“…By replacing the sharp-index interfaces between layers with rough interfaces (exponentially graded layers) of a thickness d R1 of 0mm, 0.5mm, 1.0mm, 1.5mm, and 2.0mm, the resonant dip records a shift to lower frequency from 3.3932 THz to 3.3796 THz, 3.3664 THz, 3.3543 THz, and 3.3426 THz, as clear in figure 4. This small frequency shift is due to the slight increase in the structure's thickness, according to the standing wave equation [52]. Besides, the reflectance slightly increases from 0.286 to 0.288, 0.289, 0.292, and 0.295.…”