As materials science meets solid state physics, new functional periodic structures coined phononic crystals are realized. The architecture of finite hybrid superlattices governs their band structure at high frequencies. Controlling sound propagation by structural design is a long standing goal in science and technology. A few examples are the whispering galleries that channel audible sound or the use of ultrasound for non-destructive imaging in the sub-mm regime. However, the control of even higher-pitched noise at GHz-frequencies (hypersound) demands nanofabrication of phononic structures with high precision. In this paper, we alternately deposit layers of a polymer and silica nanoparticles to fabricate well-defined one-dimensional periodic structures incorporating controlled defects. Spontaneous Brillouin light scattering and detailed theoretical calculations reveal different elastic excitations in such hybrid superlattices. The interaction of surface and cavity vibration modes is unambiguously documented and fully controlled by the layer thickness, elasticity and sequence. This soft matter based platform allows facile engineering of the vibrational density of states and opens new pathways to tunable systems that concurrently mold the flow of light and sound.

From: Dirk Schneider, Faroha Liaqat, El Houssaine El Boudouti,
Ossama El Abouti, Wolfgang Tremel,
Hans-Jürgen Butt, Bahram Djafari-Rouhani, and George Fytas,
“Defect-Controlled Hypersound Propagation in Hybrid Superlattices“,
Phys. Rev. Lett.
111, 164301 (2013).