Sharif Digital Repository

Over time, human needs more groundwater to meet agricultural, industrial, and urban uses; so the study on factor affecting groundwater and groundwater level changes are important for water resource management. However, often forgotten is the fact that accurate and reliable predictions are based on a correct diagnose of the past. One of the questions here is how the climate has changed since the last. Such a question is largely related to detection. Purpose of detecting climate variability and climate change, identifying climate variability and trends in system and describe the factors causing these changes are. Without knowing and understanding these changes and fluctuations, we are not able... 

In the last two decades, reconfigurable antennas have attracted significant research attention. Utilization of such antennas can lead to decrease in cost and power consumption in modern communication systems. The main focus of efforts is on frequency reconfigurability using different mechanisms while maintaining desirable antenna characteristics over the tuning range. In this thesis, our goal is to design, analyze and fabricate a reconfigurable antenna based on the concept of polarization standing wave (PSW) in the X band using only a simple physical tuning mechanism. Two anisotropic handedness preserving mirrors (AHPM) are placed at both ends of a cylindrical cavity resonator, thus... 

Expansion of communication and the emergence of new fields such as the Internet of Things, has led to the development of smart cars and especially self-driving cars. The antennas used in the radar of these cars are of special importance for a better understanding of the environment around the car. The important parameters for the design of antennae for automotive radar are: good resolution, narrow beam width, the ability to change the angle of the main beam, etc. Among the most common antennas used in these radars are directional antennas, for this reason phased array antennas and leaky wave antennas are suitable options for this application. In general, the complexity and difficulty in the... 

Novel Dirac Leaky Wave Antennas (DLWAs) based on Substrate Integrated Waveguide (SIW) are presented in this thesis, for millimeter wave frequencies. These DLWAs provide interesting features such as radiating through broadside, beam scanning and ease in fabrication that are well suited for emerging fifth generation (5G) and IoT applications. In this thesis, it is shown that a Dirac photonic crystal can be realized in SIW technology composed of air columns inside a host SIW waveguide, exhibiting a closed bandgap and linear dispersion around broadside. Phase and attenuation constants are controlled to obtain directive beam and scanning in a wide range of angles (from -30˚ to 20˚). This DLWA... 

This thesis formulates Green's function of periodic arrays of two-dimensional rectangular holes Perforated on a PEC film. Green's function of two-dimensional periodic free space in the spectrum domain is computed. Using Floquet theory and considering the holes as rectangular waveguides, Green's function of the structure is presented.For sub-wavelength holes in cut-off and propagating cases, numerical results are presented. The effects of changing the aperture's thickness and the number of required modes have been reported. The structure under a normal and oblique incident wave is investigated, and the high conformity of the presented model results and full-wave simulations is observed.In... 

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... 

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... 

Artificial neural networks are systems based on the brain’s functionality which in many cases are able to process highly complex computational tasks like speech recognition, image recognition ,and time series prediction. Due to the complexity of training algorithms in recurrent neural networks, reservoir computers have significant importance in machine learning.Due to low power consumption and crosstalk, high bandwidth and high-speed computing in optics, reservoir computing has proceeded to optical implementations. A reservoir computer consists of three layers: the input layer, reservoir, and output layer. A recurrent neural network is usually used as the reservoir in reservoir computers.... 

Structures with time-varying electromagnetic properties can potentially yield non-reciprocity, amplification, harmonic generation, and other interesting wave propagation phenomena. To date, some studies have dealt with time-varying media and circuits, including time-varying optical ring resonators having degenerate travelling wave resonances. In time-varying ring resonators, the clockwise and counter clockwise resonances lose their degeneracy, and non-reciprocity is observed. One of the main challenges of such research is the need to induce time-variations in the entirety of the resonator or structure, which makes their implementation very limited or impossible in practice. The purpose of...