Monolayer J‐Aggregate Crystals Strong Coupling with an All‐Dielectric Metasurface for Photonic Properties Modification

Strong coupling between light and matter, achieved by hybridizing material transitions such as bright excitons and cavity photons, offers exciting opportunities for manipulating the optical and transport properties of nanomaterials. These advancements have potential applications in technologies ranging from all‐optical switching to quantum computation. Achieving strong coupling in such hybrid systems requires high‐quality cavities and emitters with large oscillator strengths. In this work, the integration of a titanium dioxide (TiO 2 ) metasurface with a high‐quality monolayer molecular J‐aggregate crystal is presented to realize strong coupling. Leveraging the high‐Q modes (Bound States in the Continuum and guided mode resonances) of the TiO 2 metasurface and the large oscillator strength of J‐aggregate excitons, strong coupling is observed with a Rabi splitting of 78 meV in both photoluminescence (PL) and reflection spectra at room temperature. Additionally, under strong coupling, uniform modification of the emission properties across the entire sample is achieved: the polarization degree of emission is enhanced to 94% and the non‐radiative recombination ratio is reduced from 34% down to 2%. This study introduces another nanophotonic platform, extending beyond conventional photonic cavities and plasmonic structures, for achieving strong coupling with the organic molecular excitons.