Driving electromagnetic field enhancements in tailored gold surface nanostructures: Optical properties and macroscale simulations

Gold thin films with remarkable Surface Enhanced Raman scattering activity strongly dependent on their surface nanostructure were grown by nanosecond pulsed laser ablation. By changing the number of the laser pulses, keeping fixed all the other deposition parameters, we deliberately modified the surface nanostructure of the films. These nanostructured films consist of metallic islands separated from each other by inter-island channels. We observed differences in both island sizes and inter-island channel sizes as a function of the laser pulse number. The different optical properties of the films such as the position of the localized surface plasmon resonance absorption peak red shifts with increasing the laser pulse number. We performed Finite-Difference Time-Domain calculations to gain insight on how the surface nanostructure of the film affects its optical properties at the macroscale. Results indicate that the strongest localization of the electromagnetic field, and hence the strongest SERS enhancement, occurs at the channel-shaped gaps between adjacent metal islands.Larger enhancements are found in coincidence with narrower and deeper gaps. The number distribution of SERS active sites as a function of the calculated SERS enhancement factors, whose values range between 104 and 105, were obtained. Since the inter-island channels are randomly oriented they show no orientation dependence for linearly polarized radiation on a length scale larger than the typical size of the nanostructures, while larger enhancements were observed for circular polarization.