Holographic techniques allow the recording of homogeneous, high resolution, large-area light intensity patterns in photo
sensitive organic materials. The choice of proper experimental conditions easily permits the fabrication of 1D, 2D and
3D periodic and quasi-periodic structures. In this work we present a simple way to fabricate planar complex photonic
structures characterized by high dielectric contrast values. To this purpose we used polymer dispersed liquid crystals as
photosensitive material for the holographic recording, followed by the removal of the liquid crystal from the recorded
structures via a specific solvent, thus obtaining large area regular polymer-air patterns. The obtained structures have been
simulated, recorded in the substrates and optically characterized in planar light guided configurations. The relevant
optical properties have been analyzed by means of a theoretical approach formally derived from the dynamical theory of
x-ray diffraction. The presented experimental technique allows easy fabrication of optical integrated devices to be used
either as high sensitivity sensors or in the field of optical telecommunications.
A review of the recent results of our group in the field of light-induced anchoring and reorientation effects in dye-doped liquid crystals (LCs) is presented. In particular, the phenomena of photoinduced anchoring and permanent reorientation over a polymeric boundary surface of a dye-doped LC cell is reported, both in the isotropic phase and in the orientationally ordered nematic phase. The results have been interpreted microscopically in terms of adsorption and desorption of the dichroic azo-dye (methyl-red) molecules onto the illuminated surface during light irradiation. The model proposed is in agreement with recent results on the dynamic and stable grating formation in methyl-red doped LCs .
We show that light-induced modification of the anchoring conditions can lead to an extraordinarily large optical response in dye-doped nematic liquid crystals. The bulk reorientation due to the collective elastic behavior of the liquid crystal is the origin of the nonlinearity, which occurs without a direct optical torque on the molecular director in the bulk, We call this effect SINE (Surface Induced Nonlinear Effect). These results can also explain the origin of the supra-nonlinear behavior recently observed in the same composites.
We present a new type of effective photoregulation of the LC orientation on an aligning surface through light action upon the liquid crystal bulk. The illumination of an azo- containing LC bulk with polarized light is found to induce both stable high-resolution alignment onto the rigid surface and dynamic sliding of the director over the surface. Typical times of the director sliding over the aligning surface are of the order of 1 ms. Both effects are characterized by extremely small light intensities and seem to be very promising for the development of optical devices for information processing and high density information storage. Light-induced surface memory effects are also observed in these system as a consequence of induced anchoring of the molecular director. This technique has been applied to record both stationary and dynamic holograms with a spatial resolution of 7 micrometers . The surface memory effects have been applied to record both binary and gray scale images. As compared to other methods used in these materials the highest sensitivity and spatial resolution can be achieved: energy densities as low as 10-1J/cm2 were sufficient to write gratings with a resolution better than 100 lines/mm.
An x-ray investigation has been performed on five new 2,5- disubstituted 1,2,4-oxadiazoles with reversed structures containing 4-amylbiphenyl-, trans-4-amylcyclohexylphenyl- and 2-halophenyl- as substituents. The aim was to confirm the data about the phase sequence, obtained via polarizing microscopy and differential scanning calorimetry, and to measure the structural parameters. Through x-ray diffraction technique, all substances showed only the nematic mesophase. Moreover, it was proved that some enthalphy variations previously attributed to crystal-crystal transitions are in fact due either to the rearrangement of the lattice or to the coalescence of the grains. However, for some substances, a second crystalline from c' appeared at the melting point, on cooling from the isotropic phase. C' turned out to be different from the starting form C1 of the sample, which had no thermal cycle before.
An x-ray analysis of stretched oriented fibers of polyurethanes TDI-CmCn, derived from various mesogenic alkylene di[4-(w-hydroxyalkyloxy-4-oxybenzoyl)oxybenzoate]s (CmCn) and 2,4-toluenediisocyanate, is reported. The structure of two sequential nematic mesophases, a cybotactic nematic, and a conventional nematic mesophase is evidenced.