In this chapter, the mechanism of plasmonic coupling of the near-fields that takes place when nanostructure are very close to each other is described. After a theoretical introduction illustrating the main physical principles governing plasmonic coupling, several experimental systems are considered. Interestingly, experimnetal results show that both periodic and rigid, random, and flexible systems of gold nanoparticles exhibit a universal scaling behaviour and verify the plamonic ruler equation.

We have carried out an experiment on a flexible polymeric substrate, coated with a monolayer of gold nanoparticles, which demonstrates how the combined effect of nanoparticle growth and stretching influences the average normalized gap between particles, thus modifying the extinction spectra of the sample. The study paves the way for the realization of a plasmonic strain sensor based on the plasmonic coupling of gold nanoparticles deposited onto elastomeric films: application of a mechanical stretching induces a change of colour of the device and a fine control of the applied strain allows a continuous tuning of the colour.

A simple method is presented to control and trigger the coupling between plasmonic particles using both a growing process of gold nanoparticles (GNPs) and a mechanical strain applied to the elastomeric template where these GNPs are anchored. The large scale samples are prepared by first depositing and then further growing gold nanoparticles on a flexible PDMS tape. Upon stretching the tape the particles move further apart in the direction of the stretching and closer together in the direction perpendicular to it. The synergy between the controlled growth of GNPs and the mechanical strain, leads to a drastic shift of the plasmon band and a color change of the sample. Furthermore, the stretching by only a few percent of the amorphous and initially isotropic sample results in a strong polarization-dependent plasmon shift. At smaller gap sizes between neighboring particles, induced by stretching the PDMS tape, the plasmon shift strongly deviates from the behaviour expected considering the plasmon ruler equation. This shows that multipolar coupling effects significantly contribute to the observed shift. Overall, these results indicate that a macroscopic mechanical strain allows one to control the coupling and therefore the electromagnetic field at the nanoscale.