  Imaging polarimetry plays an essential role in various fields since it
imparts rich information that cannot be obtained through mere intensity and
spectral measurements. To retrieve full Stokes parameters, at least four sensor
pixels are required, each of which projects incident light to a different
polarization state in the Stokes space. Conventional full-Stokes
division-of-focal-plane (DoFP) cameras realize this function by integrating
angled polarizers and retarders on top of image sensors. Due to the inevitable
absorption at the polarizers, however, the maximum efficiency of these schemes
is limited to 50% in theory. Instead of polarizers, three sets of lossless
polarization beam splitters can be used to achieve higher-efficiency
polarimetry, however, at the cost of reduced spatial resolution due to the need
for six redundant sensor pixels. In this paper, we reveal, for the first time
to our knowledge, that low-loss four-output polarization splitting (without
filtering) is possible using a single-layer dielectric metasurface. Although
these four states are not orthogonal to each other, our metasurface enables
simultaneous sorting and focusing onto four sensor pixels with an efficiency
exceeding 50\%, which is not feasible by a simple combination of space-optic
components. The designed metasurface composed of silicon nanoposts is
fabricated to experimentally demonstrate complete retrieval of full Stokes
parameters at the near-infrared wavelength range from 1500 to 1600 nm with
$-$2.28-dB efficiency. Finally, simple imaging polarimetry is demonstrated
using a 3$\times$4 superpixel array.
