Fundamentals of Polaritons in Strongly Anisotropic Thin Crystal Layers

Polaritons in strongly anisotropic thin layers have recently captured considerable attention in nanophotonics because of their directional propagation at the nanoscale, which offers unique possibilities for nano-optical applications. However, exploiting the full potential of anisotropic polaritons requires a thorough understanding of their properties, including field confinement, energy, and phase propagation direction and losses. Here, we provide novel insights into some fundamental aspects of the propagation of anisotropic polaritons in thin biaxial layers. In particular, we introduce a novel methodology that allows us to represent isofrequency curves of polaritons in strongly anisotropic materials, considering that the real and imaginary parts of the wavevector are not parallel. In fact, we analytically show that the direction of the imaginary part of the wavevector is parallel to the group velocity, which can have different, even perpendicular or opposite, directions with respect to the phase velocity. This finding is crucial for understanding polaritonic phenomena in anisotropic media; yet, it has so far been widely overlooked in the literature. Additionally, we introduce a criterion for classifying the polaritonic modes in biaxial layers into volume and surface categories and analyze their dispersion, field structure, and losses. Finally, we discover the existence of previously unexplored anisotropic transverse-electric-like modes, which can exhibit natural canalization. Taken together, our results shed light on hitherto unexplored areas of the theory of electromagnetic modes in thin biaxial layers. Although exemplified for van der Waals alpha-MoO3 layers, our findings are general for polaritons in other strongly anisotropic biaxial hyperbolic crystals.