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Abstract

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.