Sharif Digital Repository

One of the most common devices for terahertz (THz) wave generation and detection is a Terahertz photoconductive antenna. Impedance matching between the antenna and the photoconductive material is a primary factor to obtain high optical-THz conversion efficiency. Because of the time dependant conductivity of the photoconductive material, impedance mismatching is an obstacle for generating high power THz pulses. Moreover, the range of laser power (when using Ti:Sapphire) from the excitation source varies from a few tens of milliwatts to a few watts while the radiated THz power as compared to this laser pump power is very low (in the order of a few μW); this means a low power conversion... 

The basis of this research is the use of millimeter wave frequency band, due to its unique features for standoff imaging. The millimeter wave standoff system used in this thesis faces challenges such as sensitivity to change of angle, separation of the object from the background and the inability of the system to accurately detect the edges of the object, that the purpose of defining this project is to provide solutions and their feasibility with imaging system to minimize these challenges. For this purpose, first the imaging process of the system is generally simulated by two methods of physical optics/physical theory of diffraction and equivalence method and then for different conditions... 

In recent years, the growing importance of imaging in the fields of medicine, security, food quality control, etc. has led to many investigations to find the most efficient imaging methods. In this thesis, the improvement of high frame-rate millimeter-wave stand-off imaging is investigated using the polarimetry method. The main objective of this study was to use Cx-pol scattered waves from rough surfaces, especially the edges of a Perfect Electric Conductor (PEC), to detect the edges of this object from the human body surface. Since the surface of the human body at the desired operating frequency (100 GHz) is almost flat, the background effect can be eliminated by analyzing the Cx-pol... 

In this project, we report on two waveguide structures suitable for implementing terahertz compact devices. First structure is composed of a periodic array of metallic blocks attached to a metal plate at the bottom. By integrating the metallic teeth in a silicon ridge and then evacuating the space between metal blocks, we enhanced the field confinement. At last by putting a metal plate over the structure at a proper distance in order to enhance the normal component of the electric field to the upper face of the metal blocks as main component of power transmission, in addition to even more field confinement, we isolated the structure from outside world. We assessed waveguidng properties of... 

Given the increasing need for high-speed wireless links of the 5th and 6th generations, as well as satellite links in millimeter-wave and sub-millimeter-wave bands, the use of reconfigurable intelligent surfaces (RIS) for coverage in both indoor and outdoor spaces is of significant importance. In recent years, the use of tunable arrays through various methods such as optical and electrical has become prevalent. Each of these methods can dynamically control the phase of the reflected wave by influencing the specified characteristics of each element. In this thesis, the design of a liquid crystal-based tunable reflecting array in the millimeter-wave band, for beam shaping with the capability... 

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... 

Terahertz waves have three important features that make them suitable for security imaging. 1. THz waves can penetrate or can be absorbed from many of non-metallic and non-polar materials and are perfectly reflected from most of the metals, 2. Drugs and explosives have terahertz spectra that can be detected by THz imaging, 3. THz waves do not have an ionizing effect and are harmless to the human body. Lenses can be considered as a major component of the terahertz imaging systems. Because of the relatively large terahertz wavelengths compared with wavelengths, compression, and decreasing the dimensions of the lenses in these systems matters a lot. In addition, reducing the wave reflection... 

In this thesis a waveguiding structure is introduced and analyzed for terahertz applications. The proposed structure is called Inhomogeneous Parallel-Plate Ladder Waveguide (IPPLWG). This waveguide is formed by locating a periodic structure inside parallel-plate waveguide.This periodic structure itself consists of a array of metallic posts surrounded by silicon. The combination of metallic posts and silicon have the capability of guiding and confining of EM waves in terahertz band. IPPLWG can be used in implementing the passive terahertz devices such as filters and couplers. The main objective in this thesis is to characterize the IPPLWG for potential applications. Because IPPLWG has open... 

Terahertz passive imaging has unique properties applicable to the field of surveillance and security systems due to terahertz radiation can penetrate fabrics as well as other non-metallic materials and it obtains good resolution for standoff imaging application. In this thesis, we investigate an approach to model and analyse a terahertz passive imaging system performance. We use a reported model for predicting target acquisition performance for a terahertz imaging system and also we present a ray-tracing method and a new metric to compare different quasi-optical structures and scanning systems. Eventually, to evaluate the system performance model, we investigate a commercial terahertz... 

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... 

The effect of dielectric cladding on the waveguiding characteristics of an array of metallic pillars on a metal plane in the terahertz band is explored. Firstly, the 2D structure made up of a metallic grating of infinite lateral width is studied analytically to get more insight into the problem. It is seen that by using dielectric cladding decaying length of the domino waveguide is less than one tenth of the wavelength in a frequency bandwidth.
Then the ideas inferred from the 2D structure are applied to the realistic 3D structure that has a finite lateral width. It is shown that by proper design of the dielectric medium surrounding the metallic pillars the modal field confinement can... 

In this dissertation, to improve the specifications of the available standoff imaging system in the THz and far-infrared Lab. of the electrical engineering department, we designed and analyzed some examples of reflective optical configurations as an optics of active millimeter-wave imaging systems. The superiority of reflective optics versus refractive optics due to the specifications such as loss reduction, broadband frequency response, proper aberration control, beam scanning capability, and easier transmitter and receiver positioning justifies the utilization of reflective configurations for designing millimeter-wave imaging optics. The defined design goals of this dissertation are as... 

In recent decades, the technology of utilizing millimeter wave frequency bands has made remarkable progress. This frequency band is widely employed in important areas such as security applications, medical imaging, quality control testing, and future telecommunications. The main characteristics of waves in this band are their low risk due to low-energy photons compared to X-rays, and higher image resolution in imaging due to their shorter wavelength compared to microwaves. This thesis aims to investigate the effect of irregular reflection of electromagnetic waves from the edges of a metallic body located adjacent to an dielectric background by analyzing the electromagnetic scattering. Then,... 

In recent years, given the rapid advancements in various technologies, there has been a heightened demand for establishing swift communications. Terahertz telecommunication, with its distinctive spectral characteristics, has garnered significant attention among researchers. Consequently, for the operationalization of next-generation wireless telecommunications devices, special considerations must be taken into account in their design. The selection of an appropriate substrate and possessing comprehensive information about substrate behavior at terahertz frequencies are among the crucial factors in these designs. In this thesis, we have examined the interaction of terahertz waves with... 

Terahertz waves which includes frequencies between 100 GHz to 10 THz of electromagnetic spectrum have found many applications in science, medical treatments and industry. One of the recent methods to produce these waves is using nonlinear properties of superconductors based on Josephson junctions which inherently appear in some superconductors called BSCCO. Stack of intrinsic junctions of these superconducting materials can radiate terahertz waves by only applying a DC voltage across them. This Thesis investigates some of these structures based on Josephson junctions to produce coherent radiation of terahertz waves. For the optimization of the radiation power, this study also investigates...