  The third-order optical nonlinearity in optical waveguides has found
applications in optical switching, optical wavelength conversion, optical
frequency comb generation, and ultrafast optical signal processing. The
development of an integrated waveguide platform with a high nonlinearity is
therefore important for nonlinear integrated photonics. Here, we report the
observation of an enhancement in the nonlinearity of an air-cladding silicon
pedestal waveguide. We observe enhanced nonlinear spectral broadening compared
to a conventional silicon-on-insulator waveguide. At the center wavelength of
1555 nm, the nonlinear-index coefficient of air-cladding silicon pedestal
waveguide is measured to be about 5% larger than that of a conventional
silicon-on-insulator waveguide. We observe enhanced spectral broadening from
self-phase modulation of an optical pulse in the pedestal waveguide. The
interaction of light with the confined acoustic phonons in the pedestal
structure gives rise to a larger nonlinear-index coefficient. The experimental
results agree well with the theoretical models.
