Self-assembly of liquid crystals in nanoporous solids for adaptive photonic metamaterials
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.
Funding
Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) Projektnummer 192346071, SFB 986 ”Tailor-Made Multi-Scale Materials Systems
European Union Horizon 2020 research and innovation programme under the Marie Skodowska-Curie grant agreement No 778156
History
School
- Science
Department
- Mathematical Sciences
Published in
NanoscaleVolume
11Issue
48Pages
23304 - 23317Publisher
Royal Society of Chemistry (RSC)Version
- VoR (Version of Record)
Rights holder
© The Royal Society of ChemistryPublisher statement
This is an Open Access Article. It is published by Royal Society of Chemistry under the Creative Commons Attribution-NonCommercial 4.0 International Licence (CC BY-NC). Full details of this licence are available at: https://creativecommons.org/licenses/by-nc/4.0/Acceptance date
2019-10-14Publication date
2019-11-20Copyright date
2019ISSN
2040-3364eISSN
2040-3372Publisher version
Language
- en