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 $\Lambda =625\u2009\u2009nm$ and a modulation depth of $h=50\u2009\u2009nm$. 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 $r$ and maximum diffraction orders $mmax$ taken into account. Figures 4(d)–4(f) plot the efficiencies of different diffraction orders for three distinctive angles of incidences on the grating $\Theta in,0=0\u2009\u2009deg$, $\Theta in,1(\lambda )$, and $\Theta in,2(\lambda )$, respectively. The angles of incidences $\Theta in,1(\lambda )$ and $\Theta in,2(\lambda )$ occur if normal incident photons are diffracted into the first ($m=1$) or second diffraction order ($m=2$), respectively, get reflected at the front side, and again hit the grating. One finds that for the recursion depth $r=1$ the falling slope appears if diffraction up to the third-order is taken into account ($mmax=3$). For the recursion depth $r=2$, the falling slope becomes observable if diffraction up to the second-order is considered ($mmax=2$). Accordingly, for recursion depth $r=3$ the falling slope can be found already if only first-order diffraction ($mmax=1$) 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 $\lambda >780\u2009\u2009nm$, i.e., where $\Theta out$ becomes 90 deg. The diffraction angle $\Theta out=90\u2009\u2009deg$ can be reached for normal incident light ($\Theta in,0=0\u2009\u2009deg$) by third-order diffraction ($m=3$), or for light incident with $\Theta in,1$ by second-order diffraction ($m=2$), or for light incident with $\Theta in,2$ by first-order diffraction ($m=1$). The first case can occur already for a recursion depth $r=1$ with $mmax\u22653$, whereas the second case requires a diffraction into $\Theta in,1$, which only can occur for recursion depth $r\u22652$. If the order of diffractions taken into account is limited to $|m|\u22641$, the third case can only occur for at least three interactions of the ray with the grating, i.e., $r\u22653$.