Field profile of E_x as a function of x and for different positions along z-direction. Pink solid curves: contribution of the TEM mode with no spatial cut-off equation (13); green dot-dashed curves: numerical result calculated with equation (15) with the integration range {k_x, k_y} ∈ (−∞, +∞); blue solid curves: numerical result calculated with equation (15) with the integration range {k_x, k_y} ∈ [−π/a, π/a]. The structural parameters are the same as in Figure 2.