(a) Linear, (b) nonlinear and (c) total relative absorption coefficient as a function of the incident photon energy for different interlayer distance d = 10 Å, 25 Å and 50 Å. It includes the reference value (black line) for calculations associated with the 1–0 transition in hh. The concentration of impurities in the layer is ρ_2D = 1 × 10¹³ cm⁻². We use 0.1 MW · cm⁻² for the incident light intensity.