In the following, we discuss the origin of the falling slope of the hump in the absorption spectrum in more detail for the diffraction grating with a period of and a modulation depth of . For this grating the abruptly falling slope occurs near the wavelength of 780 nm. In Figs. 4(a)–4(c) the associated absorption spectra are plotted for different recursion depths and maximum diffraction orders taken into account. Figures 4(d)–4(f) plot the efficiencies of different diffraction orders for three distinctive angles of incidences on the grating , , and , respectively. The angles of incidences and occur if normal incident photons are diffracted into the first () or second diffraction order (), respectively, get reflected at the front side, and again hit the grating. One finds that for the recursion depth the falling slope appears if diffraction up to the third-order is taken into account (). For the recursion depth , the falling slope becomes observable if diffraction up to the second-order is considered (). Accordingly, for recursion depth the falling slope can be found already if only first-order diffraction () is taken into account. This behavior can be explained by the fact that in all three cases the diffraction efficiencies of the corresponding highest considered diffraction orders become zero for wavelengths , i.e., where becomes 90 deg. The diffraction angle can be reached for normal incident light () by third-order diffraction (), or for light incident with by second-order diffraction (), or for light incident with by first-order diffraction (). The first case can occur already for a recursion depth with , whereas the second case requires a diffraction into , which only can occur for recursion depth . If the order of diffractions taken into account is limited to , the third case can only occur for at least three interactions of the ray with the grating, i.e., .