Cavity-driven arbitrary in-plane focusing of phonon polaritons in hBN with deep-subwavelength confinement

Precise control of phonon-polaritons (PhPs) in planar media is a central challenge for advancing mid-infrared nanophotonics. Here, we demonstrate cavity-driven excitation and shaping of in-plane PhPs in hBN using subwavelength resonators lithographically defined at the Au/SiO2 interface. These cavities efficiently couple far-field light into propagating PhPs and act as planar polaritonic lenses, with their shape governing the wavefront geometry and depth of field. We show that triangular and curved cavities generate “convex” and “concave” focusing behavior, enabling arbitrary spatial shaping of PhP fields. We achieve in-plane spatial confinement down to λ/70 and observe well-defined focal points. Our approach is fully compatible with standard fabrication techniques and decouples the light coupling and focusing functions-light couples at the cavity edge, while the cavity geometry governs the polariton wavefront. Unlike prior demonstrations in strongly in-plane-anisotropic media such as MoO3, our work establishes arbitrary focusing and deep-subwavelength confinement in hBN without relying on anisotropic substrates or exotic heterostructures. This cavity-driven scheme provides a distinct route for realizing polaritonic elements, including lenses, wavefront shapers, and metasurfaces operating in the mid-infrared. In-plane confinement of phonon-polaritons (PhPs) in the mid-infrared is key for compact sensors and photonic devices, but such in-plane beam control in isotropic materials requires complex nanopatterning techniques. The authors propose a scalable way to shape PhPs in hBN using lithographically defined cavities, achieving in-plane confinement down to λ/70.