M. P. De Santo

Light Control of Cholesteric Liquid Crystals Using Azoxy-Based Host Materials

The characteristics of cholesteric liquid crystals can be controlled by light irradiation if conformationally photo-active molecules are present. Recently, control of the selective reflection band (spiral pitch) in nemato-chiral mixtures was demonstrated when photosensitive molecules, namely nematic azoxy-based compounds, were used as the host material. In this report, the investigation of light induced effects in cholesterics with azoxy-based host materials is continued to highlight the mechanisms of the response. Different non-photosensitive chiral materials were added to different azoxy-nematic liquid crystals and the pitch change caused by UV irradiation was investigated. A change in the pitch of 50–210 nm was observed depending on the exposure time and the intensity of the light. This effect is reversible: under illumination at wavelengths greater than 410 nm, the pitch shifts in the opposite direction. The dependence of the selective reflection band and the full-width-at-half-maximum of the band on the exposure time and the temperature dependence of the selective reflection band were investigated. The lowering of the phase transition temperature and narrowing of the width of the selective reflection band can be explained by a decrease in the orientational order parameter. The blue shift of the selective reflection band is due to a decrease in both the order parameter and the concentration of linear nematogenic molecules.

Wide band gap materials as a new tuning strategy for dye doped cholesteric liquid crystals laser

A new tuning strategy for mirror-less liquid crystals laser is presented. A three layer cell is prepared with two cholesteric layers sandwiching a layer containing an isotropic mixture of a photoluminescent dye. One of the chiral layers contains a wide band gap material while the second layer consists of a series of small band gap materials. Through the combination of these two layers, a set of mirrors that can selectively reflect different wavelengths is obtained. A different laser wavelength is emitted from different regions of the cell under the pumping beam irradiation.

Rubbing-induced charge domains observed by electrostatic force microscopy: effect on liquid crystal alignment

The rubbing of polymer-coated substrates is one of the most frequently used techniques for liquid crystal alignment. However, the aligning mechanisms are not completely understood. The influence of friction charges induced by the rubbing process has been taken into account in theoretical publications. In this work we investigate the rubbing-induced charge domains of three polymers (PMMA, PI and PVA) with the electrostatic force microscopy technique, which allows the simultaneous determination of the surface topography and electrostatic potential. We observed a large intensity of the potential for the PMMA substrate, whereas no charge domains were observed for PVA. In addition, we followed the time evolution of the surface charge domains, and surprisingly after five days the charges were still present with a small reduction of intesity. Using polarizing optical microscopy we studied the influence of the charge domains on the LC aligning properties.

Domain orientation in ultrathin (Ba,Sr)TiO3 films measured by optical second harmonic generation

The analysis of polarization diagrams for specular and scattered second harmonic generation (SHG) was used for the structural characterization of submicron domain structures of thin (Ba,Sr)TiO3 (BST) films. It is shown that the lack of separation of these two contributions may lead to completely wrong conclusions about the domain orientation in these films. SHG studies of the thickness dependence of domain fractions (including 180° domains) reveal the presence of ferroelectric domains in ultrathin BST films (6 nm), although no domain structure was observed by atomic force microscopy. Thus the presence of ferroelectric ordering was demonstrated in perovskite films with a thickness down to 6 nm.

Switching of a ferroelectric polymer Langmuir–Blodgett film studied by electrostatic force microscopy

Ultrathin ferroelectric Langmuir–Blodgett films are studied using an electrostatic force microscope (EFM). The films were deposited by a subsequent transfer of monolayers of a ferroelectric polymer from the water surface onto Si substrates covered with Al electrodes. A film was switched locally by a voltage applied between a tip and the Al electrode. The dynamics of the electric field relief above a switched ferroelectric domain is studied by EFM imaging with simultaneous control of the topographic relief. In order to explain an image contrast a simple model for a thin ferroelectric film is discussed which takes into account the remanent polarization of the film, the surface charge, and interface work functions.Ultrathin ferroelectric Langmuir–Blodgett films are studied using an electrostatic force microscope (EFM). The films were deposited by a subsequent transfer of monolayers of a ferroelectric polymer from the water surface onto Si substrates covered with Al electrodes. A film was switched locally by a voltage applied between a tip and the Al electrode. The dynamics of the electric field relief above a switched ferroelectric domain is studied by EFM imaging with simultaneous control of the topographic relief. In order to explain an image contrast a simple model for a thin ferroelectric film is discussed which takes into account the remanent polarization of the film, the surface charge, and interface work functions.

Crystallization of PZT in Porous Alumina Membrane Channels

Composite Pb(Zr,Ti)O 3 (PZT)/alumina nanostructures are fabricated using 2-dimensi- onal porous alumina as a template. Details of pores filling as well as of crystallization are studied by atomic force microscopy and transmission electron microscopy. The presence of a ferroelectric phase is confirmed by optical second harmonic generation experiments.

Morphological and electrical investigations of lead zirconium titanate thin films obtained by sol-gel synthesis on indium tin oxide electrodes

In recent years, interest in PbZr0.53Ti0.47O3 (PZT) films has been rapidly increasing due to their technological applications as ferroelectric materials. In the present work, PZT films are obtained by sol-gel synthesis and deposited by spin coating on a transparent conductor substrate, with a perspective application as rectifying layers in asymmetric nematic liquid crystal cells. An extensive investigation is carried out on the effects of different annealing temperatures, with regard to the film texture and to the electric polarization properties, by using low vacuum scanning electron microscopy, atomic force microscopy, and electrostatic force microscopy. It has been observed that PZT domains self-organize into flower-like dendritic structures with a “rosetta” shape at the early stage of crystallization, occurring for annealing temperatures higher than 600 °C; the dimensions of such structures increase versus the annealing temperature. The ferroelectric properties of the PZT films have been related to th...

Polar diffraction gratings made by spatially periodic photopoling Langmuir–Blodgett films

Polar diffraction gratings have been prepared by spatially periodic photopoling Langmuir–Blodgett films consisting of azobenzene compounds. The gratings were made by film irradiation with two left circularly polarized Ar-ion laser beams under strong electric field from a corona discharge. Therefore, simultaneously with the spatial modulation of the linear refractive index, the local polar order of the film becomes periodically modulated (through symmetry, the latter is related to the modulation of second-order optical nonlinearity χ(2)). The local polarity is shown by an electrostatic force microscopy technique: an image of the polar grating is recorded without any trace of periodicity in the film topography.

Electrically Controlled 1D and 2D Cholesteric Liquid Crystal Gratings

Electrically induced and controlled gratings in large pitch cholesteric liquid crystals were investigated. Specially selected surface anchoring conditions together with well defined properties of cholesterics allow the formation of a stable periodic structure in the presence of a suitable external electric field. Polarization properties of first diffraction orders have been investigated as a function of the angle between the laser linear polarization and the grating direction. We propose to use two 1D gratings to create a 2D electrically controllable diffraction pattern, suitable for application in 3D optical devices.

Liquid crystal bubbles forming a tunable micro-lenses array

Cholesteric liquid crystals with long pitch confined in homeotropic cells can be used to generate stable but distorted and localized liquid crystal structures exhibiting spherulitic textures, known as “bubbles.” As these bubbles can be induced by an external electric field with a narrow range following the confinement ratio C=d/p ≈1 (d representing cell thickness and p representing cholesteric pitch), it is possible to obtain electrically controlled micro-lenses. Here we investigated the optical and electro-optical properties of such liquid crystal bubbles for creating an array of micro-lenses with electrically tunable focal length.