Ultra‐Compact, Cladding‐Free Double Hyperbolic Waveguides With Topologically Protected Boundary States

Optical waveguides, typically composed of core and cladding layers, only utilize core layers to transmit signals via the total‐internal‐reflection mechanism, with cladding layers completely wasted. While waveguides consisting of photonic‐crystal (PhC) layers can enable signal transmission in both layers through equal‐frequency‐surface (EFS) engineering, such PhC‐based waveguides only support transverse‐magnetic modes, and the interfaces between adjacent layers are still not fully exploited. Here, we propose a novel metamaterial (MTM)‐based waveguide to allow transmission of waves with two independent polarizations simultaneously, not only in all bulk layers but also at the interfaces between two types of layers, thus boosting the waveguide capacity significantly. Via carefully tuning constitutional parameters, we design a hyperbolic MTM which, for both polarizations, possesses EFSs fully isolated from those of air and exhibits topologically protected boundary states at the interfaces. We experimentally realize a waveguide by combining such hyperbolic MTM layers and air layers and confirm the existence of three types of independent waveguide modes via microwave near‐field measurements. Our work pushes the capacity limits of cladding‐free waveguide architectures and lays the foundation for cladding‐free integrated photonic devices using natural hyperbolic materials.