Fig. 2: Experimental observation of RI-dependent SPhP vibrations.

a Schematic of LSPP-SPhP coupling in the SP-PhP platform. b The dispersion relation of the SP-PhP platform plotted by experimentally measured data. Various detuning (δ) of plasmonic resonance and SPhP vibration is obtained by engineering the antenna length. The letters A, B, and C label three specific δ. c Measured spectra of the SP-PhP platform corresponding to A, B, and C. The change from δB to δC causes a variation in the RI-dependent SPhP vibrations (ΔI). d, e SEM and AFM images of the SP-PhP platform with average antenna length l = 3 μm. Scale bar, 4 μm. f Cross-sectional view (upper panel) of the SP-PhP platform using SEM. Lower panel: corresponding EDX mappings of three key elements for observing Au and SiO2 layers. g Measured spectra of the SP-PhP platform when coupled to BSA molecules of varying thicknesses. h Corresponding baseline-corrected molecular signals of the SP-PhP platform. (I) Two types of signals are observed, one being the RI-determined SPhP response and the other being the vibration mode of the bovine serum albumin (BSA) molecule (amide I, II). (II) The relationship between ΔI and BSA thickness. i Calculated two-dimensional mapping of sensitivity as a function of coupling strength and normalized frequency shift.