Direct Evidence of a Transition from Triplet Excitons to Hybrid Light-Matter States via Triplet-Triplet Annihilation

In the enclosed manuscript, we embed a triplet-triplet annihilator inaccessible molecule inside an optical cavity. The system reaches the strong coupling regime, and the formed polariton has a reduced energy compared to the molecular singlet excited state, making triplet-triplet annihilation exothermic and readily observable. By the experimentally realized on-off effect, we show unambiguously that triplet excitons can annihilate directly into exciton-polaritons, without going through the singlet manifold. We further demonstrate that the total angular momentum is conserved in exciton to polariton triplet-triplet annihilation, thus theoretically explaining the feasibility of annihilating excitonic states to hybrid light-matter states.