Orientation-dependent photonic bandgaps in gold-dust weevil scales and their titania bioreplicates

The scales of the gold-dust weevil Hypomeces squamosus are green because of three-dimensional diamond-type chitin-air photonic crystals with an average periodicity of about 430 nm and a chitin fill fraction of about 0.44. A single scale usually contains one to three crystallites with different lattice orientations. The reciprocal space images and reflection spectra obtained from single domains indicated a partial photonic bandgap in the wavelength range from 450 to 650 nm. Light reflected from {111}-oriented domains is green-yellow. Light reflected from blue, {100}-oriented domains exhibits polarization conversion, rotating the angle of linearly polarized light. The overall coloration, resulting from the reflections from many scales, is close to uniformly diffuse because of the random orientation of the domains. Using titania sol-gel chemistry, we produced negative replicas that exhibited a 70 to 120 nm redshift of the bandgap, depending on the lattice orientation. The wavelength shift in {100} orientation is supported by full-wave optical modeling of a dual diamond network with an exchanged fill fraction (0.56) of the material with the refractive index in the range of 1.55 to 2.00. The study suggests that the effective refractive index of titania in the 3D lattice is similar to that in sol-gel films. The study demonstrates the potential of replicating complex biophotonic structures using the sol-gel technique. Optimization of the sol-gel process could lead to customizable photonic bandgaps that might be used in novel optical materials.