  Sideband-resolved coupling between multiple photonic nanocavities and
propagating mechanical waves in microwave Ku-band is demonstrated. Coherent and
strong photon-phonon interaction is manifested with optomechanically induced
transparency and absorption, and phase-coherent interaction in multiple
cavities. Inside an echo chamber it is shown that a phonon pulse can interact
with an embedded nanocavity for multiple times. Our device provides a scalable
platform to optomechanically couple phonons and photons for microwave photonics
and quantum photonics.
