Cavity-enhanced structural colour in extrudeable photonic crystals

Photonic crystals remain of significant interest because of the opportunity to modify a host of optical properties by structuring the material at the sub-wavelength scale, including enhanced light emission and absorption, superprism performance, and negative refraction. However until now photonic crystals have remained expensive to fabricate and control, and inconceivable to adopt in industrial processes because their assembly requires expensive and slow lithographies. In addition, most work has concentrated on high refractive index contrast systems which offer the best chance of light localisation. Here we discuss a novel range of polymer photonic crystals that we have successfully assembled into >100 m long films using the process of shear assembly. Core-shell polymer spheres are designed in such a way that they lock into 3D fcc opals when suitably sheared, and we will demonstrate how this process can work in the extrusion geometry to give single domain samples. The optical properties of these elastomeric opals are highly unusual, since they are dominated by scattering which is enhanced by the photonic crystal environment. We will discuss how new coherent backscatter measurements taken across all wavelengths show how the photon mean free path is modified in these materials, and how scattering is forward directed in a cone at specific resonant colours. A host of applications currently under investigation will be discussed.