Room Temperature Strong Light‐Matter Coupling with Cd ₃ P ₂ Magic‐Size Clusters in a Tunable Microcavity

Abstract Light‐matter strong coupling generates polariton states, which not only are the subject of fundamental physics studies but may also enable transformative technologies in lasing, optical switching, and chemistry. The exciton polaritons of semiconductors and molecules have been extensively studied. Here, we study the strong light‐matter interaction of magic‐size nanoclusters (MSCs), which can be considered as extremely confined nanocrystals that bridge the gap between semiconductors and small molecules. It is found that Cd 3 P 2 MSCs, with superior size monodispersity and large oscillator strength, enable room‐temperature strong coupling in a tunable Fabry–Pérot microcavity, with the Rabi splitting reaching 160 meV. Importantly, the derived transition dipole moment of Cd 3 P 2 MSCs is consistent with that obtained from optical Stark effect measurements. The four orders‐of‐magnitude difference in electric field strength, however, highlights the essence of collective strong coupling in a microcavity in comparison to coupling with the light field in laser pulses.