Sharif Digital Repository

In this thesis we study the possibility of unidirectional transmission of light using reciprocal and nonreciprocal elements. Unidirectional transmission will allow us to separate the coupling of light into and out of resonators and design unidirectional waveguides. We start by studying the effects of one-sided modulations of dielectric constant of reciprocal structures. One-sided modulation will satisfy the phase matching condition for the coupling of two waves only in one direction. Using coupled mode theory, we study one sided modulation in layered media. Next, we find the exact solution to such strucures using transfer matrix method and analyze one-sided modulation in waveguide... 

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... 

Time-varying electromagnetic structures have provided the possibility of realizing new phenomena and, as a result, new applications, which include non-reciprocity, frequency conversion, amplification, and frequency comb. Among the researches that have been done on time-varying electromagnetic structures, less has been done on the rotating time-varying structure. Rotating time variations have great advantages over other time-varying structures. One of the advantages of rotating time changes is the lack of production of many additional harmonics, which originates from the principle of rotating Doppler shift in frequency. Among its other advantages, it is possible to define a time... 

Metasurfaces offer unique capabilities for the unconventional scattering and control of electromagnetic waves. By using metasurfaces, one can arbitrarily manipulate the amplitude, phase, and polarization of incident waves, enabling devices such as flat lenses, holograms, and electromagnetic wave beam deflectors. In recent years, time-varying electromagnetic structures, including time-modulated metasurfaces, have provided enhanced possibilities for controlling electromagnetic waves. By modulating the properties of metasurfaces over time, phenomena such as nonreciprocity, frequency shifts, and parametric amplification can be achieved, which traditionally require magnetic materials or nonlinear... 

In recent years, the growing research on time-varying linear media has enabled the emergence of novel and versatile wave propagation phenomena that offer significant advantages over conventional static structures. Electromagnetic effects such as non-reciprocity and isolation without the need for magnetic bias field and with optical integration capabilities, frequency conversion without the need for nonlinear elements, and parametric amplification are among the key breakthroughs in this field. These phenomena arise from the interaction of electromagnetic waves with media whose fundamental parameters—namely permittivity, permeability, or electrical conductivity—vary in time. Most existing...