Fully integrated graphene-based coupler for surface plasmon generation in the far IR regime: rigorous numerical and theoretical analysis

A fully integrated efficient surface plasmon coupler composed of a non-tapered waveguide is designed for the infrared through rigorous numerical and theoretical studies. To study such structures, a general methodology to study rigorously discontinuities in open waveguides is used. It relies on a full vector description given by Maxwell’s equations in the framework of the finite element method. In our case, the discontinuities are provided by the graphene patches that are modelled as 2D conductivity pieces. The leaky modes of the invariant structure are first computed and then injected as incident fields in the full structure with the graphene patches and sheet using a scattered field approach. This method we recently published allows to compute all the field and energy quantities needed to characterize the investigated couplers. These couplers are designed to work on the far infrared regime. TAS, a special chalcogenide glass is considered for the guiding layer. The studied waveguide is a rib waveguide. The input part of the studied device is the bare rib waveguide. Following the propagation axis, the device is composed of the coupler region, made of a finite number of graphene patches located just above the core guiding layer. The role of this coupler is to generate electromagnetic fields in the next continuous graphene sheet (also located on the top of the core layer) where the surface plasmon polariton part of the field is propagating. The graphene properties are the ones provided by the Kubo formula. The studied device is fully integrated and no out of plane waves have to be considered, only electromagnetic fields propagating inside the structures are considered. Parametric numerical studies as a function of the waveguide thickness and of the duty cycle parameter describing the coupling patches have been conducted in order to maximize the plasmonic part of the generated field near the graphene layer. Several waveguide and coupler configurations with realistic parameter have been obtained proving a highly efficient and fully integrated coupler. Among these configurations, one where the mode coupling between the main mode and higher orders modes is minimized is of particular interest for practical applications.