(a) Phase shift in the THz band for two spacer thickness t = 0.1 and 0.5 μm (d = 20 μm, a = 0.5 μm); (b) Phase shift in the infrared region for t = 10 and 30 nm (d = 2000 nm, a = 50 nm). The inset of (a) and (b) shows, respectively, the zero-phase point A as a function of t for the two frequency band, where the circles represent the theoretical results and the line a simple HFP model. The correspondence (in the infrared region) between the phase shift φ₃ (the line) and (m = 0, 1, 2) are shown in (c), where d = 2000 nm, a = 50 nm, and t = 20 nm.