Interplay of index contrast with periodicity in polymer photonic crystals

We report how the strength of resonant Bragg reflection from polymeric photonic crystals (polymer opals) varies linearly with the refractive-index contrast, Delta n, in contrast to the quadratic buildup of Fresnel reflections scaling as (Delta n)(2). This occurs due to the interplay of disorder and periodicity, in agreement with a simple 1-dimensional periodic model. Goniometry experiments show that opal films exhibit "cones" of resonantly scattered light, which extend to +/- 20 degrees angular deviation from the specular direction. The intensity of the scattering cones varies super-linearly with Delta n. Such medium contrast photonic crystals are of significant interest for understanding structural colors exhibited in nature, by structures with inherent disorder. (C) 2011 American Institute of Physics. [doi:10.1063/1.3672215]