  Internal physical structure can drastically modify the properties of
waveguides: photonic crystal fibers are able to confine light inside a hollow
air core by Bragg scattering from a periodic array of holes, while metamaterial
loaded waveguides for microwaves can support propagation at frequencies well
below cutoff. Anisotropic metamaterials assembled into cylindrically symmetric
geometries constitute light-guiding structures that support new kinds of exotic
modes. A microtube of anodized nanoporous alumina, with nanopores radially
emanating from the inner wall to the outer surface, is a manifestation of such
an anisotropic metamaterial optical fiber. The nanopores, when filled with a
plasmonic metal such as silver or gold, greatly increase the electromagnetic
anisotropy. The modal solutions in anisotropic circular waveguides can be
uncommon Bessel functions with imaginary orders.
