Ten-Micrometer-Thick Si Wafers with Ag Nanoclusters: Substrate Effects on Plasmon-Enhanced Optical Absorption

We present the optical characteristics of 10-μm-thick crystalline Si wafers with an Ag heptamer nanocluster (NC) array, using a finite-difference time-domain method. The anti-reflection properties of the Ag NC array were more pronounced at long wavelengths, with respect to a monomer array, resulting in significantly enhanced optical absorption in the underlying Si wafer. The scattering cross-section spectra of the NC on the Si wafer exhibited one broad peak with a kink, whereas those in air showed two broad peaks and a sharp Fano dip between them. The high refractive index Si wafer weakened the near-field coupling between particles in the NCs, which modified the optical cross-sections of the Ag NC more drastically than those of the Ag monomer. Therefore, the implementation of the NC nanoantennae for Si-based optoelectronic devices requires careful consideration of the substrate effects.