Absorption properties and mechanisms of metallic moth-eye structures

The antireflection and absorption properties of non-metallic moth-eye structures have been intensively studied, however the research on those of metallic moth-eye structures, especially their absorption mechanisms, is far from enough. In this paper, the absorption properties and mechanisms of a metallic Au moth-eye structure are studied in detail. The results show that broadband perfect absorption covering the entire visible to the near infrared range with polarization independence and in a wide incident angle range can be easily realized using a single-layer single-material Au moth-eye structure. As for the absorption mechanism, in the short wavelength range of 300 nm–550 nm, due to the high loss of the Au material, the antireflection effect plays a main role. While in the long wavelength range of 550 nm–1000 nm, the widely accepted absorption mechanisms (the antireflection effect as well as the cascaded multiple plasmonic resonances, and even the combination of them) cannot completely account for the broadband perfect absorption. We found that the adiabatic nanofocusing effect of the plasmonic resonances enabled by the ultra-sharp air grooves plays a key role in the absorption. It is the nanofocusing effect that effectively increases the absorption to nearly 100% in a broad wavelength band. These findings provide a deeper insight into the absorption mechanisms of metallic moth-eye structures and offer an effective guide to explore demanded excellent absorption performance.