  We explore the optical properties of La3Ni2O7 bilayer nickelates by using
density functional theory including a Coulomb repulsion term. Convincing
agreement with recent experimental ambient-pressure spectra is achieved for
U=3eV, which permits tracing the microscopic origin of the characteristic
features. Simultaneous consistency with angle-resolved photoemission
spectroscopy and x-ray diffraction suggests the notion of rather moderate
electronic correlations in this novel high-Tc superconductor. Oxygen vacancies
form predominantly at the inner apical sites and renormalize the optical
spectrum quantitatively, while the released electrons are largely accommodated
by a defect state. We show that the structural transition occurring under high
pressure coincides with a significant enhancement of the Drude weight and a
reduction of the out-of-plane interband contribution that act as a fingerprint
of the emerging hole pocket. Finally, we investigate the role of the 2-2 versus
1-3 layer stacking and compare the bilayer nickelate to La4Ni3O10, La3Ni2O6,
and NdNiO2, unveiling general trends in the optical spectrum as a function of
the formal Ni valence in Ruddlesden-Popper versus reduced Ruddlesden-Popper
nickelates.
