Suppression of the critical angle of diffraction in thin-film colloidal photonic crystals

Abstract

We present an experimental and numerical investigation of the polarization anisotropy of the zero-order backdiffracted light from three-dimensional thin-film photonic crystals assembled from colloidal spheres. In particular, we compare simulations of reflectance spectra from perfectly ordered fcc lattice of spheres with measured reflectance data from self-organized opal films and forced-assembled Langmuir-Blodgett crystal films. We identify cross-polarization couplings and interactions between photonic crystal eigenmodes as the major physical mechanisms for resonance depolarization effects. Based on this, we find that a necessary condition for the observation of critical angles of diffraction in three-dimensional lattices is that the orientation of the light's plane of incidence coincides with a high-symmetry plane of the crystal lattice. We further show that for currently achievable colloidal photonic crystals with moderate refractive index contrast, this resonance depolarization mechanisms together with the destructive influence of lattice disorder effectively renders the meaning of a critical angle of diffraction obsolete.