Anita Quadir

Characterization of Plasmonic Modes in a Low-Loss Dielectric-Coated Hollow Core Rectangular Waveguide at Terahertz Frequency

In this paper, a low-loss hollow-core rectangular plasmonic waveguide with a dielectric coating of Teflon is analyzed for terahertz (2.5 THz) propagation using a full-vectorial finite-element method (FEM). The modal properties of the waveguide, their effective indices, and power confinements have been calculated with a particular emphasis on the loss characteristics of the different modes. It has been observed that the loss characteristics of the guide are greatly affected by the thickness of the dielectric coating. It has been identified that, in contrast to the fundamental H10x mode, the H12x mode shows interesting modal properties and offers the lowest possible loss for the structure. This mode also tends to yield a near-Gaussian field profile when the dielectric coating thickness is optimized. The optimization of the loss values has been evaluated by comparing the loss characteristics for different dielectric materials, as well as by using different metal claddings.

Application of fuzzy inference and active contour model for detection of fovea and its center in a fundus image

Detecting the center of the fovea and its boundary in a human retinal image is a challenging task due to the irregularity of the avascular foveal region. Several attempts have been made in order to detect the boundary using fluorescein angiographic images. However, no attempt was made until now in detecting the boundary of fovea directly from fundus images. In this paper, a novel method for detecting the center of the fovea and its boundary in a fundus image is proposed. A fuzzy rule-based technique has been implemented along with a gradient vector flow (GVF)-based active contour technique in order to detect the boundary of the foveal avascular zone as well as its center. The proposed method exhibits to be promising in detecting fovea regions in wide range of images.

Intelligent Detection of Foveal Zone from Colored Fundus Images of Human Retina Through a Robust Combination of Fuzzy-Logic and Active Contour Model

Detection of the center of a fovea and its boundary from a retinal image is a challenging task due to the irregularity of the avascular foveal region. Several attempts have been made before in order to detect the foveal region and its boundary from fluorescein angiographic images. The irregularity and large variation in the human retinal images made the task increasingly difficult. In this current work funds images were considered instead of fluorescein angiographic images in order to ensure the applicability of the proposed algorithm in both biomedical and biometric analysis. A robust fuzzy-rule based image segmentation algorithm has been developed in order to extract the foveal region from a wide variety of images from different persons. Detection of foveal region comprised of locating the geometric center and extracting the boundary. The geometric center was evaluated by weighted averaging the grey scale intensities obtained from implementing the current algorithm. This was followed by applying gradient vector flow (GVF) based active contour technique in order to extract the boundary of the foveal region. The algorithm was applied on a several retinal images acquired from different persons with a very good success rate. The present work is considered to be an important contribution in intelligent image analysis of human retina since it incorporates a robust “fuzzy-rule” in extracting foveal region. Similar approach has not been adopted in the literature. The proposed algorithm is seen to be versatile in analyzing a wide range of retinal images.

Low-loss multimode interference couplers for terahertz waves

The terahertz (THz) frequency region of the electromagnetic spectrum is located between the traditional microwave spectrum and the optical frequencies, and offers a significant scientific and technological potential in many fields, such as in sensing, in imaging and in spectroscopy. Waveguiding in this intermediate spectral region is a major challenge. Amongst the various THz waveguides suggested, metal-clad plasmonic waveguides and specifically hollow core structures, coated with insulating material are the most promising low-loss waveguides used in both active and passive devices. Optical power splitters are important components in the design of optoelectronic systems and optical communication networks such as Mach-Zehnder Interferometric switches, polarization splitter and polarization scramblers. Several designs for the implementation of the 3dB power splitters have been proposed in the past, such as the directional coupler-based approach, the Y-junction-based devices and the MMI-based approach. In the present paper a novel MMI-based 3dB THz wave splitter is implemented using Gold/polystyrene (PS) coated hollow glass rectangular waveguides. The H-field FEM based full-vector formulation is used here to calculate the complex propagation characteristics of the waveguide structure and the finite element beam propagation method (FE-BPM) and finite difference time domain (FDTD) approach to demonstrate the performance of the proposed 3dB splitter.

Study of modal properties in gold nanowire with ZnO cladding by using the finite element method

Zinc oxide is a wide-bandgap semiconductor material, which can be grown as a single crystal and also as a thin film. A gold nanowire with zinc oxide cladding can serve as a waveguide, which can combine the plasmonic features of a metal nanowire with the sensing properties of ZnO. Using an H-field-based fully vectorial finite element method, rigorous modal solutions for the characteristics of a gold nanowire core with zinc oxide cladding are obtained. The modal properties, including the effective index, spot size, confinement factor, and modal hybridness have been analyzed to determine the fundamental plasmonic mode of this waveguide.

Emergence of THz technologies and design and optimisation low-loss waveguides and devices

THz is an emerging technology with many important applications in imaging and sensing, but due to lack of suitable low-loss waveguides future progress can be limited. A rigorous full-vectorial modal solution approach based on the computationally efficient finite element method is used to find the propagation properties of THz waveguides. Design approaches are presented to reduce the modal loss of such waveguides. Designs of several THz devices, including quantum cascade lasers, power splitters and narrow-band filters are also presented.

Characterization of graphene-based devices for THz systems

The H-field finite element method (FEM) based full-vector formulation is used in the present work to study the vectorial modal field properties and the complex propagation characteristics of Surface Plasmon modes of a hollow-core dielectric coated rectangular waveguide structures, and graphene based structures. Additionally, the finite difference time domain (FDTD) method is used to estimate the dispersion parameters and the propagation loss of such waveguides and devices.

Characterization of low-loss waveguides and devices for terahertz radiation

Abstract. A rigorous full-vectorial modal solution approach based on the finite element method is used to find the propagation properties of terahertz (THz) waveguides, such as photonic crystal fibers, quantum cascaded lasers, plasmonic waveguides, power splitters, and narrow-band filters. Design approaches to reduce the modal loss due to the material and leakage loss in photonic crystal fibers and in metal-coated hollow-glass plasmonic waveguides have also been considered. The plasmonic confinement and gain threshold of quantum cascaded lasers used as THz sources and the chromatic dispersion in plasmonic waveguides are also presented.

Design of low-loss waveguides and devices for the emerging THz technologies

A rigorous full-vectorial modal solution approach based on the computationally efficient finite element method is used to find the propagation properties of THz waveguides. Design approaches are presented to reduce the modal loss of such waveguides. Designs of several THz devices, including quantum cascade lasers, power splitters and narrow-band filters are also presented.

Dispersion characteristics of plasmonic waveguides for THz waves

Today there is an increasing surge in Surface Plasmon based research and recent studies have shown that a wide range of plasmon-based optical elements and techniques have led to the development of a variety of active switches, passive waveguides, biosensors, lithography masks, to name just a few. The Terahertz (THz) frequency region of the electromagnetic spectrum is located between the traditional microwave spectrum and the optical frequencies, and offers a significant scientific and technological potential in many fields, such as in sensing, in imaging and in spectroscopy. Waveguiding in this intermediate spectral region is a major challenge. Amongst the various THz waveguides suggested, the metal-clad waveguides supporting surface plasmon modes waves and specifically hollow core structures, coated with insulating material are showing the greatest promise as low-loss waveguides for their use in active components and as well as passive waveguides. The H-field finite element method (FEM) based full-vector formulation is used to study the vectorial modal field properties and the complex propagation characteristics of Surface Plasmon modes of a hollow-core dielectric coated rectangular waveguide structure. Additionally, the finite difference time domain (FDTD) method is used to estimate the dispersion parameters and the propagation loss of the rectangular waveguide.