Metal-Dielectric-Metal Resonators with Deep Subwavelength Dielectric Layers Increase the Near-Field SEIRA Enhancement

Plasmonic nanostructures presenting either structural asymmetry or metal-dielectric-metal (M-D-M) architecture are commonly used structures to increase the quality factor and the near-field confinement in plasmonic materials. This characteristic can be leveraged for example to increase the sensitivity of IR spectroscopy, via the surface enhanced IR absorption (SEIRA) effect. In this work, we combine structural asymmetry with the M-D-M architecture to realize Ag-Ag2O-Ag asymmetric ring resonators where two Ag layers sandwich a native silver oxide (Ag2O) layer. Their IR response is compared with the one of fully metallic (Ag) resonators of the same size and shape. The photothermal induced resonance technique (PTIR) is used to obtain near-field SEIRA absorption maps and spectra with nanoscale resolution. Although the native Ag2O layer is only 1 nm to 2 nm thick, it increases the quality factor of the resonators’ dark-mode by ≈27% and the SEIRA enhancement by ≈44% with respect to entirely Ag structures.