Self-assembly of confined liquid crystals: from nanoscale physics to designing photonic metamaterials

Liquid crystals in combination with nanoporous media, exhibiting structures significantly smaller than the wavelengths of visible light, are promising candidates to design new soft-hard photonic metamaterials. The liquid crystal provides functionality, resulting from temperature tunable anisotropic properties, i.e. optical birefringence, while the nanoporous solid provides a mechanical robust scaffold structure. Then the optical functionality of prospective metamaterials is not determined by the properties of base materials, but from newly designed multiscale structures often absent in the base materials. However, the extreme spatial and topological constraints can result in inhomogeneous molecular arrangements or even the complete suppression of liquid crystallinity in nanoconfinement hampering the optical functionality. Depending on the type of confining space (meso- and macroporous silica and anodized aluminium oxide) and as a function of temperature, pore size and pore surface chemistry, the molecular self-assembling process and the resulting metamaterial potential of a discotic liquid crystal (HAT6) in nanoconfinement is analyzed. In order to correlate orientational and translational ordering, synchrotron-based 2D X-ray diffraction in combination with high-resolution optical polarimetry measurements are performed. Resulting from a quantized phase transition a rarely measured material specific parameter giving a measure of the tendency of the liquid crystal to bend along the preferred alignment direction is found. Additionally, the experiments and complementary Monte Carlo simulations show a remarkably rich self-assembly behavior resulting in structures absent in the bulk state. These diverse configurations on the nanoscale result in integrated material systems exhibiting a temperature- and liquid-solid interaction-dependent positive and negative optical birefringence. Hence, the self-organized nanoporous solids in combination with liquid-crystalline self-assembly allow the fabrication of newly designed ...