Single‐Threshold Amplified Spontaneous Emission in the Absence of Photon Gain in MAPbBr₃ Planar Waveguides

Abstract Metal halide perovskites have emerged as promising materials for low‐threshold lasers, ideally suited for integrated optical circuits due to their easy fabrication, tunable wavelength, and seamless on‐chip integration. Exciton lasing, electron‐hole population inversion, polariton condensation and superfluorescence have been proposed to explain the low threshold. Fundamental to discriminate lasing mechanisms is the knowledge of the material's absorption/gain spectrum in the stimulated emission regime, a difficult experimental task in microcavities and metasurfaces. In this study, femtosecond tandem spectroscopy applied to MAPbBr₃ planar waveguides enables the measurement of material absorption and gain spectra at any carrier density, as well as directly monitoring the matter component of hybrid excitations. Results demonstrate that stimulated emission always involves hybrid electron‐hole‐photon excitations in the absence of photon gain, even at the highest excitations when the exciton resonance is completely bleached, a characteristic almost unique in the landscape of photonic materials. Parameter‐free numerical simulations show that MAPbBr₃ waveguides support electromagnetic modes with an exciton‐polariton‐like dispersion below the bandgap at all investigated carrier densities. These excitations combine the advantages of hybrid light‐matter states‐ namely, lower stimulated emission thresholds and higher nonlinearities‐ with the robustness of photon amplification, seamlessly extending potential applications from low to high carrier densities.