  In this work, we experimentally study the optical kerr nonlinearities of
graphene/Si hybrid waveguides with enhanced self-phase modulation. In the case
of CMOS compatible materials for nonlinear optical signal processing, Si and
silicon nitride waveguides have been extensively investigated over the past
decade. However, Si waveguides exhibit strong two-photon absorption (TPA) at
telecommunication wavelengths, which lead to a significant reduction of
nonlinear figure of merit. In contrast, silicon nitride based material system
usually suppress the TPA, but simultaneously leads to the reduction of the Kerr
nonlinearity by two orders of magnitude. Here, we introduce a graphene/Si
hybrid waveguide, which remain the optical properties and CMOS compatibility of
Si waveguides, while enhance the Kerr nonlinearity by transferring patterned
graphene over the top of the waveguides. The graphene/Si waveguides are
measured with a nonlinear parameter of 510 W-1m-1. Enhanced nonlinear
figure-of-merit (FOM) of 2.48 has been achieved, which is three times higher
than that of the Si waveguide. This work reveals the potential application of
graphene/Si hybrid photonic waveguides with high Kerr nonlinearity and FOM for
nonlinear all-optical signal processing.
