Ugo Cataldi

Growing gold nanoparticles on a flexible substrate to enable simple mechanical control of their plasmonic coupling

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.

Broad band tuning of the plasmonic resonance of gold nanoparticles hosted in self-organized soft materials

The extraordinary properties of reconfigurable soft materials are used to drive the resonance properties of noble metal nanoparticles, using an approach that puts a bridge between soft matter and plasmonics. Gold nanoparticles have been dissolved in a cholesteric liquid crystal and then infiltrated in a micro-periodic polymeric structure, realized by combining a holographic step and a microfluidic etching process. The spectral behavior of the nano-composite soft-structure has been investigated in the UV-Vis range for two different polarization directions of the impinging probe light. Correlation between the optical response and external perturbations (electric field, temperature variation) gives an outstanding example of broadband tuning of an “active” plasmon resonance.

The beginnings of plasmomechanics: towards plasmonic strain sensors

This article exposes the beginnings of a new field which could be named as “plasmomechanics”. Plasmomechanics comes from the convergence between mechanics and plasmonics. Here we discuss a relatively recent topic whose technological aim is the development of plasmonic strain sensors. The idea is based on the ability to deduce Au nanoparticles (NPs) distance distributions from polarized optical extinction spectroscopy which could thus give access to material strains. Variations of interparticle distances distributions can indeed lead to variations of plasmonic coupling and thus to material color change as shown here experimentally and numerically for random Au NP assemblies deposited onto elastomer films.

Optical control of plasmonic heating effects using reversible photo-alignment of nematic liquid crystals

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.

Preparation of Anisotropic and Oriented Particles on a Flexible Substrate.

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.

Plasmon Resonance Tunability of Gold Nanoparticles Embedded in a Confined Cholesteric Liquid Crystal Host

Gold nanoparticles have been dissolved in a cholesteric liquid crystal and then infiltrated in a micro-periodic polymeric structure. This has been realized by combining a holographic step and a microfluidic etching process. The spectral behavior of the nano-composite soft-structure has been investigated in the UV-Vis range for two different polarization directions of an impinging probe light and in presence of external perturbations (electric field, temperature variation). Obtained results show a highly tunable plasmonic response of the material.

Thermo-plasmonic effects on E7 nematic liquid crystal

ABSTRACT The photo-induced heating from a layer of uniformly distributed gold nanoparticles has been characterized to investigate the impact of a direct interaction of the metallic nanostructures with a drop of E7 nematic liquid crystal. A double effect is obtained: an increase of the photo-induced heating due to a change in the refractive index of the medium surrounding the nanoparticles (from air to E7) and a new way of exploiting this heating to induce a phase transition of the E7 NLC (from anisotropic to isotropic phase), with a significant decrease of the necessary energy density in the case of an impinging green radiation.

Control of the plasmonic resonance of a graphene coated plasmonic nanoparticle array combined with a nematic liquid crystal

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.

Plasmomechanics: A Colour-Changing Device Based on the Plasmonic Coupling of Gold Nanoparticles

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.

Active Plasmonics in Self-organized Soft Materials

In this chapter we show several possibilities for obtaining active plasmonics in self-organized amorphous materials. The starting point is a brief theoretical description of the physical mechanisms that allow such a tunability. Afterwards, it follows an overview of the up-to-date results obtained in this direction. Several proof-of-concept prototypes have been successfully fabricated and are reported as well.