Disklike molecules with aromatic cores spontaneously stack up in linear columns with high, one-dimensional charge carrier mobilities along the columnar axes, making them prominent model systems for functional, self-organized matter. We show by high-resolution optical birefringence and synchrotron-based x-ray diffraction that confining a thermotropic discotic liquid crystal in cylindrical nanopores induces a quantized formation of annular layers consisting of concentric circular bent columns, unknown in the bulk state. Starting from the walls this ring self-assembly propagates layer by layer towards the pore center in the supercooled domain of the bulk isotropic-columnar transition and thus allows one to switch on and off reversibly single, nanosized rings through small temperature variations. By establishing a Gibbs free energy phase diagram we trace the phase transition quantization to the discreteness of the layers' excess bend deformation energies in comparison to the thermal energy, even for this near room-temperature system. Monte Carlo simulations yielding spatially resolved nematic order parameters, density maps, and bond-orientational order parameters corroborate the universality and robustness of the confinement-induced columnar ring formation as well as its quantized nature.

light the whole structure is described as a metamaterial [1][2][3]. A monolithic membrane traversed by a parallel array of cylindrical nanochannels represents such a metamaterial, see illustration in Fig. 1. The advent of tailorable porosity at the nanoscale and thus of an adjustable "meta-atomic" structure offers new opportunities to tailor the optical anisotropy of such metamaterials. Effective optical properties, not achievable by base materials, are possible [2-8], specifically adjustable birefringence. This is of particular relevance for important polarisation management components, such as waveplates and compensators, that play an essential part in modern optical communication systems [9].A versatile route to fine-tune optics of porous materials is filling the pore space with a liquid crystal (LC) [10,11]. In the bulk case LCs play a dominant role for the manipulation of optical anisotropy [12]. By embedding LCs in nanoporous media the self-assembly and self-healing mechanisms can be used to design a new soft-hard metamaterial. The LC adds optical functionality, whereas the arXiv:1911.10052v1 [cond-mat.soft]