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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.

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...

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).

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 ...

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.

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.

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 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.

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.

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.

We report the observation of a bistable behavior from dye‐doped polymer dispersed liquid crystals. The interpretation of the phenomenon is given in the theoretical framework of an absorption which increases with the excitation of the material.

Among the different methods used to prepare Polymer Dispersed Liquid Crystals (PDLC), the Polymerisation Induced Phase Separation technique (PIPS) can be exploited to obtain optical recording in these materials. The possibility of performing optical storage of binary images in PDLC during the curing process and post curing holographic recording, will be described and discussed.

We Demonstrate that is possible to write and erase holographic gratings in dye-doped polymer dispersed liquid crystals (PDLC). PDLC samples were irradiated after complete curing using two beams in an interferometric geometry. In this way permanent holographic gratings were Mitten with a resolution up to 100 lines/mm. The erasure was obtained by a single beam, while successive writing were possible again by two beams interference, without any degradation after many cycles. The origin of the effect can be the onset of periodic distribution of droplets of different shape and spatial diffusion of the dye molecules.

We present measurements of picosecond laser pulses performed by exploiting the “self-diffraction” effect in two kinds of liquid crystalline materials: nematic liquid crystals (NLC) and polymer dispersed liquid crystal (PDLC). Results are in good agreement with those obtained by standard second harmonic generation techniques. The method exhibits several advantages, and the use of PDLC samples, which enable low noise measurements, makes the “self-diffraction” technique competitive with the standard ones.

An extended investigation of the phase separation process in polymer dispersed liquid crystals is presented. A detailed analysis of the experimental results shows that the nucleation and growth of the liquid crystal droplets from the polymeric matrix can be described as the coarsening process occurring in binary alloys.

We present the results of the first investigation of the excited vibronic states of the F-center by a 2-photon technique. The nonlinear induced fluorescence and the nonlinear photoconductivity are detected allowing new information on the structure of the crystal.

Clonal derivatives 8 and 11 of the T47D human breast carcinoma cell line release particles that have the biochemical characteristics of a retrovirus. Particles recovered from cultures of [3H]uridine-labeled clone 11 had a density of 1.18 g/ml and contained 60-70S and 35S RNAs associated with reverse transcriptase activity. The production of these particles was steroid-dependent. Clone 8 particles had a higher density, 1.195 g/ml, and their production was independent of steroid hormone. By RIA, antigens crossreactive with the 52,000-dalton envelope glycoprotein gp52, the major external protein of mouse mammary tumor virus, were found associated with these particles and in the media. Most of the gp52-related antigen was in soluble form, but it was enriched in the particle preparation. A lesser amount of antigen was distributed within the cultured cells. Absorption of rabbit antibody to gp52 with clone 11 particle preparations eliminated the ability of this antibody to detect immunocytochemically a crossreactive antigen previously localized in tissue sections of human breast carcinoma. These results indicate that the particle isolates from T47D contain the same gp52-related antigen found in human breast carcinomas and constitute an excellent source for the purification and characterization of this antigen.

Emission of physisorbed and chemisorbed oxygen, when stainless steel is irradiated by a high power neodymium-yttrium aluminium garnet laser in an ultrahigh vacuum system, is observed using a quadrupole mass spectrometer. Auger electron spectroscopy is used to study the laser-annealed sample after the laser shots. The results show that an etching of the target, similar to that achieved by ion gun sputtering, occurs.

The forward scattering process of motional narrowing in hydrogen gas has been investigated by an original method of nl spectroscopy of general application. The method has allowed a detailed study of the dispersion of the nl susceptibility χ(3)(ω) showing an unexpected behavior of χ(3)(ω)' near the Q01 (1) molecular eigenfrequency.

We have studied the morphology and electro-optical properties of polymer dispersed liquid crystals prepared by polymerization-induced phase separation (PIPS) under high intensity UV laser curing. The results have shown that localized high intensity curing generates a gradient of the droplet size distribution outside the directly irradiated area as a consequence of the non-uniform spatial distribution of the scattered intensity. Such a distribution of the droplet size reflects in a peculiar electro-optical behaviour that could find potential application in optical devices. Both shape and spatial extent of the droplet size gradient are affected by the spatial velocity of polymerization which in turn strongly depends on the intensity of the curing laser beam. This dependence has also been experimentally investigated.

Results are reported on the wave mixing characterisation of dye-doped dispersion of liquid crystalline droplets in a polymer matrix (PDLC) by means two beam coupling experiments and experimental observation of photoinduced DC current, which arise in these materials under cw Laser-beam illumination. We determine the TBC gain by means of the asymmetric energy transfer measurements and the amplitude and The phase shift of the phase grating and amplitude grating by means the TBC translation technique measurements. The experimental results of this first characterisation reveal the nonlocality of the effect: the gratings obtained are essentially the phase gratings spatially shifted of pi /2 respect to the interference pattern. The photoinduced currents reveal transient behaviour, i.e. a relaxation to zero value after an initial increase. A qualitative discussion of the obtained results is reported. The observation and the results of the experiments evidence the photorefractive nature of the light-induced diffraction gratings, which were earlier observed in these materials.

We have found that producing a light-induced easy axis in a liquid crystal cell doped with methyl red can result in surface director reorientation both toward and outward the polarization of the incident light. This pointed out two different mechanisms of light-induced anchoring in the system. We proposed that these mechanisms are light-induced anisotropy in the adsorbed MR-layer and light-induced adsorption of MR molecules on the substrate. The study of light-induced anchoring in the isotropic phase showed that the mechanism of light-induced anisotropy in adsorbed MR-layer prevails at small light intensities, whereas the mechanisms of light-induced adsorption dominates at high intensities.

We report an accurate analysis of an optical storage effect observed in dyedoped PDLC films. Permanent diffraction gratings are produced using standard holographic techniques with different kind of PDLC samples. For all these samples the polymerisation process is completed before the writing procedure. The analysis of the experimental results brought us to distinguish the role of the PDLC components in the observed phenomenon and propose some hypothesis about the possible physical mechanisms at the basis of the storage effect.

We report a detailed study of the optical phase shift induced by a polymer-dispersed liquid crystal subjected to a low-frequency electric field. A relationship between the optical phase shift and the droplet and sample order parameters S(d) and S(f) is worked out in order to explain the experimental data obtained using an ellipsometric technique. The measurements taken for several incidence angles are in very good agreement with the expected behavior if an increase of the droplet order parameter is also considered when increasing the field. The method appears suitable to study reorientation phenomena in polymer-dispersed liquid crystals overcoming the problems due to light scattering in these materials.

In this paper we report computer calculations of the reflectivity of a hybrid nematic cell near the condition of total incernal reflection. The Berreman matrices method allows us to drop the approximations used in previous papers. In this way we get details about the interference pattern arising when the total internal reflection occurs inside the sample. We show how the reflectivity curve is affected by the director orientation which in a hybrid cell is essentially due to the boundary conditions. Our results suggest that measurements of this reflection pattern can give detailed information about the local director orientation.

In recent papers we have studied the optical phase shift induced by a Polymer Dispersed Liquid Crystal (PDLC) sample subjected to a low frequency electric field. In the framework of the droplet model for the PDLC we have shown that the description of the sample by means of three (molecular, droplet and sample) order parameters and two (a surface elastic constant and a characteristic voltage constant) sample related parameters is able to give a good agreement with experimental data. In this paper we present further ap plications of the model showing how applied voltages controls light scattering.