Studies of the waveguide-integrated graphene electro-optical modulators. (A) Schematic of the dual-layer graphene intensity modulator [143]. (B) Schematic of the graphene-on-silicon microring intensity modulator [154]. (C) Schematic of the graphene-on-silicon slot waveguide intensity modulator [155]. (D) Schematic of the graphene phase modulator based on the MZI structure [160]. (E) Schematic of the graphene modulator based on the MZI structure. The intensity and phase modulation modes can be switched by using different driving voltages [161]. (F) Schematic of the dual-mode modulator based on the graphene-nanoribbon-on-silicon waveguide [163].