Comparison of two-dimensional periodic arrays of convex and concave nanostructures for efficient SERS templates

We describe the optical power enhancement on the surface of the 2D (two-dimensional) periodic arrays of convex and concave gold nanostructures for comparing the characteristics of the nanostructures for surface-enhanced Raman spectroscopy (SERS) templates. The optical power enhancement is due to the surface plasmon polaritons, which is calculated by the Finite-Difference Time-Domain (FDTD) method at commercially-available 532 nm pump light. A periodic array of closely-packed gold particles is defined as convex nanostructure, while a periodic array of hemispherical holes, or voids, on gold substrate is defined as concave nanostructure. The peak power enhancement factor, the average power enhancement factor and the activity rate of each structure were compared. The convex nanostructures show a strong enhancement factor in localized hotspots, while the concave nanostructures show not only the peak power enhancement factor comparable to that of convex nanostructures, but also higher spatially-averaged power enhancement factors and activity rates than those observed on the convex nanostructures, meaning that the highly enhanced near-field zone distributes densely on the substrate. We also revealed the dependence of the void diameter on the inter-void distance for the power enhancement in the concave nanostructures system, providing a guideline for the fabrication of the efficient SERS template, which shows a strong power enhancement factor with a high area density.