(a) A false-color topview SEM image of the device, displaying graphene (purple), metal leads (yellow), unetched silicon (pink), and etched region (green). Scale bar, 10 μm. (b) A zoom-in view of the region indicated by dash box in Fig. 1(a), showing graphene crimples and bulges. Scale bar, 200 nm. (c) The Raman spectrum of the device at 488 nm. (d) The cross-section view of the device structure and the optical field distribution in waveguide.