Broadband Visible and Near-Infrared Absorbers Implemented with Planar Nanolayered Stacks

Broadband light absorbers are highly desirable in various applications including solar-energy harvesting, thermo-photovoltaics, and photon detection. The Fabry–Perot (F–P) cavity comprising metal–insulator–metal (MIM) layers has attracted enormous interest as a lithography-free structure for realizing planar super absorbers. However, typical F–P cavity exhibits a narrow absorption band, and efforts have thus been made to increase the absorption bandwidth. This study demonstrates that near-perfect absorption over a broad spectral range can be obtained from the MIM structure by using thermally evaporated Ag and Au thin films whose dielectric and optical properties are much different from bulk-state properties because of their nanoscale features. A 55 nm thick SiO₂ spacer sandwiched between a 10 nm Ag top layer and a 100 nm Al back reflector exhibits absorption >95% in the visible range of 400–700 nm. The broad absorption band shifts to a near-infrared range of 650–1000 nm by replacing the top layer with a 10 nm thick Au film and increasing the SiO₂ spacer thickness to 115 nm. The experimental results are supported by finite-difference time-domain simulation. The large absorption bandwidth is attributed to the lossy nature of the nanostructured top metallic layer combined with the resonant absorption of the MIM cavity.