Tunable compact THz devices based on graphene and other 2D material metasurfaces

Since the isolation of graphene in 2004, a large amount of research has been directed at 2D materials and their applications due to their unique characteristics. Compared with the noble metal plasmons in the visible and near-infrared frequencies, graphene can support surface plasmons in the lower frequencies of terahertz (THz) and midinfrared. Especially, the surface conductivity of graphene can be tuned by either chemical doping or electrostatic gating. As a result, the idea of designing graphene metasurfaces is attractive because of its ultra-broadband response and tunability. It has been demonstrated theoretically and experimentally that the third-order nonlinearity of graphene at the THz frequency range is exceptionally strong, and graphene has smaller losses with respect to noble metals. These features make graphene a promising candidate to enhance nonlinear effects at the far-infrared and THz frequencies. In this thesis, we present several designs to explore electromagnetic applications of graphene metasurface. Theoretical and simulation studies are carried out to design tunable THz polarizers, amplifiers, coherent perfect absorbers and to achieve enhanced nonlinear effect. These studies on the applications of monolayer graphene demonstrate prospective potentials of graphene in THz sensing, imaging, modulators, and nonlinear THz spectroscopy. Adviser: Christos Argyropoulo

Analytical wideband model for strip/slit gratings loaded with dielectric slabs

This paper presents a fully analytical model to determine the transmission and reflection properties of planar 1-D distributions of metal strips or slits made in thin metal screens. In contrast with other analytical or quasi-analytical approaches, the formulation incorporates the presence of dielectric slabs and is valid over a wide frequency band, from the long wavelength limit to the grating lobes operation. The model has been adapted to the case where two 1-D planar grids are stacked or a single grid is printed on a grounded substrate. In these cases, the model rigorously takes into account higher order mode interaction between the two stacked arrays of strips/slits or with the ground plane. Oblique incidence and both TE and TM polarizations have been considered. The analytical results show a good agreement with those computed by high-performance numerical methods, accounting for very fine details of extremely complicated transmission/reflection spectra. These results are of straightforward application to a variety of practical situations from microwaves to the terahertz regime. The present methodology can still be useful at higher frequencies provided that adequate models of the planar conductors are incorporated. In general, the model provides physical insight on the nature of the expected spectra and facilitates the design of devices based on planar metallic gratings.Ministerio de Ciencia e Innovación TEC2010-16948, CSD2008-00066Junta de Andalucía P09-TIC-459

Plasmonic Devices in the Terahertz and Optical Frequency Domains

We are living in an age where the evolution of semiconductor devices and components is contingent upon their miniaturization and seamless integration with the rest of the circuitry. Unfortunately, it is anticipated that electronic systems will soon approach the theoretical design limits of size and bandwidth, and it poses to be a serious concern for the development of high-speed information technologies. Replacement of electronic pulses that act as communication signals with electromagnetic surface waves offers a very promising solution, particularly in terms of device miniaturization and the heart of this optimism are the plasmonic waves arising due to collective electron oscillations at the surface of a conductor. Surface plasmon polaritons propagating along a metal-dielectric interface at optical frequencies have lately been a subject of immense research interest, mainly due to their reduced wavelength at least by an order of magnitude. Hence, miniaturized wave devices can be created at optical frequencies. Terahertz plasma waves, on the other hand, exist in infinitesimally thin plasma regions formed inside a transistor substrate, and are observed at much lower frequencies in the far-infrared regime. Due to essentially a two-dimensional nature of the plasma region, a much higher wavelength reduction factor that can exceed well beyond 100 is achievable. Furthermore, the boundary conditions due to the transistor terminals along with electric biasing create unstable resonance conditions that eventually lead to radiation in the terahertz frequency range. Such phenomena provide bright prospects for creating highly miniaturized terahertz devices. A reliable and efficient electromagnetic (EM) analysis for multilayer geome tries has gained further significance due to the emergence of plasmonic structures in the optical as well as terahertz frequency domains. In this regard, integral equation (IE) techniques are ideally suited due to their efficient handling of mutilayer structures. Although the presence of thin layers poses a challenge to any EM analysis technique, here the procedure is simplified due to the infinitesimally thin nature of the plasma region, which can be analyzed as a conducting sheet, with the same current flowing on either side of the sheet. Essential to any IE technique is an efficient and systematic formulation of Green functions (GFs) and their subsequent computation. In this dissertation, a transmission-line network based approach is adopted to derive spectral domain GFs for an infinitesimally thin sheet in a layered medium. The associated spatial domain counterparts are then computed through the Sommerfeld integrals (SIs). The extraordinary electromagnetic properties of plasmonic devices are demonstrated by a presentation of the properties of plasmonic antennas and a super-resolution imaging scheme which is able to resolve objects separated only by a few nanometers

Engineering Metamaterials

A couple of decades have passed since the advent of electromagnetic metamaterials. Although the research on artificial microwave materials dates back to the middle of the 20th century, the most prominent development in the electromagnetics of artificial media has happened in the new millennium. In the last decade, the electromagnetics of one-, two-, and three-dimensional metamaterials acquired robust characterization and design tools. Novel fabrication techniques have been developed. Many exotic effects involving metamaterials and metasurfaces, which initially belonged in a scientist’s lab, are now well understood by practicing engineers. Therefore, it is the right time for the metamaterial concepts to become a designer’s tools of choice in the landscape of electronics, microwaves, and photonics. Answering such a demand, the book “Engineering Metamaterials” focuses on the theory and applications of electromagnetic metamaterials, metasurfaces, and metamaterial transmission lines as the building blocks of present-day and future electronic, photonic, and microwave devices

Graphene Plasmonics: A Platform for 2D Optics

2D optics is gradually emerging as a frontier in modern optics. Plasmons in graphene provide a prominent platform for 2D optics in which the light is squeezed into atomic scale. This report highlights some recent progresses in graphene plasmons toward the 2D optics. The launch, observation, and advanced manipulation of propagating graphene plasmons for 2D optical circuits are described. Representative achievements associated with graphene metasurfaces, challenges, recent progresses like photoexcited graphene metasurfaces, and the transformation optics linking 2D to bulk optics with singularity are investigated

Mid-infrared nanophotonics with hyperbolic phonon polaritons

125 p.El nitruro de boro hexagonal (h-BN) es un material especialmente interesante en óptica subdifracional en el infrarrojo medio. h-BN es un cristal polar que exhibe fonón-polaritones en sus dos bandas Reststrahlen en el infrarrojo medio. En estas bandas, debido a su estructura cristalina anisótropa, únicamente una de las componentes del tensor de permitividad (que es uniaxial) posee signo negativo. Como consecuencia, la propagación de luz a en nitruro de boro en forma de fonón-polaritones hiperbólicos. En esta tesis estudiamos dos de las nanoestructuras más fundamentales en fotónica: nanoantenas lineales y cristales polaritónicos. Las nanoantenas lineales pueden ser analizadas como guías de onda polaritónicas truncadas, que son resonantes cuando el modo polaritónico cumple la condición de Fabry-Pèrot. Las nanoantennas de h-BN son analizadas usando esta interpretación. Un cristal polaritónico, de manera análoga a un cristal fotónico, es una nanoestructura periódica que soporta polaritones, en la que el periodo del patrón es similar a la longitud de onda del polaritón. Estudiamos un cristal polaritónico bidimensional fabricado a partir de una capa fina de h-BN, cuyo modo polaritónico más relevante es un fonón-polariton hiperbólico confinado en la capa fina

Mid-infrared nanophotonics with hyperbolic phonon polaritons

125 p.El nitruro de boro hexagonal (h-BN) es un material especialmente interesante en óptica subdifracional en el infrarrojo medio. h-BN es un cristal polar que exhibe fonón-polaritones en sus dos bandas Reststrahlen en el infrarrojo medio. En estas bandas, debido a su estructura cristalina anisótropa, únicamente una de las componentes del tensor de permitividad (que es uniaxial) posee signo negativo. Como consecuencia, la propagación de luz a en nitruro de boro en forma de fonón-polaritones hiperbólicos. En esta tesis estudiamos dos de las nanoestructuras más fundamentales en fotónica: nanoantenas lineales y cristales polaritónicos. Las nanoantenas lineales pueden ser analizadas como guías de onda polaritónicas truncadas, que son resonantes cuando el modo polaritónico cumple la condición de Fabry-Pèrot. Las nanoantennas de h-BN son analizadas usando esta interpretación. Un cristal polaritónico, de manera análoga a un cristal fotónico, es una nanoestructura periódica que soporta polaritones, en la que el periodo del patrón es similar a la longitud de onda del polaritón. Estudiamos un cristal polaritónico bidimensional fabricado a partir de una capa fina de h-BN, cuyo modo polaritónico más relevante es un fonón-polariton hiperbólico confinado en la capa fina

Gradient metasurfaces: a review of fundamentals and applications

In the wake of intense research on metamaterials the two-dimensional analogue, known as metasurfaces, has attracted progressively increasing attention in recent years due to the ease of fabrication and smaller insertion losses, while enabling an unprecedented control over spatial distributions of transmitted and reflected optical fields. Metasurfaces represent optically thin planar arrays of resonant subwavelength elements that can be arranged in a strictly or quasi periodic fashion, or even in an aperiodic manner, depending on targeted optical wavefronts to be molded with their help. This paper reviews a broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised to exhibit spatially varying optical responses resulting in spatially varying amplitudes, phases and polarizations of scattered fields. Starting with introducing the concept of gradient metasurfaces, we present classification of different metasurfaces from the viewpoint of their responses, differentiating electrical-dipole, geometric, reflective and Huygens' metasurfaces. The fundamental building blocks essential for the realization of metasurfaces are then discussed in order to elucidate the underlying physics of various physical realizations of both plasmonic and purely dielectric metasurfaces. We then overview the main applications of gradient metasurfaces, including waveplates, flat lenses, spiral phase plates, broadband absorbers, color printing, holograms, polarimeters and surface wave couplers. The review is terminated with a short section on recently developed nonlinear metasurfaces, followed by the outlook presenting our view on possible future developments and perspectives for future applications.Comment: Accepted for publication in Reports on Progress in Physic