On the interpretation of small angle diffraction experiments on opaline photonic crystals

Artificial opals are the most important approach to self-assembled photonic crystals. They have been characterized optically and by electron microscopy, but detailed structural information is still missing. Classical methods fail because of the small scattering angles. Recently, interesting structure investigations with neutron reactor sources and synchrotrons have been published. Theses studies reveal a surprisingly high degree of order in the scattering pattern but also some unexpected scattering features. In this contribution we add two points to the current interpretation of the diffraction data. First, the exclusion of the allowed (002) diffraction peaks by a small atomic form factor is not fully justified and, second, surfaces scattering has to be included as a possible source for the diffraction peaks. Our neutron diffraction data indicate that surface scattering is the main reason for the lowest detected order in the diffraction patterns.