  Metasurfaces enable exceptional control over the light with surface-confined
planar components, offering the fascinating possibility of very dense
integration and miniaturization in photonics. Here, we design, fabricate and
experimentally demonstrate chip-size plasmonic spectropolarimeters for
simultaneous polarization state and wavelength determination.
Spectropolarimeters, consisting of three gap-plasmon phase-gradient
metasurfaces that occupy 120{\deg} circular sectors each, diffract normally
incident light to six predesigned directions, whose azimuthal angles are
proportional to the light wavelength, while contrasts in the corresponding
diffraction intensities provide a direct measure of the incident polarization
state through retrieval of the associated Stokes parameters. The
proof-of-concept 96-{\mu}m-diameter spectropolarimeter operating in the
wavelength range of 750-950nm exhibits the expected polarization selectivity
and high angular dispersion. Moreover, we show that, due to the circular-sector
design, polarization analysis can be conducted for optical beams of different
diameters without prior calibration, demonstrating thereby the beam-size
invariant functionality. The proposed spectropolarimeters are compact,
cost-effective, robust, and promise high-performance real-time polarization and
spectral measurements.
