Huygens' principle for hyperbolic plasmon polaritons

Huygens' principle, a cornerstone of wave propagation theory, explains how wavefronts evolve through secondary wavelets. While traditionally applied to spherical and planar waves, its extension to materials supporting hyperbolic plasmon polaritons (such as MoOCl₂), where extreme optical anisotropy governs wave behavior, unlocks new insights. In hyperbolic materials both primary wavefronts and secondary wavelets assume hyperbolic shapes, naturally leading to phenomena like negative refraction and wavevector-Poynting-vector misalignment. This extended framework provides a powerful tool for lens design and numerical modeling. By extending Huygens' principle in these media, we gain new understanding of wave hyperbolicity without any aid of reciprocal space.