We show that a metal can be turned into a broadband and omnidirectional absorber by coating a purely-dielectric thin layer of grating. An optimal design for such an absorber is proposed by putting a dielectric slot waveguide grating (SWG) on the metallic substrate. The SWG consists of two germanium nanowires (Ge NWs) separated by a sub-100 nm slot in each period. Average absorption reaches 90% when the incident angle varies between 0° and 80° over a broad wavelength range from 300 nm to 1400 nm. Multiple optical mechanisms/effects, namely, diffraction, waveguiding in the high-index Ge NWs and low-index air slot, Fabry-Perot resonances as well as surface plasmon polaritons (SPPs), are identified to govern the absorption characteristics of the present absorber. The designed absorber with such a dielectric grating is easier to fabricate as compared with other absorbers with metallic structures, and has potential applications in e.g. solar cells and photodetectors.
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