A new structure to be used as a tunable unidirectional surface plasmon source is introduced. The structure is composed of two silver films, with a nanoslit fabricated in the top Ag film and lying below is a movable Ag film. The field distribution of the structure is investigated by using the finite-difference time-domain(FDTD) method. It is found that the surface plasmon polariton intensity and the splitting ratio change periodically as the bottom film is moved, which is interpreted in terms of surface plasmon polaritons interference in two Fabry-Perot(F-P) cavities. The period obtained by the FDTD agrees well with the F-P interferometer model. The surface plasmon polaritons can be unidirectionally excited with a large intensity splitting ratio.
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