Switchable focusing of hyperbolic polariton rays in bulk anisotropic crystals

Abstract Subwavelength focusing of hyperbolic phonon polaritons (HPhPs) offers a powerful strategy for confining light beyond the diffraction limit, enabling extreme energy localization and enhanced light-matter interactions. However, the limited field enhancement, tunability, and scalability of implementations have motivated interest in polaritonic rays—high-momentum modes that offer collimated, diffraction-free propagation and an elevated optical density of states—as a promising alternative for efficient energy transport and deep subwavelength confinement. Yet, dynamic control and functional integration of such modes, particularly tunable in plane focusing, remain largely unexplored. Here, we demonstrate polarization-switchable in-plane focusing of ghost hyperbolic phonon polariton (g-HP) rays in bulk calcite—a monolithic, lithography—compatible platform that intrinsically supports ray-like modes without requiring artificial layering or phase-change engineering. By tailoring the antenna geometry, excitation frequency, and incident polarization, we realize reconfigurable focusing with up to 25-fold electric field enhancement. These results position calcite-based g-HPs as a scalable, low-loss polaritonic platform, offering new opportunities for mid infrared spectroscopy, reconfigurable photonic circuitry, and high-resolution optical sensing.