Surface Plasmon Coupling and Control Using Spherical Cap Structures

Propagating surface plasmons (PSPs) launched from a protruded silver spherical cap structure are investigated using photoemission electron microscopy (PEEM) and finite difference time domain (FDTD) calculations. Our combined experimental and theoretical findings reveal that PSP coupling efficiency is comparable to conventional etched-in plasmonic coupling structures. Additionally, plasmon propagation direction can be varied by linear rotation of the driving laser polarization. A simple geometric model is proposed in which the plasmon direction selectivity is proportional to the projection of the linear laser polarization on the surface normal. A application for the spherical cap coupler as a gate device is proposed. Overall, our results indicate that protruded cap structures hold great promise as elements in emerging surface plasmon applications.