Interference of the surface plasmon polaritons with an Ag waveguide probed by dual-probe scanning near-field optical microscopy

► We observe nanoscale behaviors of surface plasmon polaritons using dual-probe scanning near-field optical microscopy. ► We find interferences of surface plasmon polaritons only in a waveguide structure with a certain width. ► Reflective waves at side edges cause interferences, which are also confirmed from theoretical calculations.