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(a) For normal incident light, graphene has a universal absorption coefficient of 2.3%, but short interaction length. (b) By integrating graphene on a waveguide, the light-graphene interaction length is only determined by the length of the device and so complete absorption can be achieved. (c) Finite-element simulation result of the fundamental TE mode in a silicon waveguide with a layer of graphene (white dashed line) on top. The color map shows the optical intensity and the white arrows indicate the electric field. The red curve shows the profile of resistive dissipation across the width of the graphene layer.
(a) An on-chip Mach-Zehnder interferometer made of silicon waveguides to precisely measure the absorption in graphene. The two interferometer arms are covered by graphene patterned to have different lengths. (b) Cross-sectional view of the graphene/waveguide structure. Graphene is coated at a distance of h above the waveguide with a layer of annealed HSQ cladding in between. (c) Typical drain current versus backgate voltage curve measured in a field-effect transistor device made of transferred CVD graphene. The results show that typical transferred graphene layers are p-type doped with a background concentration of ∼4 × 1012 cm−2, corresponding to a value of EF of −0.23 eV. (d) Raman spectrum of typical CVD graphene showing that the transferred graphene film is single layer.
(a) Dark-field optical microscopy image of the silicon photonic MZI device used in the experiment. (b) Scanning electron microscopy image of the device after transfer and patterning of the graphene. The graphene covered area appears darker because of better discharging of the electrons, compared to the surrounding SiO2 regions. (c) Transmission spectra of two MZI devices. The first one (red) has no graphene coverage and the second one (black) has 70-μm-long graphene covering the waveguides in both arms of the interferometer. The equally high extinction ratio of 35 dB indicates that the absorption coefficient of graphene is similar in both arms and the loss in the silicon waveguide is negligible in comparison.
(a) Typical transmission spectra measured from devices with varying differential length of graphene-covered waveguide in the two arms of the MZI. A gradually decreasing extinction ratio is observed when the differential length is increased. The top three spectra (red, blue, and cyan) are offset in the vertical direction for clarity. (b) Histograms of linear attenuation for different measured from 40 devices. (c) Linear attenuation versus differential length for two values of HSQ spacing layer thickness, h. An exponential dependence is observed with the linear absorption coefficient measured to be 0.106 × (1 ± 3.3%) dB/μm and 0.0463 × (1 ± 4%) dB/μm for h = 35(±8) and 95(±6) nm, respectively. The error bars are attributed to the variation of the conditions of the transferred graphene layer. (d) Measured linear absorption coefficient (blue symbols) versus h, showing a good agreement with the theoretical result (red line).
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