Terahertz (THz) technologies have numerous applications such as biological and medical imaging, security screening, remote sensing, and industrial process control. However, the lack of practical THz sources and detectors is still a significant problem limiting the impact of these applications. In this Thesis work, three novel THz radiation mechanisms are proposed and investigated, based on the distinctive electronic properties of charge carriers in 2D single-layer graphene and related 1D conductors (i.e., graphene nanoribbons and carbon nanotubes), combined with the use of nanoscale dielectric gratings. Numerical simulations as well as fabrication and characterization activities are carried out. The first proposed radiation mechanism is based on the mechanical corrugation of a single-layer sheet of graphene or 1D carbon conductor, deposited on a lithographically-defined sinusoidal grating. In the presence of a dc voltage, carriers will therefore undergo periodic angular motion and correspondingly radiate (similar to cyclotron emission but without the need for any external magnetic field). My numerical simulations indicate that technologically significant output power levels can correspondingly be obtained at geometrically tunable THz frequencies. Initial graphene samples on sinusoidal gratings were fabricated and found to undergo significant strain redistribution, which affects their structural quality. Charge carriers moving in a flat sheet of graphene or linear 1D carbon conductor parallel to a nanoscale grating can also produce THz radiation based on the Smith-Purcell effect. The role of the grating in this case is to diffract the evanescent electromagnetic fields produced by the moving electrons and holes so that THz light can be radiated. Once again, numerical simulations indicate that this approach is promising for the realization of ultra-compact THz sources capable of room-temperature operation. Initial experimental results with ultra-high-mobility graphene samples embedded in boron nitride films show promising THz electroluminescence spectra. The last approach considered in this Thesis involves graphene plasmons at THz frequencies, which can be excited through the decay of hot electrons injected with an applied bias voltage. A nearby grating can then be used to outcouple the guided electromagnetic fields associated with these collective charge oscillations into radiation. The excitation of these THz plasmonic resonances at geometrically tunable frequencies has been demonstrated experimentally via transmission spectroscopy measurements. / 2017-06-21T00:00:00Z

We study theoretically graphene-like plasmonic metamaterials, in particular a honeycomb structured array of identical metallic nanoparticles, and examine the collective plasmonic modes that arise due to the near-field dipolar coupling between the localised surface plasmons of each individual nanoparticle. An analysis of the band structure of these eigenmodes reveals a phenomenal tunability granted by the polarisation of the dipole moments associated with the localised surface plasmons. As a function of the dipole orientation we uncover a rich phase diagram of gapped and gapless phases, where remarkably every gapless phase is characterised by the existence of collective plasmons that behave as massless chiral Dirac particles, in analogy to electrons in graphene. We consider lattices beyond the perfect honeycomb structure in two ways. Firstly, we break the inversion symmetry which leads to collective plasmons described as massive chiral modes with an energy dependent Berry phase. Secondly, we break the three-fold rotational symmetry and investigate generic bipartite lattices. In this scenario we progressively shift one sublattice away from the original honeycomb arrangement and observe a sequence of topological phase transitions in the phase diagram, as well as the merging and annihilation of Dirac points in the dispersions. After examining the purely plasmonic response we wish to address the true eigenmodes responsible for transporting electromagnetic radiation. For this reason we examine plasmon polaritons that arise from the strong light-matter coupling between the collective plasmons in a honeycomb array of metallic nanoparticles and the fundamental photonic mode of an enclosing cavity. Here we identify that the Dirac point remains robust and fixed in momentum space, irrespective of the light-matter coupling strength. Moreover, we demonstrate a qualitative modification of the polariton properties through modulation of the photonic environment, including order-of-magnitude renormalisation of the group velocity and the intriguing ability to invert the chirality of Dirac polaritons.

In our modern society, we are surrounded by numerous sensors, constantly feeding us information about our physical environment. From small, wearable sensors that monitor our physiological status to large satellites orbiting around the earth, detecting global changes. Although, the performance of these sensors have been significantly improved during the last decades there is still a demand for faster and more reliable sensing systems with improved sensitivity and selectivity. The rapid progress in nanofabrication techniques has made a profound impact for the development of small, novel sensors that enables miniaturization and integration. A specific area where nanostructures are especially attractive is biochemical sensing, where the exceptional properties of nanomaterials can be utilized in order to detect and analyze biomolecular interactions.  The focus of this thesis is to investigate plasmonic nanoparticles composed of gold or silver and optimize their performance as signal transducers in optical biosensors. Metal nanoparticles exhibit unique optical properties due to excitation of localized surface plasmons, which makes them highly sensitive probes for detecting small, local changes in their surrounding environment, for instance the binding of a biomolecule to the nanoparticle surface. This is the basic principle behind nanoplasmonic sensing based on refractometric detection, a sensing scheme that offers real-time and label-free detection of molecular interactions.  This thesis shows that the sensitivity for detecting local refractive index changes is highly dependent on the geometry of the metal nanoparticles, their interaction with neighboring particles and their chemical composition and functionalization. An increased knowledge about how these parameters affects the sensitivity is essential when developing nanoplasmonic sensing devices with high performance based on metal nanoparticles.

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.

The lipid bilayer membrane of enveloped viruses, such as human immunodeficiency virus type 1 (HIV-1), plays an important role in key steps of the infection, including cell binding and uptake. Phosphatidylserine (PS) and monosialotetrahexosylganglioside (GM1) are examples of two host-derived lipids in the membrane of enveloped virus particles that are known to contribute to virus attachment, uptake, and ultimately dissemination. A quantitative characterization of their contribution to the functionality of the virus requires information about their relative concentrations in the viral membrane. In this dissertation, a gold nanoparticle (NP) binding assay for probing relative PS and GM1 lipid concentrations in the outer leaflet of different virus-like particles (VLPs) using small sample sizes is introduced. The assay evaluates both scattering intensity and resonance wavelength and determines relative NP densities through plasmon coupling as a measure for the target lipid concentrations in the NP-labeled VLP membrane. The performed studies reveal significant differences in the membrane of HIV-1 and Ebola VLPs that assemble at different intracellular sites and pave the way to an optical quantification of lipid concentration in virus particles at physiological titers. In addition, this technique was used in another application to improve the understanding of the relationship between the membrane PS lipid and the infectivity of HIV-2 and murine leukemia virus (MLV). The composition of the membrane, in particular the cholesterol (chol) content, determines its fluidity. As differences in the membrane composition of individual virus particles can lead to different intracellular fates, biophysical tools capable of probing the membrane fluidity on the single-virus level are required. In this dissertation, we demonstrate that fluctuations in the polarization of light scattered off gold or silver nanoparticle (NP)-labeled virus-like-particles (VLPs) encode information about the membrane fluidity of individual VLPs. We developed a plasmonic polarization fluctuation tracking microscopy (PFTM) which facilitated, for the first time, the investigation of the effect of chol content on the membrane fluidity and its dependence on temperature on the single-VLP level. Chol extraction studies with different methyl-β-cyclodextrin (MβCD) concentrations yielded a gradual decrease in polarization fluctuations as function of time. The PFTM revealed chol content and fluidity heterogeneities of an HIV-1 VLP population.

In recent years, employing advanced oxidation processes (AOPs) as a means of wastewater remediation has emerged as a promising route towards maintaining a sustainable global water management program. The heterogeneous photocatalytic oxidation process has been of particular interest due to the prospective of utilizing solar radiation as the driving force behind the degradation of pollutants. Of the photocatalyst studied to date, TiO2 remains the most attractive material for environmental applications due to its affordability, stability, biocompatibility and high quantum yield. A key draw back however is roughly only 5% of solar radiation incident on earth can provide the energy required (3.0-3.2 eV) to generate the electron-hole pairs necessary for photo-oxidation. As a means to improve the process under solar irradiance, optical properties such as surface plasmon resonance of metallic nanoparticles and upconversion luminescence of rare earth ions have been exploited for improved light harvesting as well as the generation of more usable UV light from lower energy photons. In order to explore these phenomena and their role in the enhancement of this AOP, the photocatalytic degradation of organic dyes was studied under various conditions employing Degussa P25 TiO2 as the photocatalyst. Ag nanocubes, Ag-Pd core-shell nanoparticles and YAG:Yb+3,Er+3 served as the dopants for the various studies which resulted in enhanced degradation rates, insight into the applicability of utilizing Yb+3 as sensitizing ion under solar radiation and a novel core-shell nanoparticle synthesis.

The optical response of metallic structures is dominated by the dynamics of their free electron plasma. Plasmonics, the area of optics specializing in the electromagnetic behavior of heterogeneous structures with metallic inclusions, is undergoing rapid development, fueled in part by recent progress in experimental fabrication techniques and novel theoretical approaches. In this thesis I outline the behavior of four plasmonic material systems, and discuss the underlying physics that governs their optical response. First, the anomalous optical properties of solution-derived percolation films are explained using scaling theory. Second, a novel technique is developed to characterize the optics of amorphous nanolaminates, leading to the creation of a meta-material with anisotropic (hyperbolic) dispersion. The properties of such materials can be tuned by adjusting their composition. Third, the electrodynamics of vertically aligned multi-walled carbon nanotubes is derived through the development of a spectroscopic terahertz transmission ellipsometry algorithm. Lastly, a new diffraction based imaging structure based on metallic gratings is presented to have resolution capabilities which far outperform the diffraction limit. / Graduation date: 2013

Light scattering and surface Plasmon calculations were done on a variety of novel geometries using the DDSCAT software package, which simulates the scattering of objects using the discrete dipole approximation method. Calculations were done on core shell nanoparticles consisting of a silver shell and a TiO2 core in order to determine changes in the extinction spectrum and the near field patterns. Several geometries were tested, including spheres, cylinders, and hexagons, each of varying size and number. It was determined that when geometries were coupled together, there was significant near field enhancement where the geometries were in contact. This enhancement along with the increase in extinction in the visible region of the light spectrum makes these nanoparticles idea for solar cell technology, where they would increase efficiency.

Graphene, a one-atom-thick sheet of hexagonally packed carbon atoms, is a novel material with high electron mobility due to its unique linear and gapless electronic band structure. Its broadband absorption and unusual doping properties, along with superb mechanical flexibility make graphene of promising application in optoeletronic devices such as solar cell, ultrafast photodetectors, and terahertz modulators. How- ever, the current performance of graphene-based devices is quite unacceptable owning to serious limitations by its inherently small absorption cross section and low quan- tum efficiency. Fortunately, nanoscale optical antennas, consisting of closely spaced, coupled metallic nanoparticles, have fascinating optical response since the collective oscillation of electrons in them, namely surface plasmons, can concentrate light into a subwavelength regime close to the antennas and enhance the corresponding field considerably. Given that optical antenna have been applied in various areas such as subwavelength optics, surface enhanced spectroscopies, and sensing, they are also able to assist graphene to harvest visible and near-infrared light with high efficiency. Moreover, the efficient production of hot electrons due to the decay of the surface plasmons can be further implemented to modulate the properties of graphene. Here we choose plasmonic oligomers to serve as optical antenna since they pos- sess tunable Fano resonances, consisting of a transparency window where scattering is strongly suppressed but absorption is greatly enhanced. By placing them in di- rect contact with graphene sheet, we find the internal quantum efficiency of hybrid antenna-graphene devices achieves up to 20%. Meanwhile, doping effect due to hot electron is also observed in this device, which can be used to optically tune the elec- tronic properties of graphene.

Broad spectral tunability is a desired feature of many photonic and plasmonic components, such as optical filters, semiconductor lasers, and plasmonic materials. Here I show that unique properties of long-range surface plasmon polaritons (LR SPP) allow one to produce optical components with very wide tuning range using small variations in the refractive index of the dielectric cladding material. As a proof-of-concept demonstration, I present operation of LR-SPP-based bandpass optical filters in which a 0.004 variation in the refractive index of the cladding dielectric translates into 210 nm of bandpass tuning at telecom wavelengths. The tuning mechanism proposed here may be used to create monolithic bandpass filters with tuning range spanning over more than an optical octave, compact and widely-tunable diode and quantum cascade laser systems, multi-spectral imagers, and other plasmonic components with broadly-tunable optical response. / text