  Achieving non-reciprocal light propagation via stimuli that break
time-reversal symmetry, without magneto-optics, remains a major challenge for
integrated nanophotonic devices. Recently, optomechanical microsystems in which
light and vibrational modes are coupled through ponderomotive forces, have
demonstrated strong non-reciprocal effects through a variety of techniques, but
always using optical pumping. None of these approaches have demonstrated
bandwidth exceeding that of the mechanical system, and all of them require
optical power, which are both fundamental and practical issues. Here we resolve
both of these challenges through breaking of time-reversal symmetry using an
acoustic pump in an integrated nanophotonic circuit. GHz-bandwidth
optomechanical non-reciprocity is demonstrated using the action of a
2-dimensional surface acoustic wave pump, that simultaneously provides non-zero
overlap integral for light-sound interaction and also satisfies the necessary
phase-matching. We use this technique to produce a simple frequency shifting
isolator (i.e. a non-reciprocal modulator) by means of indirect interband
scattering. We demonstrate mode conversion asymmetry up to 15 dB, efficiency as
high as 17%, over bandwidth exceeding 1 GHz.
