The natural and technological worlds operate by employing waves of all sorts: electron, electromagnetic, acoustic and so on. These waves can be manipulated by blocking their propagation in certain frequency regions called gaps, while allowing the propagation in other regions called bands. Semiconductor technology is based on electron-wave propagation and photonic crystals are based on periodic structures that have bands and gaps for electromagnetic waves.
More recently, structures that can manipulate acoustic waves have been developed, and Ping Sheng and co-workers at the Hong Kong University of Science and Technology in Kowloon have demonstrated a simple idea that frees the size of the phononic crystal from being constrained by the wavelength (Z Liu et al. 2000 Science 289 1734).
In the November issue of Physics World, Eleftherios Economou of the Foundation for Research and Technology Hellas, Greece, explains how the new device works.