Studies of the optical absorption and the development of the passive devices based on 2D-material hybrid waveguides. (A) Schematic of the graphene-on-silicon optical device for measuring the graphene-introduced optical absorption coefficient [78]. (B) Optical image of a device with the ring covered by a monolayer MoS₂ over a circumference corresponding to Φ=π/4 [87]. (C) Measured Q-factor of the graphene-on-silicon nitride microring resonator as a function of the graphene length [80]. (D) Scanning electron microscope (SEM) image of the graphene-on-silicon slot waveguide [93]. The inset is the electric-field distribution of the silicon slot waveguide. (E) Measurement of the polarization-dependent transmission of the graphene-on-silicon waveguide [69] coupled by using the polarization-insensitive focusing subwavelength gratings [112]. The insets are the electric-field distributions of the TE₀ and TM₀-mode waveguides. (F) Schematic of the on-chip TE₂ mode filter based on the graphene-on-silicon multimode waveguide [109].