Polarization-dependent and -independent spectrum selective absorption based on a metallic grating structure

The absorption effect in grating-based devices is theoretically investigated. Three kinds of spectrum selective absorbers (TE polarization, TM polarization and polarization-independent) exhibiting near-unity absorption at the resonant wavelength are studied at visible frequencies. The optimized parameters of absorbers are all obtained by use of rigorous coupled-wave analysis and the simulated annealing algorithm. The underlying physics understanding of such perfect absorption effects is illustrated by investigating the field distributions and power loss density in these absorbers. The enhancement of absorption is attributed to the hybridization of cavity mode and guide mode resonance for TE polarization absorber, and attributed to the surface plasmon resonance for TM polarization absorber with ultra-thin grating structure. The relationship between the absorption spectrum and the geometric parameters of the structures is studied. In order to achieve perfect absorption, the grating should have large thickness for TE polarization, while it has not strict restriction for TM polarization. The conclusions should be useful for designing a polarization-dependent or -independent selective absorber based on a metallic grating structure in the visible region.