doi:10.1021/acs.nanolett.5c04495engLee, In-Hode León-Pérez, FernandoYoo, DaehanRyu, Eui-HyounLow, TonyCaldwell, Joshua D.Martín-Moreno, LuisOh, Sang-HyunUltrastrong Plasmon–Phonon Coupling Enabled by Acoustic Graphene PlasmonsART-2025-147436Vibrational 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.2025http://zaguan.unizar.es/record/16612010.1021/acs.nanolett.5c04495http://zaguan.unizar.es/record/166120oai:zaguan.unizar.es:166120info:eu-repo/grantAgreement/ES/MCIU/PID2023-148359NB-C21info:eu-repo/grantAgreement/ES/MICINN-AEI/PID2020-115221GB-C41info:eu-repo/grantAgreement/ES/MICIU/CEX2023-001286-SNano Letters 25, 51 (2025), 17650-17658All rights reservedhttp://www.europeana.eu/rights/rr-f/info:eu-repo/semantics/closedAccess