Modification of the refractive-index contrast in polymer opal films

Synthetic opals, based on self-assembled arrays of core-shell (bead/matrix) polymer microspheres, are a promising platform for next-generation photonic structures, coatings and sensors. The refractive index contrast (Delta n) between beads and the matrix polymer is essential for the appearance of structural colour in polymer opal films. We report how the index contrast can be modified by engineering the chemical composition of the core-interlayer-shell (CIS) precursor particles. Alternative approaches to emulsion polymerisation, using the fluorinated monomer 2,2,2-trifluoroethyl acrylate and the aromatic monomer benzyl acrylate, yield a much larger range of Delta n values than for standard systems made from styrene and ethyl acrylate. Spectroscopic studies reveal striking differences in the transmission properties of thin-films as Delta n is varied from 0.045 up to 0.18.