Hypersonic Phononic Colloidal Crystals on the Basis of Submicroscopic Silica, Polymer and Hybrid Beads

具有周期性调制介电常数的三维结构，称为光子晶体，可以显示与晶格常数相关的布拉格共振周围的带隙，在这些带隙中光的传播被禁止。理论上，类似的声子带隙已经在密度或声速周期性变化的结构中得到了预测。我们中的一些人最近确实发现了高超音速频率下的这种带隙。本项目旨在深入研究这些带隙在适当设计的具有亚微米周期性的高超声速晶体。合适的对象是具有fcc晶格的合成蛋白石。它们将通过不同的技术从胶体聚合物、二氧化硅或核-壳混合珠子中制备，这些珠子的大小和核-壳比不同。通过设计不同晶格尺寸、局部结构和弹性模量对比的多种蛋白石，可以对声子带隙进行调谐。利用布里渊光散射（BLS）研究了不同波矢量下蛋白石的声子色散关系ω(k)，并利用基于多声子散射形式的理论计算进行了解释。这些珠子的相关特性将从它们的振动特征模态中推断出来，这些振动特征模态也是由BLS测量的。目标是对实验色散ω(k)的定量理论描述，基本上没有可调参数。最后，提出了一种基于核壳胶体的高超声子晶体的计算机辅助设计方法。

3D structures with a periodically modulated dielectric constant, coined photonic crystals, can display bandgaps around Bragg resonances associated with the lattice constants where the propagation of the light is forbidden. Theoretically, similar phononic bandgaps have been predicted in structures with periodic variations of the density or sound velocity. Such a bandgap at hypersonic frequencies was indeed recently detected by some of us. This project aims at the intensive study of these bandgaps in appropriately designed hypersonic crystals with submicron periodicities. Suited objects are synthetic opals with an fcc lattice. They will be prepared by various techniques from colloidal polymer, silica or core-shell hybrid beads differing in size and core-shell ratio. A manifold of opals varying in the lattice dimensions, the local structure and the elastic modulus contrast can be designed so the phononic bandgaps can be tuned. The phonon dispersion relation ω(k) of the opals will be studied by Brillouin light scattering (BLS) at different wave vectors and interpreted using theoretical computations based on the multiple phonon scattering formalism. The relevant characteristics of the beads will be deduced from their vibration eigenmodes measured also by BLS. The goal is a quantitative theoretical description of the experimental dispersion ω(k) essentially without adjustable parameters. Eventually, a computer- aided design of hypersonic phononic crystals based on core-shell colloids is aimed at.