Ultrafast Investigation of Multiple Strong Coupling System Based on Monolayer MoS₂-Ag Nanodisk Arrays

A multiple strong coupling system comprising monolayer MoS₂ and Ag nanodisk (Ag-ND) arrays is investigated using transient absorption (TA) spectroscopy. By tuning the diameter and period of the Ag-NDs arrays, the surface plasmon polariton (SPP) resonances are made to simultaneously overlap with the A (~660 nm) and B (~608 nm) excitons of monolayer MoS₂. As a result, three distinct negative ground-state bleaching (GSB) peaks, corresponding to the upper (UP), middle (MP), and lower (LP) hybrid polariton states, were observed in the TA spectra. This confirms that a multiple strong coupling regime was achieved with both the A and B excitons of monolayer MoS₂ and SPPs modes, which was also highlighted by the anti-crossing behavior across varied Ag-NDs arrays parameters. Finally, by adding an insulating spacer layer of Al₂O₃ film, the coupling strength can be modulated from a strong coupling regime to a weak coupling regime. These results reveal a multi-exciton-plasmon strong coupling system and establish a versatile platform for ultrathin polaritonic devices, including polariton lasers and all-optical switches.