Sharif Digital Repository

Electronic processors have encountered Moore’s limitations and they can not meet expectations following computer science advances in new algorithms such as neural networks. These limitations have brought research to focus on light as a high-capacity carrier of information. Light’s degrees of freedom can be exploited whenever the structure has devices able to carry, keep, manipulate and detect each DoF's information but this exploitation may itself lead to either a large footprint or more power consumption. Moreover, there are some scientific and technological difficulties in matching different computational algorithms to these DoF’s requirements. As a result, one must consider all advantages... 

Epsilon-near-zero media have attracted considerable attention in the last decade due to their exceptional behavior in wave propagation. The purpose of this research project is to analyze time-periodic ENZ structures. To this end, layered structures consisting of plasmonic metal and dielectric layers are used to achieve the epsilon-near-zero effect in a specific frequency range, and by time-variations of the dielectric layers, an effective time-varying (TV) ENZ is achieved. A method based on transfer matrix method is developed to analyze such structures while they are undergoing time-variations, and primarily under normal incident waves. Various structures based on such TV layered structures... 

This thesis presents a resonant time-periodic structure based on the Fabry-Perot resonator for frequency comb generation. Time-periodic structures cannot generate a frequency comb by themselves because side-harmonic amplitudes are really weak. However, due to resonant structure, it is expected that the side-harmonics, which are generated because of the time-periodic part of the structure, are increased if they are close to the main modes of the structure. But the results show that, generally, the amplitude of side harmonics, which are close to the main mode, is increased and far side-harmonics don't have remarkable amplitudes. However, we will show that if the modulation frequency of the... 

Neural networks play an important role in industry nowadays. Matrix-vector multiplication (MVM) as one of the major building blocks of neural networks is conventionally implemented by electronic integrated circuits. In recent years, researchers have shown interest in implementing matrix-vector multiplication with a photonic approach to reduce the loss and increase the speed. By simultaneously using different degrees of freedom of light, such as amplitude, phase, wavelength, and angular momentum, it is possible to reduce the volume of electronic processors, increase computational accuracy, and higher processing capacity. In this research, matrix-vector multiplication is implemented by using... 

Nowadays, Diffractive lenses with the binary profile have extensive application in integrated optics and other areas photonics. These elements-because of their compatibility with integrated circuits fabrication methods- have easy and efficient fabrication processes. They also have low aberration and they offer vast degree of freedom in designing process. The main objective of this thesis is to present an effective method to analyze these binary lenses in three dimensions. From different vectorial analyses methods, Boundary Element Method (BEM) has been chosen because of its different advantages like appropriate precision and higher speed in comparison with other methods. Yet this method has... 

Luminescence down shifting (LDS) of the incident spectrum is a practical strategy that helps to increase the light harvesting efficiency by manipulating the incident spectrum instead of interfering with the active material inside the cell. In this research, for a dye-sensitized solar cell with a near-infrared sensitizer, the photovoltaic performance was enhanced remarkably with a reflective luminescent down-shifting (R-LDS) layer. Different inorganic phosphors with different luminescent quantum efficiencies have been used as the down shifting materials to increase the light-harvesting efficiency of DSSCs sensitized with TT1 and SQ1 dyes. Four different structures were examined to find the... 

Time-varying media have been in center of attention since recent years for their plethora of applications. Achieving non-reciprocity, one-way propagation, frequency modulation, and signal amplification are some applications of such media which are accessible from microwave to optical frequencies. The aim of this thesis is to first find a method for the analysis of wave propagation in homogeneous time-varying media. To this end, a new formulation is presented based on differential transfer matrices enabling us to find amplitudes, average power, and average energy of a wave in a homogeneous time-varying medium for arbitrary temporal variation of permittivity. In addition,this formulation... 

In this thesis, the Maxwell's equations are transformed into a set of ordinary differential equations and then solved in periodic structures by using the differential method (DM). The periodic structures considered in this thesis are carved in linear anisotropic, nonlinear isotropic and nonlinear anisotropic media. Different numerical issues, e.g. stability and convergence rate, are addressed by following the conventional methods, e.g. S- and R- matrix formalism, and Fourier factorization. In addition, an algorithm is proposed for fast and efficient analysis of periodic structures with identical layers. Binary gratings and photonic crystals are examples of such structures. In this algorithm,... 

To design and identify specific properties of Metamaterials we need methods that accurately and quickly provide precise information about these composite materials for engineers. In this thesis we use FMM method with ASR technique for the analysis of Metamaterials and photonic crystals in three dimensional structures. In the second chapter FMM method is described briefly and then ASR technique for applying in the FMM method is introduced. ASR approach has large convergence rates in all types of gratings particularly metallic ones and with optimization that we have done for two dimensional periodic gratings convergence speed is increased more. In the third chapter using ASR reflection and... 

Due to tremendous growth in usage of electron beams in different applications like VLSI, welding, chemical processes, Polymer industry and medical applications, precise study of these beams is of great importance. One of the most important parameters in studying electron beams characteristics is the phenomenon and effect of space charge. Although in most methods of analyzing electron optical systems, this effect is neglected, however in high density beams, this effect is bold. Space charge effect is due to interaction and electrostatic repulsion force between each of the electrons in the beam, which results in a change in shape and also divergence of beam. It is worth to mention that in... 

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... 

Achieving low group velocity of light can result progress in many fields. In this project by using complex time domain frequency, achieving method to zero group velocity is proposed. The proposed structures in this thesis consist of two lossless and lossy structures which by considering the modes with complex time frequency are excitable. Using leaky modes to analyze these structures, made them suitable as they can be excited from free space. Using slow light can lead to optimized devices because of the more interaction of light and matter which besides smaller devices too. Additionally slow light is being considered in designing optical memories, quantum computers, optical processors and... 

Researchers have long considered the phenomenon of parametric amplification. This phenomenon is introduced first in nonlinear structures and then in limited systems of time-varying linear structures. Time-modulated systems have many capabilities, such as amplification, non-reciprocity, and other phenomena in wave propagation. So far, time-varying circuits have been studied in terms of stability and analytical solution. The purpose of this project is to investigate parametric amplification in time-varying linear systems. Studying a structure under a circuit model is essential to other analyses practically implemented in various frequency ranges from microwave to light. As a result, this... 

We study the phenomenon of parametric amplification and instability in the context of time-varying electromagnetic structures, particularly dielectric slabs with periodically modulated permittivity. These structures show particular promise inasmuch as they are capable of very large amplifications when illuminated by an electromagnetic wave of half the modulation frequency. Successive studies have corroborated this finding but none have yet been able to ascertain the nature of amplification in such devices. On top of that, the problem of instability in time-periodic slabs has either been disregarded altogether or it has been subjected to loose analysis, leading to mere speculations which are... 

Reservoir computing, a computational paradigm nested within recurrent neural networks, offers a powerful framework for processing time-dependent inputs, exhibiting remarkable performance in tasks such as voice recognition and time series approximation. Recently, there has been a burgeoning interest in implementing reservoir computing in the realm of optics due to its potential for reduced power consumption, increased bandwidth, and enhanced parallelization. The crux of implementing a reservoir computer with artificial neurons lies in utilizing a nonlinear function with a delay line, which accumulates temporally delayed outputs of the function, integrates them with new inputs, and feeds them... 

Nonuniform superconducting microstrip transmission lines in microwave regime have been investigated. Nonuniform transmission lines provide the capability of controlling line parameters. Photonic structures with subwavelength features can be homogenized and thus be accurately approximated by homogeneous yet spatially dispersive structures. This idea is here applied to nonuniform superconducting transmission lines with subwavelength nonuniformities, i.e. subcentimeter features in the microwave regime. This modelling is quite useful in geometrical and optical control of quasi-TEM wave propagation and dispersion engineering along microwave superconducting lines. This approach has applications... 

Mode extraction in photonic and quantum-well structures requires solving transcendental equations and applying complex root searching methods, which incurs a diversity of numerical obstacles. In this thesis, the most effective methods for derivation of complex roots in such equations are investigated and analyzed. Thus the complex modes of an arbitrary one-dimensional photonic structure are extracted with highly adjustable precision and accuracy. The improper modes, i.e. solutions of the improper dispersion equation, are also studied and extracted by exploiting numerical methods. After this step, the migration of modes caused by changing the physical parameters of the structure is... 

Today’s technological trend towards higher performance microwave and digital circuits at increasingly smaller sizes poses new challenges to both the microwave and high-speed VLSI designers. One aspect of these challenges lies in the fact that nonuniform transmission structures must often be used to reduce circuit sizes, to increase circuit bandwidth, to reduce discontinuity effects, and to optimize the overall circuit performance. Coupled microwave transmition lines are one of most common microwave circuits, that are use to design couplers, power dividers, hybrids, baluns, filters, phase shifters, matching circuts and so on. The broadband tight couplers are one of interesting devices that... 

New technologies for three-dimensional (3D) sensing and visualization of real-world objects have been pursued by scientists and engineers for many decades. As opposed to traditional two-dimensional (2D) imaging techniques, 3D imaging technologies can potentially capture the 3D structure, range, and texture information of objects. Therefore, it can be developed for many applications, as medical imaging and diagnosis, dentistry, industrial designs, architecture and security. There are many 3D imaging technologies, such as holography, incoherent holography and integral imaging. Since holographic methods are based on the wave interferometry of two coherent beams, they demand high stability of... 

Over the past few years, time-periodic and space-time-periodic media have garnered a considerable amount of attention, thanks to their exotic and unique interactions with electromagnetic phenomena. These media do not generally comport with Lorentz's reciprocity theorem, thus can be utilized in order to implement non-reciprocal amplification and frequency transformation. One class of such media, that have been the subject of less discussion and interest, are space-time periodic diffraction gratings.In this thesis, we aim to develop a series of semi-analytical methods in order to analyze such structures. Our focus will be on a subset of methods which make use of the modal expansion of fields...