  Light-matter interactions in semiconductor systems are uniformly treated
within the electric dipole (ED) approximation, as multipolar interactions are
considered "forbidden". Here, we demonstrate that this approximation
inadequately describes light emission in novel two-dimensional hybrid
organic-inorganic perovskite materials (2D HOIPs) --- a class of solution
processable layered semiconductor with promising optoelectronic properties.
Consequently, photoluminescence (PL) spectra become strongly dependent on the
experimental geometry, a fact that is often overlooked, though critical for
correct optical characterization of materials. Using energy-momentum and
time-resolved spectroscopies, we experimentally demonstrate that low-energy
sideband emission in 2D HOIPs exhibits a highly unusual, multipolar
polarization and angle dependence. Using combined electromagnetic and
quantum-mechanical analyses, we attribute this radiation pattern to an
out-of-plane oriented magnetic dipole transition arising from the 2D character
of the excited and ground state orbitals. Symmetry arguments point toward the
presence of significant inversion symmetry-breaking mechanisms that are
currently under great debate. These results provide a new perspective on the
origins of unexpected sideband emission in HOIPs, clarify discrepancies in
previous literature, and generally challenge the paradigm of ED-dominated
light-matter interactions in novel optoelectronic materials.

Materials Science