  The discovery of many fascinating new phenomena associated with the surface
plasmon polariton (SPP) has triggered the rapid development of nanophotonics
and nanoelectronics. We report here the experimental observation of a
fundamentally new physical process, nonlinear electron scattering, stimulated
by the SPP excitation of Ag nanostructures on graphite surface in scanning
probe electron energy loss spectroscopy. The observed intensity of SPP energy
loss peak normalized to the elastic scattering intensity shows clearly a
quadratic dependence on the external electric field strength generated by the
tip-sample bias. The strong coherent nature of the SPP has made the observation
possible and a two-step scattering process is proposed to explain this novel
nonlinear effect. Our findings shed new light on the nature of SPP and pave the
way to new spectroscopic applications.
