Ultra-short coupler utilizing surface plasmon-polaritons at optical frequencies

A nano-scaled coupled-line coupler based on the guidance of surface plasmon-polaritons (SPPs) is designed and simulated at optical frequencies. The coupler comprises layered dielectric materials and silver. Comparing to conventional optical couplers, the guided modes are surface waves that can overcome the diffraction limit in the transverse direction of the guided-wave structures, therefore it has the potential to be used in nano-circuits. In addition, it operates based on the principle of contra-directional coupling between a forward and a backward wave giving rise to supermodes that are characterized by complex-conjugate eigenvalues. The resulting exponential attenuation along the coupler leads to dramatically reduced coupling lengths compared to previously reported co-directional SPP couplers (e.g. from millimeters to microns)