Hyperbolic Shear Plasmon Polaritons in a Twisted Anisotropic 2D Material.

Hyperbolic shear polaritons, recently discovered in infrared (IR) phononic systems, offer enhanced directionality and subwavelength confinement. This Letter proposes a strategy for realizing hyperbolic shear plasmon polaritons (HSPPs) in natural two-dimensional (2D) materials with time-reversal symmetry, thereby extending their operation beyond the IR range. We reveal that twisting a 2D material on an anisotropic substrate creates nondiagonalizable transverse conductivity components, leading to wave vector misalignment and the associated asymmetric loss in HSPPs. Furthermore, we highlight the tunability of these HSPPs through electrostatic doping and precise control of the twist angle. Our theoretical framework is validated using MoOCl_{2}, paving the way for achieving visible-range HSPPs in natural, gate-tunable materials.