High-efficiency couplers for graphene surface plasmon polaritons in the mid-infrared region

We numerically demonstrate a novel, to the best of our knowledge, variety of high-efficiency couplers for midinfrared graphene surface plasmon polaritons (SPPs). The proposed couplers are composed of a dual-parabolictapered transition to compress and couple the SPPs of a GaAs slab waveguide into the graphene layer. The dispersion relation, field distributions, transmittance, and coupling efficiency of the couplers are systematically analyzed. It is found that broadband coupling transmission with a transmittance around 0.3 is achieved for the coupler without a silver substrate. To further reduce radiation and enhance the transmittance, a silver substrate is introduced in the proposed coupler design, which achieves a transmittance over 0.5 ranging from 1960 to 2320 cm(-1) with a peak transmittance of 0.75. Furthermore, both couplers with and without a silver substrate achieve a similar coupling efficiency of more than 48% in the whole wavelength range of 1900 - 2400 cm(-1) and a coupling efficiency peak of over 80% around 2120 cm(-1). This Letter may provide an efficient excitation method for graphene SPPs, which is of great importance for the development of various graphene plasmonic devices. (C) 2020 Optical Society of America