Tuning plasmons of metal-coated microsphere arrays towards optimized surface-enhanced spectroscopy

Abstract

The overall optical response (transmittance, reflectance, and absorbance) of metal film over nanospheres (MFoN) is studied for a wide range of sphere diameters (200 - 1000 nm) and metal film thicknesses (40–200 nm), over the 450 - 2000 nm spectral range. Analyses are performed also in water, with microfluidic surface-enhanced Raman scattering applications in mind. Two main outcomes are the dependence of the plasmonic absorbance band on structural parameters and the behavior in aqueous environment. The parameter ranges for targeting common lasers (633 and 785 nm) are identified. Additionally, for larger sphere size and thicker films, a new absorbance band was identified, exhibiting a multipole-like electric field distribution, different than the dipole-like fields at the main absorption band. It is also shown that the fine morphology of the metal film at the inter-sphere region has a strong impact on reflectance (and absorbance) but not transmittance. The individual roles of the metal particles formed on the substrate or the dielectric sphere array on the overal optical response are discussed. Finally, the role of the metal type (Au, Ag, Cu, Al) is also analyzed.