Hybrid organic/inorganic perovskites (HOIPs) are of great interest for optoelectronic applications due to their quality electronic and optical properties and the exceptional ease of room-temperature synthesis. Layered HOIP structures, e.g., Ruddlesden-Popper phases, offer additional synthetic means to define self-assembling multiple quantum well structures. Measurements of Ruddlesden-Popper HOIP optical constants are currently lacking, but are critical for both a fundamental understanding as well as optoelectronic device design. Here, we use momentum-resolved optical techniques to measure error-constrained complex uniaxial optical constants of layered lead-iodide perovskites incorporating a variety of organic spacer molecules. We demonstrate how large optical anisotropies measured in these materials arise primarily from classical dielectric inhomogeneities rather than the two-dimensional nature of the electronic states. We subsequently show how variations among these materials can be understood within a classical effective-medium model that accounts for dielectric inhomogeneity. We find agreement between experimentally inferred dielectric properties and quantum-mechanical calculations only after accounting for these purely classical effects. This work provides a library of optical constants for this class of materials and clarifies the origins of large absorption and photoluminescence anisotropies witnessed in these and other layered nanomaterials.
Keywords: 2D materials; effective medium; electric dipoles; hybrid perovskites; momentum-resolved; optical anisotropies; optical constants.