Highly nonlinear waveguides are essential components for all-optical signal processing. Many promising nonlinear
waveguides utilize the Kerr nonlinearity, the strength of which is determined not only by the material properties,
but also by geometrical factors, quantified by the waveguide's nonlinear effective area Aeff. In an all-optical
switch, the switching threshold power is proportional to Aeff, so optimization of the nonlinear waveguide is
equivalent to minimization of Aeff. Recent studies have shown that dielectric slot waveguides can confine optical
energy far below the diffraction limit, with nonlinear effective areas considerably less than those attainable in
total internal reflection waveguides.
In this work, we instead consider the use of a gap plasmonic waveguide (GPW) for deep sub-wavelength optical
confinement. Using finite element methods, we compare optimized slot waveguides with GPWs of identical
geometry. We show that the GPW achieves a nonlinearity more than an order of magnitude superior to the
corresponding dielectric slot waveguide, and that a further optimization of the GPW is possible.