Sound and heat revolutions in phononics

The phonon is the physical particle responsible for the transmission of sound and heat; controlling the properties of phonons in materials could trigger many advances, which are reviewed here. Prepare for the age of phononics In the emerging research area of phononics, control over the mechanical vibrations that transmit sound and heat — phonons — plays a central role. Like photons and electrons, phonons can be treated as particles for many purposes, so can be harnessed and manipulated for useful applications. The phonon spectrum covers a wide range of effects, from low- frequency acoustics, to ultrasound and to heat, so that phononic techniques could enable a wide range of applications such as in earth quake protection, acoustics and heat management. In this review, Martin Maldovan discusses several approaches to the control of phonons at different length scales, for example phononic crystals, metamaterials, thermoelectrics and optomechanical devices. Today's digital revolution is underpinned by the high degree of control that can be imposed over electrons in semiconductors; Maldovan argues that precise control over phonons could have similar surprising and exciting consequences. The phonon is the physical particle representing mechanical vibration and is responsible for the transmission of everyday sound and heat. Understanding and controlling the phononic properties of materials provides opportunities to thermally insulate buildings, reduce environmental noise, transform waste heat into electricity and develop earthquake protection. Here I review recent progress and the development of new ideas and devices that make use of phononic properties to control both sound and heat. Advances in sonic and thermal diodes, optomechanical crystals, acoustic and thermal cloaking, hypersonic phononic crystals, thermoelectrics, and thermocrystals herald the next technological revolution in phononics.