Doctor of Philosophy (PhD)
In this dissertation, we first introduce compact tunable spatial mode converters for graphene parallel plate (GPP) waveguides. The converters are reciprocal and based on spatial modulation of graphene’s conductivity. The wavelength of operation of the mode converters is tunable in the mid-infrared wavelength range by adjusting the chemical potential of a strip on one of the graphene layers of the GPP waveguides. We also introduce optical diodes for GPP waveguides based on a spatial mode converter and a coupler, which consists of a single layer of graphene placed in the middle between the two plates of two GPP waveguides. \par Next, we propose an ultracompact isolator for optical frequencies employing the potential of plasmonics for miniaturizing optical structures along with the magneto-optical (MO) effect of Bismuth Iron Garnet (BIG) to achieve non-reciprocity. The proposed structure consists of a cylindrical cavity placed in the proximity of a metal-dielectric-metal (MDM) waveguide. The isolation occurs due to the frequency splitting phenomenon along with the difference in transmitted power for waves incident on the device from different directions, in the presence of MO activity. The first order perturbation theory is used to explain the frequency splitting phenomenon. The difference between the transmission spectra is described via studying the nature of the resonant modes inside the cavity. The performance of the device can be tuned via changing the parameters of the isolator such as the radius of the cavity and the distance between the MDM and the cavity. Next, we theoretically explore the notion of employing plasmonic multilayered cylindrical cavities to enhance the MO effect at optical frequencies. The frequency splitting observed in the presence of MO activity can be greatly increased in the multilayered cylindrical cavities with alternating layers of BIG and metal. We also investigated the effect of different parameters of the structures on enhancing the MO activity using transfer matrix approach as well as numerical investigations. We showed that, provided that the material and the design are chosen carefully, the MO activity can be noticeably increased. Finally, we designed an all-optical circulator based on the layered structure.
Foroughi Nezhad, Vahid, "Unidirectional and Nonreciprocal Nanophotonic Devices Based on Graphene and Magneto-Optical Materials" (2019). LSU Doctoral Dissertations. 5081.
Available for download on Sunday, October 25, 2020