Multiple Hyperbolic Dispersion Branches and Broadband Canalization in a Phonon-Polaritonic Heterostructure.

Hyperbolic polaritons in anisotropic crystals hold great promise for guiding the flow of light at deep-subwavelength scales. However, conventional hyperbolic dispersion with a single pair of symmetric branches inherently confines polaritons to propagate only within specific spatial directions. Here we demonstrate a multibranch in-plane hyperbolic dispersion in a phonon-polaritonic heterostructure composed of α-phase molybdenum trioxide (α-MoO) and 4H-silicon carbide (4H-SiC). Leveraging the in-plane hyperbolicity of α-MoO and the interlayer coupling with 4H-SiC, the polaritons manifest distinct dispersive responses along the mutually orthogonal crystal directions of α-MoO, enabling asymmetric multidirectional polariton propagation. Furthermore, the dispersion contours of polaritons along the [100] crystal direction of α-MoO evolves into flat bands as the frequency decreases, yielding broadband polariton canalization in the low-frequency region. These findings deepen our understanding of the evolution of polariton dispersions in α-MoO/4H-SiC heterostructures and highlight the potential of this phonon-polaritonic heterostructure as a versatile platform for nanolight manipulation.