Compact Low-Loss Electroabsorption Modulator Using a Graphene-Inserted Metal-Slot-Added Waveguide

Metal slots have been used to obtain strong light-graphene interaction, which usually requires them to be a few tens of nanometers wide. However, narrow metal slot waveguides have a large intrinsic loss; they are not efficiently connected to conventional silicon photonic waveguides; they are not easy to fabricate. To address the issues, a graphene-inserted metal-slot-added (GIMSA) waveguide and an electroabsorption modulator (EAM) based on it are theoretically investigated. The GIMSA waveguide consists of a silicon strip embedded in silicon dioxide and a metal slot aligned above the silicon strip with double graphene layers between them. The EAM is composed of the GIMSA waveguide and input and output couplers connecting it to silicon photonic waveguides. In order to achieve the good performance of the EAM in terms of length and insertion loss, the GIMSA waveguide and the couplers are designed. When the silicon strip and the metal slot are respectively 320 nm and 316 nm wide, the total length of the EAM with an extinction ratio of 3 dB is 6.23~\mu \text{m} , and its on-state insertion loss is 1.01 dB. Compared to previous graphene-based EAMs embedded in silicon photonic integrated circuits, this EAM is shorter and has a quite small insertion loss. The EAM's large feature size may enable fabrication using 248 nm optical lithography, and the EAM is expected to function as a compact modulator, well-integrated with silicon photonic devices.