(color online) Silicon waveguide-integrated graphene photodetectors. (a) The relative a.c. photoresponse as a function of light intensity modulation frequency shows 1 dB degradation of the signal at frequency 20 GHz. Inset (left) 12 Gbit/s optical data link test of the device, showing a clear eye opening, (right) schematic of the device. The silicon bus waveguide fabricated on an SOI wafer is planarized using SiO₂. A graphene layer is transferred onto the planarized waveguide with a spacing layer of ∼ 10 nm SiO₂. Two metal electrodes contact the graphene and conduct the generated photocurrent. One of the electrodes is closer to the waveguide to create a potential difference in the graphene to couple with the evanescent optical field of the waveguide (from Ref. ^[49]). (b) Colored scanning electron micrograph of a waveguide-integrated graphene photodetector. The “active region” of the graphene sheet is shown in violet. Inset shows a cross-section of the device. The graphene sheet coats both the top surface and sidewalls of the waveguide. A thin SiO₂ layer prevents electrical contact between the graphene and the silicon waveguide (from Ref. ^[50]).