Periodic materials cannot only control the propagation of light (photonics) but also modify the propagation of elastic (acoustic) waves. These phononic materials with variation in density and elastic constants define a rich emerging new field with both unexplored fundamental research and potential applications. In particular, hypersonic phononics are important as they provide a platform for strong phonon-photon interactions in the visible, the high frequency phonons act as a main heat carries in dielectrics and can emerge as a sensitive new characterization tool for interfaces and mechanical properties at nanoscale.
Progress in this field depends on the development of fabrication methods for patterning at the mesoscale, band structure calculation techniques, and suitable experimental techniques to record the dispersion relations. Our approach relies on the employment of the Brillouin light scattering technique, design and use of appropriate structures in close collaboration with experimental and theoretical groups. Among the current research activities are the realization of hypersonic phononic gaps in one [1], two [2], or three dimensions [3], the measurement of particle vibrations [4] and the detection of mechanical anisotropy. Long term goals are the elucidating of the role of strong resonant components in phononic materials and manipulation of the flow of heat and elastic energy.

a) Examples for 1-D (polymeric multilayers), 2-D (anodic oxidized alumina), and 3-D (colloidal crystals) mesoscopic phononic structures, b) Brillouin light scattering setup, c) phononic band diagram for a 3-D phononic crystal with two different band gaps, d) particle eigenvibrations (simulated by FEM) for spherical particles and corresponding Brillouin spectrum.

• [1] Cheng, W.; Gomopoulos, N.; Fytas, G.; Gorishnyy, T.; Walish, J.; Thomas, E. L.; Hiltner, A.; Baer, E.: Phonon Dispersion and Nanomechanical Properties of Periodic 1D Multilayer Polymer Films. Nano Lett. 2008, 8 (5), 1423-1428.
  Cheng, W.; Gorishnyy, T.; Krikorian, V.; Fytas, G.; Thomas, E. L.: In-Plane Elastic Excitations in 1D Polymeric Photonic Structures. Macromolecules 2006, 39 (26), 9614-9620
• [2] Sato, A.; Knoll, W.; Penek, Y.; Djafari-Rouhani, B.; Fytas, G.; Steinhart, M.: Anisotropic propagation and confinement of high frequency phonons in nanocomposites. J. Chem. Phys. 2009, 130, 111102.
• [3] Cheng, W.; Wang, J. J.; Jonas, U.; Fytas, G.; Stefanou, N.: Observation and tuning of hypersonic bandgaps in colloidal crystals. Nat. Mater. 2006, 5 (10), 830-836.
  Still, T.; Cheng, W.; Retsch, M.; Sainidou, R.; Wang, J.; Jonas, U.; Stefanou, N.; Fytas, G.: Simultaneous Occurrence of Structure-Directed and Particle-Resonance-Induced Phononic Gaps in Colloidal Films. Phys. Rev. Lett. 2008, 100 (19), 194301-4.
• [4] Cheng, W.; Wang, J. J.; Jonas, U.; Steffen, W.; Fytas, G.; Penciu, R. S.; Economou, E. N.: The spectrum of vibration modes in soft opals. J. Chem. Phys. 2005, 123, 121104.
  Still, T.; Sainidou, R.; Retsch, M.; Jonas, U.; Spahn, P.; Hellmann, G. P.; Fytas, G.: The Music of Core-Shell Spheres and Hollow Capsules: Influence of the Architecture on the Mechanical Properties at the Nanoscale. Nano Lett. 2008, 8 (10), 3194-3199.
  Still, T.; D'Acunzi, M.; Vollmer, D.; Fytas, G.: Mesospheres in nano-armor: Probing the shape-persistence of molten polymer colloids. J. Coll. Int. Sci. 2009, 340 (1), 42-45.