Impact of Propagative Surface Plasmon Polaritons on the Electromagnetic Enhancement by Localized Gap Surface Plasmons Between Metallic Nanoparticles and Substrate

The nanoparticle-on-mirror system as a surface-enhanced Raman scattering substrate is sufficient for single molecule detection and possesses advantages of high reproducibility and ease of assembly. In this paper, one single spherical gold nanoparticle (NP) placed on a flat gold substrate with a gap size of 10 nm is firstly studied. Then, two NPs with separations in order of wavelengths is investigated. The enhanced field of the localized gap surface plasmon (LGSP) in the NP-substrate nanogap is analyzed quantitatively with the finite element method, and a simplified model is proposed to describe the impact of the propagative surface plasmon polariton (SPP) on the LGSP. A 34% improvement of the enhancement factor of the Raman signal is achieved compared to a single NP. The field distribution of SPPs is found to play an important role in determining the optimal positions of NPs to generate the strongest hot spots. Then, the case of a single NP or a NP doublet in a gold groove is considered, and an 8.22-fold increase of the enhancement factor of the Raman signal is obtained compared to the case without the groove. The interference among the groove-excited SPPs, the NP-excited SPPs, and the LGSP determines the optimal positions of the NPs in the groove to generate the strongest hot spots. The present work reveals the great impact of the propagative SPPs on the field enhancement of the LGSP in the NP-substrate gap, and provides a theoretical basis for generating multiple strong hot spots by arranging NPs' positions according to the field distribution of the propagative SPPs.