Gain Narrowing in Low‐Dimensional PEA:CsPbBr 3 : Superfluorescence versus Amplified Spontaneous Emission

ABSTRACT Quantum phenomena typically occur for low temperatures. Moreover, single‐molecule and single‐particle spectroscopy have revealed a plethora of quantum phenomena in photonics. Superfluorescence (SF) is a macroscopic quantum effect where a macroscopic ensemble of emitters become coherent above a certain excitation threshold and cooperatively emit a burst of coherent photons. Amplified spontaneous emission (ASE), on the other hand, is a collective emission process where a spontaneously emitted photon propagates inside the emitter material and induces stimulated emission. Both SF and ASE lead to a narrow peak rising out of the photoluminescence spectrum. Recently, SF at room temperature was invoked for the explanation of spectral narrowing in low‐dimensional PEA:CsPbBr 3 under high excitation. Here, we investigate the emission characteristics of this material and clarify whether ASE or SF is responsible for the emission narrowing. We do so by testing the effect on samples processed on different substrates, thereby manipulating the optical waveguide characteristics of the stack. We show, that the narrow emission peak is suppressed for substrates with a refractive index higher than the active material. This results from the absence of waveguide modes. These findings show that ASE—not SF—is the dominant mechanism responsible for the observed spectral narrowing.