Giovanna Palermo

Templating gold nanorods with liquid crystalline DNA

A liquid crystalline, negatively charged, whole-genome DNA is exploited to organize positively charged gold nanorods (GNRs) by means of electrostatic interaction. A mesoscopic alignment of the composite system along a preferred direction is obtained by casting a droplet of the DNA-nanorods solution onto an untreated glass substrate. Gel electrophoresis analysis enables evaluating the effective electric charge of the system, thus minimizing the DNA fragmentation. Polarized optical microscopy, combined with transmission and scanning electron microscopy, shows that, up to 20% in weight of GNR solution, the system exhibits both a long range order, induced by the liquid crystalline phase of the DNA, and a nanoscale organization, due to the DNA self-assembly. These evidences are confirmed by a polarized spectral analysis, which also points out that the optical properties of GNRs strongly depend on the polarization of the impinging probe light. The capability to organize plasmonic nanoparticles by means of DNA material represents a significant advance towards the realization of life science inspired optical materials.

Liquid Crystals as an Active Medium:Novel Possibilities in Plasmonics

Abstract The peculiar properties of Liquid Crystals (LCs) foster new possibilities in plasmonics. The combination of the intrinsic tunability of LCs with the plasmonic properties of metallic nanoparticles (NPs) provides novel and intriguing features of systems commonly identified as active plasmonics. Being LCs, one of the media whose refractive index can be controlled through the application of external stimuli, they represent a convenient host for enabling plasmonic tunability. On the other hand, the localized plasmonic resonance, typical of NPs, can strongly influence and control the behaviour of LCs. In this paper, we overview several systems of NPs combined with LCs arranged in different configurations. The properties of the resulting systems suggest novel, intriguing outcomes in both fundamental and applied research.

A conformal silk-azobenzene composite for optically switchable diffractive structures

The use of biomaterials as optical components has recently attracted attention because of their ease of functionalization and fabrication, along with their potential use when integrated with biological materials. We present here an observation of the optical properties of a silk-azobenzene material (Azosilk) and demonstrate the operation of an Azosilk/PDMS composite structure that serves as a conformable and switchable optical diffractive structure. Characterization of thermal and isomeric properties of the device, along with its overall performance, is presented in terms of diffractive characteristics and response times. The ease of manufacturing and functionalization opens a promising avenue for rapid device prototyping and interfaces of expanded utility.

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.

Plasmonic Thermometer Based on Thermotropic Liquid Crystals

Localized plasmon resonance (LPR) of noble Metal Nanoparticles (MNPs) opens up a new horizon for nanoscale materials able to convert light into heat, since the strong electric field generated around the MNPs can transform them into original heat nanosources. Thus, investigation of the heat transport mechanism, from the heated MNPs to their surrounding medium, is fundamental for realizing applications in nanotechnology and thermal-based therapies, and a challenge is definitely represented by the possibility of measuring temperature variations at the surface of the MNPs undergoing optical illumination. In this framework, we show that an ingenious combination of characteristics of short pitch liquid crystalline compounds and MNPs has demonstrated effective to provide an advanced tool to monitor nanoscale temperature variations.

Developing novel liquid crystal technologies for display and photonic applications

Abstract Modern liquid crystal displays (LCDs) require novel technologies, such as new alignment methods to eliminate alignment layers, fast response and long operation time. To this end, we report an overview of recent efforts in LCD technologies devoted to realize more display modes having no alignment layer, faster switching time and low battery consumption. In particular, we overview recent advances on the liquid crystals (LCs) alignment for display applications, which includes superfine nanostructures, polymeric microchannels and polymer stabilized LCs. Furthermore, we analyze the main optical and electro-optical properties of new generation LCDs displays addressing a particular attention to LCs blue phase hosting gold nanoparticles. Moreover, we focus on the progress of electrofluidic displays, which demonstrates characteristics that are similar to LCDs, with attention on various pixel designs, operation principles and possible future trends of the technology.

Electro and pressure tunable cholesteric liquid crystal devices based on ion-implanted flexible substrates

We report an electro-responsive and pressure sensitive device based on conductive polydimethylsiloxane (PDMS) combined with a short pitch Cholesteric Liquid Crystal (CLC). Ion-implantation and surface chemistry in PDMS enable both the induction of conducting properties in this elastomer as well as long range organization of the CLC. Sample electro-optical and pressure dependent optical properties are explored by applying a modulated electric field through the conducting PDMS and a uniform pressure on the top cover substrate. We show that both an electric field of a few V μm−1 and an external pressure of up to 128 kPa can tune the reflection band by about 100 nm.

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.

Nematic liquid crystals used to control photo-thermal effects in gold nanoparticles

We report on photo-thermal effects observed in gold nanoparticles (GNPs) dispersed in Nematic Liquid Crystals (NLCs). Under a suitable optical radiation, GNPs exhibit a strong light absorption/scattering; the effect depends on the refractive index of the medium surrounding the nanoparticles, which can be electrically or optically tuned. In this way, the system represents an ideal nano-source of heat, remotely controllable by light to adjust the temperature at the nanoscale. Photo-induced temperature variations in GNPs dispersed in NLCs have been investigated by implementing a theoretical model based on the thermal heating equation applied to an anisotropic medium; theoretical predictions have been compared with results of experiments carried out in a NLC medium hosting GNPs. Both theory and experiments represent a step forward to understand the physics of heat production at the nanoscale, with applications that range from photonics to nanomedicine.

Flexible Structures Based on a Short Pitch Cholesteric Liquid Crystals

We report on the realization and characterization of electro-responsive and pressure sensitive polydimethylsiloxane (PDMS) conductive photonic structures combined with the reconfigurable properties of short pitch cholesteric liquid crystals (aligned in Grandjean configuration). By combining ion-implantation process and surface chemistry functionalization, we have overcome the insulating properties of PDMS and induced long range organization of cholesteric liquid crystals, thus controlling both diffraction and selective Bragg reflection of light by means of external perturbations (electric field, pressure). We have characterized our devices in terms of morphological, optical and electro-optical properties.