  Measuring light's state of polarization is an inherently difficult problem,
since the phase information between orthogonal polarization states is typically
lost in the detection process. In this work, we bring to the fore the
equivalence between normalized Stokes parameters and diffraction contrasts in
appropriately designed phase-gradient birefringent metasurfaces and introduce a
concept of all-polarization birefringent metagratings. The metagrating, which
consists of three interweaved metasurfaces, allows one to easily analyze an
arbitrary state of light polarization by conducting simultaneous (i.e.,
parallel) measurements of the correspondent diffraction intensities that reveal
immediately the Stokes parameters of the polarization state under examination.
Based on plasmonic metasurfaces operating in reflection at the wavelength of
800 nm, we design and realize phase-gradient birefringent metasurfaces and the
correspondent metagrating, while experimental characterization of the
fabricated components convincingly demonstrates the expected functionalities.
We foresee the use of the metagrating in compact polarimetric setups at any
frequency regime of interest.
