OPPROMUS: A software tool to analyze optical properties of flat multilayer systems with arbitrarily shaped nanoscale inclusions

• Software for convenient analyses of optical transmission and reflection spectra. • The mathematical framework of the program is based on the transfer-matrix method. • The Maxwell-Garnett and Bruggeman approaches are employed to account for inhomogeneities. • Modeling the optical response of multilayer systems comprising nanoscale inclusions. • Optimization of the parameters by adapting optical response to the experimental results. This article presents a simulation program to conveniently analyze optical transmission, reflection and absorption spectra. It enables the visualization of large optical datasets, the calculation of optical spectra of complex multilayer systems and the iterative optimization of such systems to adapt their properties to measured optical data. The mathematical framework is based on the transfer-matrix method combined with the Maxwell-Garnett and Bruggeman formalisms for structurally inhomogeneous samples. The quality of the simulations is evaluated by the root-mean square error, that can be systematically minimized by adapting the properties of the model system, for instance the number/thickness of optical layers and the geometry/filling factor of the inclusions. The capabilities of the software are demonstrated at the example of a large optical dataset acquired during Cu oxidation at a well-defined temperature and oxygen pressure. The reaction is monitored in situ by recording transmission spectra of the thin-film sample with high time resolution. As the oxidization proceeds, the transmittance increases first, as metallic Cu transforms to dielectric Cu 2 O with 2.15 eV band gap, but then decreases again due to deeper oxidation to CuO (gap size 1.35 eV). The resulting transmission spectra are relatively complex, as Cu oxidation does not follow a layer-by-layer scheme but proceeds along grain boundaries in the initial film. The software still enables excellent modelling of the experimental data, thus providing unique insights into the mechanisms of Cu oxidation. In general, our simulation program opens versatile pathways to study time-dependent processes that systematically alter the optical response of samples, e.g. deposition and growth phenomena, chemical reactions and magnetic reorganizations.