  The interference patterns of the surface plasmon polaritons(SPPs) on the
metal surface from a "point" source are observed. These interference patterns
come from the forward SPPs and the reflected one from the obstacles, such as
straightedge,corner, and ring groove structure. Innovation to the previous
works, a "point" SPPs source with diameter of 100 nm is generated at the freely
chosen positions on Au/air interface using near field excitation method. Such a
"point" source provides good enough coherence to generate obvious interference
phenomenon. The constructive and destructive interference patterns of the SPPs
agree well with the numerical caculation. This "point" SPPs source may be
useful in the investigation of plasmonics for its high coherence, deterministic
position and minimum requirement for the initial light source.
