Hypersonic hybridization band gaps in structures based on polymer-tethered colloids.*

Phononic hybridization gaps, originating from the anti-crossing between local resonant and propagating modes, are robust to structural disorder and occur at wavelengths much larger than the size of the resonant unit. Here, polymer-tethered colloidal particles (particle brush) were used to fabricate hybridization gap structures harnessing the anisotropic elasticity across their solid particle-polymer interface. Brillouin light spectroscopy was employed to record the dispersion diagram for longitudinal and transverse phonons as a function of grafting density and polymer length. Frequency band gap width and position decreased with polymer volume fraction at roughly constant grafting density but increased with decreasing grafting density at constant volume fraction. Modeling of the phononic band diagrams using the layered-multiple scattering formalism revealed the importance of the boundary conditions and differences of the polymer elastic moduli from their isotropic bulk values. These intriguing findings were discussed in terms of structural and particle brush topology effects and complemented by the elastic excitations of the individual particle brush systems. The results point to new opportunities to harness efficient self-assembly methods for the fabrication of phononic materials.