Elucidating heterogeneity in nanoplasmonic structures using nonlinear photon localization microscopy

Using nonperturbative photon localization microscopy and electromagnetic simulation, it is observed that localized modes in plasmonic devices are significantly impacted by small, and frequently time-dependent, structural variations on the nanometer scale. This is important because many such devices rely on the concentration of electromagnetic energy at the ∼10 nm length scale and below for applications ranging from ultrasensitive molecular spectroscopy and detection, to chemical nano-imaging and plasmo-catalysis. In all devices, but particularly those based on noble metals, structural heterogeneity at these length scales is unavoidable, emphasizing the need for characterizing and understanding its effects. By exploiting the two-photon photoluminescence signal, one addresses the specific challenge of probing local electromagnetic fields inside the metal, which directly determine hot carrier generation and photoemission. It is found that heterogeneous nanoscale asperities serve as energy localization centers, and that functional impact is influenced primarily by two factors: position relative to a plasmonic mode volume, and how the asperity affects the smallest critical dimension, such as the size of a nanogap, in the structure.