Exciton-Polaritons with Size-Tunable Coupling Strengths in Self-Assembled Organic Microresonators

Self-assembled nano/microcrystals of organic semiconductors with regular faces can serve as optical microresonators, which hold a promise for studying the light confinement and the light-matter interaction. Here, single crystalline microribbons of 1,4-bis­(2-(4-(<i>N</i>,<i>N</i>-di­(<i>p</i>-tolyl)­amino)­phenyl)-vinylbenzene (DPAVB) are synthesized with well-controlled sizes by a facile solution-exchange method. We find that individual microribbon can work as Fabry-Pérot (FP) resonator along its width (<i>w</i>), in which strong coupling of optical modes with excitons results in the formation of exciton polaritons (EPs). The dispersion relation of <i>E</i> ∼ <i>k</i>_<i>z</i> of EPs is constructed by extracting the energies (<i>E</i>) of FP resonances at integer multiples of π/<i>w</i> in the wavevector (<i>k</i>_<i>z</i>) space. By simulating the significantly curved dispersion of EPs with a two coupled harmonic oscillator model, a coupling strength between 0.48 and 1.09 eV are obtained. Two coupling regimes are classified: in regime I, the coupling strength is constant at 0.48 eV for microribbons with the cavity length of <i>w</i> ≥ 2.00 μm; in regime II, the coupling strength increases dramatically from 0.48 to about 1 eV with decreasing the resonator length from <i>w</i> = 2.00 to 0.83 μm. More significantly, our results suggest that the exciton-photon coupling strength could be modulated by varying the size of microribbon cavities, providing an effective method for engineering the light–matter interaction in organic single crystalline microstructures