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 lifetimes^{1,2,3,4,5,6,7}. 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.

范德华材料中的声子极化子由于其极强的电场限制和^{1,2,3,4,5,6,7的}长寿命而可以在中红外频率上大大增强光-物质的相互作用。因此，声子极化子具有与分子振动强耦合的潜力。尽管用声子-极化子纳米谐振器在光谱上观察到了振动强耦合的发生⁸，没有实验解决现实空间中和传播模式下的振动强耦合。在这里，我们通过纳米成像证明可以在范德华薄晶体（六方氮化硼）中传播的声子极化子与相邻的薄分子层中的分子振动之​​间实现振动强耦合。我们进行了近场极化子干涉测量，结果表明，振动强耦合导致形成传播混合模式，在其分散体中具有明显的反交叉区域，在该模式中发现了负群速度的传播。数值计算可预测几层范德华材料中纳米级分子层和声子极化子的振动强耦合，