Multipole Plasmonic Optical Properties of the Au Cone-On-Plate Nanostructure: Change the Effective Mean Free Path to Improve Refractive Index Sensing

In this study, a novel structure, the Au cone-on-plate nanostructure, has been proposed, which can substantially improve the refractive index (RI) sensitivity of the Au nanocone. The plasmonic properties of the Au cone-on-plate nanostructure have been investigated by the discrete dipole approximation (DDA). The presence of the Au nanoplate can effectively reduce the full width at half maximum (FWHM) of local surface plasmon resonance (LSPR) from the Au nanocone. The analysis of the local electric fields indicates that the Au nanoplate can influence the FWHM of multipole resonance (l = 3) peak by changing the effective mean free path of polarized charges. We use three parameters to evaluate the RI sensitivity: the figure of merit (FOM), the bulk sensitivity (ηB), and the ηB × FOM. By adjusting the thickness of Au nanoplate (T) and the space between Au nanoplate and Au nanocone (S), the RI sensitivity could be well tuned. When T = 5 nm and S = 2 nm, the FWHM can be reduced from 220 to 80 nm, ηB = 500 nm/RIU, FOM = 6.25 RIU⁻¹, and ηB × FOM = 3125 nm/RIU², which is seven times that of the Au nanocone. It shows excellent potential in the RI sensitivity of Au nanoparticles. The above results suggest the Au nanoplate can influence the distribution of polarized charges thereby substantially improving the RI sensitivity. This expands more possibilities for the Au nanocone in plasmonic sensing.