Demonstrating the antinode loss effect as the operational mechanism of broadband multilayer film absorbers

Broadband multilayer film absorbers (BMFAs) consisting of alternating metallic and dielectric layers allow low-cost large-scale fabrication, which can be effectively applied in radiative-cooling, photodetector and photovoltaic, etc. The efficient design of BMFAs requires a clear physical understanding. Most studies indicate that Fabry-Perot resonance is the absorption mechanism of BMFAs. We challenge this view, arguing that the absorption of BMFAs is caused by the non-resonant antinode loss effect (ALE): placing a high-loss metal film with subwavelength thickness near the antinodes of the optical field significantly enhances absorption. Our theoretical analysis indicates that achieving near-perfect absorption of BMFAs requires two conditions: amplitude matching and phase matching of the standing wave reflection coefficient. We demonstrate that using ALE with an appropriately thick high-loss metal film can achieve amplitude matching, while phase matching can be achieved by adjusting the thickness of the surface dielectric layer. Using this efficient two-step design method, we design BMFAs for the visible, near-infrared, and visible-near-infrared wavelength ranges, respectively, and their experimental results align with theoretical expectations. Additionally, we demonstrate that ALE can also guide the design of metamaterial absorbers. Our research corrects the misunderstandings about the mechanisms of broadband absorbers and provides an efficient design methodology for these devices.