A high figure of merit of phonon-polariton waveguide modes with hBN/SiO 2 /graphene/hBN ribs waveguide in THz range

Abstract

The utilization of THz waveguides has enabled a variety of applications in integrated optics and communications. To effectively control THz signals on photonic chips, it is essential to design a waveguide that offers both a high figure of merit (FOM) and low loss. In this paper, we propose a THz waveguide composed of a dielectric rib and a graphene layer sandwiched between two hexagonal boron nitride (hBN) ribs on a gold layer. Numerical simulations demonstrate the existence of two type guided modes of the proposed waveguide in the second reststrahlen band (1360.0cm− 1 < ω < 1609.8cm− 1) of hBN. These modes are formed by coupling the hyperbolic phonon polariton (HPhP) of two hBN rib in the middle dielectric rib and are then modulated by a graphene layer. Interestingly, we found that four waveguide transmission parameters, including an effective length, a figure of merit, a device length and a propagation loss of the guided modes, vary with operation frequency and gate voltage. By altering geometry parameters and dielectric permittivity, the modal properties were analyzed. Simulation results demonstrate that a high FOM of 4.0×107 is achieved by optimizing waveguide size parameters. The proposed waveguide offers a promising approach for designing a tunable THz waveguide on photonic chips, which can be extended to other hyperbolic materials and 2D materials such as MnO3, silicone and germanene.