Thickness Tuned Ultra-Strong Light-Matter Coupling and Self-Grouping of Exciton-Ensemble in Single Crystals of Metal-Organic Framework

Exciton-polaritons are coupled particles resulting from a strong interaction between electron-hole pairs (excitons) inside material and photonic modes of a microcavity. The unique features of these particles can be engineered for new technological applications such as inversionless lasers, all-optical logic gates and others. Recently researchers have been exploring ultra-strong coupling (USC) which is a new light-matter interaction regime that exceeds both weak and strong coupling schemes. Here, we introduce a solid-state organic polariton light source, for the first time, based on pristine and single-crystalline microplates of a dye-coordinated Metal-Organic Framework (MOF) operating under the USC regime. From optical measurements, we extract a record-high Rabi splitting value of 1.15 eV, which is 51% of the molecular transition energy. Further, the percentage of virtual photons in the polariton ground state is determined to be 1.79 %, indicating MOF as a promising platform for vacuum emission. A miniaturized form of such light sources has a wide variety of applications in science, technology and medicine. More importantly, we propose a novel physical mechanism for the first time, named ‘self-grouping of exciton-ensemble’, to explain the anomalous thickness dependence of multimode Rabi splitting. All the experimental results can only be explained using the correction factor introduced by this new mechanism.<br/>An ultra-low threshold and a high degree of emission polarization have been achieved by stimulated polariton scattering occurred in single-crystallites of as-synthesized MOF microplates. With combined experiments and theoretical modelling, we found that the exciton-polaritons are formed at room temperature because of the strong coupling between Fabry-Perot cavity modes inherently formed by two parallel surfaces of a microplate and Frenkel excitons provided by the 2D layers of the dye linkers in the MOF. Such a cost-effective exciton-polariton cavity is realized at room temperature, without the support of any external mirrors made by metal coatings, or distributed Bragg reflectors (DBRs). Our findings confirm that MOFs, which can be synthesized readily from easily available materials under relatively mild conditions, are the potential candidates to unveil the previously unexplored fundamental physics and advanced applications of the USC regime without needing any complicated material fabrication procedures.