Dispersion curves and effective refractive indexes of symmetric (red lines) and antisymmetric (blue lines) modes with d = 10 nm. Panels (a) and (b) show the dispersion curves of SPP modes in structures A and B, respectively. The insets show the dispersion curves of the antisymmetric SPP modes by amplifying the longitudinal-coordinate axis. (c) The effective refractive index of symmetric mode with ω = 2.5 × 10¹⁵ rad/s and asymmetric mode with ω = 2.989 × 10¹⁵ rad/s in structure A. (d) The effective refractive index of symmetric mode with ω = 5.5 × 10¹⁵ rad/s and asymmetric mode with ω = 6.124 × 10¹⁵ rad/s in structure B.