000166120 001__ 166120 000166120 005__ 20260128154426.0 000166120 0247_ $$2doi$$a10.1021/acs.nanolett.5c04495 000166120 0248_ $$2sideral$$a147436 000166120 037__ $$aART-2025-147436 000166120 041__ $$aeng 000166120 100__ $$aLee, In-Ho 000166120 245__ $$aUltrastrong Plasmon–Phonon Coupling Enabled by Acoustic Graphene Plasmons 000166120 260__ $$c2025 000166120 5203_ $$aVibrational 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. 000166120 536__ $$9info:eu-repo/grantAgreement/ES/MICIU/CEX2023-001286-S$$9info:eu-repo/grantAgreement/ES/MICINN-AEI/PID2020-115221GB-C41$$9info:eu-repo/grantAgreement/ES/MCIU/PID2023-148359NB-C21 000166120 540__ $$9info:eu-repo/semantics/closedAccess$$aAll rights reserved$$uhttp://www.europeana.eu/rights/rr-f/ 000166120 655_4 $$ainfo:eu-repo/semantics/article$$vinfo:eu-repo/semantics/publishedVersion 000166120 700__ $$0(orcid)0000-0003-0381-3448$$ade León-Pérez, Fernando$$uUniversidad de Zaragoza 000166120 700__ $$aYoo, Daehan 000166120 700__ $$aRyu, Eui-Hyoun 000166120 700__ $$aLow, Tony 000166120 700__ $$aCaldwell, Joshua D. 000166120 700__ $$0(orcid)0000-0001-9273-8165$$aMartín-Moreno, Luis 000166120 700__ $$aOh, Sang-Hyun 000166120 7102_ $$12003$$2395$$aUniversidad de Zaragoza$$bDpto. Física Materia Condensa.$$cÁrea Física Materia Condensada 000166120 773__ $$g25, 51 (2025), 17650-17658$$pNano lett.$$tNano Letters$$x1530-6984 000166120 8564_ $$s10208548$$uhttps://zaguan.unizar.es/record/166120/files/texto_completo.pdf$$yVersión publicada 000166120 8564_ $$s3149101$$uhttps://zaguan.unizar.es/record/166120/files/texto_completo.jpg?subformat=icon$$xicon$$yVersión publicada 000166120 909CO $$ooai:zaguan.unizar.es:166120$$particulos$$pdriver 000166120 951__ $$a2026-01-28-15:42:25 000166120 980__ $$aARTICLE