Lee, In-Ho de Leon-Perez, Fernando Yoo, Daehan Ryu, Eui-Hyoun Low, Tony Caldwell, Joshua D. Martin-Moreno, Luis Oh, Sang-Hyun 2026-01-13T07:30:34Z 2026-01-13T07:30:34Z 2026-01-12 2025-12 1530-6984 https://pubs.kist.re.kr/handle/201004/153993 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. English American Chemical Society Ultrastrong Plasmon–Phonon Coupling Enabled by Acoustic Graphene Plasmons Article 10.1021/acs.nanolett.5c04495 1 Nano Letters, v.25, no.51, pp.17650 - 17658 Nano Letters 25 51 17650 17658 N scie scopus 001638372800001 2-s2.0-105025594890 Chemistry, Multidisciplinary Chemistry, Physical Nanoscience & Nanotechnology Materials Science, Multidisciplinary Physics, Applied Physics, Condensed Matter Chemistry Science & Technology - Other Topics Materials Science Physics Article Graphene plasmons Acoustic plasmons Opticalphonons Ultrastrong coupling Midinfrared nanophotonics