Ultrastrong plasmon-phonon coupling in double-layer graphene intercalated with a transition-metal dichalcogenide

We pursue the premise that plasmon-phonon coupling and hybrid plasmon-phonon modes can be broadly tailored in van der Waals (vdW) heterostructures. While the coupling between optical plasmons in graphene and phonons of substrate materials has already been widely investigated, the coupling of acoustic plasmons to phonons has remained elusive to date. Here we demonstrate that double-layer graphene intercalated with a transition-metal dichalcogenide (TMD) can harbor acoustic plasmon-phonon resonances with particularly high coupling strength. Using the quantum-electrostatic heterostructure method, which takes into account the contribution of each vdW monolayer at the ab initio level, we present the dependence of the plasmon-phonon coupling strength on the thickness of the TMD, as well as on the graphene doping. Our results reveal optimal and experimentally feasible conditions to achieve ultrastrong plasmon-phonon coupling, and thus enable further advances in nanoscale thermal and optical devices of high sensitivity.