  Optomechanical structures are well suited to study photon-phonon
interactions, and they also turn out to be potential building blocks for
phononic circuits and quantum computing. In phononic circuits, in which
information is carried and processed by phonons, optomechanical structures
could be used as interfaces to photons and electrons thanks to their excellent
coupling efficiency. Among the components required for phononic circuits, such
structures could be used to create coherent phonon sources and detectors.
Complex functions other than emission or detection remain challenging and
addressing a single structure in a full network proves a formidable challenge.
Here, we propose and demonstrate a way to modulate the coherent emission from
optomechanical crystals by external optical pumping, effectively creating a
phonon switch working at ambient conditions of pressure and temperature and the
working speed of which (5 MHz) is only limited by the mechanical motion of the
optomechanical structure. We additionally demonstrate two other switching
schemes: harmonic switching in which the mechanical mode remains active but
different harmonics of the optical force are used, and switching to- and from
the chaotic regime. Furthermore, the method presented here allows to select any
single structure without affecting its surroundings, which is an important step
towards freely controllable networks of optomechanical phonon emitters.
