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 highefficiency 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.

The rapid developments in nanotechnology and plasmonics allow the manipulation of light at nanometer scales, such as light propagation and resonances. Differing from the symmetric Lorentzian‐like profiles in the conventional resonances, Fano resonances, which originate from the interference of different resonant modes, exhibit obviously asymmetric spectral profiles. Based on lineshape engineering, the Fano resonances with sharp asymmetric profiles exhibit a small linewidth and a high spectral contrast by exploiting different mechanisms and designing various metallic nanostructures. Both of the above properties in the sharp Fano resonances have significant applications in nanoscale plasmonic sensors and modulators. This review summarizes the underlying mechanism of the Fano resonances in various metallic nanostructures. Then, practical applications of the Fano resonances in nanoscale plasmonic sensing and modulation are reviewed. At last, the development and challenges of plasmonic sensing and modulation based on Fano resonances are discussed.

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