Polarization-independent almost-perfect absorber controlled from narrowband to broadband.

Achieving perfect absorption and controlling the absorption bandwidth are highly desirable for many applications. In this work, we design a narrowband almost-perfect absorber by using a metal-insulator-metal thin-film stack with absorption up to 99.67% at 0.58μm incident wavelength. The peak of absorption can be totally controlled by adjusting the thickness of the insulator layer. When the top metal layer is patterned by crossed grating nanostructure with optimized parameters, the absorber becomes broadband over 150nm bandwidth with average absorption exceeding 97% from 0.5μm to 0.65μm in the visible region. Both the narrowband and broadband absorbers are independent on polarization in specific incident angle range. This work opens up a promising new approach to control bandwidth of perfect absorption, which implicates many potential applications.