P. Maddalena

Wavelength dependence of optical reorientation in dye-doped nematics

Abstract The wavelength dependence of the optical reorientation of nematic liquid crystals is investigated in the presence of two dyes, using the z-scan technique. A strong correlation is found between the magnitude of the dyeinduced amplification of the optical torque and the dichroism of the system.

Interferometric technique for the determination of thermal nonlinearities in semiconductor glasses

Abstract An experimental method for the determination of thermal nonlinearities of semiconductor doped glasses induced by cw laser radiation is presented. An estimation of the intensity dependent refractive index is given, at steady state conditions, which is in good agreement with experimental data reported in the literature for similar materials.

Dynamical Regimes and Motion Intermittence Induced by a Laser Beam in a Nematic Liquid Crystal Film

When an s-polarised laser beam shines on a horneotropically aligned film of nematic liquid crystal at small incidence angle, undamped oscillations of the molecular director may be produced. At high beam intensities, the oscillations break up into deterministic chaos. Although this effect has been known for a long time and the route to chaos has been qualitatively analysed, no detailed study of the director motion has been carried out. We designed an experimental apparatus to monitor the dynamics of the director, as described by its two polar angles. We observed different dynamical regimes, depending on laser intensity: steady states, oscillations, rotations, and, at the highest laser intensities, deterministic chaos. Moreover, the transitions between the oscillation and rotation regimes are characterized by intermittency.

A New Optical Method for the Measurement of Viscoelastic Coefficient in Nematic Liquid Crystals

Abstract A new method to measure the ratio between orientational viscosity and the bend elastic constant of nematic liquid crystals is presented. The method is based on the observation of the molecular rotation induced in the sample by the angular momentum transfer from a circularly polarized laser beam. Backflow is also considered. PACS numbers: 61.30.Gd, 42.65.-k, 64.70.Md

Pyroelectric manipulation of liquid crystal droplets

Very interesting effects can be observed in maneuvering nematic liquid crystal (NLC) droplets onto functionalized polar lithium niobate (LN) crystal surfaces, covered with thin films of Polydimethylsiloxane (PDMS). It has been discovered that pyroelectric effect is able to drive a reversible fragmentation process in liquid crystal drops, starting from nanoliter drops and obtaining pico/femtoliter droplets. These small droplets are patterned according to the geometry of the substrate and aligned along the electric field lines. This novel approach for manipulating different classes of liquids by exploiting the pyroelectric effect, where the strong electric fields generated allow to manipulate liquids in 2D on a substrate or even in 3D, has been recently discovered and exploited for different purposes. In particular, manipulation of liquid crystals by a thermal stimulus could be suitable for applications such as spatial modulation of the wettability (i.e. wettability patterning), or, in principle, a dynamical optical element able to switch from a diffuser (fragmentation) state to a microlens array. Moreover, the biocompatibility of some kinds of nematic or cholesteric liquid crystals makes them suitable as biomaterials for applications in biology and tissue engineering.

Manipulation of Nematic Liquid Crystal Microdroplets by Pyroelectric Effect

We present experimental results concerning liquid crystals microdroplets behavior onto functionalized lithium niobate substrates covered with Polydimethylsiloxane (PDMS) polymer. Droplets are fragmented, driven, and reassembled by electric fields generated by pyroelectric effect. We analyze the dynamics of the observed phenomena and suggest possible technological applications.

A FOURIER-TRANSFORM-BASED INTERFEROMETRIC TECHNIQUE FOR MEASURING THE ELASTIC ANISOTROPY OF A NEMATIC LIQUID CRYSTAL

We propose a method based on an interferometric technique to measure the molecular distortion induced by an incident light beam on a nematic liquid crystal film. An analysis of the fringe pattern generated by means of a Fourier transform method allows comparison of the measurements with the theoretical molecular distribution. From this analysis an estimation of the elastic anisotropy of the liquid crystal is derived which is in good agreement with previous results.

Experimental investigation of lateral wave contribution to the shift of a reflected beam at surface plasmon resonance

Abstract Reflected field intensity distributions are reported for surface plasmon wave excitation in an attenuated total internal reflection configuration for different metal film thickness. The measured lateral shift is compared with theoretical predictions.

Self-assembling of liquid crystal droplets on lithium niobate substrates driven by pyroelectric effect

It is known that one of the major attractive feature of liquid crystals (LCs) lies in the fact that their optical properties can be modulated by electric, optical or magnetic fields. Recently, it was discovered that liquid crystal droplets, in particular conditions, can be driven in desired locations following electric field lines. The pyroelectric properties of the periodically poled lithium niobate (PPLN) crystal are exploited and PPLN crystal is used as substrate. In fact, heating and/or cooling this material, it has already been demonstrated that surface charges can appear, by pyroelectric effect, leading to very interesting phenomena. Then, being a polar LC molecule, it undergoes a force due to the existing fields, able to move the material. Results show that fragmented droplets coalesce to form bigger droplets in fixed locations, on a longer time scale. In some cases, one single drop can be observed onto each hexagonal domain. These drops behave as microlenses and the whole sample could be viewed as a dynamical optical micro-element able to switch from a diffuser state (fragmentation state) to a microlens array (coalescence state), without the need of an external voltage. Moreover, the birefringent properties of liquid crystals can make such microlenses also tunable.

Manipulating liquid crystals by pyroelectric effect

Liquid crystals (LCs) are substances that flow like liquids but have molecules ordered in a crystal-like way. These properties allow LCs to organize themselves in mesophases, states where they present features of both liquid and solid states, under particular conditions. This peculiarity has allowed these materials to find wide applicability in many fields, from display systems to optics and photonics.1, 2 An attractive property of LCs is the ability to modulate their optical properties using electric, optical, or magnetic fields. At present, different techniques exist for manipulating LCs that have found applications in various areas. For example, researchers have extensively studied LC tunable lenses and have designed and experimentally demonstrated many successful configurations.3 Other groups have proposed optical microresonators (light-trapping microspheres that have applications in laser sources)4 and LC-based optical devices such as electrooptic switches and beam scanners.5 Nevertheless, faster and more versatile approaches to modulate the properties of LCs are desirable, particularly in emerging fields of technology. The selective patterning of LCs into optical devices is often achieved by ink-jet printing approaches where a nozzle is scanned onto the target support for precise delivery of LC droplets. However, these techniques make use of expensive systems based on external voltage generators and, due to the scanning-mode operation, are relatively time-consuming. We found that the spatial self-assembly of LC droplets can be an efficient alternative to these techniques since, in particular conditions, the droplets can be driven to desired locations following electric-field lines (see Figure 1). Our approach is easier to accomplish than the ink-jet printing methods, even over relatively large areas, thus providing a cost-effective and rapid manipulation of LC droplets. Figure 1. Liquid-crystal (LC) droplets aligned onto a lithium niobate (LN) substrate following electric-field lines that were pyroelectrically generated. Droplet dimensions are of the order of microns or submicrons.