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

• Applicants: Privatdozent Dr. Goetz P. Hellmann; Professor Dr. Ulrich Jonas
• Subject Area: Experimental and Theoretical Physics of Polymers
• Term: from 2008 to 2011
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 61437418

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.

• DFG Programme: Research Grants