Long-Lived Dynamics Enables Exciton-Polariton Upconversion in CdSe Nanoplatelets

Strong coupling of electronic transitions of molecules with confined electromagnetic fields inside an optical cavity creates hybrid light-matter states known as exciton-polaritons, which can lead to altered cavity-mediated chemical reactions and provide a platform for investigating quantum electrodynamics in chemical physics. Here, we strongly couple colloidal CdSe nanoplatelets in a high Q-factor Fabry-Pérot optical cavity giving rise to enhanced upper polariton photoluminescence (PL) emission. Through resonant lower polariton excitation, we observe upconverted PL emission from repopulation of the upper polariton states, thus providing mechanistic insights into the role of dark states for collectively coupled polaritonic systems. Finally, by resolving lower and upper polariton lifetimes, we report timescales on the order of 100 picoseconds, suggesting the potential for polariton systems to enhance photochemical reaction rates. This work, along with developed theoretical quantum dynamical simulation tools, paves the way towards practical polariton photochemistry and provides important insight into the photophysics of nanocrystal-based exciton-polariton systems.