  Surface plasmon polaritons (SPPs) have been widely exploited in various
scientific communities, ranging from physics, chemistry to biology, due to the
strong confinement of light to the metal surface. For many applications it is
important that the free space photon can be coupled to SPPs in a controllable
manner. In this Letter, we apply the concept of interfacial phase discontinuity
for circularly polarizations on a metasurface to the design of a novel type of
polarization dependent SPP unidirectional excitation at normal incidence.
Selective unidirectional excitation of SPPs along opposite directions is
experimentally demonstrated at optical frequencies by simply switching the
helicity of the incident light. This approach, in conjunction with dynamic
polarization modulation techniques, opens gateway towards integrated plasmonic
circuits with electrically reconfigurable functionalities.
