Sharif Digital Repository

Millimeter wave imaging systems have unique features and have been used extensively in a variety of applications including medical imaging, construction operations, places with high security such as airports, Nondestructive testing and etc. These systems provide high resolution, due to the order of wavelength which is several millimeter or centimeter in this band. Also in places with high security and for detecting the concealed objects under people’s clothing, these waves can easily pass through the materials and investigate the person’s body without causing health problems. A wide variety of these imaging systems , each with different design parameters and resolutions, have been proposed.... 

Subwavelength metallic slit structures allow extraordinary transmission of light (EOT) at certein frequencies.In recent years, the effect of graphene layer on EOT has been investigated, which is used in the design of terahertz modulators. The electrostatic tuning ability of graphene electrical conductivity controls the resonance frequency and the amount of incident radiation absorption. Because of the single atomic thickness of graphene, the wide range control of incident light can not be achieved alone. However, by placing this layer with various 1-D and 2-D patterns on periodic metallic layers with EOT ability, high light modulations can be obtained. In particular, the structure of the... 

This thesis examines a new electromagnetic mode called line mode. This mode occurs when the boundaries have complementary surface impedances. It is possible to control the mode confinement by changing the surface impedance values. Impedance surfaces with non-complementary impedances guides the quasi-line mode, which is a mode between the line mode and the edge mode. Essential features of the line wave are its singularity over the line, its broad bandwidth, and its robustness to certain defects. Another significant characteristic of a line wave is that unlike surface waves in which it is not possible to change the spin orientation, in a line wave, the spin orientation can be altered by... 

Optical computing is a new approach to the hardware implementation of devices that were previously implemented digitally and electronically. It has attracted a great deal of interest due to its benefits, which include high bandwidth, extensive internal connections, the possibility of parallel processing, high calculation speed, and low power consumption. Consequently, this type of implementation is regarded as an appropriate substrate for optical neural networks. Compact and low-power CMOS-compatible hardware can be used for on-chip optical neural networks (ONNs), enabling affordable and portable image classification solutions for applications like autonomous vehicles, healthcare, and... 

In this thesis we introduce line wave. Which propagates only in one direction and dissipate in other direction. Line wave can propagates at the interface of two complementary impedance surfaces. There are various ways to build a waveguide which can support line waves like complementary structures. In this thesis we try to find a way which will not use complementary structures. Hence we use Spoof Surface Plasmon Polariton concept to mimic complementary structure characteristics. A complementary surface-based waveguide consists of two parts; one part supports TE polarization waves and the other one supports TM polarization waves. When these TE and TM waves face each other in the boundary, line... 

Analog computing has emerged as a promising candidate for Neural networks' implementation due to its high Interconnectivity, high bandwidth, parallel computing, high-speed processing, and low power consumption. Artificial Neural Networks have a wide range of applications; however, the Implementation of complicated Neural Networks on traditional computers would encounter two fundamental obstacles: limited processing speed and non-optimal energy consumption. This thesis's primary focus is on designing and simulating a whole-passive planar Optical neural network(ONN) based on silicon photonics technology. Firstly, the concept of ONN is studied using some lately proposed work. The device is... 

In recent years, thin-film photovoltaic cells with thicknesses of less than 1-2 µm have been developed with potentially lower production costs. Due to the small thickness of the absorbing semiconductor in these cells, the absorption is inevitably low at energies close to the electronic band gap of the semiconductor. This is particularly a problem for thin-film devices. Recently، periodic metallic nanostructures supporting surface plasmons have been introduced as alternative solutions to achieve light trapping in thin-film solar cells.Full numerical methods are usually used for the analysis of these periodic structures. The main drawback of these methods is that they are time-consuming and... 

In its most general form, analysis of macroscopic electromagnetic behaviour of a medium must include the dependence of magnetic flux density on the electric field and electric flux density on the magnetic filed. This new degree of freedom can lead to new propagation phenomena and result in novel microwave and optical devices. In recent years, numerous theoritical and exprimental studies have been carried out to fabricate these bi-anisotropic materials and understand their behaviour. A particular class of bi-anisotropic materials is electromagnetically equivalent to an anisotropic medium with gyrotropic permittivity and permeability tensors that operate on pseudo-electromagnetic fields. In... 

In this dissertation, we design and simulate Electromagnetic (EM) wave absorbers using periodic array of ultra-thin metallic ribbons, metallic disks and graphene disks based on circuit theory. The equivalent circuit for EM structures significantly eliminates the requirement of systems with advanced hardwares. Firstly, we categorize the metallic absorbers in the literature in two types: narrowband absorbers and broadband absorbers. Surface plasmon polariton, magnetic resonance, interference theory and localized surface plasmon are employed to illustrate the mechanism behind the narrowband absorption. Therefore, they will be the basis for classifying the narrowband absorbers in this work.... 

An important and interesting aspect of graphene plasmons (GPs) is their tunability via gating or chemical doping. This property can be used for controlling the propagation of GPs by using spatially non-uniform conductivity profiles in a single flake of graphene. Conductivity patterns are made by locally adjusting the chemical potential of graphene. The chemical potential in turn can be locally tuned by means of chemical doping or gate voltage. Therefore, it is important to know how GPs propagate on a graphene layer with patterned conductivity. In this thesis we will investigate this problem. First, we consider the scattering problem of GPs normally incident on an arbitrary conductivity... 

Conventional imaging systems composed of two linear arrays of antennas as a transmitter and receiver. These arrays of antennas electronically scan along their antenna elements and are mechanically moved along a second orthogonal direction for scanning large two-dimensional areas quickly. In this thesis, the transmitter antenna elements are Tapered slot-line Antenna operating at 20-30GHz. A multiplexer circuit is used to transmit wideband signals to each antenna in the array. The receiver is composed of Schottky diode and pick up antenna that they are printed onto the radiating end of the antennas on the edge of the transmitter circuit board.In the available imaging systems, for achieving the... 

Digital computers, comprising universal logic gates, are tremendously versatile. This versatility associated with the increasing integrability of digital electronics due to Moore’s law has left nearly no room for analog computers in the last few decades. However, in the case of some specific problems such as modeling complex and nonlinear systems or pro-cessing large amounts of data, computation time and power is still a serious restriction of digital computing. Optical phenomena are fast enough and offer novel approaches to overcome the mentioned restrictions about the system speed and power consumption. Therefore, optical structures make possible to perform ultra-fast spatial analog... 

Metagrating is a new concept for wavefront manipulation that, unlike phase gradient metasurfaces, does not suffer from low efficiency and also has a less complicated fabrication process. However, they have some drawbacks, which we are trying to resolve in this thesis. First, a compound metallic grating is proposed for anomalous reflection. We propose an analytical method for analyzing the electromagnetic response of this grating and show that the structure can have unprecedented near-to-unitary efficiency. Next, two-dimensional (2D) metagratings are proposed for diffraction pattern control. We first present an analytical method for diffraction analysis of 2D compound metallic grating. As a... 

In several electro-optic and optoelectronic devices, we require the structures with simultaneous high electrical conductivity and optical transparency so as to transmit the incident light. Transparent Conductive Electrodes (TCEs) that are both highly conductive and transparent were introduced to be an answer to the mentioned challenge. Periodic arrays of metallic holes are amongst the most important structures that are used as transparent electrodes. Using the metals with low electrical resistivity, such as Au and Ag, provides these structures with high electrical conductance. On the other hand, since these micro/nano structures support the propagation of guided electromagnetic waves, light... 

Although digital computers have been developed enormously in recent years, analog computing is still of great importance in some specialized application. There are basically two different methods to perform analog computing: metasurface approach (MS) and Green’s function (GF) method. In this contribution, we have focused our attention to the second method. In this approach, the associated Green’s function of the operator of choice is directly performed using a multilayered slab. Despite the applicability of this method, there are two major drawbacks regarding it. The first one is that it is only feasible to perform operators whose Green’s functions have an odd symmetry in the spatial Fourier... 

In this article some systems wich have a resolution beyond the diffraction limit and much more than the conventional lenses (like convex lenses, optical microscopes that have a resolution in order of wavelength) have been reviewed. These structures can be used in many different areas like Electron-Beam lithography (Photoleptography and Nano lithography), printing integrated optical circuits and chips, accurate cell imagings (entering and exiting of proteins in cells) and etc. This kind of imaging is called ”subwavelength imaging”. Generally some remarkable properties of these structures are propagation of all the spatial frequencies or amplifying the evanescent waves. One of the most known... 

By the advance of wireless communication systems, requirements to more advanced systems is increased. Circularly polarized antennas are considered as a robust solution to the demands of contemporary systems because of their unique attributes. This polarization is being used in various systems like satellite communication, global navigation system, mobile communications, wireless sensors, radio frequency identification, wireless power transmission and etc. Different designs are proposed to achieve circular polarization which among them the ones using single feeding techniques are more favored because of their ease of fabrication and excitation. In this thesis, three circularly polarized... 

A phase shifter is a microwave element that changes the phase of the input signal at a specific center frequency and bandwidth and, ideally, produces a signal with the same amplitude and a modified phase at the output. In practice, this phase change in the passive phase shifter is accompanied by a decrease in amplitude due to different factors. Various use cases can be described for phase shifters in telecommunication circuits, but its most important application is in regulating the relative phase of each antenna in a phased array system, which is the most common method of beam steering and forming. A phased array system is one of the leading solutions to achieve beamforming and high... 

Early detection of diseases has always been critical, especially during the outbreak of infectious and epidemic diseases plays an essential role in controlling the spread of the disease. Finding diagnostic methods with high sensitivity, especially in the early stages of the disease, is an important challenge in the face of new diseases. periodic structures biosensors are accurate sensors and have been very popular in recent years. In this thesis, two types of sensors that are very sensitive are investigated using a circuit model. The first structure is the array of sub-wavelength apertures perforated on a PEC film. In these structures, the extraordinary transmission phenomena are used for... 

In this dissertation, some computational operators are designed and simulated using plasmonic metastructures. Superiority of graphene over its plasmonics counterparts in terms of exceptional features such as tunability, thinness, and also highly confined graphene plasmons intrigued us to employ graphene-based meta-transmit-array for designing efficient devices in the infrared regime. First off, we propose a transmit-array of metasurfaces containing subwavelength graphene ribbons and evaluate the structure capability in full controlling over amplitude and phase of the impinging wave locally. By considering optics nanocircuits concept, an equivalent admittance is assigned to each metasurface...