Near-field Coupling Effect between Individual Au Nanospheres and their Supporting SiO2/Si Substrate

We studied the far-field optical reflection contrast spectroscopy (FORCS) properties of the following system: individual Au nanospheres (radius R) immobilized above Si substrate with different thicknesses (d) SiO2 between them. We found that peaks in the FORCS red-shift exponentially with d decreasing. The near-field coupling between the Au nanosphere and its supporting substrate is revealed to contribute to this, while the coupling strength is demonstrated to decrease exponentially with a decay length of 0.30 in units of d/R. It qualitatively agrees well in magnitude with the near-field coupling between two noble metal nanoparticles consisting of a dimer. Our results demonstrate that the FORCS can provide insight into the near-field coupling, which is significant for their applications in nano-photonics, sensing, surface-enhanced spectrascopies, etc.