Fabricating opals to control the flow of light

Photonic crystals are structures that can control light in ways analogous to the control that semiconductor materials have over electrons. The opal gemstone is a naturally occurring example of a photonic crystal, and its iridescence in an example of its control over the flow of light. A common method for fabricating opals is self-assembly. This involves taking a sample of tiny spheres, usually less than 1μm in diameter, and arranging them into a periodic lattice. We report on both the development of fabrication methods, including vertical deposition and sedimentation, and experimental results for polystyrene and silica based self-assembled opal structures. High magnification images show near perfect order of spheres over large regions (~100×100 μm for polystyrene and ~50×50 μm for silica). Good control over sample thickness is achieved through controlling the concentration of microspheres in the initial solutions. Photonic bandgaps were measured and compared well to theoretically modelled bandgaps. Self-assembled opals were also grown around tapered fibre structures. Tapered fibres are fabricated by heating and drawing a standard optical fibre, causing a section of the fibre to taper down such that the core and cladding now have a significantly smaller diameter (~20μm). This means that light travelling down the fibre will have an evanescent field that exists outside of the tapered region, and hence will interact with the environment outside the fibre. By growing an opal structure around a tapered fibre, we can probe the resonant features of the opal by launching light down the fibre and observing transmission and reflection spectra.