Terahertz hybrid plasmonic waveguides with ultra-long propagation lengths based on multilayer graphene-dielectric stacks.

To develop on-chip photonic devices capable of transmitting terahertz signals beyond the propagation distance of millimeter while keeping deep subwavelength field confinement has been a challenging task. Herein, we propose a novel multilayer graphene-based hybrid plasmonic waveguide (MLGHPW) consisting of a cylindrical dielectric waveguide and hyperbolic metamaterials. The device is based on alternating graphene and dielectric layers on a rib substrate, operating in the terahertz range (f = 3 THz). We couple the fundamental dielectric waveguide mode with the fundamental volume plasmon polarition modes originated from the coupling of plasmon polaritons at individual graphene sheets. The resulting hybrid mode shows ultra-low loss compared with the conventional GHPW modes at the comparable mode sizes. The present MLGHPW demonstrated a few millimeters of propagation length while keeping the mode area of 10A, where A is the diffraction-limited area, thus possessing a thirty times larger figure of merit (FoM) compared to other GHPWs. The additional degree of freedom (the number of graphene layers) makes the proposed MLGHPW more flexible to control the mode properties. We investigated the geometry and physical parameters of the device and identified optimal FoM. Moreover, we analyzed the crosstalk between waveguides and confirmed the potential to construct compact on-chip terahertz devices. The present design might have the possible extensibility to other graphene-like materials, like silicene, germanen, stanene etc.