Ultrastrong Plasmon–Phonon Coupling Enabled by Acoustic Graphene Plasmons

Vibrational ultrastrong coupling (USC) opens new opportunities for controlling chemical reactivity and advancing mid-infrared nanophotonics. Here, we demonstrate USC in an ultrathin polar dielectric using acoustic graphene plasmons (AGPs). Unlike conventional graphene plasmons, whose coupling weakens with a reduced dielectric thickness, AGPs maintain strong coupling through enhanced field confinement in narrow dielectric gaps. This enables strong coupling in a polar film as thin as λ/8,000 and USC at λ/2,000. While USC is usually attributed to transverse optical phonons, we identify interface phonon polaritons at polar-nonpolar boundaries as an additional contributor. By tuning the AGP coupling to both phonon types, we observe a transition from a single anticrossing to two distinct anticrossings. This platform, sustaining USC in an extremely small volume, provides a promising basis for mid-infrared nanophotonics and quantum optics.