Search

We show here that light polarization of a beam propagating through a heliconical cholesteric cell can be controlled by tuning the Bragg resonance of the structure. We demonstrate that this control is achieved by varying either the low-frequency electric field or the intensity of a pump beam impinging on the sample. The study confirms the recently reported phenomenon of optical tuning of the heliconical cholesterics and opens the door for the development of simple and efficient polarization modulators controlled electrically or optically.

We report the realization and characterization of porous nanostructures where a periodic refractive index modulation is achieved by stacking layers with different nano-architectures. One multilayer photonic crystal has been fabricated starting from colloidal dispersion of silicon dioxide and zirconium dioxide using spin coating technique. Improved efficiency of Bragg reflectivity (up to 85%) has been obtained by a new bottom-up fabrication technique of photonic hierarchical nanostructures based on self-assembly from the gas-phase at low temperature whit a very thin (≈ 1 μm) photonic crystal devices. Due to the high porosity, these systems can be infiltrated with nematic liquid crystals leading to tuning of the Bragg reflection band by applying low voltages to the structure.

We report on investigation of a light-induced anchoring of dye-doped liquid crystal(LC)in nematic phase. Contributions of light-induced dye adsorption and desorption processes as well as of the bulk-induced reorientation effect were separated by choosing specific geometries of the light irradiation. We propose a model that explains peculiarities of light-induced easy axis producing in nematic phase which differs from one in isotropic phase due to presence of anisotropic dark-adsorbed layer and effective bulk-induced director reorientation.

In this letter we demonstrate that the measurement of the reflectivity of a hybrid aligned nematic sample vs. the incidence angle give us detailed information on the local orientation of the molecular director, when the condition of total internal reflection is achieved inside the sample. Moreover theoretical fit of the experimental data shows that the method is suitable to obtain the tilt angles on the boundary walls with a sensitivity < 3-degrees.

In this article, the thermo-optical properties of polymer dispersed liquid crystals (PDLCs) in confined geometry are experimentally investigated to demonstrate the feasibility of a fiber optic sensing device based on PDLCs. Since an unexpected behavior of PDLCs in confined geometry has been experienced, a systematic study of PDLCs’ thermo-optical properties in bulk is presented also to point out principal differences. Finally, bistable all fiber optic temperature sensors, in which a PDLC permits at the same time the opto-mechanical interconnection of two fibers and the modulation of light crossing the device, has been realized and characterized, for the first time. Being the modulation controlled by external temperature, the device has been proven to be suitable for the realization of a heat flow sensor. The sensor presents the typical advantages of both fiber optic sensors and liquid crystal technology. Moreover, due to its small thermal capacity, it should exhibit little influence on thermal equilibrium...

X-ray microdiffraction (μ-XRD) is used for the first time to probe the liquid crystal (LC) ordering in confined LCs. We report on the direct observation of the director field in single droplets of nematic LCs embedded in a solid polymeric matrix (polymer-dispersed LCs). The bipolar configuration is identified in droplets with radius R ≤ 1 μm. Both the director distribution function and the droplet order parameter are obtained from diffraction patterns and the results are in excellent agreement with theoretical calculations. This technique is unique in its application to dispersed mesophases in that it allows to probe the local (μm length scale) LC ordering. The possibility of focusing measurements on single droplets eliminates the problems connected with averaging, which are typical of other techniques. Our results prove that μ-XRD is a novel and valuable tool in the study of confined LCs and open new exciting opportunities for future investigations.

Among the different methods used to prepare polymer dispersed liquid crystals (PDLCs), polymerization induced phase separation can be successfully exploited to obtain optical recording of high resolution holographic gratings and binary images in these materials. In this paper we report a new method that allows hidden images to be obtained in PDLCs that are not detectable by light in the visible range. The possibility of storing invisible images during the curing process will be described and discussed. The binary images obtained can be detected by illuminating them with low power UV radiation, thus opening the way to interesting applications in the field of optical storage of reserved information.

A detailed study of the light-induced gratings in Methyl-Red-doped 4-n-pentyl-4′-cyanobiphenyl (5CB) homeotropically aligned is reported. In particular, the dependence of diffraction efficiency upon the pump and probe polarizations has been investigated both for transient and permanent gratings. Our results confirm the earlier observations on the role of light-induced adsorption of MR molecules on the cell boundaries and demonstrate the common origin of the supranonlinear response and the permanent recording effect.

Pump-probe and TIR experiments have been performed on the dye-doped nematic liquid crystal 5CB, in order to study the basic mechanisms of the extraordinarily large nonlinear response recently reported. Experimental data show that light-induced modifications of the anchoring conditions can be the origin of the observed effect. The bulk reorientation due to the collective elastic behaviour of the liquid crystal gives then rise to the nonlinearity, which occurs without a direct optical torque on the molecular director in the bulk. We have called this effect SINE (Surface Induced Nonlinear Effect).

Medium size HPC clusters play an important role in the HPC landscape in that they provide both the training environment for system scalability and a flexible production field for a large class of numerical problems. In this poster we present CRESCO4, the latest medium size HPC cluster purchased by ENEA, in operation since few months. CRESCO4 is part of a family of HPC systems, all integrated within ENEAGRID, a large infrastructure for cloud computing, which includes all the computational facilities installed at several ENEA sites in Italy.

Microbial degradation rates in the subsurface are not only limited by the physiological capacity of the organisms, but also by inefficient supply of nutrients to the microbes. Although mass transfer limitation of biodegradation in the subsurface has been postulated for years, experimental evidence is still scarce. In the column experiments described here, diffusive transport of 4-nitroanisole from the bulk solution to cells of Rhodococcus opacus strain AS2 immobilized on glass beads or sand appeared to be responsible for the slow transformation rates observed. Assuming steady state, we applied a coupled transformation/transport equation to these data (Best equation) and apparent bead-related mass transfer coefficients were found to increase in proportion to the surface area covered with bacteria. This implies that mass transfer coefficients for individual cells remained constant. In an idealized oligotrophic environment where cells are only loosely clustered and do not shield each other, we would therefore expect biodegradation rates to be independent from the longitudinal distribution of the total biomass along a given flow path. Moreover, apparent mass transfer coefficients increased with the grain size of the column fillings, but did not change upon varying the flow rate. With a limiting external transport step, overall transformation fluxes do not become saturated at concentrations as low as predicted for Michaelis-Menten-type kinetics. Mass transfer limitation thus offers a justification for the common assumption that biodegradation rates in the subsurface follow first order kinetics in a wide concentration range.

As it is well known, the final morphology of PDLCs samples depends on a number of parameters among which light intensity, curing time and temperature are probably the most important once one has choose a particular mixture to cure. In this work, we show the influence of these different external curing parameters on the film morphology of PDLCs obtained by strong UV laser irradiation of the homogeneous prepolymer-liquid crystal mixture. In particular, we show that some of these parameters can affect the morphological uniformity of the samples and can give rise to a highly interconnected‘channel’ pattern, while the classic‘droplets’ morphology is almost completely lost.

We have examined the morphology of UV cured PDLCs made by the standard mixture NOA65/E7, for different liquid crystal content, UV dose and intensity and exposure time. The distribution function for the liquid crystal droplets size has been determined in order to provide much understanding in the UV light-induced phase separation process.

The need to understand important factors affecting the spread of bacteria in groundwater aquifers is evident for fields as diverse as drinking water safety or environmental engineering concerned with bioremediation of polluted sites. For example, increasing concentrations of dissolved minerals tend to increase the deposition efficiency of bacteria in porous media. As bacteria and mineral surfaces are mostly negatively charged, this is generally assumed to be a consequence of the higher ionic strength, which leads to stronger shielding of the surface charges by the counterion-cloud in solution. We found Mg2+ to enhance deposition of Pseudomonas sp. strain B13 in sand columns with respect to a solution of identical ionic strength containing Na+. Hence bivalent cations are likely to affect microbial deposition specifically, for example due to specific binding to the cell surface. Moreover, low concentrations of Pb2+ or Cu2+ reverted the surface potential of strain B13, thus providing additional evidence for ...

Attraverso alcuni esempi (Codro, Bocchi, Adrovandi, Mrsili), il contributo propone il tema del viaggio e tra XV e XVIII secolo nella cultura filosofica e scientifica bolognese

We studied the effective interaction of the angular momentum of light with a liquid crystal (LC) doped with azo-dye. The experiments were carried out in a combined cell with one substrate providing strong unidirectional planar alignment of LC and the other covered by iso-tropic polymer layer providing a degenerated planar alignment. The effect manifests itself as azimuth reorientation of the liquid crystal director under the action of circularly polarized light in the absorption band of azo-dye. The new orientation of LC was captured by the command surface because of a light-induced alignment memory effect. To describe the experimental dependencies we solved the self-consistent problem of the propagation of a circularly polarized light in the absorbing nonlinear media using the geometric optics approximation. Comparing the experimental data with numerical evaluation we obtain the basic parameters of the system: the orientation nonlinearly coefficient and the value of anchoring of the director wit...

We present Raman studies on various substituted polyvinylcinnamates, [-CH2CH(O2CCH-CHC6H5)-]n, which are well known for undergoing cycloaddition when exposed to UV radiation. Our comparative studies on these unphototransformed polymers show that the substitutions on the aryl cycle play a fundamental role in the vibrational dynamics of these molecules. We have been able to identify the characteristic modes of each type of fluoro-substitution (mono, para and ortho), and as a result, it is possible to determine whether the two isomers and two rotating conformers coexist in the samples. In order to proceed to further investigations on the phototransformation process, we have determined how the C-C and C-O double-bond stretching modes behave versus the type of substitution.

Travel distances of bacteria in groundwater aquifers often exceed predictions based on filtration theories. These findings have mostly been ascribed to structural heteroge-neities in the subsurface, but variations in the adhesive properties within the microbial populations have been observed too. In laboratory experiments with Pseudomonas sp. strain B13, we found that only a fraction of the cells was efficiently deposited in sand columns while the remainder passed a second column identical to the first without hindrance. Upon rinsing the columns with deionized water, between 10 and 35% of the deposited cells were flushed out, thus showing that increased electrostatic repulsion between sand and bacteria partially reverted the deposition. Lipopolysaccharides (LPS) extending from the cell surface into the medium as well as estimated DLVO-type interaction energy curves indicate that cells were trapped at a distance of more than 20 nm from the sand surface. We hypothesize that differences in the LPS coat were responsible forthe fractionation of the bacterial population. Our results indicate that the high travel distances of microorganisms might be due not only to the complex structure of aquifer material but also to heterogeneity in the adhesion properties within the bacterial populations.

In this paper the 'unconventional' optical non-linearities of liquid crystals are considered. The basic ideas for the interpretation of collective thermal origin are reviewed. After reporting the which are the major challenges for scentists in this field are recalled. reorientation and of other phenomena of main results of this approach, the issues

We report first measurements of optical second harmonic generation in filled nematics. They were performed in order to clarify open questions about the anchoring conditions in this material, which consists of low portions of pyrogenic silica dispersed in a nematic liquid crystal. We compare the SHG behaviour with that of Polymer Dispersed Liquid Crystals and show that in FN two contributions to SHG with different origins have to be considered.

Using an Attenuated Total Internal Reflection (A.T.I.R.) technique which is herein fully detailed, we have determined the director distribution in some hybrid nematic films at the vicinity of the interface where the nematic is homeotropically aligned. This latter alignment is achieved using two different surfactants: either n-Hexadecyl-Trimethyl-Ammonium-Bromid or lecithin. These experimental measurements are confronted with the results obtained from the elastic theory: a discrepancy close to the interface is observed for the films treated with the lecithin. An exponentially decaying correction to the elastic theory is found to fit the experimental datas.

We report lasing from a simple wafer structure made of a thin layer of rhodamine 6G as active material and of a switchable holographic mirror, based on a reflection grating made of polymer dispersed liquid crystals. It is shown that switching-off the mirror reflectivity by application of a suitable voltage allows wavelength flipping of the laser emission by 10 nm.

We report a study of the temperature dependence of the optical phase shift induced by a polymer dispersed liquid crystal subjected to a low frequency electric field. In the framework of a recently introduced model we point out the role of two parameters strictly depending on the PDLC configuration: an “effective elastic constant” K d and a droplet “voltage-constant” V d. We show that a suitable temperature dependence of such parameters is able to give a good agreement between experimental data and theoretical values.

Three different results on optically distorded nematic films are presented. They have been derived from T.I.R. experiments and graphical discussion on the observed far field diffraction pattern. First, the optically induced director reorientation profile (radial dependence) in a nematic film can be much more larger than the pump beam profile. Second, a maximum reorientation tilt angle as large as 80 degrees can be obtained and third, the same sample can be focusing, diffracting and defocusing depending on the pump power. The later is a pure thermal effect and occurs as the boundary conditions are very weak.

Self-transparency due to thermal nonlinearities is presented as basic switching effect in polymer dispersed liquid crystals. The main consequences of this phenomenon on wave mixing are opto-optical switch and self-diffraction with threshold, How this threshold behavior can be exploited to drive self-diffraction by an external voltage applied to the sample, coupling in an original way thermal nonlinearities of the liquid crystal with its orientational properties is explained here. The possibility of obtaining a transient amplitude grating in these materials is also discussed.

According to the model of optical reorientation of the molecular director, we report a study of the effect of temperature variations on the nonlinear optical behavior of a homeotropic liquid crystal cell. We show that the model is in good agreement with our experimental data obtained for the K18 nematic liquid crystal. By numerical simulation we also show what is the role played by the different material parameters in determining the temperature behavior

We discuss the dynamics of the light induced nematicisotropic transition underlying the role that can be plaied by nuclei of critical size. It is found that critical time for droplet formation can have a very rare dependence on the overheating temperature which can explain the self-pulsing phenomenon recently observed at a glassliquid crystal interface.

We report detailed measurements performed on a sample of a dye doped Polymer Dispersed Liquid Crystal (PDLC). It is shown that the laser induced phase transition of the liquid crystal droplets to the isotropic state can be exploited to get an optical switching and an optical bistable behavior. These effects can be satisfactory explained within the theoretical framework of an absorption which increases with the degree of excitation of the material.

In a twisted nematic liquid crystal cell we report and study the self-transparency effect induced by the creation of an isotropic droplet. A simple theoretical model is presented and good agreement with experimental data is found. In a two beams configuration transistor-like behavior is observed and theoretically discussed.

We report the observation and study of optical bistability in the power transmitted by a twisted nematic liquid crystal bounded by crossed dichroic polarizers. A detailed study of the dependence of the phenomenon on the sample initial temperature has been performed. The effect is a good test for a simple theoretical model which describes the isotropic droplet formation in a nematic sample under laser illumination. A good agreement is found between experimental results and theoretical predictions. Applications of the effect to realize temperature sensors are envisaged.

We report on the possibility of observing the amazing ring pattern typical of self phase modulation (SPM) in dye doped liquid crystals with quite low impinging intensity. The rings formation is controlled by an external dc field applied perpendicular to the cell substrates.

We present a new computational method which allows to work out the director orientation in each location of a nematic liquid-crystal cell from a measurement of its reflectivity. We show how it is possible to extract this information from the fringe pattern due to the onset of total internal reflection inside the sample. The very good agreement with the expected results makes the method suitable to study several kinds of deformations occurring in liquid crystals.

We report the observation of orientational gratings induced by the photorefractive effect in dye-doped polymer-dispersed liquid crystals. The photorefractive origin of the grating-induction effect is supported by the results of two-beam coupling experiments and by the possibility of erasing the grating by uniform illumination. For all the samples a stable memory effect was observed.

Light-induced director reorientation in dye-doped nematic liquid crystals was recently reported to be an efficient method of writing permanent holographic gratings with high sensitivity [Phys. Rev. Lett. 82, 1855 (1999)]. We report the achievement of stable director reorientation in the same materials by means of a single 4-ns pulse of the second harmonic of a Nd:YAG laser. Fast recording of high-resolution holographic gratings (more than 500 lines/mm) can be obtained with an energy density as low as 7x10(-3)J /cm(2) .

Objetivou-se com este trabalho verificar a quebra de dormência, utilizando ácido sulfúrico (H2SO4) e escarificação mecânica, e o efeito da luz na germinação das espécies Euphorbia heterophylla, Ipomoea nil, Sida glaziovii e Brachiaria plantaginea. Para avaliar a germinação, os tratamentos foram realizados no escuro e no claro. Não foram constatadas diferenças entre eles nas condições avaliadas, indicando que as espécies em estudo germinam em ambiente com diferentes luminosidades. Quanto a quebra de dormência, observou-se que o tratamento submetido à quebra de dormência aumentou a porcentagem de germinação em algumas espécies. Desse modo, concluiu-se que a quebra de dormência viabiliza condições para germinação de algumas espécies de plantas daninhas e, sob qualquer condição de luminosidade, há germinação das sementes das espécies estudadas. This research evaluated dormancy break using sulfuric acid (H2SO4), mechanical scarification and the effect of light on the seed germination rate of Euphorbia heterophylla, Ipomoea nil, Sida glaziovii and Brachiaria plantaginea. To evaluate the germination rate, treatments were performed in the dark and light; no differences were observed between both treatments under these conditions, indicating that these species germinate under different light conditions. Dormancy break increases the percentage of germination rate in some species. Thus, dormancy break makes the germination of some species viable. The germination can occur both under light or dark conditions.

Nas áreas de colheita de cana-de-açúcar sem queima prévia (cana-crua), a presença da palha afeta a germinação de plantas daninhas e a dinâmica dos herbicidas. O presente trabalho foi conduzido com o objetivo de avaliar os efeitos do sulfentrazone e imazapic, associado à palha de cana-de-açúcar, com chuva após a aplicação dos herbicidas, no controle de Cyperus rotundus. O delineamento experimental adotado foi o de blocos casualizados no esquema fatorial 3 x 3 x 2 x 2, com quatro repetições, totalizando 144 parcelas. Os tratamentos foram conseqüência da combinação fatorial entre três níveis do fator palha de cana-de-açúcar (20, 10 t ha-1 e ausência de cobertura morta sobre o solo); três níveis do fator herbicida (sulfentrazone a 800 g ha-1, imazapic a 147 g ha-1 e ausência de herbicida); dois níveis do fator intensidade de chuva (10 e 20 mm); e dois níveis do fator épocas em que foram simuladas as chuvas após a aplicação dos herbicidas (24 e 168 horas). Avaliou-se o número de plantas por vaso, a massa seca da parte aérea e das estruturas do sistema subterrâneo e o número de tubérculos e bulbos sadios. Constatou-se que, no caso do herbicida sulfentrazone, a presença de 20 t ha-1 de palha de cana-de-açúcar diminuiu-lhe a eficácia. O herbicida imazapic teve bom desempenho tanto na ausência quanto na presença de palha e causou redução das variáveis estudadas, independentemente da intensidade de chuva. A intensidade de chuva de 10 mm não foi suficiente para transpor o herbicida sulfentrazone na quantidade de 20 t ha-1 de palha. Já a intensidade de 20 mm foi suficiente para lixiviar o herbicida até mesmo na maior quantidade de palha. O herbicida sulfentrazone suportou mais a permanência na palha, pois apresentou melhor eficácia em relação ao imazapic, quando a chuva foi simulada 168 horas após a aplicação dos herbicidas.

We report an extended investigation performed on novel polymeric materials aimed at finding the most suitable one to be used as the recording medium for microholographic data storage. Different experimental setups developed at Politechnic University of Marche (UPM) and at Technical University Berlin (TUB) have been used to this aim.

In this communication we report lasing effects from three plastic structures operating in different geometric configurations. The three devices are based on high resolution gratings recorded in a photosensitive organic mixture by means of holographic techniques. The first device has been realized integrating the holographic mirror in a layered structure which consists of two separate regions. The first region contains a dye as active material, the second one the holographic mirror. The third configuration consists of a standard distributed feedback (DFB) arrangement in which the active material is placed directly in contact with the periodic structure