The experimentally determined absorption coefficient of a CaCuP film (Film C in Figure ) is plotted in Figure . The spectrum can be divided into three regions. Absorption below 1.0 eV photon energy can be attributed to free carrier absorption, i.e. intraband absorption of free holes in the valence band. We note that this absorption mechanism is not considered in the calculated coefficient of pristine CaCuP without free carriers. Absorption starts to rise above 1.0 eV photon energy, with indirect transitions from the Γ point to between L and M likely responsible for this (Figure ). † Absorption increases more intensely at a second onset around 2.5 eV, which is likely due to direct transitions at the Γ point into the conduction band minimum (CBM) and possibly the CBM+1 (Figure ). This onset is particularly obvious when plotting the extinction coefficient on a linear scale (Figure ). Experimental estimates of the indirect and direct band gaps are 0.9 eV and 2.5 eV respectively (Figure ). † The refractive index of CaCuP below the direct band gap is between 3.0 and 3.5, as extracted by ellipsometry (Figure ).