  Recent years have seen increased interest to plasmonic enhancement of
nonlinear optical effects, yet there remains an uncertainty of what are the
limits of this enhancement. We present a simple and physically transparent
theory of plasmonic enhancement of third order nonlinear optical processes
achieved in plasmonic structures and show that while huge enhancement of
effective nonlinear index can be attained, the most relevant figure of merit,
the of phase shift per one absorption length remains very low. This means that
while on one hand nonlinear plasmonic materials are not well suitable for
applications requiring high efficiency, e.g. all-optical switching and
wavelength conversion, on the other hand they can be very useful for the
applications where the overall high efficiency is not a must, such as sensing.
