Slotted gap-surface plasmon resonator as an efficient platform for sensing.

Film-coupled plasmonic resonators offer efficient platforms for light enhancement due to the excitation of gap surface plasmons (GSPs) at metal-insulator-metal interfaces, where electromagnetic energy is stored within the spacer. In applications like biosensing and spontaneous emission control, spatial overlap between the target molecule and plasmonic hotspots is essential. Here, we propose utilizing the controllable, efficient light enhancement capabilities of a specifically designed GSP disk resonator for biosensing and spontaneous emission enhancement. To create an external plasmonic hotspot and make the strong field stored in the spacer accessible to nearby molecules, we introduce a nanoslot in the top metallic disk with its long axis oriented perpendicular to the incident field polarization. This orientation ensures significant electric field enhancement due to boundary conditions, while the resonant modes of the GSP and nanoslot are further tailored to optimize the field distribution. Finite element method-based simulations reveal the simultaneous excitation of electric-dipole modes due to the nanoslot alongside GSP modes, resulting in a more than two-order magnitude increase in total electromagnetic energy. Additionally, varying the slot length allows precise control over resonances, revealing different modes of the system. The external hotspot in the nanoslot ensures direct interaction with nearby molecules, enhancing the radiative decay rate by nearly three orders of magnitude. The suggested configuration of a plasmonic disk combined with a rectangular nanoslot extends the degree of freedom for designing external electromagnetic hot spots.