Zero-dimensional molecular exciton-polaritons in cavity-free solutions

The hybridization of a molecular excitation with an oscillating electromagnetic field has been performed in a confined structure, typically a Fabry-Perot cavity. The operation of light-matter interac-tions in free space has been a challenging task. Here, we report the strong coupling of electronic transitions in quadruply bonded Mo2 complexes with visible light in cavity-free solution under ambient conditions, evolving the true zero-dimensional exciton-polaritons. The formation of molecular exciton-polaritons is demonstrated by the Rabi doublets of the 8/8* and metal-to-ligand charge transfer excitations in steady-state and time-resolved absorption spectra and by the Mollow triplet in photoluminescence spectra. Ultra -strong coupling is readily achieved in this molecule-light hybrid system, resulting in the dimensionless coupling constants g/u0 reaching values up to 0.14. This work shows that the integrated Mo2 unit, acting as an atomic resonator, drives the coherent light -molecule coupling, opening a new avenue for the study of light -mat-ter interactions.