Robust Absorption in a Four-Layer Dielectric-Metal Structure

This letter predicts that robust absorption will appear in a four-layer dielectric-metal structure, and our experimental results verify this prediction well. The mechanisms are as follows: 1) The four layers form a cavity, and zero phase shift at the interface between the first metallic layer and the second dielectric layer is designed to make the backward light form a destructive interference and minimize the reflection. 2) The slow-light effect near the cutoff frequency in this structure increases the absorptive length many times. 3) It also can be explained as an asymmetric resonator, which satisfies the critical coupling condition. Many applications can be developed from this work, such as ensuring laser welding high quality for infrared transparent optoelectronic components.