Image Dipole Method for the Beaming of Plasmons from Point Sources

A point dipole source models the electrical excitation of surface plasmon polaritons (SPPs), and is promising as a compact source for the beaming of plasmons in optical nanocircuits. However, conventional design approaches rely on iterative numerical simulations to achieve beaming from dipole sources, and they consume significant computing resources. Here, we introduce a universal semianalytical approach to solve the reflection of dipole-excited SPPs from the edge of a metal film by developing the image dipole method for SPPs. This approach achieves the directional propagation of SPPs through a multielement dipole array formed by a single dipole source and its reflections. Doing so mitigates the challenges in integrating and achieving coherent excitation among independently driven electrical sources. In addition, we provide design parameters for tuning the amplitude and phase of the image dipoles to engineer the directivity of SPP propagation. The configurations discussed can be readily implemented in the setting of tunnel-junction-based plasmon sources