Borrmann effect in photonic crystals.

We present the experimental observation of the optical Borrmann effect in 1D porous silica-based photonic crystals (PhC) composed of several hundreds of dielectric layers with different porosity. Two mechanisms of the effect are demonstrated, which involve the optical losses associated with light scattering and absorption in nanoporous layers. Absorption is introduced by a small amount of silicon in partially annealed porous silicon PhC. We show that the PhC transmittance increases substantially under the Bragg diffraction condition, which is a manifestation of the Borrmann effect. The observed effects along with their polarization sensitivity are confirmed by numerical calculations.