Real-space observation of vibrational strong coupling between propagating phonon polaritons and organic molecules

Phonon polaritons in van der Waals materials can strongly enhance light–matter interactions at mid-infrared frequencies, owing to their extreme field confinement and long lifetimes1,2,3,4,5,6,7. Phonon polaritons thus bear potential for vibrational strong coupling with molecules. Although the onset of vibrational strong coupling was observed spectroscopically with phonon-polariton nanoresonators8, no experiments have resolved vibrational strong coupling in real space and with propagating modes. Here we demonstrate by nanoimaging that vibrational strong coupling can be achieved between propagating phonon polaritons in thin van der Waals crystals (hexagonal boron nitride) and molecular vibrations in adjacent thin molecular layers. We performed near-field polariton interferometry, showing that vibrational strong coupling leads to the formation of a propagating hybrid mode with a pronounced anti-crossing region in its dispersion, in which propagation with negative group velocity is found. Numerical calculations predict vibrational strong coupling for nanometre-thin molecular layers and phonon polaritons in few-layer van der Waals materials, which could make propagating phonon polaritons a promising platform for ultrasensitive on-chip spectroscopy and strong-coupling experiments.We acknowledge financial support from the Spanish Ministry of Science, Innovation and Universities (national projects MAT2017-88358-C3, RTI2018-094830-B-100, RTI2018-094861-B-100, and the project MDM-2016-0618 of the Maria de Maeztu Units of Excellence Program), the Basque Government (grant numbers IT1164-19 and PIBA-2020-1-0014) and the European Union’s Horizon 2020 research and innovation programme under the Graphene Flagship (grant agreement numbers 785219 and 881603, GrapheneCore2 and GrapheneCore3). F. Calavelle acknowledges support from the European Union H2020 under the Marie Skłodowska-Curie Actions (766025-QuESTech). J.T.-G. acknowledges support through the Severo Ochoa Program from the Government of the Principality of Asturias (number PA-18-PF-BP17-126). P.A.-G. acknowledges support from the European Research Council under starting grant number 715496, 2DNANOPTICA. Further, support from the Materials Engineering and Processing program of the National Science Foundation, award number CMMI 1538127 for h-BN crystal growth is greatly appreciated.Peer reviewe