Phononic Crystals at Various Frequencies

Over the past three decades, phononic crystals have attracted scientific interest by demonstrating a variety of unique manipulations of sound and elastic wave propagation, through control of wave dispersion by Bragg scattering. The high freedom in the choice of materials and designs of phononic crystal systems and the wide available frequency range provide researchers with a vast playground in physics and applications. The field started with studies in the sonic and the ultrasonic frequency range, with applications in acoustic metamaterials, sound shielding, sound absorption, medical imaging, micro-electro-mechanical systems, surface acoustic waves, acousto-optics, and optomechanics. Concepts have then been extended to nonlinear physics and non-reciprocal acoustics. Recently, the downsizing of phononic crystals has broadened the research area toward higher frequencies and has enabled the manipulation of heat conduction in solids. The new concept of thermocrystal opens a new possibility of the phononic crystal assisted by material science and nanofabrication technology. Following fundamental advances in topological insulators made in solid state physics, topological phononics now promotes protection against radiation loss and spurious scattering. This special topic aims at providing a comprehensive overview of the most recent advances in research and applications of phononic crystals at various frequencies, from sound to heat.