Self-assembly of liquid crystals in nanoporous solids for adaptive photonic metamaterials.
Self-assembly of liquid crystals in nanoporous solids for adaptive photonic metamaterials.
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DOI：
10.1039/c9nr07143a
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发表时间：
2019-11
期刊：
影响因子：
6.7
通讯作者：
K. Sentker;Arda Yildirim;Milena Lippmann;A. Zantop;Florian Bertram;Tommy Hofmann;O. Seeck;A. V. Kityk;Marco G Mazza;A. Schönhals;Patrick Huber
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文献类型：
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作者：
K. Sentker;Arda Yildirim;Milena Lippmann;A. Zantop;Florian Bertram;Tommy Hofmann;O. Seeck;A. V. Kityk;Marco G Mazza;A. Schönhals;Patrick Huber
纳米多孔介质的结构比可见光的波长小得多，因此可以作为光子超材料。它们的光学功能不是由基础材料的性质决定的，而是由量身定做的多尺度结构决定的，就精确的孔形状、几何形状和取向而言。在孔隙空间中嵌入液晶提供了在单孔、超原子尺度上控制光-物质相互作用的额外机会。在这里，我们提出了基于同步加速器的X射线衍射和高分辨率双折射实验相结合的依赖温度的3D倒易空间映射，该实验吸收在自排列的平行圆柱孔阵列中的盘状介元(HAT6)，直径从17 nm到160 nm，在单块阳极氧化铝(AAO)中。与蒙特卡罗计算机模拟一致，我们观察到了一种非常丰富的自组装行为，这是从体态未知的。它包括线性柱中各向同性液体状态和盘状堆积之间的转变，以及垂直和平行于孔轴的圆形同心环的形成。这些织构转变支撑了光学双折射功能，可通过孔径、孔表面嫁接和温度从正到负的大小和符号进行调节。我们的研究表明，在整体固体中出现大规模的自组织纳米孔，以及液晶物质在单孔尺度上的限制可控的相行为，为设计具有可调节光学各向异性的材料提供了可靠和可访问的工具，从而为微调材料中偏振相关的光传播速度提供了多种途径。这种可裁剪性是新兴的变革性光学领域的核心，允许例如可调节的吸光器和极薄的金属透镜。
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.