  Recently, an achromatic metasurface was successfully demonstrated to deflect
light of multiple wavelengths in the same direction and it was further applied
to the design of planar lenses without chromatic aberrations [Science, 347,
1342(2015)]. However, such metasurface can only work for normal incidence and
exhibit low conversion efficiency. Here, we present an ultrawide-angle and
high-efficiency metasurface without chromatic aberration for wavefront shaping
in visible range. The metasurface is constructed by multiple metallic
nano-groove gratings, which support enhanced diffractions for an ultrawide
incident angle range from 10o to 80o due to the excitations of localized gap
plasmon modes at different resonance wavelengths. Incident light at these
resonance wavelengths can be efficiently diffracted into the same direction
with complete suppression of the specular reflection. This approach is applied
to the design of an achromatic flat lens for focusing light of different
wavelengths into the same position. Our findings provide a facile way to design
various achromatic flat optical elements for imaging and display applications.
