Ultraviolet Polariton Formation in CdS Magic-Size Clusters

Exciton-polaritons have recently emerged as a powerful new concept in materials science. Through the strong coupling between material excitations and confined photonic modes, it is possible to create new hybrid light-matter states that combine the properties of both parent particles. Reported polaritonic effects range from the formation of room-temperature Bose-Einstein condensates with laser-like emission, to dramatic enhancements to charge or energy transport, to changes in the rate or selectivity of thermally driven chemical reactions. Such results have inspired widespread proposals for polariton-enhanced materials, where light-harvesting and -emitting materials could be pushed into new regimes with their functional properties re-engineered, simply through incorporation into appropriate photonic structures. These goals have yet to be realized, not least due to the limited materials space for polariton studies. This is particularly the case in the technologically important UV range, where few polaritonic systems aside from GaN have been reported. Here, we report a versatile new platform for UV polariton studies based on CdS magic-size clusters. These colloidal nanomaterials are amenable to solution processing, exhibit large oscillator strengths and highly confined excitons at 3.8 eV, and can be induced to assemble into hierarchical mesophases through control of film processing. Incorporating these materials into Fabry-Perot cavities, we realize strong coupling with Rabi splittings up to 390 meV and tunable band edge over a 570 meV range. Using ultrafast spectroscopy, we assess how these changes in MSC emissive properties are reflected in changes to the system’s photophysics.