Biexcitonic Optical Gain in CsPbBr<sub>3</sub> Quantum\nDots

Lead halide perovskite\nquantum dots (QDs) are a promising material\nfor light amplification devices. In order to improve their optical\ngain threshold and lifetime, it is essential to understand the underlying\ngain mechanism. However, there is still debate on the nature of gain\nin perovskite QDs, which has been attributed to different origins\nsuch as biexcitons, trions, and single excitons. Here we study amplified\nspontaneous emission and optical gain of monodisperse spherical CsPbBr<sub>3</sub> QDs and conclusively assign the gain to biexcitons. This\nis based on the gain threshold and its spectral position which we\nstudy via femtosecond transient absorption spectroscopy. Furthermore,\nthe optical gain vanishes within 30 ps, matching the biexciton lifetime,\ndemonstrating the strong correlation to the biexciton population.\nBy identifying the intrinsic mechanism of optical gain in CsPbBr<sub>3</sub> QDs and its limiting factors, our findings show the direction\nfor future work on optimizing their gain threshold and lifetime.