Highly efficient ultra-broadband perfect absorber based on a simple four-layer structure Al–SiO2–Si–Ti in visible to near-infrared region

We propose a highly efficient ultra-broadband perfect absorber with a double-layer continuous spacer of SiO2 and Si based on a simple four-layer structure Al–SiO2–Si–Ti. The absorber shows an average absorption over 98.1% at 780-nm bandwidth from 440 to 1220 nm, where two successive absorption bands are over 99%, and the absorption peak is up to 99.9%. Highly efficient broadband absorption benefits from the surface plasmon resonance (the propagating surface plasmon resonance and the localized surface plasmon resonance). Comparing the absorbers with different spacers, the proposed perfect absorber can be regarded as the superposition of two triple-layer structure absorbers. The absorber exhibits nearly perfect polarization independence and a wide incident angle up to 40 deg for transverse magnetic polarization and transverse electric polarization. In addition, the absorber is not only simple in structure and cheap in Al and Ti materials but also compatible with complementary metal oxide semiconductor technology, which means low cost and high efficiency. We believe that the proposed absorber has tremendous application prospects in areas such as solar energy, optical imaging, and infrared detection.