Shang Guoliang, Furlan Kaline Pagnan, Zierold Robert, Blick Robert H, Janßen Rolf, Petrov Alexander, Eich Manfred
Institute of Optical and Electronic Materials, Hamburg University of Technology, Eissendorfer Strasse 38, 21073, Hamburg, Germany.
Institute of Advanced Ceramics, Hamburg University of Technology, Denickestrasse 15, 21073, Hamburg, Germany.
Sci Rep. 2019 Aug 6;9(1):11379. doi: 10.1038/s41598-019-47963-2.
Self-assembled periodic structures out of monodisperse spherical particles, so-called opals, are a versatile approach to obtain 3D photonic crystals. We show that a thin conformal coating of only several nanometers can completely alter the reflection properties of such an opal. Specifically, a coating with a refractive index larger than that of the spherical particles can eliminate the first photonic band gap of opals. To explain this non-intuitive effect, where a nm-scaled coating results in a drastic change of optical properties at wavelengths a hundred times bigger, we split the permittivity distribution of the opal into a lattice function convoluted with that of core-shell particles as a motif. In reciprocal space, the Bragg peaks that define the first Brillouin zone can be eliminated if the motif function, which is multiplied, assumes zero at the Bragg peak positions. Therefore, we designed a non-monotonic refractive index distribution from the center of the particle through the shell into the background and adjusted the coating thickness. The theory is supported by simulations and experiments that a nanometer thin TiO coating via atomic layer deposition (ALD) on synthetic opals made from polystyrene particles induces nearly full transparency at a wavelength range where the uncoated opal strongly reflects. This effect paves the way for sensing applications such as monitoring the thicknesses growth in ALD in-situ and in real time as well as measuring a refractive index change without spectral interrogation.
由单分散球形颗粒构成的自组装周期性结构，即所谓的蛋白石，是获得三维光子晶体的一种通用方法。我们表明，仅几纳米厚的薄保形涂层就能完全改变这种蛋白石的反射特性。具体而言，折射率大于球形颗粒折射率的涂层可以消除蛋白石的第一个光子带隙。为了解释这种非直观效应，即纳米级涂层会导致波长比其大一百倍处的光学性质发生剧烈变化，我们将蛋白石的介电常数分布分解为一个晶格函数与作为基元的核壳颗粒的介电常数分布的卷积。在倒易空间中，如果相乘的基元函数在布拉格峰位置处为零，则可以消除定义第一布里渊区的布拉格峰。因此，我们设计了一种从颗粒中心穿过壳层到背景的非单调折射率分布，并调整了涂层厚度。模拟和实验支持了该理论，即通过原子层沉积（ALD）在由聚苯乙烯颗粒制成的合成蛋白石上沉积纳米级薄的TiO涂层，会在未涂层蛋白石强烈反射的波长范围内产生几乎完全的透明度。这种效应为传感应用铺平了道路，例如原位实时监测ALD中的厚度增长以及在无需光谱询问的情况下测量折射率变化。
