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Seminar 2/8/2012 - George Fytas, Max Planck Institute for Polymer Research, University of Crete: High Frequency Soft Phononics

Abstract: Phononic crystals, the acoustic equivalents of the photonic crystals, are controlled by a larger number of material parameters1,2. The study of hypersonic crystals imposes substantial demand on fabrication and characterization techniques. Colloid and polymer science offer methods to create novel materials that possess periodic variations of density and elastic properties at mesoscopic length scales commensurate with the wave length of hypersonic phonons and hence photons of the visible light. Polymer-and colloid-based phononics is an emerging new field at the interface of soft materials science and condensed matter physics with rich perspectives ahead. Here, examples from fabricated structures will be highlighted.
The key quantity is the dispersion of high frequency (GHz) acoustic excitations which is nowadays at best measured by high resolution spontaneous Brillouin light scattering. Depending on the components of the nanostructured composite materials, the resolved vibration eigenmodes (their music) of the individual particles sensitively depend on the particle architecture and their thermo-mechanical properties3,4. In periodic structures (the concert) of polymer based colloids, the dispersion relation ω(k)  between the frequency and the  phonon wave vector k has revealed hypersonic phononic band gaps of different nature2,5,6: Bragg gap for propagation near the edge of the first Brillouin zone due to destructive interference and hybridization gap due to the interaction of particle eigenmodes with the effective medium acoustic branch; the latter is therefore robust against structural. Boosting the strength of the phonon scattering by the individual spheres, e.g., elastically hard SiO2 particles can activate additional mechanisms for tunability of the phononic band structure.

The elucidation of all important parameters towards the general design of optimal phononic structures remains complicated due to the vector nature of elastic wave propagation. In this regard, 1D phononic crystals (SiO2/PMMA) multilayer films turns out to be a model system7. Since hypersonic crystals can simultaneously exhibit phononic and photonic band gaps in the visible spectral region, and phonons are the main heat carriers in dialectics, many technological applications are feasible.

All seminars are held on Wednesdays from 12:00 noon-1:00 p.m. in the Bowen Hall Auditorium Room 222. A light lunch is provided at 11:30 a.m. in the Bowen Hall Atrium immediately prior to the seminar.

Location: Bowen Hall Auditorium

Date/Time: 02/08/12 at 12:00 pm - 02/08/12 at 01:00 am

Category: PRISM/PCCM Seminar Series

Department: PRISM