  The grating magneto-optical trap (GMOT) requires only one beam and three
planar diffraction gratings to form a cloud of cold atoms above the plane of
the diffractors. Despite the complicated polarization arrangement, we
demonstrate sub-Doppler cooling of 87Rb atoms to a temperature of 7.6(0.6) uK
through a multi-stage, far-detuned GMOT in conjunction with optical molasses. A
decomposition of the electric field into polarization components for this
geometry does not yield a mapping onto standard sub-Doppler laser-cooling
configurations. With numerical simulations, we find that the polarization
composition of the GMOT optical field, which includes sigma- and pi-polarized
light, does produce sub-Doppler temperatures.
