On the Influence of Spatial Dispersion on the Performance of Graphene-Based Plasmonic Devices

We investigate the effect of spatial dispersion phenomenon on the performance of graphene-based plasmonic devices at THz. For this purpose, two different components, namely a phase shifter and a low-pass filter, are taken from the literature, implemented in different graphene-based host waveguides, and analyzed as a function of the surrounding media. In the analysis, graphene conductivity is modeled first using the Kubo formalism and then employing a full-$k_\rho$ model which accurately takes into account spatial dispersion. Our study demonstrates that spatial dispersion up-shifts the frequency response of the devices, limits their maximum tunable range, and degrades their frequency response. Importantly, the influence of this phenomenon significantly increases with higher permittivity values of the surrounding media, which is related to the large impact of spatial dispersion in very slow waves. These results confirm the necessity of accurately assessing non-local effects in the development of practical plasmonic THz devices.Comment: 5 pages, 18 figures, 2 table

Lowpass lter design for space applications in waveguide technology using alternative topologies

The main goal of this project is to study the possibility of utilizing topologies based on curved surfaces (metallic posts) to realize low pass lters in waveguide technologies, as an alternative to the traditional implementation based on rectangular irises, in order to obtain devices that present a higher multipaction threshold. In the rst chapters, the theoretical synthesis techniques utilized are explained. Understanding the synthesis of the di erent types of lter polynomials and prototype networks is necessary in order to precisely design lters with a predictable frequency response. The commercial package HFSS will be used for the design and veri cation of the lters, controlling its operation with scripts in order to automate the design process.Escuela Técnica Superior de Ingeniería de TelecomunicaciónUniversidad Politécnica de Cartagen

Filtros plasmónicos reconfigurables y fenómenos de dispersión espacial en tecnología de grafeno en la banda de terahercios

[ENG]The exceptional electrooptical, thermal, and mechanical properties of graphene has motivated an enormous interest from the scienti c community in a wide variety of elds in recent years. In particular, the capability of mono- and multilayer graphene to support highly con ned recon gurable surface plasmon polaritons in the terahertz (THz) and infrared regime has motivated an explosive growth of graphene plasmonics, a discipline which is paving the way towards fully integrated THz transceivers and sensing systems. In this project, we rst present the novel design and analysis of planar recon gurable THz lters hosted in graphene nanoribbons, which are e ciently designed taking advantage of the quasistatic nature of graphene surface plasmon polaritons (SPPs) in nanostructures and graphene's eld e ect. The proposed lters are highly miniaturized and present recon guration capabilities not possible with other technologies in the THz band. Spatial dispersion in graphene sheets is then reviewed. This e ect is closely related to the quantum capacitance of graphene and strongly a ects surface wave propagation under certain circumstances. This phenomenon is studied in the THz and near infrared frequency bands, and accurate equivalent circuits that provide deep physical insight and simplify design tasks are developed. The practical implications of spatial dispersion regarding THz graphene-based plasmonic devices like the lters mentioned above are discussed.[SPA]Las excepcionales propiedades térmicas, mecánicas y electro-ópticas del grafeno han atraído un enorme interés de las comunidades cientí cas de diversas áreas en los últimos años. La capacidad del grafeno de soportar la excitación y propagación de plasmones de super cie en la bandas de terahercios (THz) e infrarrojos han motivado un crecimiento explosivo del estado del arte en cienca y tecnología de plasmones en estas bandas de frecuencias, una disciplina que podría ser crucial para el futuro desarrollo de sistemas integrados y altamente miniaturizados de comunicación, detección y sensores. En este proyecto, se presenta en primer lugar la síntesis y análisis de ltros planares recon gurables en la banda de THz mediante control electrostático de plasmones en tiras de grafeno. Se ha desarrollado una técnica de diseño e ciente, explotando la naturaleza cuasi-estática de este tipo de ondas electromagnéticas en tiras de ancho mucho menor que la longitud de onda. Se ilustra el rendimiento de estos ltros con múltiples ejemplos, demostrando capacidades de recon guración que no son posibles con otras tecnologías. Posteriormente se estudia de forma analítica el fenómeno de dispersión espacial en guías de onda mono- y multicapa de grafeno. Se establece una conexión entre este fenómeno y la capacidad cuántica intrínseca del material, y se estudia cómo afecta a las propiedades electromagneticas del grafeno en la banda de THz. Se han desarrollado circuitos equivalentes capaces de modelar la propagación de plasmones en estas estructuras, proporcionando una importante comprensión de los diferentes mecanismos de propagación una herramienta útil para el diseño de dispositivos. Por último, se analizan mediante ejemplos numéricos las implicaciones prácticas de la dispersión espacial en la respuesta de los filtros diseñados.Escuela Técnica Superior de Ingeniería de TelecomunicaciónUniversidad Politécnica de CartagenaUniversidad Politécnica de Cartagen

Spatially Dispersive Graphene Single and Parallel Plate Waveguides: Analysis and Circuit Model

The propagation of surface waves along spatially dispersive graphene-based 2-D waveguides is investigated in detail. Graphene is characterized using a full-k(rho) conductivity model under the relaxation-time approximation, which allows to obtain analytical and closed-formed expressions for the wavenumber of plasmons supported by sheets and parallel plate waveguides, respectively. Per unit length equivalent circuits are introduced to accurately characterize the propagation in different waveguides, and analytical relations between the effective TM-mode circuit lumped elements and graphene conductivity are derived. The proposed circuits allow identifying the different mechanisms involved in spatially dispersive plasmon propagation, explaining their connection with the intrinsic properties of graphene. Results demonstrate that spatial dispersion, which significantly decreases the confinement and the losses of slow surface plasmons, must be accurately assessed in the design of graphene-based plasmonic components at millimeter-waves and low terahertz frequencies

Graphene-Based Plasmonic Tunable Low-Pass Filters in the Terahertz Band

We propose the concept, synthesis, analysis, and design of graphene-based plasmonic tunable low-pass filters operating in the terahertz band. The proposed structure is composed of a graphene strip transferred onto a dielectric and a set of polysilicon dc gating pads located beneath it. This structure implements a stepped impedance low-pass filter for the propagating surface plasmons by adequately controlling the guiding properties of each strip section through graphene's field effect. A synthesis procedure is presented to design filters with desired specifications in terms of cutoff frequency, in-band performance, and rejection characteristics. The electromagnetic modeling of the structure is efficiently performed by combining an electrostatic scaling law to compute the guiding features of each strip section with a transmission line and transfer-matrix framework, approach further validated via full-wave simulations. The performance of the proposed filters is evaluated in practical scenarios, taking into account the presence of the gating bias and the influence of graphene's losses. These results, together with the high miniaturization associated with plasmonic propagation, are very promising for the future use and integration of the proposed filters with other graphene and silicon-based elements in innovative terahertz communication systems