Linear and nonlinear optical properties of gold nanospheres immobilized on a metallic surface

We studied linear and nonlinear optical properties of surface-immobilized gold nanospheres (SIGNs) above a metallic surface with a gap distance of a few nanometers. The nanogap is supported by self-assembled monolayers (SAMs); they are used as spacers. A localized surface plasmon resonance (LPR) band (lambda=600-700 nm) redshifted from the LPR band of isolated gold nanospheres (lambda similar to 520 nm) is observed in the p-polarized reflection absorption spectra with oblique incidence. This band originates from the electromagnetic interaction of the gold nanospheres with their images produced in the metallic substrate. The amount of redshift depends on the gap distance, which is controlled by the thickness of the SAMs. These optical properties can be simulated using theoretical calculations obtained by considering multipolar interactions based on a quasistatic approximation. Further, in SIGNs above a metal, we observed a significantly enhanced optical second-harmonic generation (SHG). The field enhancement factor was found to be greater than 10 at 1064 nm. This is due to an increase in local electric fields in SIGN systems at both fundamental and SHG wavelengths (lambda(2 omega)=532 nm). Local electric field calculations imply that the SHG from the nanogap does not dominate even under the LPR condition because the nanogap region is almost centrosymmetrical.