Engineering Polaritonic Topology with Composite Grating Metasurfaces

Metasurfaces offer powerful means to control polaritons, yet achieving sophisticated manipulation of their dispersion topology remains challenging with conventional homogeneous designs. Here, a composite grating metasurface (CGM) paradigm is introduced and theoretically validated enabling precise topological control over mid‐infrared phonon polaritons. This design comprises alternating, deep‐subwavelength nanostrips of an anisotropic hyperbolic material‐α‐molybdenum trioxide (α‐MoO 3 ) and an isotropic polaritonic material‐silicon carbide (SiC), operating within their overlapping Reststrahlen bands. It is shown that coupling between the distinct polariton modes supported by each constituent leads to the formation of volume‐surface hybrid phonon polaritons (VS‐HPhPs). Crucially, the topological nature of the resulting hybrid polariton dispersion—transitioning between elliptical and hyperbolic isofrequency contours (IFCs)—can be deterministically engineered by tailoring the grating geometry, specifically its orientation relative to the α‐MoO 3 crystal axes or the material filling factors. This composite metasurface approach provides expanded degrees of freedom for manipulating polariton propagation and topology, offering a versatile platform for developing advanced nanophotonic devices for topological optics and on‐chip light manipulation.