Directional strong coupling at the nanoscale between hyperbolic polaritons and organic molecules

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.