Figure 2

(a) Schematic of the structure comprising a thin film of LiF with thickness t that is centered at z = 0 and placed on a lossless substrate. (b) Real (solid blue line) and imaginary (dashed red line) parts of εLiF within 1–20 THz; vertical dashed lines correspond to fTO = 9.22 THz and fLO = 19.1 THz of LiF, being the boundaries of the polaritonic gap (RS band).