  Achieving nonreciprocal light propagation in photonic circuits is essential
to control signal crosstalk and optical back-scatter. However, realizing
high-fidelity nonreciprocity in low-loss integrated photonic systems remains
challenging. In this paper, we experimentally demonstrate a device concept
based on nonlocal acousto-optic light scattering to produce nonreciprocal
single-sideband modulation and mode conversion in an integrated silicon
photonic platform. In this process, a traveling-wave acoustic phonon driven via
optical forces in a silicon waveguide is used to modulate light in a spatially
separate waveguide through a linear inter-band Brillouin scattering process. We
demonstrate up to 38 dB of nonreciprocity with 37 dB of single-sideband
suppression. In contrast to prior Brillouin- and optomechanics-based schemes
for nonreciprocity, the bandwidth of this scattering process is set through
optical phase-matching, not acoustic or optical resonances. As a result,
record-large bandwidths in excess of 125 GHz are realized, with potential for
significant further improvement through optical dispersion engineering.
Tunability of the nonreciprocal modulator operation wavelength over a 35 nm
bandwidth is demonstrated by varying the optical pump wavelength. Such
traveling-wave acousto-optic modulators provide a promising path toward the
realization of broadband, low-loss isolators and circulators in integrated
photonic circuits.
