Amplified spontaneous emission (ASE) has been reported in different kinds of MHPs, including MAPbBr₃, a bright emitter in the visible-green range, which is the focus of this study. When the material is optically pumped, when the excitation fluence exceeds a certain threshold, ASE appears as a narrow peak on the low-energy side of the spontaneous PL spectrum that, when threshold is reached, grows that grows superlinearly with excitation fluence. Despite being widely reported, the origin of ASE is still debated, being unclear whether ASE arises from direct stimulated photon emission or stimulated emission of exciton-polaritons.

Here we report the results of a new experimental approach, based on tandem ultrafast spectroscopy, to investigate the microscopic origin of ASE in MAPbBr₃ planar waveguides consisting of a 100-nm-thick perovskite layer on top of a glass substrate and covered with an antireflective PMMA coating. To this aim, we configured a femtosecond spectroscopy station to measure both transient transmission and reflection spectra, from which transient absorption coefficient (𝛼) spectrograms were determined, allowing to detect photon gain with unprecedented sensitivity, down to a few tens of cm⁻¹. This result was complemented by time resolved photoluminescence (TRPL) measurements monitoring the coherent light emission from the same excitation spot as the transient pump-probe setup, with comparable time resolution.

ASE was found to occur without population inversion and photon gaina and, even at high excitation densities above threshold, negative 𝛼(𝜔) coefficient was never observed. Just above the threshold, a clear exciton peak was present, with a cross-section only slightly reduced compared to the unpumped waveguide. These results indicate that ASE mainly involves the stimulated emission of exciton-polaritons rather than photons. To confirm our hypothesis, we also solved Maxwell’s equations for the waveguide structure using the measured optical permittivity, finding that light-matter interaction in the highest excitation regime leads to hybrid modes with an energy-momentum dispersion below the bandgap, like that of the lower exciton-polariton band.

In summary, our results show that ASE happens without optical gain, it involves exciton-polaritons at low excitations and new hybrid states at high excitations, that we call band-edge polaritons. Our findings represent an important step toward understanding the fundamental processes governing light amplification in hybrid perovskites that, owing their compatibility with nanopatterning techniques, makes extremely promising candidates for integrated energy-efficient light sources.