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

## TL;DR

This paper reveals that 2D lead-halide perovskites emit light through magnetic dipole transitions, challenging the traditional electric dipole approximation and highlighting the importance of multipolar interactions in their optical properties.

## Contribution

It demonstrates the presence of magnetic dipole emission in 2D perovskites and shows that light emission depends on experimental geometry, which was previously overlooked.

## Key findings

- Low-energy sideband emission shows multipolar polarization dependence
- Magnetic dipole transitions are out-of-plane oriented in 2D HOIPs
- Inversion symmetry-breaking mechanisms influence emission properties

## Abstract

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.