Manipulation of In-Plane Hyperbolic Phonon Polaritons\nfor Configurable Focusing

In-plane\nhyperbolic phonon polaritons (HPhPs) in biaxial α-phase\nmolybdenum trioxide (α-MoO<sub>3</sub>) hold great promise for\nlight manipulation at the deeply subwavelength scale. However, achieving\nconfigurable focal lengths for these in-plane HPhPs remains a significant\nchallenge due to the intrinsically dispersive nature of α-MoO<sub>3</sub>. In this study, a vertically stacked van der Waals heterostructure\nis fabricated by placing α-MoO<sub>3</sub> on the top of tellurium\n(Te), a dielectric with a large refractive index in the mid-infrared\nregion, to engineer the in-plane hyperbolic dispersion of polaritons\nin α-MoO<sub>3</sub>. HPhPs in this heterostructure exhibit\nTe-thickness-dependent dispersion, enabling configurable in-plane\nfocusing with focal lengths ranging from 2.29 to 5.61 μm. Moreover,\nthis configurable in-plane focusing consistently maintains the focal\nspot size within a deeply subwavelength range, approximately 1/20\nof the free-space wavelength. We envision that these findings provide\na feasible approach to effectively manipulate in-plane HPhPs via dispersion\nengineering, with potential applications in planar meta-optics and\noptical sensing.