Tunable topological transition of ghost surface polaritons in hBN/α-MoO₃ hyperbolic metamaterials

The development of mid-infrared photonic devices with conventional surface polaritons is often hindered by the high losses and limited tunability. In this work, we demonstrate the dispersion engineering and tunable topological transitions of ghost surface polaritons (GSPs) in a hyperbolic metamaterial (HMM) composed of hexagonal boron nitride (hBN) and α-phase molybdenum trioxide (α-MoO₃). By modulating the filling ratios or deflection angle, we observe two distinct types of surface polaritons: the low-momentum GSP-I undergoes a reversible hyperbolic-to-elliptic transition near the epsilon-near-zero (ENZ) of the HMM, whereas the high-momentum GSP-II exhibits stable hyperbolic dispersion and pronounced monopole diffraction characteristics around the epsilon-near-pole (ENP) of the HMM. COMSOL simulations confirm that both GSP-I and GSP-II display the oscillatory decay of both the electric field and energy flow perpendicular to the interface. Our results highlight the role of hyperbolic reshaping in controlling polariton modes and indicate a possible path for designing dynamically tunable mid-infrared polariton-based devices with directional excitation and tailored propagation.