Hyperbolic localized plasmons and twist-induced chirality in an anisotropic 2D material.

Two-dimensional (2D) van der Waals materials with strong in-plane anisotropy are emerging as fertile platforms for nanophotonics beyond conventional isotropic noble metals and dielectrics. Here, we demonstrate hyperbolic localized plasmon resonances (H-LPRs) in MoOCl2, a representative anisotropic 2D crystal. Unlike conventional plasmons, H-LPRs arise directly from the crystal anisotropy and show unprecedented properties: (i) one-dimensional resonances in circularly symmetric nanodisks, (ii) Z-gap independence in metal-insulator-metal heterostructures, and (iii) twist-induced chirality with circular dichroism values exceeding 0.65. The H-LPRs are characterized by both far-field spectra and near-field imaging. By stacking twisted MoOCl2 flakes, we bridge H-LPRs with concepts of moiré photonics and twistronics, introducing a new degree of freedom in plasmonic design. These findings establish H-LPRs in anisotropic 2D materials as a generalizable and versatile platform for polarization engineering, ultrasensitive chiral sensing, and integration into compact on-chip and quantum nanophotonic devices.