Exciton-polaritons are quasiparticles with mixed photon and exciton character that demonstrate rich quantum phenomena, novel optoelectronic devices and the potential to modify chemical properties of materials. Organic materials are of current interest as active materials for their ability to sustain exciton-polaritons even at room temperature. However, within organic optoelectronic devices, it is often the 'dark' spin-1 triplet excitons that dominate operation. These triplets have been largely ignored in treatments of polaritons, which instead only consider the role of states that directly and strongly interact with light. Here we demonstrate that these 'dark' states can also play a major role in polariton dynamics, observing polariton population transferred directly from the triplet manifold via triplet-triplet annihilation. The process leads to polariton emission that is longer-lived (>μs) even than exciton emission in bare films. This enhancement is directly linked to spin-2 triplet-pair states, which are formed in films and microcavities by singlet fission or triplet-triplet annihilation. Such high-spin multiexciton states are generally non-emissive and cannot directly couple to light, yet the formation of polaritons creates for them entirely new radiative decay pathways. This is possible due to weak mixing between singlet and triplet-pair manifolds, which-in the strong coupling regime-enables direct interaction between the bright polariton states and those that are formally non-emissive. Our observations offer the enticing possibility of using polaritons to harvest or manipulate population from states that are formally dark.
|Number of pages||12|
|State||Published - 2020|
Bibliographical noteFunding Information:
This work was supported by the Engineering and Physical Sciences Research Council, U.K. (Grant Number EP/M025330/1, ‘Hybrid Polaritonics’), the University of Sheffield and EU project 679789-455 ‘CONTREX’. DGB was supported by the EPSRC Centre for Doctoral Training in New and Sustainable Photovoltaics (EP/L01551X/1). TL and AIT thank the nancial support of the European Graphene Flagship Project under grant agreement 785219 and EPSRC grant EP/P026850/1. LAMM and JYZ were supported by the US Department of Energy, Office of Science, Basic Energy Sciences, CPIMS Program under Early Career Research Program award DE-SC0019188. AJM gratefully acknowledges LR Weiss for useful discussion about exchange coupling. The spectroscopic work was performed at the EPSRC Lord Porter Laser Facility, EP/L022613/1 and EP/R042802/1.
© 2020 The Royal Society of Chemistry.
ASJC Scopus subject areas
- Chemistry (all)