Investigation of the Nonlinear Properties of Plasmonic Gold Nanocrescents for Surface-Enhanced Spectroscopies

Plasmonic nanocrescents are of interest as substrates for surface-enhanced spectroscopies due to their tunable resonance in the visible to mid-IR region and high optical near-fields. Due to the asymmetric nature of the nanocrescent structure multiple resonance modes arise, and can be selectively excited by the polarization of the incident light. To date only the linear properties of nanocrescents have been studied; however, we present enhanced second harmonic emission from gold nanocrescent arrays fabricated by nanosphere template lithography. Samples were designed to have both short and long axis resonances overlap with the fundamental wavelength. Results show that SHG intensity is dependent on the polarization of incident light, with maximum signal occurring for light polarized along the long axis. This data implies that the near-field enhancement of the long axis is more intense than the short axis. These preliminary results indicate that the near-fields of nanocrescents can enhance nonlinear processes, which is promising for mapping the near-field distribution of nanocrescents through two photon absorption of a photopolymer. Two photon absorption is an inherently weak process that can be enhanced by optical near-fields of nanostructures, leading to selective polymerization of the photopolymer where localized near-fields of the nanostructure are most intense.