Real-space observation of flat-band ultrastrong coupling between optical phonons and surface plasmon polaritons

Nature Materials (2025)Cite this article Strong and ultrastrong coupling are pivotal phenomena in science and technology, where light–matter hybridization opens new avenues for manipulating quantum states, material properties or chemical reactions. Here we use pump–probe nanospectroscopy for real-space mapping of vibrational ultrastrong coupling between optical phonons in a thin SiC layer and surface plasmon polaritons in a semiconductor (InAs) substrate. By adjusting the InAs carrier density through photoexcitation, we align the flat dispersion limit of the surface plasmon polaritons to the SiC transverse optical phonon, yielding hybridized modes in an intriguingly wide wavevector range. This flat-band ultrastrong coupling contrasts conventional ultrastrong coupling, where hybridization typically occurs in a narrow wavevector range. We further predict flat-band coupling for weak oscillators, illustrated by strong coupling of molecular vibrations with low-loss surface phonon polaritons at their dispersion limit. Achieving strong and ultrastrong coupling over a large wavevector range, and thus many hybrid modes, may benefit polariton chemistry and phase transitions induced by strong and ultrastrong coupling.This is a preview of subscription content, access via your institution Access Nature and 54 other Nature Portfolio journals Get Nature+, our best-value online-access subscription $32.99 / 30 days cancel any timeSubscribe to this journal Receive 12 print issues and online access $259.00 per yearonly $21.58 per issueBuy this articleUSD 39.95Prices may be subject to local taxes which are calculated during checkoutThe data, plotting scripts and complementary description of the calculations used in this study are openly available via Zenodo at https://doi.org/10.5281/zenodo.17233543 (ref. 57). Source data are provided with this paper.Galego, J., Garcia-Vidal, F. J. & Feist, J. Cavity-induced modifications of molecular structure in the strong-coupling regime. Phys. Rev. X 5, 041022 (2015).

Google Scholar Garcia-Vidal, F. J., Ciuti, C. & Ebbesen, T. W. Manipulating matter by strong coupling to vacuum fields. Science 373, eabd0336 (2021).Article CAS PubMed Google Scholar Forn-Díaz, P., Lamata, L., Rico, E., Kono, J. & Solano, E. Ultrastrong coupling regimes of light-matter interaction. Rev. Mod. Phys. 91, 025005 (2019).Article Google Scholar Hutchison, J. A., Schwartz, T., Genet, C., Devaux, E. & Ebbesen, T. W. Modifying chemical landscapes by coupling to vacuum fields. Angew. Chem. Int. Ed. 51, 1592–1596 (2012).Article CAS Google Scholar Nagarajan, K., Thomas, A. & Ebbesen, T. W. Chemistry under vibrational strong coupling. J. Am. Chem. Soc. 143, 16877–16889 (2021).Article CAS PubMed Google Scholar Xiang, B. & Xiong, W. Molecular polaritons for chemistry, photonics and quantum technologies. Chem. Rev. 124, 2512–2552 (2024).Article CAS PubMed PubMed Central Google Scholar Frisk Kockum, A., Miranowicz, A., De Liberato, S., Savasta, S. & Nori, F. Ultrastrong coupling between light and matter. Nat. Rev. Phys. 1, 19–40 (2019).Article Google Scholar Baranov, D. G. et al. Ultrastrong coupling between nanoparticle plasmons and cavity photons at ambient conditions. Nat. Commun. 11, 2715 (2020).Article CAS PubMed PubMed Central Google Scholar Stassi, R. et al. Quantum nonlinear optics without photons. Phys. Rev. A 96, 023818 (2017).Article Google Scholar Sentef, M. A., Ruggenthaler, M. & Rubio, A. Cavity quantum-electrodynamical polaritonically enhanced electron-phonon coupling and its influence on superconductivity. Sci. Adv. 4, eaau6969 (2018).Article CAS PubMed PubMed Central Google Scholar Stassi, R. & Nori, F. Long-lasting quantum memories: extending the coherence time of superconducting artificial atoms in the ultrastrong-coupling regime. Phys. Rev. A 97, 033823 (2018).Article CAS Google Scholar Chikkaraddy, R. et al. Single-molecule strong coupling at room temperature in plasmonic nanocavities. Nature 535, 127–130 (2016).Article CAS PubMed PubMed Central Google Scholar Tame, M. S. et al. Quantum plasmonics. Nat. Phys. 9, 329–340 (2013).Article CAS Google Scholar Ruggenthaler, M., Tancogne-Dejean, N., Flick, J., Appel, H. & Rubio, A. From a quantum-electrodynamical light–matter description to novel spectroscopies. Nat. Rev. Chem. 2, 0118 (2018).Article CAS Google Scholar Vergauwe, R. M. A. et al. Modification of enzyme activity by vibrational strong coupling of water. Angew. Chem. Int. Ed. 58, 15324–15328 (2019).Article CAS Google Scholar Thomas, A. et al. Tilting a ground-state reactivity landscape by vibrational strong coupling. Science 363, 615–619 (2019).Article CAS PubMed Google Scholar Ashida, Y. et al. Quantum electrodynamic control of matter: cavity-enhanced ferroelectric phase transition. Phys. Rev. X 10, 041027 (2020).CAS Google Scholar Autore, M. et al. Boron nitride nanoresonators for phonon-enhanced molecular vibrational spectroscopy at the strong coupling limit. Light Sci. Appl. 7, 17172 (2018).Article CAS PubMed PubMed Central Google Scholar Menghrajani, K. S., Nash, G. R. & Barnes, W. L. Vibrational strong coupling with surface plasmons and the presence of surface plasmon stop bands. ACS Photon. 6, 2110–2116 (2019).Article CAS Google Scholar Dayal, G., Morichika, I. & Ashihara, S. Vibrational strong coupling in subwavelength nanogap patch antenna at the single resonator level. J. Phys. Chem. Lett. 12, 3171–3175 (2021).Article CAS PubMed Google Scholar Autore, M. et al. Enhanced light–matter interaction in 10B monoisotopic boron nitride infrared nanoresonators. Adv. Optical Mater. 9, 2001958 (2021).Article CAS Google Scholar Yoo, D. et al. Ultrastrong plasmon–phonon coupling via epsilon-near-zero nanocavities. Nat. Photon. 15, 125–130 (2021).Article CAS Google Scholar Bylinkin, A. et al. Real-space observation of vibrational strong coupling between propagating phonon polaritons and organic molecules. Nat. Photon. 15, 197–202 (2021).Article CAS Google Scholar Liu, K. et al. Vibrational strong coupling between surface phonon polaritons and organic molecules via single quartz micropillars. Adv. Mater. 34, 2109088 (2022).Article CAS Google Scholar Dolado, I. et al. Remote near-field spectroscopy of vibrational strong coupling between organic molecules and phononic nanoresonators. Nat. Commun. 13, 6850 (2022).Article CAS PubMed PubMed Central Google Scholar Muller, E. A. et al. Nanoimaging and control of molecular vibrations through electromagnetically induced scattering reaching the strong coupling regime. ACS Photon. 5, 3594–3600 (2018).Article CAS Google Scholar Hirschmann, O., Bhakta, H. H., Kort-Kamp, W. J. M., Jones, A. C. & Xiong, W. Spatially resolved near field spectroscopy of vibrational polaritons at the small N limit. ACS Photon. 11, 2650–2658 (2024).Article CAS Google Scholar Ni, G. X. et al. Ultrafast optical switching of infrared plasmon polaritons in high-mobility graphene. Nat. Photon. 10, 244–247 (2016).Article CAS Google Scholar Wehmeier, L. et al. Landau-phonon polaritons in Dirac heterostructures. Sci. Adv. 10, eadp3487 (2024).Article CAS PubMed PubMed Central Google Scholar Wagner, M. et al. Ultrafast dynamics of surface plasmons in InAs by time-resolved infrared nanospectroscopy. Nano Lett. 14, 4529–4534 (2014).Article CAS Google Scholar Knoll, B. & Keilmann, F. Infrared conductivity mapping for nanoelectronics. Appl. Phys. Lett. 77, 3980–3982 (2000).Article CAS Google Scholar Hillenbrand, R., Taubner, T. & Keilmann, F. Phonon-enhanced light–matter interaction at the nanometre scale. Nature 418, 159–162 (2002).Article CAS PubMed Google Scholar Stiegler, J. M. et al. Nanoscale free-carrier profiling of individual semiconductor nanowires by infrared near-field nanoscopy. Nano Lett. 10, 1387–1392 (2010).Article CAS PubMed Google Scholar He, M. et al. Polariton design and modulation via van der Waals/doped semiconductor heterostructures. Nat. Commun. 14, 7965 (2023).Article CAS PubMed PubMed Central Google Scholar Hillenbrand, R., Abate, Y., Liu, M., Chen, X. & Basov, D. N. Visible-to-THz near-field nanoscopy. Nat. Rev. Mater. 10, 285–310 (2025).Article CAS Google Scholar Xu, R. et al. Highly confined epsilon-near-zero and surface phonon polaritons in SrTiO3 membranes. Nat. Commun. 15, 4743 (2024).Article CAS PubMed PubMed Central Google Scholar Schwennicke, K., Giebink, N. C. & Yuen-Zhou, J. Extracting accurate light–matter couplings from disordered polaritons. Nanophotonics 13, 2469–2478 (2024).Article CAS PubMed PubMed Central Google Scholar Mancini, A. et al. Near-field retrieval of the surface phonon polariton dispersion in free-standing silicon carbide thin films. ACS Photon. 9, 3696–3704 (2022).Article CAS Google Scholar Fei, Z. et al. Infrared nanoscopy of Dirac plasmons at the graphene–SiO2 interface. Nano Lett. 11, 4701–4705 (2011).Article CAS PubMed Google Scholar Dai, S. et al. Tunable phonon polaritons in atomically thin van der Waals crystals of boron nitride. Science 343, 1125–1129 (2014).Article CAS PubMed Google Scholar Passler, N. C. et al. Strong coupling of epsilon-near-zero phonon polaritons in polar dielectric heterostructures. Nano Lett. 18, 4285–4292 (2018).Article CAS PubMed Google Scholar Hauer, B., Engelhardt, A. P. & Taubner, T. Quasi-analytical model for scattering infrared near-field microscopy on layered systems. Opt. Express 20, 13173–13188 (2012).Article PubMed Google Scholar Huber, M. A. et al. Femtosecond photo-switching of interface polaritons in black phosphorus heterostructures. Nat. Nanotechnol. 12, 207–211 (2017).Article CAS PubMed Google Scholar Chen, S. et al. Real-space nanoimaging of THz polaritons in the topological insulator Bi2Se3. Nat. Commun. 13, 1374 (2022).Article CAS PubMed PubMed Central Google Scholar Chen, S. et al. Real-space observation of ultraconfined in-plane anisotropic acoustic terahertz plasmon polaritons. Nat. Mater. 22, 860–866 (2023).Article CAS PubMed Google Scholar Yoxall, E. et al. Direct observation of ultraslow hyperbolic polariton propagation with negative phase velocity. Nat. Photon. 9, 674–678 (2015).Article CAS Google Scholar Barra-Burillo, M. et al. Microcavity phonon polaritons from the weak to the ultrastrong phonon–photon coupling regime. Nat. Commun. 12, 6206 (2021).Article PubMed PubMed Central Google Scholar Muniain, U., Aizpurua, J., Hillenbrand, R., Martín-Moreno, L. & Esteban, R. Description of ultrastrong light–matter interaction through coupled harmonic oscillator models and their connection with cavity-QED Hamiltonians. Nanophotonics 14, 2031–2052 (2025).Article PubMed PubMed Central Google Scholar Schubert, M., Bundesmann, C., Jacopic, G., Maresch, H. & Arwin, H. Infrared dielectric function and vibrational modes of pentacene thin films. Appl. Phys. Lett. 84, 200–202 (2004).Article CAS Google Scholar Bryxin, V. V., Mirlin, D. N. & Reshina, I. I. Surface plasmon-phonon interaction in n-InSb.

Solid State Commun. 11, 695–699 (1972).Article Google Scholar Dunkelberger, A. D. et al. Active tuning of surface phonon polariton resonances via carrier photoinjection. Nat. Photon. 12, 50–56 (2018).Article CAS Google Scholar Hagenmüller, D., Schachenmayer, J., Genet, C., Ebbesen, T. W. & Pupillo, G. Enhancement of the electron–phonon scattering induced by intrinsic surface plasmon–phonon polaritons. ACS Photon. 6, 1073–1081 (2019).Article Google Scholar Deutsch, B., Hillenbrand, R. & Novotny, L. Near-field amplitude and phase recovery using phase-shifting interferometry. Opt. Express 16, 494–501 (2008).Article CAS PubMed Google Scholar Woessner, A. et al. Highly confined low-loss plasmons in graphene–boron nitride heterostructures. Nat. Mater. 14, 421–425 (2015).Article CAS PubMed Google Scholar Lorimor, O. G. & Spitzer, W. G. Infrared refractive index and absorption of InAs and CdTe. J. Appl. Phys. 36, 1841–1844 (1965).Article CAS Google Scholar Esteban, R., Aizpurua, J. & Bryant, G. W. Strong coupling of single emitters interacting with phononic infrared antennae. New J. Phys. 16, 013052 (2014).Article Google Scholar Vicentini, E. Real-space observation of flat-band ultrastrong coupling between optical phonons and surface plasmon polaritons. Zenodo https://doi.org/10.5281/zenodo.17233543 (2025).Download referencesE.V. acknowledges funding from grant FJC2021-046779-I, financed by MICIU/AEI/10.13039/501100011033 and the European Union NextGenerationEU/PRTR, and from the European Union’s Horizon Europe research and innovation programme under Marie Skłodowska-Curie grant agreement no. 101063152. The work also received financial support from grant CEX2020-001038-M, grant PID2021-123949GB-I00 (NANOSPEC) and grant PID2024-156602NB-I00 (CEDANO), funded by MICIU/AEI/10.13039/501100011033 and by ERDF/EU. J.A., R.E. and X.A. acknowledge funding from grant PID2022-139579NB-I00, funded by MICIU/AEI/10.13039/501100011033 and ERDF/EU; from project IT1526-22 from the Department of Science, Universities and Innovation of the Basque Government; and from Elkartek project ‘u4smart’ from the Department of Industry of the Basque Government. X.A. acknowledges the Spanish Ministerio de Ciencia, Innovación y Universidades for his grant, FPU21/02963. M.S. acknowledges financial support from grant PID2023-148359NB-C22 (AI-NANOSPEC), funded by MICIU/AEI/10.13039/501100011033 and ERDF/EU, and grant RYC2023-044188-I, funded by MICIU/AEI/10.13039/501100011033 and by FSE+.CIC nanoGUNE, Basque Research and Techology Alliance, Donostia–San Sebastián, SpainEdoardo Vicentini, Martin Schnell, Nicolas Pajusco, Felix Begemann, Maria Barra Burillo, Maria Ramos, Andrei Bylinkin & Rainer HillenbrandCentro de Física de Materiales (CFM-MPC), Consejo Superior de Investigaciones Científicas (CSIC) and Universidad del Pais Vasco – Euskal Herriko Unibertsitatea (UPV/EHU), Donostia–San Sebastián, SpainXabier Arrieta, Maria Ramos & Ruben EstebanDepartment of Electricity and Electronics, University of the Basque Country, Leioa, SpainXabier Arrieta, Javier Aizpurua & Rainer HillenbrandDonostia International Physics Center (DIPC), Donostia–San Sebastián, SpainMartin Schnell, Ruben Esteban & Javier AizpuruaIkerbasque, the Basque Foundation for Science, Bilbao, SpainMartin Schnell, Javier Aizpurua & Rainer HillenbrandLaboratoire d’Acoustique de l’Université du Mans (LAUM), Institut d’Acoustique–Graduate School (IA-GS), CNRS and Le Mans Université, Le Mans, FranceNicolas PajuscoSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarE.V. and R.H. conceived the experiment. M.B.B. and M.R. prepared the samples. E.V., N.P. and F.B. performed the experiments. E.V. and X.A. developed and implemented the theoretical models and analysed the data. A.B. contributed to the dispersion calculations. M.S. performed numerical simulation. R.H. led the project. E.V., X.A., R.E., J.A. and R.H. co-wrote the paper with input from all co-authors. All authors contributed to discussions.Correspondence to Rainer Hillenbrand.R.H. is a co-founder of Neaspec GmbH, which is now a part of Attocube Systems GmbH, a company producing s-SNOM systems such as the one used in this study. The other authors declare no competing interests.Nature Materials thanks Wei Xiong and Joel Yuen-Zhou for their contribution to the peer review of this work.Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.Supplementary Figs. 1–9, Notes 1–9 and Tables 1 and 2.Source data for Fig. 1b–h.Source data for Fig. 2b–g.Source data for Fig. 3a–i.Source data for Fig. 4a–f.Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.Reprints and permissionsVicentini, E., Arrieta, X., Schnell, M. et al. Real-space observation of flat-band ultrastrong coupling between optical phonons and surface plasmon polaritons. Nat. Mater. (2025). https://doi.org/10.1038/s41563-025-02412-6Download citationReceived: 25 October 2024Accepted: 17 October 2025Published: 18 December 2025Version of record: 18 December 2025DOI: https://doi.org/10.1038/s41563-025-02412-6Anyone you share the following link with will be able to read this content:Sorry, a shareable link is not currently available for this article. Provided by the Springer Nature SharedIt content-sharing initiative