  The visualization of pure phase objects by wavefront sensing has important
applications ranging from surface profiling to biomedical microscopy, and
generally requires bulky and complicated setups involving optical spatial
filtering, interferometry, or structured illumination. Here we introduce a new
type of image sensors that are uniquely sensitive to the local direction of
light propagation, based on standard photodetectors coated with a specially
designed plasmonic metasurface that creates an asymmetric dependence of
responsivity on angle of incidence around the surface normal. The metasurface
design, fabrication, and angle-sensitive operation are demonstrated using a
simple photoconductive detector platform. The measurement results, combined
with computational imaging calculations, are then used to show that a standard
camera or microscope based on these metasurface pixels can directly visualize
phase objects without any additional optical elements, with state-of-the-art
minimum detectable phase contrasts below 10 mrad. Furthermore, the combination
of sensors with equal and opposite angular response on the same pixel array can
be used to perform quantitative phase imaging in a single shot, with a
customized reconstruction algorithm which is also developed in this work. By
virtue of its system miniaturization and measurement simplicity, the phase
imaging approach enabled by these devices is particularly significant for
applications involving space-constrained and portable setups (such as
point-of-care imaging and endoscopy) and measurements involving freely moving
objects.
