Theory and modelling of functional photonic opals.

In this dissertation, we explore the optical properties of various opal-based photonic crystal structures. Particular attention is paid to the tunability of the optical properties, especially the photonic band gaps (PBG), with the motivation to apply these opal-based photonic crystals (PCs) to the design of functional surfaces and switchable windows. After reviewing the basic optical properties of inverse opals, two different types of opal-based photonic crystals, namely the Double-Shell Photonic Crystal (DSPC) and the Double-Inverse-Opal Photonic Crystal (DIOPC) are successively introduced and throroughly studied. In the DSPC structure, each sphere in the periodic photonic crystal structure is made of a hollow core, along with an accompanying shell of a different dielectric material; the resulting spherical structure is embedded in a high-index dielectric background. By contrast, the DIOPC is designed with an inverse opal backbone, in which the air pores are partially filled with a dielectric core sphere. In our work, two types of photonic bandgap tuning are examined, namely geometrical/positional, and tailoring of the materials' properties. A comparison of the proposed structures regarding their potential for experimental realization is also performed. Considering that the air shells in the DIOPC structure allow for relative movement of the dielectric cores inside them, we propose and study a completely novel approach to obtain a switchable complete band gap achieved, by shifting the spheres inside the air shells. After demonstrating that the complete photonic band gap is open for certain core sphere positions and is closed for others, we propose ways to optimise this new switching process. This optimisation leads to a maximum switching of the complete photonic band gap of 3.5%.Taking into account that in real-world situations, all photonic crystals possess a certain amount of structural imperfections, the last step of the study concerns the effects of disorder on the optical properties of opal-based photonic crystals. After discussing different types of disorder, in both the backbone and the core spheres, we conclude that to retain a complete PEG in the DIOPC structure, and thus the switching process, a maximum backbone disorder of 1 % should be reached experimentally, whereas the disorder on the core sphere does not affect the PEG as much. These investigations have been performed using both a plane-wave expansion method and a finite-difference time-domain method