Improvement of surface-enhanced Raman scattering by dipolar resonance mode of silver half-shell array

In this paper, we study the tuning of the optical responses and surface-enhanced Raman scattering (SERS) of the silver nanostructure film corrugated with a two-dimensional (2D) colloidal crystal. Using rhodamine 6G as the probe molecules, we show that the SERS signal can be efficiently improved by controlling the silver layer thickness or the microsphere size. Under the excitation of a laser with 514 nm in wavelength, the largest spatially average enhancement factor of SERS signal is on the order of ~3 × 107 when the silver nanostructure is patterned by sputtering ~100-nm-thick silver layer onto 2D colloidal microspheres of 360 nm in diameter. We further demonstrate that the Raman intensity is proportional to the product of the absorptances of the as-prepared metallic SERS substrates at excitation and Stokes wavelengths. Numerical simulations revealed that this huge enhancement of Raman signal is attributed to the excitation of a dipolar plasmon resonance of the silver nanostructure.