MODELLING AND DESIGN OF APPLICATION- SPECIFIC PHOTONIC CRYSTAL WAVEGUIDES AND DEVICES

Abstract

Photonic crystals are a unique category of nanomaterials characterized by a periodic arrangement of regions with varying refractive indices. This structured pattern, comparable in scale to the wavelength of light, creates distinctive optical properties. Over the past few years, photonic crystals (PhCs) have become a central focus in nanophotonics research due to their potential for advancing optical technologies and deepening our understanding of light- matter interactions. Their promising applications continue to drive significant research efforts globally. This thesis delves into the design and modelling of photonic crystal-based waveguides and devices, primarily focusing on sensing applications. Silicon-based photonic crystal designs have been emphasized owing to the excellent material properties of silicon (Si), including a large refraction index, low loss at the infrared wavelength, strong Kerr and Raman effects, high thermal conductivity and high optical threshold damage. Silicon is a reliable material, and its processing is well-developed by the electronics industry for most integrated optical applications, prompting intensified research in silicon photonics. The proposed thesis investigates the effect of temperature variations on the photonic bandgap (PBG) characteristics of two-dimensional (2D) and three-dimensional (3D) PhC structures. The 2D structure comprises a hexagonal lattice photonic crystal waveguide (PCW) of air holes in a Si slab. In contrast, the 3D structure is composed of dielectric Si spheres in air in a diamond lattice configuration. The tuning of band-edge and midgap wavelengths with temperature based on the thermo-optic effect in Si has been demonstrated for temperature sensing applications. The 2D and 3D structures have also been proposed for refractive index (RI) sensing, where the 2D PhC is used as an RI sensor for different liquids and the 3D PhC is used as a biosensor for different blood components. Further exploring the tunability of PhCs by shifting focus from temperature to pressure, the effect of pressure variations on the PBG characteristics of different semiconductor-based PhCs has been analysed. Pressure-sensitive 2D PhC designs with a hexagonal lattice of air holes in dielectric slabs based on gallium arsenide (GaAs), germanium (Ge) and Si have been achieved by exploiting the stress optic effect in these materials. A comprehensive XII comparative analysis of the pressure sensitivities and properties of the three semiconductors- based PhCs is presented proposing the structures for high-pressure sensing applications. Moreover, the impact of temperature variation on the coupling characteristics of a Si-PCW coupler has been thoroughly examined. The influence of temperature changes, based on the thermo-optic effect in Si, on the coupling parameters of the PCW coupler composed of Si rods in air has been studied extensively. The variation in coupling length with temperature is evaluated computationally and corroborated mathematically based on the coupled-mode theory.