In-Plane Optical Absorption and Free Carrier Absorption in Graphene-on-Silicon Waveguides

We experimentally study the in-plane optical absorption and free carrier absorption (FCA) in graphene-on-silicon waveguides using a pump-probe measurement over microsecond timescales. The silicon waveguide is fabricated using complementary metal-oxide-semiconductor compatible processes, and directly covered by a graphene layer. Saturable absorption in the graphene is observed at the beginning of the pump pulse followed by an increase in absorption. The increase in absorption builds up over several microseconds, and is experimental evidence that free carriers generated by the pump absorption in graphene can transfer into silicon waveguides. The FCA in silicon waveguides eventually dominates the optical loss, which reaches ~9 dB, after several microseconds. All-optical modulations of the probe light are thus demonstrated. There is also a large thermally induced change in waveguide effective refractive index because of the optical absorption in the graphene.