  The generation of propagating acoustic waves is essential for
telecommunication applications, quantum technologies, and sensing. Up to now,
the electrical generation has been at the core of most implementations, but is
technologically limited to a few gigahertz. Overcoming this frequency limit
holds the prospect of faster modulators, quantum acoustics at higher working
temperatures, nanoacoustic sensing from smaller volumes. Alternatively, the
optical excitation of acoustic resonators has unlocked frequencies up to 1 THz,
but in most cases, the acoustic energy cannot be efficiently extracted from the
resonator into a propagating wave. Here, we demonstrate a quasi-continuous and
coherent source of 20 GHz acoustic phonons, based on a ridge waveguide,
structured in the vertical direction as a high-Q acousto-optic resonator. The
high frequency phonons propagate up to 20 $\mu$m away from the source, with a
decay rate of $\sim$1.14 dB/$\mu$m. We demonstrate the coherence between
acoustic phonons generated from two distant sources through spatio-temporal
interference. This concept could be scaled up to a larger number of sources,
which enable a new generation of optically programmed, reconfigurable
nanoacoustic devices and applications.
