Metal-Insulator-Metal Nanoislands with Enhanced Local Fields for SERS-Based Detection

Metallic nanogap dimers can improve surface-enhanced Raman scattering and a variety of nonlinear optical effects by utilizing near-field enhancement effects based on localized surface plasmon resonance. Unfortunately, the strong dipole-dipole interaction between two metallic nanostructures causes the metallic nanogap dimers to exhibit severe light scattering. As a result, they do not always have a structural design that exhibits the highest near-field enhancement because of the radiation loss. Here, we suggest employing dark plasmon modes to further improve the near field on metallic nanogap nanoantennas by coupling the large metal-insulator-metal (MIM) nanocylinders and small Ag nanorod arrays. We found that this heterogeneous MIM nanoisland with a wide nanogap can support the high near-field enhancement effect with strong scattering suppression based on the excitation of the dark plasmon mode. Furthermore, we reveal the relevant mechanism: (1) the suppression of scattering intensity by excitation of a dark plasmon based on the strong near-field interaction between the large MIM nanocylinders and small Ag nanorods and (2) the enhancement effect of the local near fields by the hybrid gap mode between the large MIM nanocylinders and small Ag nanorods. As a result, the local electric field amplitude of the heterogeneous MIM nanoisland nearby nanogaps is enhanced up to about 50 times as compared to that measured using the silver nanogap dimer with the same gap width. This discovery is expected to be particularly beneficial for SERS applications.