Phys. Rev. X 5, 031011 (2015)

Topological states, also known as chiral edge states, are special robust states of matter that are protected against scatter due to the engineering of a material's band structure. Such states have been observed for electrons, atoms and photons but not yet for phonons in a solid-state material. Now Florian Marquardt's group in Erlangen, Germany report that cavity optomechanics in a photonic crystal may provide a means of implementation. In particular, they demonstrate that the topological properties of sound waves (phonons) in such a material can be tuned by adjusting the amplitude and frequency of a driving laser to control the optomechanical interaction between light and sound. Theoretical analysis suggests that the approach should yield chiral, topologically protected phonon states. The research team emphasizes that no time-dependent modulation of the laser is required and that a single laser field with a suitable phase pattern is sufficient to create the phonon edge states. They also comment that by sweeping the laser frequency a regime is entered where it is no longer possible to view phonons and photons as separate due to the existence of a hybrid band structure.