  We experimentally demonstrate enhanced spectral broadening of femtosecond
optical pulses af-ter propagation through silicon-on-insulator (SOI) nanowire
waveguides integrated with two-dimensional (2D) graphene oxide (GO) films.
Owing to the strong mode overlap between the SOI nanowires and the GO films
with a high Kerr nonlinearity, the self-phase modulation (SPM) process in the
hybrid waveguides is significantly enhanced, resulting in greatly improved
spectral broadening of the femtosecond optical pulses. A solution-based,
transfer-free coating method is used to integrate GO films onto the SOI
nanowires with precise control of the film thickness. Detailed SPM measurements
using femtosecond optical pulses are carried out, achieving a broadening factor
of up to ~4.3 for a device with 0.4-mm-long, 2 layers of GO. By fit-ting the
experimental results with theory, we obtain an improvement in the waveguide
nonlin-ear parameter by a factor of ~3.5 and the effective nonlinear figure of
merit (FOM) by a factor of ~3.8, relative to the uncoated waveguide. Finally,
we discuss the influence of GO film length on the spectral broadening and
compare the nonlinear optical performance of different integrated waveguides
coated with GO films. These results confirm the improved nonlinear optical
per-formance for silicon devices integrated with 2D GO films.
