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

Abstract

In this paper, a novel structure for graphene-based directional coupler in 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 coupler (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 (15.1dB), high isolation (25%) and small footprint (about 100[nm]) for high-coupling coefficients while the double-gap coupler shows better directivity (>27.7dB) and isolation (<-42.8dB) for low-coupling coupler. So it is superior to other structure reported in literature. The propagation loss and dimensions of the coupler waveguides have been efficiently controlled to be small by optimizing the imaginary and real parts of effective mode index of the surface plasmon polariton mode. The full-wave simulations show that the presented approach gives very good results for the designing of graphene-based directional couplers with different coupling coefficients. These structures are analyzed and optimized by commercial COMSOL Multiphysics electromagnetic solver.