Bright magnetic dipole radiation from two-dimensional lead halide perovskites

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) comprise a versatile class of solution-processable materials with outstanding optoelectronic properties. Analogous to conventional semiconductors, light-matter interactions in 2D HOIPs have been completely described within the electric dipole (ED) approximation. Here, using energy-momentum spectroscopies, we demonstrate that the ED approximation insufficiently describes light-matter interactions in 2D HOIPs. We identify an optical transition in 2D HOIPs that exhibits striking multipolar radiation patterns at an energy 90 meV below the primary exciton emission. The transition is evidently intrinsic in origin and arises from an unconventionally fast magnetic dipole (MD) transition from a polaron-like excited state. These results suggest the presence of even-parity excited state symmetries that have not yet been established, and represent the first observation of MD optical interactions in a bulk crystal. In addition to being of fundamental interest, this demonstration may be crucial for resolving open questions regarding the previously observed bounty of complex optical signatures in 2D HOIPs, and may have implications for quantum information applications based on “dark” excitons in 2D materials.