Sharif Digital Repository

Nowadays high temperature superconductors (HTS) are very efficient and useful materials for microwave devices such as planar filters, resonators, hybrids, antennas and delay lines. The advantages of these devices are high quality factor, low insertion loss and reduced size. However the nonlinear effects of superconductors in high incident powers has limited usage of HTS in some applications and causes disadvantages such as reducing quality factor, intermodulation distortion and third harmonic generation of signal. The nonlinear effects of superconductor are due to the dependency of penetration depth and normal conducting coefficient to the current density of superconductor. Nonlinr effect... 

In this thesis, first, random lasing in a three dimensional disordered medium has been simulated using FDTD method and some properties of random lasers which were observed by others in the previous experiments have theoretically been shown. Our simulation results show that when the density of particles increases to an upper limit, the intensity of lasing modes is enhanced. Also, the effects of pumping rate and particle size on the number of lasing modes and their intensity are studied. Lasing threshold of laser modes in different disordered systems is calculated and it is shown that by an appropriate selection of the central frequency of gain line-shape, the output power intensity of random... 

In this thesis periodic plasmonic nanostructures are studied for detection of biological specious. The behavior of metal nanostructure arrays under normal incidence has been widely reported. However, simulation of periodic dispersive structures under oblique incidence requires newer formulations. Formulations, and algorithms based on modified split-field finite-difference time-domain (SF-FDTD) method are introduced, permitting analysis of metallic nanostructures arrays under oblique incidence. These novel algorithms are practically implemented on a parallel processing system based on graphics processing unit (GPU). Test and verification of these formulations are done by analyzing referenced... 

Different methods of shifting and shaping of space-time electromagnetic pulses are in this work investigated by nonlinearity and/or dispersion. Developing two dimensional (2D) finite difference time domain (FDTD) codes in the nonlinear regime and taking the optical dispersion of ideal metals into account, nanophotonic and plasmonic structures are analyzed in this thesis.
Goos-Hänchen shift, superprism effect, and optical bistability are particularly emphasized. A heuristic approximation is presented to extract the Goos-Hänchen shift at the interface of 1D and 2D photonic crystals. The superprism effect for electric polarization – where the electric field vector is perpendicular to the 2D... 

Finite dielctric structures are commonly used in optical devices, Microwave Integrated Circuitc (MICs) and printed antennas. To analyze these structures, full-wave analysis methods cannot be employed easily. They need huge computer resources and are time-consuming especially for electrically large structures. On the other hand, asymptotic techniques may not be exact enough for these structures. In this thesis, MPIE technique is chosen as an accurate and efficient technique for analyzing these structres. To use this technique, the magnetic vector potential and electric scalar potential are required. Uniform and closed-form spatial Green's function for finite dielectric structures is... 

The Transparent electrode (transparent conductive layer) is an essential component of electro-optical devices, especially screens, solar cells and touch screens. Today, most semiconductor oxides such as ITO are used for this purpose, which have problems such as inflexibility, high cost, environmental degradation and ion penetration in organic displays. In this research, the design, simulation and fabrication of a transparent electrode with a network of metal nanowires with low resistance between cross fibers and high aspect ratio during the production stages of polymer mold, metal coating and characterization has been done. Using the reducing atmosphere in the sputtering chamber, the... 

Ever-increaing demand for wireless communication systems impels an efficient system planning before the system deployment. The very first step of the planning is radio-wave channel modeling. Generally, all propagation models can be categorized in three types of models: Emprical models, semi-emprical models and deterministic models. The statistical and semi-emprical models can give a quite prompt signal level prediction over the vast propagation area using few computations. In the recent wireless services which wider bandwidth is required, only the signal level prediction cannot give a proper perspective of design and planning. The parameters such as the power-delay profile, rms delay spread;...