  Artificial metasurfaces are capable of completely manipulating the phase,
amplitude, and polarization of light with high spatial resolutions. The
emerging design based on high-index and low-loss dielectrics has led to the
realization of novel metasurfaces with high transmissions, but these devices
usually operate at the limited bandwidth, and are sensitive to the incident
polarization. Here, for the first time we report experimentally the
polarization-independent and high-efficiency dielectric metasurfaces spanning
the visible wavelengths about 200 nm, which are of importance for novel flat
optical devices operating over a broad spectrum. The diffraction efficiencies
of the gradient metasurfaces consisting of the multi-fold symmetric
nano-crystalline silicon nanopillars are up to 93% at 670 nm, and exceed 75% at
the wavelengths from 600 to 800 nm for the two orthogonally polarized
incidences. These dielectric metasurfaces hold great potential to replace
prisms, lenses and other conventional optical elements.
