  Photonic integrated circuit based optical phased arrays (PIC-OPA) are
emerging as promising programmable processors and spatial light modulators,
combining the best of planar and free-space optics. Their implementation in
silicon photonic platforms has been especially fruitful. Despite much progress
in this field, demonstrating steerable two-dimensional (2D) OPAs scalable to a
large number of array elements and operating with a single wavelength has
proven a challenge. In addition, the phase shifters used in the array for
programming the far field beam are either power hungry or have a large
footprint, preventing implementation of large scale 2D arrays. Here, we
demonstrate a two-dimensional silicon photonic phased array with high-speed
(~330 KHz) and ultralow power microresonator phase-shifters with a compact
radius (~3 {\mu}m) and 2{\pi} phase shift ability. Each phase-shifter consumes
an average ~250 {\mu}W static power for resonance alignment and ~50 {\mu}W
power for far field beamforming. Such PIC-OPA devices can enable a new
generation of compact and scalable low power processors and sensors.
