Bidirectional band-switchable nano-film absorber from narrowband to broadband.

We propose a switchable perfect absorber with broadband and narrowband absorption based on alternating dielectric and metal nano-film structures in this paper. The lithography-free pattern is equipped with polarization insensitivity, good ductility and manufacturability, which has great significance in practical device development and applications. The quasi-complete selective absorption of incident light can be originated from asymmetric Fabry-Perot resonance, which combines the destructive interference in dielectric layers with inherent absorption in metal layers. When the light incidents on the surface covered with ultra-thin metal film of this structure, it acts as a narrowband absorber with over 99.90% absorption at 771 nm wavelength and a full wave at half maximum of 20 nm. When the light incidents on other surfaces covered with anti-reflective dielectric film, it achieves broadband perfect absorption with an average absorption exceeding 96.02% in a 500-1450 nm wavelength range. The absorption spectrum of oblique incidence shows that the broadband absorption behaves big angle range tolerance while the narrowband absorption exhibits angular dependence. The band-switchable performance of this absorber makes it valuable for energy harvesting/re-radiation applications in solar thermal photovoltaic systems.