MODELING OF PHOTONIC CRYSTAL BASED LOGIC GATES AND OPTICAL DEVICES

Abstract

Recently, light guidance in photonic crystal structures has become one of the most fascinating technologies for the design and development of various optoelectronic devices. These structures provide an interesting platform for building the photonic circuitry due to their unique possibilities of molding the flow of light. Analogous to the electronic band gap in atomic crystals, photonic crystals exhibit photonic band gap, which is a wavelength range over which electromagnetic radiations are inhibited. The density of photon states in the photonic band gap region goes to zero. The two dimensional (2D) photonic crystals have attracted considerable attention due to their on-chip applications and the comparative ease of fabrication. They are periodic along a plane and extruded in the third dimension i.e. they include either periodically arranged dielectric rods in air or equally spaced air holes in a dielectric block. This thesis addresses the design of all optical photonic crystal (PhC) devices and circuits. In this thesis, some novel designs of all optical logic gates have been presented. The two dimensional PhC composed of hexagonal array of silicon rods in air has been used for devising ultra compact photonic crystal waveguide based AND optical logic gate. Further, the AND optical logic gates have been designed in the PhC consisting of hexagonal array of air holes in silicon with the material waveguide as well as with the air waveguide. The proposed gates are operational at a wavelength of 1.55μm as also indicated by their spectral response. Further, all-optical logic gates such as AND, OR, NOT, NAND, NOR, XOR, XNOR have been designed by introducing an appropriate initial phase difference between the input waveguides and the reference waveguide in the photonic crystal air bridge structure working for single polarization. However, these structures are mechanically unstable and require specific polarization leading to their unsuitability for on-chip integration. To overcome these problems, polarization independent all-optical logic gates have been designed on silicon on insulator (SOI) substrate. The contrast ratio, response period and bit rate have been calculated for all the proposed designs. These devices have Abstract x high switching rates as they require low power consumption of about micro watts and response time less than few pico seconds. Moreover, these devices are easier to fabricate and could be a strong candidate for future polarization independent all optical integrated circuits. Another unique property of the photonic crystal structures is their ability to slow down the group velocity of light due to the strong light matter interaction. Slow light in slotted photonic crystal waveguide with slow down factor of 18.50 and its application as an optical buffer without the use of nonlinear material has been investigated in the thesis. The structure has been analysed for its application as time and wavelength division demultiplexer. Complete photonic band gap is exhibited by a few specifically designed photonic crystal structures only. A photonic crystal with honey comb lattice arrangement of air holes of two different radii on SOI substrate exhibits complete photonic band gap for a narrow range of frequencies. This feature has been exploited to design a polarization beam splitter. The characteristics of the proposed polarization beam splitter such as extinction ratio, insertion loss, excess loss, coupling loss and degree of polarization indicate the separation of two polarizations. Hence, a suitably tailored photonic crystal structure can have a broad spectrum of applications ranging from the development of all-optical logic families to slow light photonic devices and polarization splitters, operational at a wavelength of 1.55 μm.