  Nanoporous media exhibit structures significantly smaller than the
wavelengths of visible light and can thus act as photonic metamaterials. Their
optical functionality is not determined by the properties of the base
materials, but rather by tailored, multiscale structures, in terms of precise
pore shape, geometry, and orientation. Embedding liquid crystals in pore space
provides additional opportunities to control light-matter interactions at the
single-pore, meta-atomic scale. Here, we present temperature-dependent 3D
reciprocal space mapping using synchrotron-based X-ray diffraction in
combination with high-resolution birefringence experiments on disk-like
mesogens (HAT6) imbibed in self-ordered arrays of parallel cylindrical pores 17
to 160 nm across in monolithic anodic aluminium oxide (AAO). In agreement with
Monte Carlo computer simulations we observe a remarkably rich self-assembly
behaviour, unknown from the bulk state. It encompasses transitions between the
isotropic liquid state and discotic stacking in linear columns as well as
circular concentric ring formation perpendicular and parallel to the pore axis.
These textural transitions underpin an optical birefringence functionality,
tuneable in magnitude and in sign from positive to negative via pore size, pore
surface-grafting and temperature. Our study demonstrates that the advent of
large-scale, self-organised nanoporosity in monolithic solids along with
confinement-controllable phase behaviour of liquid-crystalline matter at the
single-pore scale provides a reliable and accessible tool to design materials
with adjustable optical anisotropy, and thus offers versatile pathways to
fine-tune polarisation-dependent light propagation speeds in materials. Such a
tailorability is at the core of the emerging field of transformative optics,
allowing, e.g., adjustable light absorbers and extremely thin metalenses.
