Self-Assembly of Liquid Crystals in Nanoporous Solids for Adaptive Photonic Metamaterials

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.

arXiv commentSupporting information (SI) available as ancillary file. The SI movies are available at the repository TORE of Hamburg University of Technology (https://doi.org/10.15480/336.2515). This work is dedicated to Prof. Peter S. Pershan (Harvard University), a pioneer in the field of x-ray scattering studies of soft matter, in particular of liquid surfaces and liquid crystals, on his 85th birthday

References in corpus (14)

• Parallel Tempering: Theory, Applications, and New Perspectives
• Accurate determination of crystal structures based on averaged local bond order parameters
• Complete amplitude and phase control of light using broadband holographic metasurface
• Switchable Imbibition in Nanoporous Gold
• Continuous Paranematic-to-Nematic Ordering Transitions of Liquid Crystals in Tubular Silica Nanochannels
• Elastic Response of Mesoporous Silicon to Capillary Pressures in the Pores
• Quantized Self-Assembly of Discotic Rings in a Liquid Crystal Confined in Nanopores
• Preferred orientation of n-hexane crystallized in silicon nanochannels: A combined x-ray diffraction and sorption isotherm study
• Melting and freezing of argon in a granular packing of linear mesopore arrays
• Thermotropic Orientational Order of Discotic Liquid Crystals in Nanochannels: An Optical Polarimetry Study and a Landau-de Gennes Analysis
• A ferroelectric liquid crystal confined in cylindrical nanopores: Reversible smectic layer buckling, enhanced light rotation and extremely fast electro-optically active Goldstone excitations
• Liquid n-hexane condensed in silica nanochannels: A combined optical birefringence and vapor sorption isotherm study
• Inhomogeneous Relaxation Dynamics and Phase Behaviour of a Liquid Crystal Confined in a Nanoporous Solid
• Entropy-driven enhanced self-diffusion in confined reentrant supernematics

Cited by in corpus (7)

• Wafer-Scale Electroactive Nanoporous Silicon: Large and Fully Reversible Electrochemo-Mechanical Actuation in Aqueous Electrolytes
• Anisotropic Confinement of Chromophores Induces Second-Order Nonlinear Optics in a Nanoporous Photonic Metamaterial
• On the Issue of Textured Crystallization of Ba(NO${}_3$)${}_2$ in Mesoporous SiO${}_2$: Raman Spectroscopy and Lattice Dynamics Analysis
• Straight versus Spongy -- Effect of Tortuosity on Polymer Imbibition into Nanoporous Matrices Assessed by Segmentation-Free Analysis of 3D Sample Reconstructions
• Multiple glassy dynamics of a homologous series of triphenylene-based columnar liquid crystals -- A study by broadband dielectric spectroscopy and advanced calorimetry
• Adsorption from Binary Liquid Solutions into Mesoporous Silica: A Capacitance Isotherm on 5CB Nematogen/Methanol Mixtures
• Nematic liquid crystal flow driven by time-varying active surface anchoring