Cavity Controlled Upconversion in CdSe Nanoplatelet Polaritons

Strong coupling between electronic transitions of matter and confined electromagnetic fields inside an optical cavity creates hybrid, light-matter states known as polaritons. Polaritons provide a versatile platform for investigating quantum electrodynamics effects in chemical systems, such as polariton-altered chemical reactivity. However, using polaritons in chemical contexts will require a better understanding of the photophysical properties of polaritons, especially at ambient temperature and pressure, where chemistry is typically performed. Here, we investigated leveraging strong light-matter interactions to control the excited state dynamics of colloidal CdSe nanoplatelets (NPLs) coupled to a Fabry-Pérot optical cavity. Importantly, changes in the cavity quality (Q) factor, which is a measure of photon loss from the cavity, were used to profoundly change the polariton dynamics. As the Q-factor was increased, we observed significant population of the upper polariton (UP) state, exemplified by the rare observation of substantial UP photoluminescence (PL) at room temperature. In fact, with low energy excitation at the lower polariton (LP), which is not absorbed by uncoupled nanoplatelets, we observed upconverted PL emission from the UP branch, due to efficient exchange of population between the LP, UP and the reservoir of dark states present in collectively coupled polaritonic systems. Critical physical insight into the mechanism of PL enhancement of the UP, and PL upconversion, was provided by state-of-the-art quantum dynamics simulations that account for multiple cavity electromagnetic modes and cavity loss. In addition, by resolving lower and upper polariton PL lifetimes, we found timescales for polariton dynamics on the order of 100 picoseconds, implying great potential for NPL based polariton systems to affect photochemical reaction rates. This work provides important insight into the photophysics of nanocrystal-based exciton-polariton systems and is a significant step towards the development of practical polariton photochemistry platforms.