Real 3D Photonic Band Gap Crystals are Perfect Specular Reflectors

It is generally assumed that 3D photonic band gaps behave as perfect reflectors, which would be striking in view of their complex 3D nanostructures. Therefore, we developed an interferometric optical reflectivity microscope to observe phase-sensitive complex reflectivity and intensity reflectivity of nanophotonic structures. We define a measure of the specularity of a sample, where we ratio the complex reflectivity amplitude to the intensity reflectivity. We experimentally observe that the specularity in the gap of 3D Si woodpile crystals is near unity. Thus, 3D band gap crystals are perfect specular reflectors, and remarkably well outside the gaps.