Anodic aluminum oxide (AAO) has long been known to display self-organized structure. Here is a nice SEM image. Since alumina is a durable material, AAO can be used as masks and molds for patterning materials. For example, polycyanurate nanorods were recently polymerised in AAO molds for sensing applications.
While self-organization yields largely periodic structures, long-range order must be induced by other means. This turns out to be quite simple: one embosses the aluminum starting layer before starting the anodization. See this article. Click through to the downloadable Supplementary info for a process pictorial.
The same Berkeley group now reports a variant of the technique to yield dual-diameter pores. They used these to grow germanium arrays which absorb 99% of all incoming radiation in the wavelength range of 300 nm – 900 nm. This is almost as absorptive as the SWNT carpet reported some time ago, (SEM image) but a good deal more useful for photonics.