  We have performed an x-ray holotomography study of a three-dimensional (3D)
photonic band gap crystal. The crystals was made from silicon by
CMOS-compatible methods. We manage to obtain the 3D material density throughout
the fabricated crystal. We observe that the structural design is for most
aspects well-realized by the fabricated nanostructure. One peculiar feature is
a slight shear-distortion of the cubic crystal structure. We conclude that 3D
X-ray tomography has great potential to solve many future questions on optical
metamaterials.
