Exploring Hybrid States and Their Ultrafast Dynamics in Exciton-Plasmon Strong Coupling Systems

To enhance the interaction between light and matter, it is crucial to confine light into minute spaces while simultaneously slowing it down. Plasmon resonance has been a principle used to amplify the interaction between light and matter by acting as a nanoscale optical resonator. However, their light confinement capability is limited, indicating a short phase relaxation time. Here, we explored the possibility of extending this phase relaxation time through strong coupling to long-lived excitons. Initially, estimation from the width of the far-field spectrum suggested that the spectral width of the exciton-plasmon strong coupling system narrowed compared to the plasmon bandwidth, hinting at an extension of the phase relaxation time. In the excitation spectrum measurements, we not only demonstrated the extended phase relaxation time similar to the analysis results from the far-field spectrum but also successfully highlighted the clear formation of hybrid states based on strong coupling. Ultrafast time-resolved measurements and electromagnetic simulations employing the finite-difference time-domain method further revealed the extended lifetime of the exciton-plasmon hybrid structure compared to the precoupled plasmon, foreseeing applications in nonlinear photochemical reaction fields based on enhanced electromagnetic field derived from the extension of phase relaxation time.