Fig. 2.

Dispersion analysis and figure of merit of the topological insulator thin film. a Real (red colour) and imaginary (blue colour) parts of the permittivity spectra of the bulk (solid curves) and surface (dashed curves) states of the Sb₂Te₃ thin film, respectively. The thickness of the bulk and the surface layer of Sb₂Te₃ thin film is 100 and 1.5 nm, respectively. b The numerically simulated normalized intensity distribution of the SPPs field excited at the interface between the Sb₂Te₃ thin film and the air. The thickness of the Sb₂Te₃ film is 100 nm. The width of air slits is 250 nm. The x-linearly polarized incident light has a wavelength of 640 nm. The period of the interference fringes of evanescent SPPs standing waves is 302 nm, which suggests the SPP wavelength of 604 nm in the numerical simulation. c, d Dependence of plasmonic figure of merit for propagation length (c) and degree of confinement (d) on the ultraviolet-visible wavelength for the Sb₂Te₃ thin film and the gold material, respectively