abstract: This thesis summarizes modeling and simulation of plasmonic waveguides and nanolasers. The research includes modeling of dielectric constants of doped semiconductor as a potential plasmonic material, simulation of plasmonic waveguides with different configurations and geometries, simulation and design of plasmonic nanolasers. In the doped semiconductor part, a more accurate model accounting for dielectric constant of doped InAs was proposed. In the model, Interband transitions accounted for by Adachi's model considering Burstein-Moss effect and free electron effect governed by Drude model dominate in different spectral regions. For plasmonic waveguide part, Insulator-Metal-Insulator (IMI) waveguide, silver nanowire waveguide with and without substrate, Metal-Semiconductor-Metal (MSM) waveguide and Metal-Insulator-Semiconductor-Insulator-Metal (MISIM) waveguide were investigated respectively. Modal analysis was given for each part. Lastly, a comparative study of plasmonic and optical modes in an MSM disk cavity was performed by FDTD simulation for room temperature at the telecommunication wavelength. The results show quantitatively that plasmonic modes have advantages over optical modes in the scalability down to small size and the cavity Quantum Electrodynamics(QED) effects due to the possibility of breaking the diffraction limit. Surprisingly for lasing characteristics, though plasmonic modes have large loss as expected, minimal achievable threshold can be attained for whispering gallery plasmonic modes with azimuthal number of 2 by optimizing cavity design at 1.55µm due to interplay of metal loss and radiation loss. / Dissertation/Thesis / M.S. Electrical Engineering 2014

A plasmônica é a área que faz a junção entre fotônica e nanoestruturas. As implicações tecnológicas resultantes do acoplamento entre campos eletromagnéticos e oscilações eletrônicas em um material condutor fazem desta área uma das mais excitantes da óptica atualmente. Neste contexto, o objetivo deste trabalho é o projeto, fabricação e caracterização de nanoestruturas metálicas visando aplicações em óptica difrativa, incluindo filtros e lentes. Inicialmente, uma extensa revisão bibliográfica permitiu definir quais tipos de estruturas seriam abordadas, levando em conta tanto a capacidade computacional para fazer a modelagem numérica quanto a infraestrutura necessária na fabricação dos elementos. A primeira estrutura analisada foi um filtro óptico baseado em ressonância de modo guiado e ressonância plasmônica. Foram projetados e fabricados três filtros operando no azul, verde e vermelho. Resultados experimentais mostraram eficiência acima de 80% e largura de banda em torno de 20 nm, consideravelmente menor que os ~60 nm obtidos previamente na literatura considerando estrutura semelhante. Foi possível verificar as cores puras associadas à ressonância de modo guiado. Além disso, foi demonstrado como gerar as três cores primárias - azul, verde e vermelho - usando apenas o filtro vermelho. A segunda estrutura proposta consiste em uma lente tipo zonas de Fresnel integrada com um filme metálico. Resultados numéricos identificaram uma estrutura ressonante do tipo Fabry-Perot que possibilita uma redução dos lóbulos laterais gerada pela lente por um fator 3.0 na polarização TM e 4.8 na polarização TE. A estrutura foi fabricada usando litografia por nanoimpressão. Por fim, a terceira estrutura analisada foi um holograma binário baseado em metassuperfície, cuja célula básica é composta de um ressoador tipo nanorod. Foi proposta uma geometria na qual a diferença de fase entre os elementos é igual a π independente do comprimento de onda. Assim, o holograma pode operar em uma faixa espectral definida pela largura de banda transmitida. É descrito o inicio da fabricação do elemento usando litografia por feixe de elétrons. / Plasmonics is a field of study that merge photonics and nanostructures. The advanced technological implications makes it one of the most exciting field in Optics in current days. Therefore the objective of this study is the design and fabrication of metallic nanostructures aiming at applications in diffractive optics. Firstly, an extensive literature review allowed to define what types of structures would be addressed, taking into account both software simulations and the require infrastructure for the elements\' fabrication. The first analyzed structure was an optical color filter based on guided mode resonance and surface plasmon resonance. Three filters, operating in blue, green and red, were designed and fabricated using interferometric lithography. Experimental results show above 80% efficiency and ~20 nm bandwidth, which is significantly smaller than ~60 nm previously obtained in the literature with similar structures. It was possible to show the pure colors associated with the modal resonance. Furthermore, it was shown how to obtain the primary red, blue, and green colors using only the red filter. The second structure proposed consists of Fresnel zones plates integrated with a metallic film. Numerical results show a resonant structure which enables side lobe reduction by a factor 3.0 in the TM polarization and 4.8 in the TE polarization. This structure was fabricated using nanoimprint lithography. The third analyzed structure was a binary hologram based on metasurface whose basic cell is composed of a nanorod metallic resonator. The phase difference between two elements is equal to π, regardless of the wavelength; thus, the hologram operates in a spectral band defined by transmitted bandwidth. The first steps of its fabrication process using electron beam lithography are presented and described.

O interesse por aplicações que utilizam efeitos de plásmons poláritons de superfície (SPP) vem crescendo, pois as ondas SPPs apresentam enorme potencial no desenvolvimento de filtros e biossensores ópticos. A sensibilidade da ressonância de plásmons em nanoestruturas permite o estudo em tempo real de variações mínimas em índice de refração, solutos e antígenos. Neste trabalho foram aplicadas técnicas de nanofabricação (litografia por feixe de elétrons e íons) para o desenvolvimento de estruturas plasmônicas e sua posterior caracterização. As estruturas foram utilizadas para verificar propriedades de absorção e fluorescência em moléculas opticamente ativas - Porfirina e Rodamina 6G. As estruturas - conjuntos de fendas e matrizes de buracos circulares com diversos períodos - foram fabricadas em um filme fino de ouro (Au) sobre substrato de vidro (Borofloat 33 - Schott), usando um feixe de íons de Gálio (FEI Quanta Quanta 3D 200i). A transmissão óptica foi estudada na região de 400nm a 900nm (VIS-NIR). Os resultados experimentais foram comparados com simulações computacionais. O estudo da absorção molecular da porfirina foi conduzido observando-se a variação na intensidade da transmissão. Ao alterar a concentração da porfirina sobre as estruturas, foi possível caracterizar a curva de absortividade ε(λ) da porfirina para concentrações entre 100 μg/ml e 500 μg/ml em quantidades mínimas de analito (20 μl). A técnica de microscopia confocal foi empregada no estudo da fluorescência da Rodamina 6G diluída num filme fino de PMMA sobre as estruturas. Ao avaliar a fluorescência da Rodamina 6G na reflexão das estruturas, observou-se o efeito de quenching devido a emissão de plásmons. Os resultados obtidos poderão ser utilizados de apoio a trabalhos futuros, desenvolvidos em plasmônica aplicada a biossensores. / The interest in applications that use the effects of surface plasmon polaritons (SPP) has been increasing. SPPs waves have an enormous potential for the construction of optical filters and biosensors. The sensitivity of plasmon resonance in nano-structures allows studying in real-time minimal variations in the refractive index, solutes and antigens. In this work, we have studied nanofabrication techniques (electron and ion beam lithography) and the characterization of plasmonic structures. Plasmonic effects were used as biosensors of absorption and fluorescence in optically active molecules - Porphyrin and Rhodamine 6G. The structures - sets of slits and arrays of circular holes with different periods - were manufactured in gold (Au) thin film on a glass substrate (Borofloat 33 - Schott) using a galium ion beam equipment (FIB FEI Quanta Quanta 3D 200i). Optical transmission was studied in the region of 400 nm to 900 nm (VIS-NIR). The characterization of structures was realized used the Ocean Optics USB-2000 spectrometer. The experimental results were compared to computer simulations. The study of molecular absorption of porphyrin was conducted by observing the variation in intensity of transmission. By changing the porphyrin concentration in the structures, it was possible to characterize the porphyrin absorptivity curve ε(λ) in concentrations between 100 μg/ml and 500 μg/ml in minimum amounts of analyte (20 μl). Confocal microscopy was used to study the fluorescence of Rhodamine 6G on plasmonic structures. The plasmon quenching effect was observed in the evaluation of the fluorescence of Rhodamine 6G in the reflection of the structures. The results will support future works linking plasmonics and biosensors.

Driven by exciting new research and applications, top-down and bottom-up fabrication techniques are producing ever more intricate, reproducible, plasmonic nano-architectures with gaps and junctions approaching the single nanometre and atomic scales. Such atomic-sized features promote the intersection of physics, chemistry and biology in plasmonics. Consequently, understanding light-matter interactions in such closely spaced, electromagnetically coupled, metallic nanosystems is of vital importance to a tremendous variety of current and future nanophotonic technologies. This thesis describes the first dynamically controlled, optically broadband, experimental investigations of light-driven plasmonic coupling between two metal nanostructures with sub-nanometre separation. A new experimental apparatus and nanosystem alignment technique was developed to enable the required sub-nanometre inter-nanoparticle geometry to be created and probed. Two conducting atomic force microscopy tips with nanoparticle functionalised apices are brought into nanoscale `tip-to-tip' axial alignment with dynamically-controlled spacing and ultra-wide optical access. Resonant electrical parametric mixing, created by oscillating the electromechanically coupled tips, is utilised to extract an electronic signal due to nanoscale changes in inter-tip position. Experimental results match theory confirming the viability of the technique. By functionalising the tip apices, this unique multi-functional observation platform allows the plasmonic response of nanoparticle dimers with sub-nanometre separations to be characterised. By simultaneously capturing both the electrical and optical properties of tip-mounted gold nanoparticles with controllable sub-nanometre separation, the first evidence for the quantum regime of optically driven tunnelling plasmonics is revealed in unprecedented detail. It is demonstrated that quantum mechanical effects are critically important at approximately the 0.3 nm scale where spatially non-local tunnelling plasmonics controls the optical response. All observed phenomena are in good agreement with a recently developed quantum-corrected model of plasmonic systems. The findings imply that tunnelling establishes a quantum limit for plasmonic field enhancement and confinement. Additionally, the work suggests the highly enhanced local density of photonic states in nanoscale cavities could enable coherent plasmon-exciton coupling. This thesis prompts new experimental and theoretical investigations into quantum-domain plasmonic systems, and impacts the future of nanoplasmonic device engineering, nanoscale photochemistry and plasmon-mediated electron tunnelling.

Specific detection of biological matter is one of the key elements in a wide range of modern fields such as food industry, medicine, environmental and pharmaceutical industries. Generally, current common methods of detection (e.g. ELISA) involve molecular labelling, requirements for well-trained personnel and lengthy experimental procedures such as bacteria culture. All of the above issues result in high costs for biological analysis, and consequently, high costs for medical service, therapeutic drugs and various food products. Biosensors, on the other hand, can provide quick and cheap solutions to these problems. The field of optical biosensors is dominated by the method of surface plasmon resonance, which so far has attracted a lot of attention in the pharmaceutical industry. Investigation of long-range surface plasmon-polariton waveguides as an application for biosensing is still very novel, and most of it exists in the venue of theoretical discussions and modelling. The objective of this thesis is to demonstrate the capability of the novel optical biosensor based on plasmonic waveguides to selectively detect various biological entities in solutions. The experiments were conducted on photolithographically fabricated sensors consisting of straight gold waveguides embedded in low-refractive index fluoropolymer CYTOP and a microfluidic channel. As a proof-of-concept, a demonstration of basic sensing experiments such as detection of change in refractive index of bulk solution and non-specific adsorption of bovine serum albumin is provided. Further investigation of the sensor capabilities involved specific detection of human red blood cells and leukemia markers. Red blood cell detection was based on ABO blood grouping and included the estimation of limit of detection and signal-to-noise ratio for single cell detection. Finally, a clinically relevant problem of B-cell leukemia marker detection was targeted. The sensor demonstrated the ability to detect the relative abundance of similar proteins (immunoglobulin kappa and lambda) in a complex fluid (human serum). In addition, an experimental study on the optimization of the sensor for sensitivity was conducted.

Nanophotonics is a booming field of research with the promise of chip-scale devices which harness the tremendous potency of light. In this regard, surface plasmons have shown great potential for confining and manipulating light at extreme sub-wavelength scales. Advances in fabrication technology have enabled the scientific community to realize metasurfaces with unconventional properties that push the limits of possible applications of light. This thesis is comprised of computational and experimental studies on plasmonic metasurfaces. The computational study presents efficient design principles for plasmonic half-wave plates using L-shaped nanoantennas. These principles can be used to design waveplates at an operating wavelength of choice and for specific application requirements. The impact of this study goes beyond the efficient design of waveplates: it provides useful insights into the Physics of L-shaped nanoantenna arrays which have been proposed as building blocks for plasmonic metasurfaces. The experimental work investigates the interaction of a plasmonic metasurface, composed of dipole antenna arrays, with an epsilon-near-zero (ENZ) material. This work thus forms a bridge between plasmonics and ENZ materials science, which is a rapidly advancing field in its own right. The first experimental study investigates the exciting unconventional response of plasmonic dipole antennas when placed on a thin indium tin oxide (ITO) film near its ENZ wavelength of 1417 nm. The antenna-on-ITO system has split resonances whose spectral positions are largely independent of the antenna dimensions. The resonance splitting occurs due to coupling between the antenna resonance and the ENZ mode of the ITO film. This coupling results in field intensity enhancements on the order of a 100 in the ITO film. The second experimental study demonstrates, using the z-scan method, that this large field enhancement in the antenna-on-ITO structure further enhances the already strong nonlinearity of ITO around its ENZ wavelength. In particular, the antenna-on-ITO structure exhibits an extremely large nonlinear absorption coefficient, which is two orders of magnitude larger than that of a bare ITO film, and three to five orders of magnitude larger than that of many other nonlinear materials. This thesis thus constitutes a beautiful blend of three thriving areas of research: plasmonics, ENZ materials science and nonlinear optics. The findings reported here have the potential to contribute to all of these fields, and thus have relevance to a broad spectrum of optical scientists.

archives@tulane.edu / 1 / Robert Mackin

In the framework of the quasi-static approximation (QSA), some theoretical studies were conducted within the local response approximation (LRA). In these studies, certain plasmonic and plexcitonic systems were proposed, and their spectroscopic properties investigated. The QSA allows us to study metal nanoparticles (MNPs) and inter-particle distances that are small compared to the wavelength of light in the medium surrounding the MNPs, while the LRA enables us to utilize the bulk dielectric response of the metal in consideration. We have studied the following properties in detail: localized surface plasmon resonances (LSPRs), plasmon-induced transparency (PIT), and plasmon-enhanced fluorescence (PEF), while exciton-induced transparency (EIT) has only been partly studied. LSPR and PIT are properties of plasmonic systems while PEF and EIT are properties of plexcitonic systems. Both PIT and EIT are forms of electromagnetically-induced transparency. We started by constructing a geometry-based theoretical model that predicts the LSPR formula of any member of a certain group of single MNPs, using the LSPR for the most complex MNP geometry in the group. The model shows that from the LSPR of a nanorice, one could predict the LSPRs of concentric nanoshells, solid and cavity nanorods and nanodisks, respectively, and solid and cavity nanospheres. These formulae serve as quick references for predicting LSPRs since they can easily be compared to LSPRs obtained from spectral analysis. Likewise, we studied LSPR in addition to PIT in a nanoegg-nanorod dimer. We proposed this dimer in order to investigate how the interplay between plasmon coupling and MNP sizes affects PIT in complex geometries such as nanoeggs. Our result shows that the formation of PIT dips — regions in the dimer spectra where little or no incident radiation is absorbed by the dimer — are strongly-dependent on the nanorod size, due to the dependence of the plasmon coupling strength on the half-length of the nanorod. We investigated the phenomenon of PEF using a nanoegg-emitter system and a nanorod-emitter system, respectively. Emitters are organic or inorganic materials whose radiative decay rates increase dramatically when placed near a MNP subjected to plasmon excitation. Our theoretical results show that the choice of the MNP-emitter system to use depends on both the intrinsic quantum yield of the emitter and the antenna efficiency of the MNP. Theory shows that PEF is more substantial when the former is very low, and it will always occur if the latter is greater than the former. A nanorod-emitter system should serve as the preferred choice, due to the relatively easier synthesis of nanorods compared to nanoeggs, and the large longitudinal polarizability of nanorods as a result of the lightning rod effect. However, our theoretical model also shows that a nanoegg-emitter system can rival the PEF parameters obtained in a nanorod-emitter system, due to an increase in the Purcell factor of the emitter with increasing core-offset of the nanoegg, resulting from the presence of dipole-active modes in the nanoegg. / Thesis (PhD (Physics))--University of Pretoria, 2020. / University of Pretoria / National Research Foundation (NRF) / Physics / PhD (Physics) / Unrestricted

Plasmonic structures are utilized in many applications ranging from bio-medicine to solar energy generation and transfer. Numerical schemes capable of solving equations of classical electrodynamics have been the method of choice for characterizing scattering properties of such structures. However, as dimensions of these plasmonic structures reduce to nanometer scale, quantum mechanical effects start to appear. These effects cannot be accurately modeled by available classical numerical methods. One of these quantum effects is the tunneling, which is observed when two structures are located within a sub-nanometer distance of each other. At these small distances electrons “jump" from one structure to another and introduce a path for electric current to flow. Classical equations of electrodynamics and the schemes used for solving them do not account for this additional current path. This limitation can be lifted by introducing an auxiliary tunnel with material properties obtained using quantum models and applying a classical solver to the structures connected by this auxiliary tunnel. Early work on this topic focused on quantum models that are generated using a simple one-dimensional wave function to find the tunneling probability and assume a simple Drude model for the permittivity of the tunnel. These tunnel models are then used together with a classical frequency domain solver. In this thesis, a time domain surface integral equation solver for quantum corrected analysis of transient plasmonic interactions is proposed. This solver has several advantages: (i) As opposed to frequency domain solvers, it provides results at a broad band of frequencies with a single simulation. (ii) As opposed to differential equation solvers, it only discretizes surfaces (reducing number of unknowns), enforces the radiation condition implicitly (increasing the accuracy), and allows for time step selection independent of spatial discretization (increasing efficiency). The quantum model of the tunnel is obtained using density functional theory (DFT) computations, which account for the atomic structure of materials. Accuracy and applicability of this (quantum corrected) time domain surface integral equation solver will be shown by numerical examples.

Plasmonics is an important branch of nanophotonics and is the study of the interaction of electromagnetic fields with the free electrons in a metal at metallic/dielectric interfaces or in small metallic nanostructures. The electric component of an exciting electromagnetic field can induce collective electron oscillations known as surface plasmons. Such oscillations lead to the localization of the fields that can be at sub-wavelength scale and to its significant enhancement relative to the excitation fields. These two characteristics of localization and enhancement are the main components that allow for the guiding and manipulation of light beyond the diffraction limit. This thesis focuses on developing plasmonic devices for near and far-field applications. In the first part of the thesis, we demonstrate the detection of single point mutation in peptides from multicomponent mixtures for early breast cancer detection using selfsimilar chain (SCC) plasmonic devices that show high field enhancement and localization. In the second part of this work, we investigate the anomalous reflection of light for TM polarization for normal and oblique incidence in the visible regime. We propose gradient phase gap surface plasmon (GSP) metasurfaces that exhibit high conversion efficiency (up to ∼97% of total reflected light) to the anomalous reflection angle for blue, green and red wavelengths at normal and oblique incidence. In the third part of the thesis, we present a theoretical approach to narrow the plasmon linewidth and enhance the near-field intensity at a plasmonic dimer gap (hot spot) through coupling the electric localized surface plasmon (LSP) resonance of a silver hemispherical dimer with the resonant modes of a Fabry-Perot (FP) cavity. In the fourth part of this work, we demonstrate numerically bright color pixels that are highly polarized and broadly tuned using periodic arrays of metal nanosphere dimers on a glass substrate. In the fifth and final part of the thesis, we propose numerically an approach to narrow the plasmon linewidth and enhance the magnetic near field intensity at a magnetic hot spot in a hybridized metal-insulator-metal (MIM) structure. The computational method used throughout the thesis is the finite-difference time-domain method (FDTD).