  Chip-scale optical phased arrays could enable compact beam steering and LIDAR
for autonomous vehicles, precision robotics, and free-space optical
communications. Because these applications demand wide angle beam steering as
well as high optical power in the output beam, a natural design choice would be
to space the array emitters at a half-wavelength pitch, as is common in
radiofrequency phased arrays. Optical phased arrays, however, unlike RF phased
arrays, have been limited by the tradeoff between field of view (i.e. angle
steering) and beamforming efficiency (i.e. optical power in the output beam).
This tradeoff exists because optical phased arrays rely on waveguides as
emitters, which suffer from strong crosstalk when placed in close proximity
relative to their mode size. Here we overcome these limitations and demonstrate
a platform for optical phased arrays with 180{\deg} field of view, where more
than 72 percent of the power is carried in a single diffraction-limited beam
even when steered up to 60{\deg} off-axis. Our platform leverages high
index-contrast, dispersion-engineered waveguides spaced one half-wavelength
apart without incurring crosstalk.
