Sharif Digital Repository

The purpose of this thesis is the analysis, design and fabrication of polarization converters in the millimeter-wave and terahertz regions using metasurfaces in order to generate different polarizations in the transmitter of terahertz stand-off imaging systems. In this project, the design and fabrication of two linear cross-polarizers and a circular polarization converter have been done at the frequency of 100 GHz. These polarizers are easy to fabricate by printed circuit board (PCB) technology and are capable of converting the polarization of a linear incident wave to the cross-polarized transmission wave and circular polarization, respectively. Linear polarizers are double-layer and... 

Optical metasurfaces are 2D structures of a repetitive arrangement of a number of nanoparticles that are artificially constructed and do not exist in nature. Important applications of optical metasurfaces are meta lenses, flexible solar cells and extremely small antennas. Fabricating surfaces with nanoparticles of proper geometry under light radiation along with elastic deformation, we can have optical metasurfaces with selective reflection or transparency in the visible spectrum. The aim of this project is modeling of an optical metasurface with elastic deformation leading to change of color and transparency. For this purpose, assuming vertical light radiation and linear polarization, we... 

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... 

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... 

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... 

Beam shaping, the transformation of radiated Gaussian beam from the millimeter wave source (conical horn antenna) into a flat-top beam at a certain distance, in the Millimeter wave active stand-off imaging system can be done in different ways, among them, beam shaping with a concave-convex lens (double-sided lens) in transmission mode (previous research done) can be mentioned. This lens consists of a convex-plane and a plane-concave lenses. when the electromagnetic wave passes through the convex-plane part, the wave front of the incident electric field changes from spherical to plane so that the divergent incident rays become parallel, and then the phase profile of the electric field changes... 

Electromagnetic metamaterials are designed artificial materials with sub-wavelength resonant inclusions. They can exhibit extraordinary properties such as negative permittivity, negative permeability, and anomalous reflection/refraction. Metasurfaces are 2D counterparts of metamaterials. Here, we proposed a framework for the multiscale multiphysics analysis of deformable metasurfaces. Nonlinear mechanical analysis (Geometry and material behavior), periodic boundary conditions, homogenization, multiscale analysis, and electromagnetic analysis are implemented in this framework. Benefiting from the framework, we proposed a multifunctional hyperelastic structured surface that can generate... 

Optical computing has emerged as a promising candidate for real-ime and parallel continuous data processing. Optical devices that perform spatial and spatiotemporal transformations of optical signals are of great interest for a wide range of applications including ultrafast all-optical information processing and analog optical computations. Although plasmonic metasurfaces(for analog computing applications) was investigated in the past, they are not realizable. Consequently the silicon photonics is a good alternative for realizing these applications.Firstly, the optical computing and its important features explained and a common architecture was introduced. in addition, another two structures... 

Due to the gyrotropic properties of ferrites, they have the ability to create nonreciprocal structures and have been used in microwave frequencies for a long time. Despite the use of these magnetic materials in ferrite devices, circulators, antennas, insulators, etc. The investigation of wave scattering by ferrite materials has been very limited and has only been limited to finding the radar cross-section. On the other hand, metasurfaes have been the subject of many studies in recent years in order to control propagation waves and create unusual scattering (reflection or propagation) from a surface.In this thesis, electromagnetic wave scattering by a grounded ferrite disk and the phenomena... 

Ultrafast signal processing in time-domain with high resolution and reconfigura-bility is a challenging task. This paper, for the first time, introduces a time-varying metasurface consisting of graphene microribbon array for implementing time-lens in the terahertz domain. Given that the surface conductivity of graphene is proportional to the Fermi energy level in the THz regime, it is possible to change the phase property of the incident electromagnetic pulse by changing the Fermi level while the Fermi level itself is a function of voltage. Upon this fact, a quadratic temporal phase modulator, namely time-lens has been realized. This phase modulation is applied to the impinging signal in the...