TAILORING LOCALIZED SURFACE PLASMON RESONANCES IN METALLIC NANOANTENNAS

Abstract

The strong localized electromagnetic field achievable with metallic nanoantennas provides new opportunities for harmonics generation and label-free chemical sensing. In this work, the localized surface plasmon resonances (LSPRs) of metallic nanoarcs on dielectric substrates have been systematically investigated with visible and infrared spectroscopy, with the goal of elucidating the relationship between the structural and material parameters of the nanoarcs and their resonances. The transmission spectra provide rich information regarding the fundamental and higher order LSPR modes. Experimental results and numerical simulations demonstrate that the LSPR wavelengths are governed by the mid-arc length of the nanoarcs, and the extinction cross-sections of the different order modes are controlled by the central angle of the nanoarc and the symmetry of the mode. The fundamental and second order LSPR wavelengths can be tuned independently through the design of a non-uniform arc-width profile. Several relationships between features of the LSPR modes and the geometric parameters of nanoarcs are also confirmed by transformation optics analysis. The newly found relationships are then utilized as guidelines for the realization of plasmonic nanoarc antennas exhibiting efficient second harmonic generation (SHG). In another application, strong coupling between LSPRs and molecular vibrations is evident in the IR spectra of plasmonic nanoarcs placed in contact with a thin film of polymer, a native oxide layer or a thiol monolayer, enhancing the vibrational mode signals. This observation suggests that by appropriately tuning the frequency of the LSPR modes, the localized electromagnetic field around nanoarcs can resonantly couple to another emitter to boost its far-field radiation, which could benefit applications requiring highly localized, sensitive and selective chemical detection.