  Optical metasurfaces have emerged as a new generation of building blocks for
multi-functional optics. Design and realization of metasurface elements place
ever-increasing demands on accurate assessment of phase alterations introduced
by complex nanoantenna arrays, a process referred to as quantitative phase
imaging. Despite considerable effort, the widefield (non-scanning) phase
imaging that would approach resolution limits of optical microscopy and
indicate the response of a single nanoantenna still remains a challenge. Here,
we report on a new strategy in incoherent holographic imaging of metasurfaces,
in which unprecedented spatial resolution and light sensitivity are achieved by
taking full advantage of the polarization selective control of light through
the geometric (Pancharatnam-Berry) phase. The measurement is carried out in an
inherently stable common-path setup composed of a standard optical microscope
and an add-on imaging module. Phase information is acquired from the mutual
coherence function attainable in records created in broadband spatially
incoherent light by the self-interference of scattered and leakage light coming
from the metasurface. In calibration measurements, the phase was mapped with
the precision and spatial background noise better than 0.01 rad and 0.05 rad,
respectively. The imaging excels at the high spatial resolution that was
demonstrated experimentally by the precise amplitude and phase restoration of
vortex metalenses and a metasurface grating with 833 lines/mm. Thanks to
superior light sensitivity of the method, we demonstrated, for the first time
to our knowledge, the widefield measurement of the phase altered by a single
nanoantenna, while maintaining the precision well below 0.15 rad.
