On the origin of bulk-related anisotropies in surface optical spectra

Reflection anisotropy spectroscopy is widely used to probe the optical properties of surfaces, yet the origin of the ’bulk-related’ features has been debated for decades. It is often argued that these features are related to surface-induced bulk anisotropy because they coincide with critical energies of the bulk dielectric function. Here, we show that a quantitative understanding of these features requires an explicit analysis of excitonic contributions. We introduce a layer-resolved exciton localization measure within the framework of many-body perturbation theory, enabling a direct assessment of the spatial origin of optical anisotropies. Applying this approach to arsenic-modified silicon reconstructions, we find that, depending on the surface structure, the apparent ’bulk-related’ features originate predominantly from surface-localized states. Supported by low-temperature reflection anisotropy measurements, our findings challenge the traditional explanation of surface-induced bulk anisotropy and suggest that bulk-enhanced surface anisotropies also contribute to, or are responsible for, these features.