Abstract
Strong coupling is a fundamental concept in physics that describes extreme interactions between light and matter. Recent experiments have demonstrated strong coupling at the nanometre scale, where strongly confined polaritons, rather than photons, couple to quantum emitters or molecular vibrations. Coupling with the latter is generally referred to as vibrational strong coupling (VSC) and is of substantial fundamental and technological interest, as it can be an effective tool for modifying molecular properties. However, the implementation of VSC, especially at the nanoscale, depends on the development of tuning mechanisms that allow control over the coupling strength and, eventually, its directionality, opening the door for the selective coupling of specific molecular vibrations. Here we report the observation of directional VSC, which we carried out at the nanoscale. Specifically, we show the nanoscale images of propagating hyperbolic phonon polaritons coupled to pentacene molecules, revealing that the fingerprint of VSC for propagating polaritons—a marked anticrossing in their dispersion at the vibrational resonance—can be modulated as a function of the direction of propagation. In addition, we show that VSC can exhibit an optimal condition for thin molecular layers, characterized by the maximum coupling strength along a single direction. This phenomenon is understood by analysing the overlap of the polariton field with molecular layers of varying thicknesses. Apart from their fundamental importance, our findings promise novel applications for directional sensing or local directional control of chemical properties at the nanoscale.
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Data availability
All data supporting the findings of this study are available in the Article and the Supplementary Information. Additional data can be obtained from the corresponding authors upon reasonable request.
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Acknowledgements
We acknowledge support from the Spanish Ministry of Science and Innovation (grants PID2022-141304NB-I00 and PID2023-147676NB-I00). A.I.F.T.-M. and J.Á.-C. acknowledge support through the Severo Ochoa programme from the Government of the Principality of Asturias (numbers PA-21-PF-BP20-117 and PA-22-PF-BP21-100). K.V.V. acknowledges support from a fellowship from ‘la Caixa’ Foundation (ID 100010434); the fellowship code is LCF/BQ/DI21/11860026. J.T.-G. acknowledges support from the Swiss National Foundation (grant number 200020_201096). A.M. and P.D.-N. acknowledge support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Consolidator grant agreement number 865590, Programmable Matter). J.M.-S. acknowledges financial support from the Spanish Ministry of Science and Innovation (grant number PID2023-148457NB-I00, funded by MCIN/AEI/10.13039/501100011033 and FSE+), PCI2022-132953 (funded by MCIN/AEI/10.13039/501100011033 and the EU ‘NextGenerationEU’/PRTR) and CNS2024-154342 (funded by MICIU/AEI/10.13039/501100011033). P.A.-G. acknowledges financial support from the ERC under Consolidator grant number 101044461, TWISTOPTICS. A.Y.N. acknowledges the Basque Department of Education (grant number PIBA-2023-1-0007) and the IKUR strategy.
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P.A.-G. and A.Y.N. conceived the study. A.I.F.T.-M. performed the sample fabrication and near-field experiments with the help of J.T.-G., A.T.M.-L. and J.D. O.G.M., C.L. and J.Á.-C. performed the analytical study and simulations with input from K.V.V. and G.A.-P., under the supervision of V.S.V. and A.Y.N. P.D.-N. and A.M. performed the nano-Fourier transform infrared (nano-FTIR) spectroscopy measurements. F.C. and G.A. performed the fabrication of the pentacene films under the supervision of L.E.H. A.B., J.M.-S. and R.H. participated in the data analysis. A.I.F.T.-M., C.L. and P.A.-G. wrote the manuscript with input from the rest of the authors. P.A.-G. and A.Y.N. supervised the work. All authors contributed to the scientific discussion and manuscript revisions.
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R.H. is a co-founder of Neaspec GmbH, a company that manufactures s-SNOM systems, including the one used in this study. The other authors declare no competing interests.
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Supplementary Figs. 1–20, Table 1 and Notes I–VII.
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F. Tresguerres-Mata, A.I., Matveeva, O.G., Lanza, C. et al. Directional strong coupling at the nanoscale between hyperbolic polaritons and organic molecules. Nat. Photon. (2025). https://doi.org/10.1038/s41566-025-01762-6
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DOI: https://doi.org/10.1038/s41566-025-01762-6
