  Acoustic waves can serve as memory for optical information, however, acoustic
phonons in the GHz regime decay on the nanosecond timescale. Usually this is
dominated by intrinsic acoustic loss due to inelastic scattering of the
acoustic waves and thermal phonons. Here we show a way to counteract the
intrinsic acoustic decay of the phonons in a waveguide by resonantly
reinforcing the acoustic wave via synchronized optical pulses. This scheme
overcomes the previous constraints of phonon-based optical signal processing
for light storage and memory. We experimentally demonstrate on-chip storage up
to 40 ns, four times the intrinsic acoustic lifetime in the waveguide. We
confirm the coherence of the scheme by detecting the phase of the delayed
optical signal after 40 ns using homodyne detection. Through theoretical
considerations we anticipate that this concept allows for storage times up to
microseconds within realistic experimental limitations while maintaining a GHz
bandwidth of the optical signal. The refreshed phonon-based light storage
removes the usual bandwidth-delay product limitations of e.g. slow-light
schemes.
