Phononic crystals are structures with periodic variations in density and elastic constants that possess band gaps for sound and other mechanical waves. This opens the possibility to control properties and propagation of sound, ultrasound and phonons, as well as the random thermal vibrations of atoms.

We have fabricated hypersonic crystals with band gaps in GHz frequency range using interference lithography and measure their phononic dispersion relation with Brillouin light scattering (BLS).