Near-UV Tunable Polaritons from Magic-Size Clusters

Stronglight-matter coupling to form polaritons has gained significant attention for its applications in materials engineering, optoelectronics, and beyond. The combined properties of their underlying states allow for numerous advantages such as delocalization over long distances, room-temperature Bose–Einstein condensation, and tunability of energy states. Few exciton-polariton systems, however, reach into the UV, and identifying ideal materials that possess large oscillator strengths, large exciton binding energies, ease of processing, and that are stable for device integration has proven challenging. Here, we demonstrate that CdS magic-size clusters (MSCs) combine all these traits. Simple solution processing in metallic Fabry–Perot (FP) cavities enables the MSCs to exhibit room-temperature strong coupling, as demonstrated by the square root dependence of Rabi splitting on chromophore concentration. Rabi splitting as large as 390 meV can be achieved, with emission from polariton states spanning from 3.07 eV (403 nm) to 3.64 eV (340 nm). When Rabi splittings are normalized by the excitonic line width, this system is comparable with high-performing systems in the visible range and surpasses reported UV polariton systems. The strong UV absorption of these MSCs establishes a platform to develop stable polaritonic devices with tunability across the near-UV.