Numerical prediction of the effect of nanoscale surface roughness on film-coupled nanoparticle plasmon resonances

Abstract

Plasmon resonant metal nanoparticles on substrates have been considered for use in several nanophotonic applications due to the combination of large field enhancement factors, broadband frequency control, ease of fabrication, and structural robustness that they provide. Despite the existence of a large body of work on the dependence of the nanoparticle plasmon resonance on composition and particle-substrate separation, little is known about the role of substrate roughness in these systems. This is in fact an important aspect, since particle-substrate gap sizes for which large resonance shifts are observed are of the same order of typical surface roughness of deposited films. In the present study, the plasmon resonance response of 80 nm diameter gold nanoparticles on a thermally evaporated gold film are numerically calculated based on the measured surface morphology of the gold film. By combining the measured surface data with electromagnetic simulations, it is demonstrated that the plasmon resonance wavelength of single gold nanoparticles is blueshifted on a rough gold surface compared the response on a flat gold film. The anticipated degree of spectral variation of gold nanoparticles on the rough surface is also presented. This study demonstrates that nanoscale surface roughness can become an important source of spectral variation for substrate tuned resonances that use small gap sizes.