Light-Induced Activation of Forbidden Exciton Transition in Strongly Confined Perovskite Quantum Dots.

We report the strong light-induced activation of forbidden exciton transition in CsPbBr perovskite quantum dots mediated by the symmetry-breaking polaron that modifies the optical selection rule of the confined exciton transition. The activated forbidden transition results in an intense pump-induced absorption in the transient absorption spectra above the bandgap, where the original parity-forbidden transition was located. In contrast to many other semiconductor quantum dots, photoexcitation of an exciton in CsPbBr quantum dots creates a sufficiently large perturbation via a lattice-distorting polaron, which turns on the formally forbidden transition. Compared to the bulk or weakly confined CsPbBr, the activation of the forbidden transition in strongly confined quantum dots is much more prominent due to the stronger influence of the polaron on exciton transitions in the confined space. This nonlinear optical property highlights the intimate coupling of the photoexcited charge carriers with the lattice in the CsPbBr quantum dots, allowing access to the forbidden exciton transitions with light.