Bright magnetic dipole radiation from two-dimensional lead-halide perovskites

Hybrid organic-inorganic perovskites (HOIPs) constitute a promising class of versatile, solution-processable materials for optoelectronic applications, such as photovoltaic devices, light-emitting diodes, and lasers. Light-matter interactions within HOIPs have been uniformly treated, as in conventional semiconductor systems, within the electric dipole (ED) approximation; multipolar interactions are considered "forbidden". Here, using energy-momentum and time-resolved spectroscopies, we experimentally demonstrate bright multipolar photoluminescence (PL) in two-dimensional (2D) lead-halide HOIPs at an energy 90 meV below the primary ED-mediated exciton. Using combined electromagnetic and quantum-mechanical analyses, we attribute the multipolar PL to an out-of-plane oriented magnetic dipole transition arising from the significant 2D character of the excited and ground state orbitals. Symmetry considerations point toward the presence of odd-parity ground-state electronic symmetries that have note yet been established. These results elucidate the origins of unexpected sideband emissions in 2D HOIPs, motivate significant reconsideration of the electronic states participating in optical transitions, and generally challenge the paradigm of ED-dominated light-matter interactions in novel optoelectronic materials.