Nanoslit-microcavity-based narrow band absorber for sensing applications.

We propose an ultranarrow bandwidth perfect infrared absorber consisting of a metal periodic structured surface with nanoslits, a spacer dielectric, and a metal back plate. Its bandwidth and aborption are respectively about 8 nm and 95%. The thickness of the nanobars and the spacer, and the width of the nanoslits are primary factors determining the absorption performance. This structure not only has narrow bandwidth but also can obtain the giant electric field enhancement in the tiny volume of the nanoslits. Operated as a refractive index sensor, this structure has figure of merit as high as 25. It has potential in biomedical and sensing applications.