Localized Surface Plasmon Resonance in Gold Nanocluster Arrays on Opaque Substrates

We report on the investigation of the localized surface plasmon resonance (LSPR) in periodical Au nanostructures. The arrays of Au nanoclusters and dimers were fabricated on Si and Si/SiO2 surfaces by electron beam lithography. Diameters and periods of nanoclusters with disk shape vary in the range of 30–150 and 130–200 nm, respectively. Because of the opaque nature of the substrates, optical reflection spectroscopy was chosen to probe the plasmonic properties of the metal nanostructures. From a comparison of experimental reflection spectra with those numerically simulated by the Finite Difference Time Domain (FDTD) method, we determined the model structural parameters of the plasmonic nanostructures. These parameters were further used for the calculation of absorptance spectra of the plasmonic structures for which absorptance in the substrate was subtracted. LSPR positions were determined from the maxima of the absorptance spectra. This study reveals a strong dependence of the LSPR position on nanocluster size, distance between nanoclusters, as well as on the SiO2 layer thickness in the nanometer range. In the case of dimer arrays, the plasmon anisotropy in the dimers leads to a splitting of the LSPR plasmon into two modes with orthogonal polarizations. The absorptance spectra reveal a transverse LSPR mode corresponding to the excitation of plasmons in nanoclusters induced by scattered fields from the neighboring ones. This research provides a pathway for a fast and cost-effective determination of the LSPR position from optical reflection spectra. A broad field of potential applications of metal structures with well-controlled plasmonic properties includes surface-enhanced infrared absorption, photoluminescence, and Raman scattering as well as signal transmission in silicon photonics.