Optical gain of lead halide perovskites measured via the variable stripe length method: what we can learn and how to avoid pitfalls

In the search for novel photonic materials, the recent focus on Metal halide perovskites (MHPs) has revealed their promise to become groundbreaking low-threshold, tunable coherent light sources. An accurate determination of the optical gain coefficient (g) would help to screen for materials and design highly efficient perovskite lasers. Nevertheless, contradictory numbers are continuously reported, making this figure of merit unreliable. To address this issue, the present work outlines a meticulous analysis to retrieve g of MAPbI₃, based on the variable stripe-length (VSL) method. This method is often preferred due to its apparent simplicity; however, one can arrive at incorrect conclusions without the adequate considerations. Therefore, here the experimental implementation and numerical treatment of the data are thoroughly discussed to establish a robust VSL methodology. The obtained power dependence and spectral gain evolution point to the role of electron-hole bimolecular recombination dictating the stimulated emission properties of MAPbI₃, with a behavior resembling that of bulk GaAs. Beyond providing further knowledge on the procedure to carry out pertinent VSL measurements, this work also outlines a meticulous methodology to study the underlying photophysical gain properties of MHPs and consequently, to obtain a deeper understanding of the lasing properties of these complex materials.