Phonon Bottleneck Slowing Down Exciton Dynamics in 2D Perovskites and Transition Metal Dichalcogenides

Monolayer transition metal dichalcogenides (TMDs) and 2D perovskites exhibit a rich exciton physics. The combination of spin-orbit and exchange coupling leads to a complex excitonic landscape of bright and dark states in 2D perovskites, which dictates not only optical signatures but also exciton formation, relaxation and decay dynamics. Despite having a lowest energy dark state, a surprisingly large photoluminescence from the higher-energy bright states has been observed. Combining low temperature magneto-optical measurements with a sophisticated many-particle theory we reveal the microscopic origin of the unexpected enhanced emission of bright excitons in 2D perovskites. We attribute the emission to a pronounced phonon-bottleneck effect, which considerably slows down the relaxation of excitons towards to energetically lowest dark states at low temperatures. We show that this bottleneck can be controlled by changing the bright-dark splitting and the energy of optical phonons by considering different perovskite materials and dielectric surroundings.<br/><br/>We demonstrate that a similar phonon bottleneck can also occur for exciton polaritons created in MoSe₂ monolayers integrated into a Fabry-Perot cavity. Efficient scattering into the exciton polariton ground state is a key prerequisite for generating Bose-Einstein condensates and low-threshold polariton lasing. However, this can be challenging to achieve at low densities due to the appearanche of the polariton bottleneck effect that impedes phonon-assisted scattering into the lowest polariton states. However, the rich exciton landscape of TMDs provides potential intervalley scattering pathways to rapidly populate these polariton states. By exploiting phonon-assisted transitions between momentum-dark excitons and the lower polariton branch, we demonstrate that it is possible to bypass the bottleneck region and efficiently populate the low-momentum states in the lower polariton branch at room temperature.<br/><br/>Our work provides new insights on exciton dynamics in atomically thin semiconductors offering strategies for tuning the exciton emission in layered perovskites and populating the polariton groundstate in TMD monolayers that is crucial for many technological applications based on these materials.