Simultaneous strong coupling of multiple vibrational transitions of organic molecules enabled by acoustic graphene plasmon resonances with three-dimensional optical confinement

Vibrational strong coupling (VSC) of organic molecules has demonstrated its ability to modulate the chemical properties and reactivity of materials. Currently, the VSC research faces challenges related to weak near-field confinement or narrow operational bandwidth. Here, we first present a theoretical model to reveal the formation mechanism of localized acoustic graphene plasmon resonances (LAGPRs) with three-dimensional deep-subwavelength confinement. Furthermore, we simultaneously access the nanoscale VSC effect arising from the interaction of the LAGPRs with multiple molecular vibrations within an ultrathin organic film measuring just 10 nm. Moreover, modulating graphene’s Fermi level allows us to track the distinct anti-crossing behavior by analyzing changes in the absorption spectrum associated with the VSC effect. Notably, the proposed theoretical model also reveals the excitation conditions of VSC modes and predicts the frequencies of hybridized vibrational polaritons, in agreement with results from the coupled harmonic oscillator model (CHOM) and numerical simulations. As the LAGPR mode is excited across a continuous range from infrared to terahertz, the proposed nanocavity structure can be used to realize the nanoscale VSC effect of a wide variety of organic materials.