Manipulation of In-Plane Hyperbolic Phonon Polaritons for Configurable Focusing

In-plane hyperbolic phonon polaritons (HPhPs) in biaxial α-phase molybdenum trioxide (α-MoO₃) hold great promise for light manipulation at the deeply subwavelength scale. However, achieving configurable focal lengths for these in-plane HPhPs remains a significant challenge due to the intrinsically dispersive nature of α-MoO₃. In this study, a vertically stacked van der Waals heterostructure is fabricated by placing α-MoO₃ on the top of tellurium (Te), a dielectric with a large refractive index in the mid-infrared region, to engineer the in-plane hyperbolic dispersion of polaritons in α-MoO₃. HPhPs in this heterostructure exhibit Te-thickness-dependent dispersion, enabling configurable in-plane focusing with focal lengths ranging from 2.29 to 5.61 μm. Moreover, this configurable in-plane focusing consistently maintains the focal spot size within a deeply subwavelength range, approximately 1/20 of the free-space wavelength. We envision that these findings provide a feasible approach to effectively manipulate in-plane HPhPs via dispersion engineering, with potential applications in planar meta-optics and optical sensing.