To confirm the interband nature of the absorption involved in the PL process, we compare in the following the PLE with a numerical model of the interband absorption. We used a well-established method by Rosei to calculate the dissipative imaginary permittivity, (see SI for details). Figure presents a comparison of the experimental PLE spectrum with the simulated imaginary permittivity, representing the interband absorption. The data show a remarkable correspondence to the simulation, up to the form of the characteristic M 1-type van-Hove singularity at the X -point. This close correspondence excludes the contribution of intraband absorption in the PL and unambiguously proves that only interband absorption is involved in the PL process.