doi:10.1038/s41566-020-00731-5engYoo, DaehanLeón Pérez, Fernando dePelton, MatthewLee, In-HoMohr, Daniel A.Raschke, Markus B.Caldwell, Joshua D.Martín-Moreno, LuisOh, Sang-HyunUltrastrong plasmon–phonon coupling via epsilon-near-zero nanocavitiesART-2021-121334Vibrational ultrastrong coupling, where the light–matter coupling strength is comparable to the vibrational frequency of molecules, presents new opportunities to probe the interactions between molecules and zero-point fluctuations, harness cavity-modified chemical reactions and develop novel devices in the mid-infrared spectral range. Here we use epsilon-near-zero nanocavities filled with a model polar medium (SiO2) to demonstrate ultrastrong coupling between phonons and gap plasmons. We present classical and quantum-mechanical models to quantitatively describe the observed plasmon–phonon ultrastrong coupling phenomena and demonstrate a modal splitting of up to 50% of the resonant frequency (normalized coupling strength η > 0.25). Our wafer-scale nanocavity platform will enable a broad range of vibrational transitions to be harnessed for ultrastrong coupling applications2021http://zaguan.unizar.es/record/10615610.1038/s41566-020-00731-5http://zaguan.unizar.es/record/106156oai:zaguan.unizar.es:106156info:eu-repo/grantAgreement/ES/MINECO/MAT2017-88358-C3-1-Rinfo:eu-repo/grantAgreement/ES/MINECO/MAT2017-88358-C3-2-RNature Photonics 15 (2021), 125–130All rights reservedhttp://www.europeana.eu/rights/rr-f/info:eu-repo/semantics/openAccess