Biexcitonic Optical Gain in CsPbBr₃ Quantum Dots

Lead halide perovskite quantum dots (QDs) are a promising material for light amplification devices. In order to improve their optical gain threshold and lifetime, it is essential to understand the underlying gain mechanism. However, there is still debate on the nature of gain in perovskite QDs, which has been attributed to different origins such as biexcitons, trions, and single excitons. Here we study amplified spontaneous emission and optical gain of monodisperse spherical CsPbBr3 QDs and conclusively assign the gain to biexcitons. This is based on the gain threshold and its spectral position which we study via femtosecond transient absorption spectroscopy. Furthermore, the optical gain vanishes within 30 ps, matching the biexciton lifetime, demonstrating the strong correlation to the biexciton population. By identifying the intrinsic mechanism of optical gain in CsPbBr3 QDs and its limiting factors, our findings show the direction for future work on optimizing their gain threshold and lifetime.