3D Nanophotonics in Artificially Structured Chalcogenide Materials

The iridescent colours often seen in nature in butterfly wings, beetle carapaces and cuttlefish mating displays are a result of the wave nature of light which can show constructive and destructive interference effects for different colours. We can make in the lab repeated structures where the repeat period is close to the wavelength of light and see similar effects; a well-known example being the reflection from a compact disc but also the opposite effect is seen in the anti-reflection coating applied to spectacles which can be seen in the violet coloured high angle reflections. In fact, the angular sensitivity of these diffractive reflection effects can lead to wonderful rainbow reflective colour displays, while some three-dimensional structures such as butterfly wings suppress this angular change; as in the case of the famous blue morphospecies which maintains a largely blue wing colour while flying.

In this project, we aim to build three-dimensional repeating structures that can lead to very strong reflection effects while reducing the angular changes normally seen with two-dimensional gratings and mirrors. These 3D periodic materials can effectively reflect light incident from any angle for a particular range of colours (wavelengths) and essentially block light from passing through the material in any direction. These materials are known as photonic bandgap materials because they block a band of colours and because they have properties analogous to the semiconductor bandgaps that block electrons travelling in certain energy bands. Although difficult to fabricate, these materials could exhibit quite striking and useful effects. For instance, blocking all transmission in all directions could be used as protection against bright light (e.g. lasers) or the bandgap could be made sensitive to certain molecular species or pollutants providing a sensing modality. In our team, we have been looking at the light trapping properties of these materials and their effect on light emission. For instance, if a fluorescent dye molecule with emission band entirely within the bandgap is excited inside such an ideal material then it will have no route by which to emit light and remain in its excited state until decaying by a non-radiative route. However, if we create a cavity by removing a small amount of material, the light emission will occur but will be trapped in this cavity until absorbed or leaking through the finite barrier to the edge of the material. Theoretically, these storage times can be very long while the cavity volumes can be made very small which can lead to a strong enhancement of emission and absorption of light by the fluorophore, so-called 'strong coupling' predicted by the full quantum mechanical treatments of the light-matter interaction.

Finally, in this project, we will develop reliable techniques to make these 3D light confining materials and exploit their novel properties to trap light in tiny 'cavities' and waveguides thus showing the strongest light-matter interactions possible. These results will have an impact across the board from creating new light sources containing single 'atom' like emitters through to the smallest lasers and materials mimicking the reflectivity of butterfly wings.