  We present a full study of an improved nonlinear plasmonic slot waveguides
(NPSWs) in which buffer linear dielectric layers are added between the Kerr
type nonlinear dielectric core and the two semi-infinite metal regions. For TM
polarized waves, the inclusion of these supplementary layers have two
consequences. First, they reduced the overall losses. Secondly, they modify the
types of solutions that propagate in the NPSWs adding new profiles enlarging
the possibilities offered by these nonlinear waveguides. Our structure also
provides longer propagation length due to the decrease of the losses compared
to the simple nonlinear slot waveguide and exhibits, for well-chosen refractive
index or thickness of the buffer layer, a spatial transition of its main modes
that can be controlled by the power. We provide a full phase diagram of the TM
wave operating regimes of these improved NPSWs. The stability of the main TM
modes is then demonstrated numerically using the FDTD. We also demonstrate the
existence of TE waves for both linear and nonlinear cases (for some
configurations) in which the maximum intensity is located in the middle of the
waveguide. This kind of structures could be fabricated and characterized
experimentally due to the realistic parameters chosen to model them.
