Coupling-induced spectral splitting for plasmonic sensing with ultra-high figure of merit

We investigate a kind of spectral splitting effect in a plasmonic multilayer system, which consists of stacked Al₂O₃ and SiO₂ layers, a thin metal film, and a dielectric prism substrate. The results illustrate that an obvious peak appears in the center of the surface plasmon resonance (SPR)-induced reflection spectral dip in the structure with the SiO₂/Al₂O₃/SiO₂ layers. This spectral splitting response can be regarded as an electromagnetically induced transparency (EIT) like effect, which is attributed to the coupling and interference between the SPR on the metal film and guided-mode resonance (GMR) in the Al₂O₃ layer. The theoretical calculations agree well with the numerical simulations. It is also found that the reflection spectrum will be further split by the introduction of another Al₂O₃ layer into the multilayer structure. The reintroduced GMR in the Al₂O₃ layer changes the coupling and interference process between the SPR and GMR field, giving rise to the generation of ultra-narrow reflection dip. Especially, the spectral splitting can facilitate the realization of plasmonic sensors with ultra-high figure of merit (583), which is about 5 times larger than that of traditional SPR sensors. These results will provide a new avenue to the light field manipulation and optical functionalities, especially biochemical and environmental sensing.