et une source (ou un absorbeur) localisée en son sein. L'utilisation de résonateurs de taille inférieure à la longueur d'onde permet de réaliser cette fonction de manière efficace, sur une bande spectrale relativement étendue, et d'avoir une antenne compacte.La bonne connaissance des propriétés optiques de ces résonateurs, pris séparément, et de leurs couplages entre eux, est nécessaire pour pouvoir proposer des designs d'antenne efficaces.Dans cette thèse, en se basant sur la décomposition multipolaire des champs et sur la méthode de la matrice-T, on obtient des solutions analytiques rigoureuses pour des résonateurs sphériques et homogènes, dont on tire des modèles simplifiés, intuitifs, et proches de la solution exacte des équations de Maxwell.Entre autre résultats, ces modèles nous ont permis de proposer un design d'antenne optique compacte, directive, à taux de désexcitation et rendement quantique élevés en utilisant une structure hybride métal-diélectrique. Des collaborations avec des expérimentateurs ont permis de valider, d'une part les caractéristiques de chromophores auto-assemblés par ADN (S. Bidault à Paris), et d'autre part, la possibilité d'utiliser plusieurs résonances électriques et magnétiques combinées (supportées par des sphères diélectriques d'indice modéré, n=2,45) pour réfléchir ou bien collecter le rayonnement d'un émetteur dipôle électrique placé à proximité (expérience menée dans le régime micro-ondes par R. Abdeddaim et J-M. Geffrin). / Optical antennas are structures able to convert, in both ways, electromagnetic energy between a light beam and a source (or absorber) placed in the structure. The use of sub-wavelength resonators enables one to realize this function in an efficient way, on relatively broad bandwidths, and to have a compact design. A good understanding of the optical properties of such resonators, taken individually, and of their couplings, is thus necessary in order to propose efficient optical antenna designs. In this manuscript, using a multipole decomposition of the fields and a T-matrix method, we obtain rigorous analytical solutions for spherical, homogeneous resonators, from which we deduce simplified, intuitive models that are still very close to the exact resolution of the Maxwell equations.Among other results, those models enabled us to propose a nanoantenna design that is at once compact, radiative and efficient, by using a hybrid metallo-dielectric structure. Some collaborations with experimental groups enabled us to validate, on the one hand, the optical characteristics of hybrid chromophores that are self-assembled using a DNA template (S. Bidault, Paris), and on the other hand, the possibility of using multiple combined electric and magnetic resonances (supported by dielectric spheres of moderate refractive index, n=2.45) in order to reflect, or more importantly collect, radiation coming from an electric dipole emitter placed nearby (the experiment was realized in the microwave regime by R. Abdeddaim and J-M. Geffrin).

The harvesting of solar light is one of the main challenges in science. The outstanding optical properties of plasmonic in the visible and near-infrared ranges due to the localized surface plasmon resonance (SPR) has emerged as a promising approach for the solar-tochemical energy conversion. Specifically, it has been demonstrated that the SPR excitation in the visible range in silver (Ag) and gold (Au) nanoparticles can drive and accelerate chemical transformations. This field, coined plasmonic catalysis, enables one to merge catalytic and optical properties in the nanoscale and use visible or near-infrared light as a sustainable energy input to accelerate molecular transformations. In the first part of this thesis. we developed Au branched nanostructures to be employed as plasmonic catalysts. In this case, we aimed at investigating the effect of the sharp tips at their surface over their plasmonic catalytic performance, as it is established that tips can concentrate higher electric field enhancements relative to rounded surfaces as a result of the lightning rod effect, which, in turn, can translate into higher plasmonic catalytic performances. Here, the plasmonic-catalytic performances were tested using the SPR mediated oxidation of paminothiophenol and benzylamine as model transformations. While the Ag and Au nanoparticles support LSPR excitation in the visible and near-infrared ranges, their catalytic properties are limited in terms of versatility. Conversely, metals that are important in catalysis, such as palladium Pd, do not support SPR excitation in the visible or near-infrared range. In the second part of this thesis, we developed multimetallic nanoparticle morphologies, composed of both Au and Pd, that enabled us to marry catalytic and plasmonic component in order to address this challenge. We focused on plasmonic core-catalytic shell structures, in which the shell displayed a branched morphology. Parameters such as shell thickness could be controlled, and structure performance relationships were established towards the methanol electro-oxidation under plasmonic excitation. / O aproveitamento da luz solar é um dos principais desafios da ciência. As excepcionais propriedades óticas plasmônicas nas regiões do visível e do infravermelho próximo, devido a ressonância plasmônica de superfície localizada (SPR), surgiram como uma abordagem promissora para conversão de energia solar em energia química. De maneira mais específica, vem sendo demonstrado que a excitação SPR na região do visível em nanopartículas de prata (Ag) e ouro (Au) podem conduzir e acelerar transformações químicas. Esse campo, chamado catálise plasmônica, permite a fusão de propriedades óticas e catalíticas na nanoescala e a utilização de luz visível ou infravermelha próxima como uma fonte de energia para acelerar transformações moleculares. Na primeira parte desta dissertação, nós desenvolvemos nanoestruturas de ouro ramificadas para serem empregadas em catálise plasmônica. Neste caso, nosso foco era investigar o efeito de pontas afiadas em sua superfície sobre seu desempenho catalítico plasmônico, visto que está bem estabelecido que pontas podem concentrar maiores intensificações de campo elétrico em relação a superfícies arredondadas como resultado do \"efeito para-raios\" o que, por sua vez, pode se traduzir em maiores desempenhos em catálise plasmônica. O desempenho da catálise plasmônica foi testado através da oxidação mediada por SPR do p-aminotiofenol e da benzilamina como reações modelo. Contudo, enquanto nanopartículas de prata e ouro apresentam excitação SPR nas regiões do visível e infravermelho próximo, suas propriedades catalíticas são limitadas em termos de versatilidade. Por outro lado, metais que são importantes em catálise, como o paládio, não apresentam excitação SPR no visível e infravermelho próximo. Por isso, na segunda parte desta dissertação, nós desenvolvemos nanopartículas multimetálicas, compostas de Au e Pd, que nos permitem unir os componentes catalíticos e plasmônicos com o objetivo de enfrentar este desafio. Nós focamos em estruturas do tipo core-shell, com núcleos plasmônicos e cascas catalíticas, na qual a casca apresenta morfologia ramificada. Paramêtros como a espessura da casca puderam ser controlados, e a relação estruturaperformance foi estabelecida através da eletro-oxidação do metanol sobre excitação plasmônica.

Diminuer l'épaisseur de la couche d'absorbeur est une solution attractive pour produire des cellules photovoltaïques à coût réduit. Cela permet également de réduire la quantité de matériau actif utilisé ainsi que d'améliorer la collection du courant dans la cellule. Cette thèse s'est focalisée sur la conception de nanostructures pour exalter l'absorption de la lumière dans des couches de semiconducteur d'épaisseur réduite et ainsi proposer des cellules ultraminces efficaces.Dans un premier temps, nous avons proposé une approche originale pour piéger la lumière dans une cellule ultra-fine (≤ 100 nm) en silicium amorphe. Un réseau métallique est placé en face avant de la cellule déposée sur un miroir métallique afin d'obtenir une absorption multi-résonante large bande pour les deux polarisations de la lumière. Nous proposons aussi d'utiliser le réseau métallique comme une électrode transparente alternative afin de réduire les pertes optiques dans le contact avant de la cellule. Une analyse numérique approfondie des mécanismes résonants en jeu a été menée ainsi que la fabrication et la caractérisation optique de démonstrateurs.Dans un deuxième temps, nous avons appliqué ce concept de contact avant multi-résonant à des couches ultra-fines en arsenure de gallium (GaAs). Nous avons montré numériquement et expérimentalement le potentiel d'une nanogrille métallique bi-dimensionnelle pour le confinement efficace de la lumière dans 25 nm de GaAs.Enfin, nous avons étudié la possibilité de réduire l'épaisseur de cellules en silicium cristallin d'un facteur 10 à 100 par rapport à l'état de l'art. Nous avons développé un procédé pour transférer des couches de silicium cristallin de quelques microns d'épaisseur épitaxiées par PECVD sur un substrat hôte bas coût. Nous avons également travaillé à la structuration contrôlée de nanopyramides en vue d'un piégeage optique efficace dans ces couches minces. / Reducing the absorber thickness is an attractive solution to decrease the production cost of solar cells. Furthermore, it allows to reduce the amount of material needed and improve the current collection in the cell. This thesis has been focused on the design of nanostructures to enhance light absorption in very small semiconductor volumes in order to achieve efficient ultra-thin solar cells. First, we have proposed an original light-trapping concept for ultra-thin amorphous silicon (a-Si:H) solar cells. A one-dimensional metallic grating is patterned on the front surface of the cell deposited on a metallic mirror. Broadband multi-resonant absorption has been demonstrated for both light polarizations. The metallic grating is also used as an alternative transparent electrode in order to reduce optical losses in the front contact. A detailed analysis of the multi-resonant absorption mechanism has been carried out through numerical calculations. The fabrication and optical characterization of ultra-thin a-Si:H solar cells with metallic gratings have validated the multi-resonant approach.Second, we have proposed a design with a two-dimensional metallic grid as a resonant front contact for very thin (25 nm) gallium arsenide (GaAs) layers. We have shown through the design and fabrication of a proof-of-concept structure the potential of metallic nanogrids to confine efficiently light absorption with an ultra-thin GaAs layer.Finally, advanced light-trapping structures could also allow a thickness reduction of crystalline silicon wafers of a factor 20 to 100 with respect to state-of-the-art cells. We have developed a process to transfer micron-thick epitaxial crystalline silicon (c-Si) layers onto a low-cost host substrate. Inverted nanopyramids have also been fabricated in crystalline silicon in order to achieve a broadband anti-reflection effect. It opens promising perspectives towards the realization of double-sided nanopatterned ultra-thin c-Si cells.

A plasmônica tem chamado atenção nos últimos anos como um candidato viável para substituir a indústria eletrônica, assim como interação dos plásmons com a matéria devido a suas propriedades exóticas. O confinamento destes plásmons de superfície em nanoestruturas metálicas fabricadas com técnicas de litografia óptica, eletrônica e de íons cada vez mais avançadas, abriu a possibilidade de desenvolver vários modelos de dispositivos ópticos que trabalham na região do visível. Além disso, o estudo da interação de plásmons poláritons de superfície com emissores quânticos nas proximidades de nanoestruturas metálicas permite manipular as propriedades tanto dos plásmons como dos emissores quânticos. Tanto a preparação como a análise de amostras em plasmônica necessitam de técnicas capazes de investigar sistemas em nanoescala. Neste trabalho, investigamos a interação de plásmon poláritons confinados numa superfície de ouro com átomos artificiais, i.e. os emissores quânticos são pontos quânticos numa matriz de InAs/GaAs. Para isso, empregamos a análise da interação dos plásmons confinados numa grade metálica, com dimensões características abaixo do comprimento de onda da luz utilizada, assim como um sistema simples composto por uma na camada de ouro capaz de confinar plásmons em duas dimensões. A análise da interação com os estados de energia dos éxcitons nos pontos quânticos foi feita empregando medidas de micro-fotoluminescência a 77K e medidas de tempo de vida. Nos sistemas compostos pelas grades metálicas, observamos que é possível manipular a relação do espectro de luminescência correspondente a cada estado de energia do éxciton. Já no sistema composto pelo filme metálico simples, foi possível modificar o tempo de vida do estado fundamental do éxciton apenas modificando o cap layer da camada de pontos quânticos. / Plasmonics has drawn attention in recent years as a viable candidate to replace the electronics industry, as well as the interaction of plasmons with matter due to its exotic properties. The confinement of these surface plasmons in metal nanostructures made of increasingly advanced optical, electronic and ionic lithography techniques, opened the possibility of developing various models of optical devices working in the visible spectrum. Moreover, the study of interaction of surface plasmon polaritons with quantum emitters nearby metallic nanostructures opens a path to manipulate the properties of both plasmons and the quantum emitters. Both the preparation and analysis of samples in plasmonics require techniques capable of investigating nanoscale systems. In this thesis, we investigate the interaction of plasmon polaritons confined to a golden metallic surface with artificial atoms, i.e. quantum emitters consisting of quantum dots in a matrix of InAs/GaAs. For this, we used the analysis of the interaction of plasmons confined in a metallic grating with characteristic dimensions below the wavelength of light used, as well as a simple system composed of a thin gold layer which can confine plasmons in two dimensions. The analysis of the interaction with the exciton energy states in quantum dots was made using micro-photoluminescence measurements at 77 K and lifetime measurements. In systems composed by metal gratings, we note that it is possible to manipulate the relationship of the corresponding luminescence spectrum for each exciton energy state. In the system composed of the simple metal lm, it was possible to modify the ground state lifetime of the exciton only modifying the cap layer of the quantum dot layer.

Thesis advisor: Michael J. Naughton / The nanoscale morphology of metals can enable special functionality in plasmonic and electrochemical devices, with applications in energy conversion and storage, sensors, and computers. In particular, sharp edges on metal nano and microstructures are understood to affect the density of electrons on the metal surface. The associated concentration of electric field can concentrate surface plasmon polaritons (SPPs) and enable waveguiding of the SPPs, as we show in this thesis for sharp ridges along aluminum nanowires. Also important is the presence of facets on the metal structures, which determines the orbitals that electrons occupy on the metal surface. Changes in both the electron density and orbitals can affect the binding of molecules to the metal, which can improve reaction kinetics in catalysis. We demonstrate this on gold dendrite and plate electrocatalysts for CO2 electrolysis. Regarding metal nanostructure fabrication, electrochemical deposition and corrosion have demonstrated promising control over the morphology, including the topography, crystallinity, grain boundaries, and crystal faceting. This is important, because existing methods for metal nanostructure fabrication can only produce a circumscribed assortment of morphologies. In contrast, semiconductors and insulators have many new deposition techniques that produce a wide range of controlled morphologies. Of further appeal, electrochemical techniques are solution-based and typically operate at room temperature and pressure, allowing facile scale-up to industrial production. Here we demonstrate and discuss the mechanisms of two new techniques, which produce the aluminum nanowires and gold dendrites and plates discussed above. / Thesis (PhD) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Thesis advisor: Krzysztof Kempa / Thesis advisor: Zhifeng Ren / In the past few decades, accompanied by the fascinating development of micro- and nano-fabrication techniques, the successful integration of subwavelength optics and multilayer structures has led to a number of remarkable discoveries. In this work, I present both experimental and theoretical investigations of the optics of thin metallic films with micro-/nano-scale perforations in the UV-VIS-IR ranges. Different fabrication techniques are employed, including nanosphere lithography, grain boundary lithography, crack templates, and sintered nanoparticles. The optical properties these films are studied, revealing important relation between optical response and the film geometry. This includes the evolution of plasmonic resonances in a series of periodic arrays of holes in a metallic film, with hole sizes increasing gradually until an array of islands is achieved. This evolution is an analog of the percolation problem, and critical phenomena are observed at the percolation threshold. Multilayer broad-band electromagnetic absorbers are also designed and fabricated based on the study of these perforated films. Parallel with these observations, an analytical coherence model is proposed to bridge the subwavelength and superwavelength limits. Such a model also provides an alternative way to handle thin random structures, avoiding large quantity of numerical computation. These studies can find applications in the design of sensors, ultrathin solar cells and transparent electrodes, as well as in applications where random structures are widely used. / Thesis (PhD) — Boston College, 2014. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Thesis advisor: Krzysztof Kempa / In my work presented in this dissertation, I have focused on simulation studies of light interaction with nanostructures made of metals and dielectrics. Of particular interest have been plasmonic effects. The structures included the wire and coaxial nanowaveguides, as well as periodic arrays of planar quasi-triangles, and periodic arrays of nanoholes in thin metallic films. In the nanowaveguides I focused on plasmon polariton modes which resemble the TEM modes propagating in the corresponding conventional radio transmission lines. This collaborative research, involving an experimental effort, showed how the nanoscopic plasmon polariton modes reduce in the retarded limit to the TEM modes, and in the non-retarded limit to the corresponding surface plasmon modes. My simulations explained details of recent experimental results involving plasmonic waveguiding in metallic nanowires. Similar results have been obtained for nanocoaxial waveguides. My simulations of the optical absorption in the arrays of nano quasi-triangles, recently observed experimentally, helped identify those as due to Mie plasmonic resonances in these nanoparticles. They also explained the peak shifts in terms of the 2D surface plasmon dispersion, and the plasmon momentum quantization. In the study of the arrays evolution from holes to quasi-triangles, my simulations provided the clue to the critical behavior of the peak position for structures approaching the percolation threshold (the transitional structure in the series, for which film resistance diverges), and allowed to identify the series of structures as an analog of the percolation threshold problem. Finally, I have simulated optical performance of nanorod arrays (or multi-core nanocoax), which have been employed as platform for novel solar cells. My simulations have been employed to predict and optimize these cells. My work resulted in 5 publications and 2 manuscripts in preparation. / Thesis (PhD) — Boston College, 2011. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Working with nanoscale optics requires methods and equipment designed for the purpose. This thesis describes the development of techniques and a system for performing highly localised spectroscopy. The system consists of a nanonics multiview 2000 scanning near-field optical microscope, a grating spectrometer and a photonic crystal fibre supercontinuum light source. Discussion of the microscope includes its modes of operation and development of software to collect and analyse data. In order to demonstrate the setup, an example of localised spectroscopy is presented in the form of an investigation of hollow core photonic crystal fibre. Taking spectra of the components of the cladding of these fibres makes it possible to investigate the origins of bandgap guidance. A core focus of nanoscale optics is the interaction of light with metal structures. This field is called plasmonics. Fabrication of structures is presented and requires special facilities and processes. These processes are both time consuming and expensive, both factors that emphasise the need for prior modelling. Forward difference time domain modelling of a proposed structure comprising of a concentrically arranged ring and disk is explored using home written code and a commercial package called CST Microwave Studio. The investigation of this concentric design through modelling shows a very highly localised field enhancement which can be engineered to have a narrow spectral resonance in the near infrared. The interaction of the two components which govern this resonance is explained using a theory called plasmon hybridization. Once the optical behaviour of small metal objects is understood they can be used in other ways. An example of this is shown in Porous Silicon. As a material it provides an excellent template for formation of metal nano-particles. Embedded in a high surface area network of silicon these particles can be used as very effcient catalysts.