Investigation of LSPR and Raman Sensing Sensitivities of Circular Metal-insulator-metal (MIM) Cavity Arrays

Abstract

We investigate the performance of vertical metal-insulator-metal (MIM) structures as refractive index (RI) sensors and surface enhanced Raman scattering (SERS) substrates. The disc-on-film structures with different geometrical parameters and different spacer configurations were studied systematically by experiments and simulations. The circular Fabry-Pérot (FP) cavity model was applied to describe the cavity plasmon modes, which were divided into edge cavity modes and surface cavity modes, showing different optical response to wet etching, RI sensing and SERS. The dependencies of cavity plasmon modes as a function of periodicity, cavity radius and cavity height were studied. The reasons for different line shapes of far-field spectra were revealed. The influences of varying spacers on the locations of 'hot spots' and resonance wavelengths were systematically studied. The 'hot spots' of cavity plasmon modes were exposed by wet etching. The RI sensitivities of MIM structures were thoroughly studied including different types of plasmonic modes, sensing based on spectral shifts and intensity changes, and bulk/molecular RI sensing configurations. Notable SERS enhancement factors (EFs) were achieved by underetching. EF peaks attributed to different orders of cavity plasmon modes were observed in both experiments and simulations. Good qualitative agreement was found between the experimental and simulated results. A similar dependency of the effective SERS EF as a function of cavity radius was found for vertical disc dimers compared to disc-on-film substrates.