A Review of Metamaterial Invisibility Cloaks

The exciting features of metamaterial in conjunction with transformation optics leads to various applications in the microwave regime with such examples as invisible cloak, frequency selective surfaces (FSS), radomes, etc. The concept of electromagnetic invisibility is very much important in aerospace platform. Hence to study the feasibility of implementation of this concept for stealth, an extensive literature survey of metamaterial cloaks has been carried out and reported in this paper along with the basic concept of cloaking. To make the review more effective, the technical papers are classified into three broad sections viz. mathematical modeling, design and simulations, and fabrications and experimental demonstration. Further the design and simulation is focused on different techniques implemented such as finite difference time domain (FDTD), finite element method (FEM), finite integration technique (FIT), inductor-capacitor representation of metamaterial (LC MTM) etc. The review also reports the methods implemented for analysis of metamaterial cloaks with possibility of application to the specific frequency rang

Implementation of the Finite Difference Time Domain algorithm for the analysis of terahertz waves

The aim of this report is to present results related to the application of the Finite Difference Time Domain (FDTD) method for the study of various devices at terahertz frequencies. The FDTD method has emerged as one of the most widely used numerical analysis methods used for electromagnetic simulations. The FDTD method can be used to solve numerous types of problems ranging from modelling the behaviour of microwave circuits, waveguides, and photonics to simulating terahertz devices and plasmas. In this study the FDTD approach is derived from first principles (finite differences) and explicit equations are shown based on Maxwell's equations. Furthermore, absorbing boundary conditions (ABCs) are discussed for the FDTD method. The algorithm is tested against a range of problems to ensure its validity and accuracy. The FDTD method is then applied for the numerical analysis of a range of devices at terahertz frequencies. The numerical results obtained from the application of the FDTD method to examine a microstrip circuit with filter loading and a Multi-mode Interference (MMI) coupler at terahertz frequencies are discussed in detail. Moreover, the results of the application of the FDTD method for the calculation of the dispersion characteristics of plasmonic waveguides are also presented. Finally, a summary of the work carried out is presented and an outline is given for future research which can be carried out by using the FDTD method for the study of terahertz frequency devices

Terahertz Plasmonic Devices

Terahertz (THz) devices are designed to operate from 0.1-10 THz. The THz spectra have unique properties such as penetration through soft materials and reflecting from hard materials, which make THz technologies, a prime candidate for imaging. Plasmons are longitudinal charge oscillations in carrier rich materials. Plasmons can be generated over the channel of transistors inducing a voltage between the source-drain when conditions are satisfied. In this thesis, plasmonic devices operating in the THz region have been studied both theoretically and experimentally investigating GaN/AlGaN and Graphene based transistors. First, we report on a detailed study of dispersion properties of uniform grating gate THz plasmonic crystals, asymmetric dual grating gate plasmonic crystals and with symmetry-breaking defect-like cavities in order to understand the physics behind THz plasmons. For the first time, we defined the dispersion of plasmons in terms of effective plasmonic index. By adding an additional grating on top of the grating gate with a different periodicity, doubles the amount of absorption. Plasmons can be excited when polarization is perpendicular to the gate. We then showed focusing and exciting of THz plasmons polarization independent using circular grating lenses. Sub-micron THz ring resonators are presented showing THz guiding in plasmonic waveguides. So far, resonant sensing has been observed only at cryogenic temperatures since electron mobility is high enough at low temperatures to sustain resonant plasmonic excitation at the channel of the detector. Recently, graphene attracted the attention of the researchers because of its high mobility at room temperature. Room temperature detection has been attempted and achieved, however the detectors have very small responsivity with non-resonant behavior since the graphene is sandwiched and fabrication of such detectors in large scale is impossible with the methods used. Here, we present a resonant room temperature detection of THz with upside down free standing graphene FETs having more than a 400 quality factor, a record high number in the field which is up to 50 times higher than GaN detectors and hundreds of responsivity values with a maximum around 400 V/W which is record high for graphene (10,000 times higher than previously reported graphene detector)

Doctor of Philosophy

dissertationThis dissertation describes our work on design, fabrication and characterization of plasmonic metamaterials and tapered structures, with primary focus on their applications at terahertz (THz) frequencies. The phenomena associated with these structures rely on surface plasmon polaritons (SPPs), which may allow for high field enhancement and tight field confinement. We have investigated the underlying mechanisms of these structures and used that knowledge to develop unique and practical applications. We first studied two-dimensional periodic and random lattices based on aperture arrays, and modified the model to describe the effective dielectric response of the perforated metallic medium. Using two layers of the perforated stainless steel films, we demonstrated the emergence of an additional resonance and reproduced the transmission spectra using the effective dielectric model of the single-layer medium. Also, we improved the filtering performance of the multilayer periodic aperture arrays by adjusting the relative distance and angle between the layers, and demonstrated its application as a high quality bandpass filter. Then, we examined the transmission properties of graphite and carbon nanotube (CNT) films, and then the same films perforated with periodically distributed aperture arrays. The extracted dielectric constants of the graphite and CNT films demonstrate their availability for THz surface plasmonic devices. Moreover, we developed a narrow band/multiband THz detector in which the photoconductive antenna was surrounded by periodically corrugated gratings. This detector not only enhanced the sensitivity of detection at the specific frequencies, but also efficiently collected the radiation within the structure area, which obviated the need for a substrate lens. Finally, we improved the concentration properties of conically tapered apertures. Based on the optimal taper angle we determined, we introduced various modifications to the individual tapered aperture, e.g., to form an array and insert a gap spacing, and further enhanced the concentration capabilities and realized complete broadband transmission. Based on these studies and results, we are currently extending our work towards development of more reconfigurable and active devices that could enrich the available pool of THz and optical devices. Furthermore, such THz devices have great promise for the development of THz systems level applications and even a THz-based world in the future

Metamaterial

In-depth analysis of the theory, properties and description of the most potential technological applications of metamaterials for the realization of novel devices such as subwavelength lenses, invisibility cloaks, dipole and reflector antennas, high frequency telecommunications, new designs of bandpass filters, absorbers and concentrators of EM waves etc. In order to create a new devices it is necessary to know the main electrodynamical characteristics of metamaterial structures on the basis of which the device is supposed to be created. The electromagnetic wave scattering surfaces built with metamaterials are primarily based on the ability of metamaterials to control the surrounded electromagnetic fields by varying their permeability and permittivity characteristics. The book covers some solutions for microwave wavelength scales as well as exploitation of nanoscale EM wavelength such as visible specter using recent advances of nanotechnology, for instance in the field of nanowires, nanopolymers, carbon nanotubes and graphene. Metamaterial is suitable for scholars from extremely large scientific domain and therefore given to engineers, scientists, graduates and other interested professionals from photonics to nanoscience and from material science to antenna engineering as a comprehensive reference on this artificial materials of tomorrow

Electromagnetic phenomena based on surface modes and their transfer to atom optics

Tesis doctoral inédita. Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Teórica de la Materia Condensada. Fecha de lectura: 14-05-0

Characterization of multi-wall carbon nanotubes and their applications

PhDCarbon nanotubes (CNT) and their applications is a field which has attract a lot of interest in the past two decades. Since the first invention of CNTs in 1991, and in view of utilising nanoantennas, the focus in many laboratories around the world has shifted to trying to lengthen nanotubes longer from nanometers to few centimeters. Eventually this could lead to CNTs’ use in sub-millimeter, millimiter wave and microwave antenna applications. In this thesis, fundamental properties of carbon nanotube films are investigated, and some applications such as the use of CNTs as absorbers or CNT doped liquid crystals are considered. The concept of frequency tunable patch antennas is also presented. Simulation and measurement results of the liquid crystal based antenna show that frequency tuning is possible, through the use of a liquid crystal cell as a substrate. Additionally, greater tuning can be achieved using liquid crystals with higher dielectric anisotropy at microwave frequencies. This can be achieved by using CNT doped liquid crystals. As mentioned, microwave and terahertz measurements of vertically aligned carbon nanotube arrays placed on the top surface of a rectangular silicon substrate are presented. The S-parameters are calculated allowing the extraction of the complex permittivity, permeability and conductivity of the samples. Theoretical models are being introduced delineating the behaviour of the multi-walled nanotube (MWNT) samples. The material properties of this film provide useful data for potential microwave and terahertz applications such as absorbers. Finally, finite-difference time-domain (FDTD) modelling of CNTs is introduced, verifying the measurements that have been performed, confirming that CNT arrays can be highly absorptive. A novel estimation of the permittivity and permeability of an individual carbon nanotube is presented and a periodic structure is simulated, under periodic boundary conditions, consisting of solid anisotropic cylinders. In addition, the optical properties of vertically aligned carbon nanotube (VACNT) arrays, when the periodicity is both within the sub-wavelength and wavelength iii regime are calculated. The effect of geometrical parameters of the tube such as length, diameter and inter-tube distance between two consecutive tubes are also examined

Teraherts Waveguiding on Metamaterials

Terahertz time-domain spectroscopy (TTDS) is a powerful spectroscopic technique, combining pulsed broadband operation with high sensitivity coherent detection at room temperature. This thesis describes studies of terahertz surface plasmon polariton (SPP) guidance on a range of metamaterial structures using TTDS. Metamaterials are artificial media constructed from sub-wavelength dimension conducting elements which have an electromagnetic response that can be engineered by creating geometrical plasma-like resonances. In this work, high-confinement terahertz waveguiding is achieved by binding SPPs to cavity resonances which spoof the behaviour of intrinsic surface plasmon resonances found at much higher frequencies. The main aim of these studies is to investigate their properties with regard to potential applications in waveguiding and sensing. The first two chapters of this thesis describe the background to the subject. In chapter 3, the construction of a novel, flexible geometry, fibre-coupled TTDS system using hollow-core photonic crystal fibre (HC-PCF) is described. The extension of the system to include a near-field probe for evanescent field characterisation is also discussed. In chapter 4, we present the first direct observation of terahertz SPP propagation on plasmonic metamaterials consisting of copper sheets patterned with two-dimensional arrays of square copper-lined holes. Wavelength-scale field confinement is experimentally observed over an octave in frequency close to the band edge, representing a two order of magnitude increase in confinement compared to a flat metal sheet. In chapter 5, metamaterials consisting of two-dimensional arrays of coaxial apertures are shown to support two spoof plasmon modes below the band edge, enabling wavelength-scale field confinement to be experimentally realised at two distinct frequencies. In chapter 6, we present the first experimental results for terahertz SPP propagation on helical and discretely grooved cylindrical metamaterials termed metawires. In each case the results are compared with numerical simulations.EThOS - Electronic Theses Online ServiceGBUnited Kingdo