166120 20260128154426.0 doi 10.1021/acs.nanolett.5c04495 sideral 147436 ART-2025-147436 eng Lee, In-Ho Ultrastrong Plasmon–Phonon Coupling Enabled by Acoustic Graphene Plasmons 2025 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. info:eu-repo/grantAgreement/ES/MCIU/PID2023-148359NB-C21 info:eu-repo/grantAgreement/ES/MICINN-AEI/PID2020-115221GB-C41 info:eu-repo/grantAgreement/ES/MICIU/CEX2023-001286-S info:eu-repo/semantics/closedAccess All rights reserved http://www.europeana.eu/rights/rr-f/ info:eu-repo/semantics/article info:eu-repo/semantics/publishedVersion de León-Pérez, Fernando Universidad de Zaragoza (orcid)0000-0003-0381-3448 Yoo, Daehan Ryu, Eui-Hyoun Low, Tony Caldwell, Joshua D. Martín-Moreno, Luis (orcid)0000-0001-9273-8165 Oh, Sang-Hyun 2003 395 Universidad de Zaragoza Dpto. Física Materia Condensa. Área Física Materia Condensada 25, 51 (2025), 17650-17658 Nano lett. Nano Letters 1530-6984 10208548 http://zaguan.unizar.es/record/166120/files/texto_completo.pdf Versión publicada 3149101 http://zaguan.unizar.es/record/166120/files/texto_completo.jpg?subformat=icon icon Versión publicada oai:zaguan.unizar.es:166120 articulos driver 2026-01-28-15:42:25 ARTICLE