Vibrational strong coupling of organic molecules embedded within graphene plasmon nanocavities facilitated by perfect absorbers.

The strong coupling between infrared photonic resonances and vibrational transitions of organic molecules is called vibrational strong coupling (VSC), which presents attractive prospects for modifying molecular chemical characteristics and behaviors. Currently, VSC studies suffer from limited bandwidth or enormous mode volumes. In addition, in certain instances, the absorption spectrum of VSC is weaker, thus impeding the effective monitoring of the VSC effect. Here, we theoretically study the VSC effect by embedding 5-nm-thick organic molecules into a graphene plasmon nanocavity (GPNC). Pronounced anti-crossing characteristics with Rabi splitting exceeding 80 cm-1 are disclosed from the spectra of the coupled molecular system, benefiting from the ultra-small mode volume provided by the GPNC. Further assembling the GPNC into a perfect absorber configuration can significantly enhance the spectral peaks of the VSC effect, thus maximizing the reachability of the VSC phenomenon. Furthermore, the tunability of graphene enables monitoring of spectral changes by electrically adjusting graphene's Fermi level in a structure with fixed geometric parameters. In addition, we establish an analytical framework in alignment with computational simulations to elucidate the triggering criteria for the VSC mode, thereby giving a clear picture for understanding the physical processes that form the VSC mode. Given that graphene supports plasmon modes across an extensive range extending from infrared to terahertz, the suggested GPNC presents a suitable framework for investigating the VSC effect of diverse organic materials.