L. Pezzi

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. A new set of experimental measures are also presented.

Non-Linear Effects in NLC Media Undergoing Two Beams Irradiation

A Nematic Liquid Crystal crossed by the overlap of multiple laser beams gives rise to different phenomena, such as the cancellation of reorientation and critical reorientation. A nonlinear 2D theoretical model is presented to describe these effects in the case of two laser beams impinging on a homeotropically aligned cell. By comparing the theoretical estimation of the re-orientational effect to the observed optical divergence of one beam, it is shown that they depend on the same phenomenon, although they are not directly correlated. Solutions given by the model show a good agreement with experimental results.

A command layer for anisotropic plasmonic photo-thermal effects in liquid crystal

ABSTRACT Photo-anisotropic properties of a particular command layer for Liquid Crystals (LCs), based on azo-benzene material, are exploited to control the photo-thermal response of a single layer of homogeneously and uniformly distributed Au nanoparticles, immobilised on a glass substrate. Experiments demonstrate that the intrinsic anisotropy of materials can influence the photo-thermal response of plasmonic systems. Indeed, the resonant absorption of radiation by plasmonic subunits is followed by a noticeable increase of their temperature. However, the thermal response observed in presence of a homogeneous and random array of AuNPs directly exposed to air or embedded in ice is typically isotropic; on the contrary, a homogenous, yet thin, coating made of a particular command layer for LCs, deposited on a large-area carpet of AuNPs, influences their thermal response in an anisotropic way. In particular, the temperature increase, induced by pumping with a laser source of resonant wavelength with the plasmonic AuNPs, strongly depends on the alignment direction of the command layer. This effect makes the command layer of particular interest for its capability to drive intriguing optically induced ‘thermal-reorientational’ effects in a liquid crystal film. Graphical Abstract

Determination of NLC refractive index dispersion in wavelength and temperature for plasmonic applications

ABSTRACT Nematic liquid crystals (NLCs) have proved to be good surrounding materials to investigate variations of physical properties produced by plasmonic resonance in metallic nanoparticles (NPs). In this framework, availability of a simple and reliable tool enabling a complete analysis of dispersion in wavelength and temperature of the refractive index of a NLC containing NPs has been developed, which could reveal of great utility. By using the Extended Cauchy equations system applied to a particular NLC (E7), we have here implemented a simple formula that yields a fine evaluation of the NLC refractive indices dispersion in the wavelength range [450, 656] nm and in the temperature range [15, 55]°C. This allow also to estimate with quite high accuracy temperature variations around the metal NPs produced by the plasmonic resonance.

Nematic liquid crystal cells for optical spatial solitons (nematicons)

The study of optical solitons and light filaments steering in liquid crystals requires utilization of particular cells designed for top view investigation and realized with an input interface which enables both to control the molecular director configuration and to prevent light scattering. Up to now, the director orientation imposed by this additional interface has been only estimated by experimental observations. In this paper, we report on the design and characterization of liquid crystal cells for investigation of optical spatial solitons as well as on a simple model describing the configuration of the molecular director orientation under the anchoring action of multiple interfaces. The model is based on the elastic continuum theory and only strong anchoring is considered for boundary conditions. Controlling of the director orientation at the input interface, as well as in the bulk, allows to obtain configurations that can produce distinct optical phenomena in a light beam propagating inside the cell. For a particular director configuration, it is possible to produce two waves: the extraordinary and the ordinary one. With a different director configuration, the extraordinary wave only is obtained, which propagates inside the cell at an angle of more than 7° with respect to the impinging wave vector direction. Under this peculiar configuration and by applying an external voltage, it is possible to have a good control of the propagation direction of the optical spatial soliton.