Modellization of optical radiation - Liquid crystal interaction in complex geometries

Dipartimenti di Chimica e Fisica, Dottorato di Ricerca in “Scienze e Tecnologie delle Mesofasi e dei Materiali Molecolari, XX Ciclo a.a. 2007Università della Calabri

Different reorientational regimes in a liquid crystalline medium undergoing multiple irradiation

We present a numerical approach to the nemato-elasticity differential equation in a nematic liquid crystal cell when irradiated with multiple gaussian beams. Solutions have been carried out on a configuration with two coplanar beams illuminating the sample in order to compare it with particular nonlinear phenomena experimentally studied in the past. A new set of experimental measures were realized confirming the validity of the numerical model. Solutions for an instable case showing nonlocal effects are also presented as an example of the broader class of systems this approach can describe. (c) 2006 Optical Society of America

Photo-induced heating in plasmonic nanoparticles trapped in thermo-sensitive liquid crystals

Thermo-sensitive liquid crystals may result, for some aspects, good host materials for plasmonic nanoparticles. In particular they are suitable to study and measure the temperature variations produced by photo-induced plasmonic joule effect in the metallic nanoparticles. Combining the properties of liquid crystals and metallic nanoparticles, allows to measure temperature variations in different ways by exploiting the optical properties of thermotropic liquid crystals: In a first attempt, by combining nematic liquid crystals and spherical metallic nanoparticles, we have predicted and measured temperature changes, under a suitable (resonant) optical illumination, by measuring the photo-thermal induced birefringence variation. In a different experiment, we have combined cholesteric liquid crystals and gold nanorods: Light-induced variations of structural colorations exhibited by cholesteric liquid crystals has been used as a new methodology to measure nanoscale heat variation with a very high sensitivity (0.03 K)

Thermo-plasmonic effects on E7 nematic liquid crystal

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

Photo-Thermal Effects in 1D Gratings of Gold Nanoparticles

This work investigates the heat delivered by a mono-layer 1D grating of gold nanoparticles (GNPs) created by photo-reduction through two-photon direct laser writing (2P-DLW) in a poly-vinyl alcohol (PVA) matrix doped with HAuCl4, under resonant laser radiation. We drop cast a film of a PVA + HAuCl4 mixture onto a glass substrate, in which we create gratings of 1 mm2 made by stripes of GNPs characterized by high polydispersivity. We demonstrate that, by controlling the pitch of the GNP stripes, we obtain different values of the photo-induced temperature variations. In the framework of thermo-plasmonics, the experimental investigation of the heat generation from a monolayer of gold nanoparticles represents a key element as a starting point to design thermo-smart platforms for sensing, solar energy harvesting and thermo-catalysis

Resonant Gain Singularities in 1D and 3D Metal/Dielectric Multilayered Nanostructures

We present a detailed study on the resonant gain (RG) phenomena occurring in two nanostructures, in which the presence of dielectric singularities is used to reach a huge amplification of the emitted photons resonantly interacting with the system. The presence of gain molecules in the considered nanoresonator systems makes it possible to obtain optical features that are able to unlock several applications. Two noticeable cases have been investigated: a 1D nanoresonator based on hyperbolic metamaterials and a 3D metal/dielectric spherical multishell. The former has been designed in the framework of the effective medium theory, in order to behave as an <i>epsilon-near-zero-and-pole</i> metamaterial, showing extraordinary light confinement and collimation. Such a peculiarity represents the key to lead to a RG behavior, a condition in which the system is demonstrated to behave as a self-amplifying perfect lens. Very high enhancement and spectral sharpness of 1 nm of the emitted light are demonstrated by means of a transfer matrix method simulation. The latter system consists of a metal/doped-dielectric multishell. A dedicated theoretical approach has been set up to finely engineer its doubly tunable resonant nature. The RG condition has been demonstrated also in this case. Finite element method-based simulations, together with an analytical model, clarify the electric field distribution inside the multishell and suggest the opportunity to use this device as a <i>self-enhanced loss compensated</i> multishell, being a favorable scenario for low-threshold SPASER action. Counterintuitively, exceeding the resonant gain amount of molecules in both systems causes a significant drop in the amplitude of the resonance

Antimicrobial Effects of Chemically Functionalized and/or Photo-Heated Nanoparticles

Antibiotic resistance refers to when microorganisms survive and grow in the presence of specific antibiotics, a phenomenon mainly related to the indiscriminate widespread use and abuse of antibiotics. In this framework, thanks to the design and fabrication of original functional nanomaterials, nanotechnology offers a powerful weapon against several diseases such as cancer and pathogenic illness. Smart nanomaterials, such as metallic nanoparticles and semiconductor nanocrystals, enable the realization of novel drug-free medical therapies for fighting against antibiotic-resistant bacteria. In the light of the latest developments, we highlight the outstanding capabilities of several nanotechnology-inspired approaches to kill antibiotic-resistant bacteria. Chemically functionalized silver and titanium dioxide nanoparticles have been employed for their intrinsic toxicity, which enables them to exhibit an antimicrobial activity while, in a different approach, photo-thermal properties of metallic nanoparticles have been theoretically studied and experimentally tested against several temperature sensitive (mesophilic) bacteria. We also show that it is possible to combine a highly localized targeting with a plasmonic-based heating therapy by properly functionalizing nanoparticle surfaces with covalently linked antibodies. As a perspective, the utilization of properly engineered and chemically functionalized nanomaterials opens a new roads for realizing antibiotic free treatments against pathogens and related diseases

Thermoplasmonic‐Activated Hydrogel Based Dynamic Light Attenuator

AbstractThis work describes the morphological, optical, and thermo‐optical properties of a temperature‐sensitive hydrogel poly(N‐isopropylacrylamide‐co‐N‐isopropylmethacrylamide) [P(NIPAm‐co‐NIPMAm]) film containing a specific amount of gold nanorods (GNRs). The light‐induced thermoplasmonic heating of GNRs is used to control the optical scattering of an initially transparent hydrogel film. A hydrated P(NIPAm‐co‐NIPMAm) film is optically clear at room temperature. When heated to temperatures over 37 °C via light irradiation with a resonant source (λ = 810 nm) to the GNRs, a reversible phase transition from a swollen hydrated state to a shrunken dehydrated state occurs. This phenomenon causes a drastic and reversible change in the optical transparency from a clear to an opaque state. A significant red shift (≈30 nm) of the longitudinal band can also be seen due to an increased average refractive index surrounding the GNRs. This change is in agreement with an ad hoc theoretical model which uses a modified Gans theory for ellipsoidal nanoparticles. Morphological analysis of the composite film shows the presence of well‐isolated and randomly dispersed GNRs. Thermo‐optical experiments demonstrate an all‐optically controlled light attenuator (65% contrast ratio) which can be easily integrated in several modern optical applications such as smart windows and light‐responsive optical attenuators