  We propose and demonstrate purely optical feedback cooling of neutral
nanoparticles in an optical lattice to an occupation number of $0.85\pm0.20$.
The cooling force is derived from the optical gradients of displaced optical
lattices produced with two sidebands on the trapping laser. To achieve highly
accurate position observations required for cooling near the ground state, we
reduce the laser intensity noise to a relative power noise of
$\unit[6\times10^{-8}]{/Hz}$ in a frequency band of $\unit[30]{kHz}$ to
$\unit[600]{kHz}$. We establish a reproducible method for neutralizing
nanoparticles at high vacuum via a combination of discharging and irradiating
an ultraviolet light. Our results form an important basis for the investigation
of quantum mechanical properties of ultracold nanoparticles and are also useful
for precision measurements with neutral nanoparticles.
