Anisotropic strong coupling between two propagating polariton modes

Interface polaritons offer extreme electromagnetic confinement and enhanced light–matter interactions. Although polaritonic strong coupling has been extensively studied, it has largely been confined to isotropic systems with limited coupling strength and tunability. Here, we demonstrate anisotropic strong coupling between hyperbolic volume polaritons and isotropic surface polaritons in cavity-free van der Waals heterostructures. Through near-field nano-imaging and nano-spectroscopy, we directly observe anisotropic strong coupling with normalized coupling strength up to 16.4 ± 1.4, which can be well-described by the multimode Hopfield model. The coupling characteristics are further controlled by adjusting the material thickness and interlayer separation, culminating in epsilon-near-zero strong coupling. These results establish a paradigm for manipulating strong light–matter interactions at the nanoscale, with promising implications for molecular orientation sensing, on-chip optical routing, and directional nanolasing. Various material systems exhibit coupling between distinct classes of polaritons. Here, the authors demonstrate anisotropic strong coupling between hyperbolic volume polaritons and isotropic surface polaritons in α-MoO3/AlN heterostructures.