Gold thin-film based narrow-band perfect absorbers for near-IR frequencies

Perfect absorbers have received significant attention due to their tunable and extraordinary absorptive behaviour of light at selected wavelengths. Such absorption can be achieved by means of nanostructured materials with various types of flexibility. Despite these advantages, nanofabrication is a challenging process. In this study, we manufacture and characterize a gold thin-film-based ultra narrow-band perfect absorber that does not require nanostructures for full absorption of the incident light. Perfect absorption is based on Fabry-Perot interference created using a metal-insulator-metal triple layer. The fabrication involves precise deposition of a gold film (35 nm) and silicon nitride dielectric film (110-170 nm) in a controlled fashion. Optical characterization is achieved using a portable, simple reflection probe. The fabricated multi-layer thin film demonstrates ultra-narrow bandwidth (25 nm) and 99.4% maximum absorption. The use of gold thin film ensures inertness and makes the proposed system robust under harsh environmental conditions, unlike other metals. Our findings underscore the potential of this engineered perfect absorber for applications in photovoltaics and as optical filters; but not in sensing. We show that the electric field is bound between the metallic thin films and not affected by the change of refractive index outside the system.