Phononic crystals are structures possessing periodic variations in density and/or elastic constants, which result in band gaps for sound and other mechanical waves. Sonic crystals can be used for sound and vibration isolation, ultrasonic crystals for acoustic imaging. Our interest is in hypersonic crystals with 100nm feature sizes for the enhancement of acousto-optical interactions: engineering of phonon –photon as well as electron-phonon interactions.
We fabricated hypersonic crystals with band gaps in GHz frequency range using interference lithography and measured their phonon dispersion relation with Brillouin light scattering (BLS). We reported the first experimentally measured band gap at hypersonic frequencies in a single crystalline hypersonic phononic crystal.
Fig. 2. (a) Single Crystal Hypersonic Reflector
(b) Theoretically Calculated Dispersion relation of the epoxy-air phononic crystals along (c) Experimental Phononic dispersion relation along the  direction showing a partial band gap between 1.21 and 1.57 GHz (in grey)
Fig. 1. A sound wave is incident on the surface of a two dimensional phononic crystal consisting of air cylinders on a triangular lattice in a solid film. As the sound wave has a frequency within the bandgap, propagation is not allowed and the wave is reflected backwards.
E. Thomas et al., Nature Materials, 5(10), 773, 2006
J.-H. Jang et al., Applied Physics Letters (submitted)
Hypersonic phononic crystals variations in density and/or elastic constants, which result in band gaps for sound and other mechanical waves. Edwin L. Thomas, Massachusetts Institute of Technology, DMR 0308133