Credit: © 2006 Nature Materials

Optical bandgap materials — or ‘photonic crystals’ — are engineered to forbid the propagation of certain frequencies of light (photons). Analogously, ‘phononic crystals’ are designed to forbid the propagation of certain frequencies of elastic vibrations (phonons). Existing phononic crystals have forbidden frequencies in the sonic or ultrasonic range and can be used as acoustic shields or filters. The potential to simultaneously manipulate elastic energy and light, however, calls for phononic crystals to operate at hypersonic (gigahertz) frequencies.

George Fytas and colleagues1 of the Max Planck Institute for Polymer Research in Mainz, Germany have designed and measured the first hypersonic bandgap in a phononic crystal. The synthetic crystals consist of 256-nm polystyrene nanoparticles, self-assembled on a glass slide. The space between the particles is filled with a viscous liquid, such as silicon oil.

The authors used light scattering to determine the allowed energies and momenta of elastic vibrations in the colloidal crystal and found a gap in the allowed vibrations in the hypersonic range. The properties are easily tuned as the spacing between the nanoparticles determines the frequency of the gap. Moreover, the difference between the densities and sound velocities of the liquid and particles determines the gap width.