  Controlling and manipulating quanta of coherent acoustic vibrations - phonons
- in integrated circuits has recently drawn a lot of attention, since phonons
can function as unique links between radiofrequency and optical signals, allow
access to quantum regimes and offer advanced signal processing capabilities.
Recent approaches based on optomechanical resonators have achieved impressive
quality factors allowing for storage of optical signals. However, so far these
techniques have been limited in bandwidth and are incompatible with
multi-wavelength operation. In this work, we experimentally demonstrate a
coherent buffer in an integrated planar optical waveguide by transferring the
optical information coherently to an acoustic hypersound wave. Optical
information is extracted using the reverse process. These hypersound phonons
have similar wavelengths as the optical photons but travel at 5-orders of
magnitude lower velocity. We demonstrate the storage of phase and amplitude of
optical information with GHz-bandwidth and show operation at separate
wavelengths with negligible cross-talk.
