A phononic or sonic crystal is the acoustic equivalent of a photonic crystal. Just as the periodic variation of the refractive index in a photonic crystal means that only certain wavelengths of light are able to pass through it, a periodic variation in the acoustic properties of a phononic crystal means that only phonons with frequencies outside the phononic band gap can propagate. Such crystals are made by embedding cylinders of one material in a different background medium, with the properties of the phononic band gap depending on the size and periodicity of the cylinders.

As the periodicity of the crystal become smaller the gap moves to higher frequencies, and at hypersonic frequencies - between 1 and 100 gigahertz - the period becomes comparable with the wavelength of visible light. This means that such crystals should exhibit both phononic and photonic band gaps. However, hypersonic crystals are difficult to fabricate and characterise.

The MIT-lead team has now developed a complete "tool set" for designing and making hypersonic crystals, and for studying the motion of phonons in them. Gorishnyy and colleagues used a technique called holographic interference lithography to grow high-quality, defect-free single crystals that consisted of triangular arrays of cylindrical air holes in an epoxy polymer matrix about 6 microns thick. The team measured the phononic band gaps in the materials and followed the movement of phonons through them with a technique called Brillouin light scattering.

Controlling phonons in sonic crystals could be used to reduce noise in electronic circuits, control heat flow in nanostructures and enhance the interactions between light and sound waves in materials. "Acousto-optical interactions in hypersonic crystals are predicted to lead to a number of intriguing effects, such as optical cooling and shock-wave-mediated light frequency shifts," Gorishnyy told PhysicsWeb. "Our results suggest a novel way to design a variety of acousto-optical devices, such as optical modulators and optically pumped acoustic oscillators."