Insights into second-harmonic generation mechanisms in plasmonic nanostructures: Interplay between surface and bulk contributions

The origin of second-harmonic generation (SHG) in centrosymmetric materials, such as plasmonic metals, remains a subject of long-standing debate. In this work, the relative roles of local surface- and nonlocal bulk-induced nonlinear sources in the SHG of gold nanostructures are investigated using full-wave simulations and experimental SHG microscopy. The transition from the dipolar to the quadrupolar regime is studied using multipolar decompositions. SHG measurements of fabricated nanocylinders support the simulation results, indicating the increasing role of bulk nonlinearities in high-aspect ratio structures. Far-field scattering diagrams suggest that different SHG sources may be distinguished through directional emission. This study provides a comprehensive framework for understanding and dissecting the microscopic origins of SHG in plasmonic systems.