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

Second-Harmonic Generation from Sub‑5 nm Gaps by Directed Self-Assembly of Nanoparticles onto Template-Stripped Gold Substrates

Strong field enhancement and confinement in plasmonic nanostructures provide suitable conditions for nonlinear optics in ultracompact dimensions. Despite these enhancements, second-harmonic generation (SHG) is still inefficient due to the centrosymmetric crystal structure of the bulk metals used, e.g., Au and Ag. Taking advantage of symmetry breaking at the metal surface, one could greatly enhance SHG by engineering these metal surfaces in regions where the strong electric fields are localized. Here, we combine top-down lithography and bottom-up self-assembly to lodge single rows of 8 nm diameter Au nanoparticles into trenches in a Au film. The resultant “double gap” structures increase the <i>surface-to-volume</i> ratio of Au colocated with the strong fields in ∼2 nm gaps to fully exploit the surface SHG of Au. Compared to a densely packed arrangement of AuNPs on a smooth Au film, the double gaps enhance SHG emission by 4200-fold to achieve an effective second-order susceptibility χ⁽²⁾ of 6.1 pm/V, making it comparable with typical nonlinear crystals. This patterning approach also allows for the scalable fabrication of smooth gold surfaces with sub-5 nm gaps and presents opportunities for optical frequency up-conversion in applications that require extreme miniaturization