  Rapid and low-power control over the direction of a radiating light field is
a major challenge in photonics and a key enabling technology for emerging
sensors and free-space communication links. Current approaches based on bulky
motorized components are limited by their high cost and power consumption,
while on-chip optical phased arrays face challenges in scaling and
programmability. Here, we propose a solid-state approach to beam-steering using
optomechanical antennas. We combine recent progress in simultaneous control of
optical and mechanical waves with remarkable advances in on-chip optical phased
arrays to enable low-power and full two-dimensional beam-steering of
monochromatic light. We present a design of a silicon photonic system made of
photonic-phononic waveguides that achieves 44$^{\circ}$ field of view with
$880$ resolvable spots by sweeping the mechanical wavelength with about a
milliwatt of mechanical power. Using mechanical waves as nonreciprocal, active
gratings allows us to quickly reconfigure the beam direction, beam shape, and
the number of beams. It also enables us to distinguish between light that we
send and receive.
