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Publisher DOI: 10.1039/C9NR07143A
Title: Self-assembly of liquid crystals in nanoporous solids for adaptive photonic metamaterials
Language: English
Authors: Sentker, Kathrin 
Yildirim, Arda 
Lippmann, Milena 
Zantop, Arne 
Bertram, Florian 
Hofmann, Tommy 
Seeck, Oliver H. 
Kityk, Andriy V. 
Mazza, Marco G. 
Schönhals, Andreas 
Huber, Patrick 
Issue Date: 2019
Publisher: RSC Publ.
Source: Nanoscale (11) (2019)
Journal or Series Name: Nanoscale 
Abstract (english): 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 160nm 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.
DOI: 10.15480/882.2517
ISSN: 2040-3372
Institute: Werkstoffphysik und -technologie M-22 
Type: (wissenschaftlicher) Artikel
Funded by: Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)
Project: Diskotische Flüssigkristalle in Nanoporösen Festkörpern: Von der Struktur und Dynamik zum lokalen Ladungtransport 
SFB 986: Teilprojekt B7 - Polymere in grenzflächenbestimmten Geometrien: Struktur, Dynamik und Funktion an planaren und in porösen Hybridsystemen 
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