We report on the fabrication and characterization of a switchable plasmonic device based on a conductive graphene oxide (cGO) coated plasmonic nanoparticle (NP) array, layered with nematic liquid crystal (NLC) as an active medium. A monolayer of NPs has been immobilized on a glass substrate through electrostatic interaction, and then grown in place using nanochemistry. This monolayer is then coated with a thin (less then 100nm) cGO film which acts simultaneously as both an electro-conductive and active medium. The combination of the conductive NP array with a separate top cover substrate having both cGO and a standard LC alignment layer is used for aligning a NLC film in a hybrid configuration. The system is analysed in terms of morphological and electro-optical properties. The spectral response of the sample characterized after each element is added (air, cGO, NLC) reveals a red-shift of the localized plasmonic resonance (LPR) frequency of approximately 62nm with respect to the NP array surrounded by air. The application of an external voltage (8Vpp) is suitable to modulate (blue shift) the LPR frequency by approximately 22nm.

We demonstrate and characterize an optical control of the plasmonic heat delivered by a monolayer substrate of gold nanoparticles, obtained by modulating the effective refractive index of the neighboring dielectric medium. The effect, which exploits the dependence of the nematic liquid crystal (NLC) refractive index on the molecular director orientation, is realized by using a polarization dependent, light-induced molecular reorientation of a thin film of photo-alignment layer that the NLC is in contact with. For a suitable alignment, plasmonic pumping intensity values ranging from 0.25 W/cm² to 6.30 W/cm² can induce up to 17.4 °C temperature variations in time intervals of the order of seconds. The reversibility of the optically induced NLC molecular director orientation enables an active control of the plasmonic photo-induced heat.

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.

Elongated plasmonic nanoparticles show superior optical properties when compared to spherical ones. Facile, versatile and cost-effective bottom-up approaches for fabrication of anisotropic nanoparticles in solution have been developed. However, fabrication of 2-D plasmonic templates from elongated nanoparticles with spatial arrangement at the surface is still a challenge. We used controlled seed-mediated growth in the presence of porous and functionalized surface of flexible polydimethylsiloxane (PDMS) templates to provide directional growth and formation of elongated gold nanoparticles (AuNPs). Atomic force microscopy (AFM) and spectroscopy revealed embedding of the particles within the functionalized porous surface of PDMS. Nanoparticles shapes were observed with transmission electron microscope (TEM), UV–Vis spectroscopy, and X-ray powder diffraction (XRPD) measurements, which revealed an overall orientation of particles at the surface. Anisotropic and oriented particles on a flexible substrate are of interest for sensing applications.

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.