Compact, low-loss, and wideband graphene-based directional coupler in the terahertz and infrared frequency ranges

In this paper, a novel structure for a graphene-based directional coupler in the THz frequency region is presented. This new configuration consists of two graphene-based single-mode waveguides, placed side by side with some connection gaps between them to allow coupling. Two different types of directional couplers (single-gap and double-gap) are designed at the frequency of 50[THz]. The simulation results show that the designed single-gap coupler has the advantages of low insertion loss ( < − 1.4 d B ), high directivity ( > 15 d B ), high isolation ( < − 19.4 d B ), wide bandwidth ( > 25 % ), and small footprint (about 100[nm]) for high-coupling coefficients, while the double-gap coupler shows better directivity ( > 30.17 d B ) and isolation ( < − 41.5 d B ) for a low-coupling coupler, so it is superior to other structures reported in the literature. The propagation loss and dimensions of the coupler waveguides have been efficiently controlled to remain small by optimizing the imaginary and real parts of the effective mode index of the surface plasmon polariton mode. The full-wave simulations show that the presented approach gives very good results for designing graphene-based directional couplers with different coupling coefficients. These structures are analyzed and optimized by the commercial COMSOL Multiphysics electromagnetic solver.