  We experimentally demonstrate enhanced self-phase modulation (SPM) in silicon
nitride (Si3N4) waveguides integrated with 2D graphene oxide (GO) films. GO
films are integrated onto Si3N4 waveguides using a solution-based,
transfer-free coating method that enables precise control of the film
thickness. Detailed SPM measurements are carried out using both picosecond and
femtosecond optical pulses. Owing to the high Kerr nonlinearity of GO, the
hybrid waveguides show significantly improved spectral broadening compared to
the uncoated waveguide, achieving a broadening factor of up to ~3.4 for a
device with 2 layers of GO. By fitting the experimental results with theory, we
obtain an improvement in the waveguide nonlinear parameter by a factor of up to
18.4 and a Kerr coefficient (n2) of GO that is about 5 orders of magnitude
higher than Si3N4. Finally, we provide a theoretical analysis for the influence
of GO film length, coating position, and its saturable absorption on the SPM
performance. These results verify the effectiveness of on-chip integrating 2D
GO films to enhance the nonlinear optical performance of Si3N4 devices.
