Karen Asatryan

Polymer-stabilized liquid crystal for tunable microlens applications

We investigate the electro-optical properties of polymer stabilized nematic liquid crystals produced by in situ photopolymerization technique using Gaussian laser beam. The distribution of refractive index in such structure under the action of a homogeneous electric field reveals a non-homogeneous lens-like character, approximately reproducing the intensity transverse distribution in the photopolymerizing beam.

Optical polarization grating induced liquid crystal micro-structure using azo-dye command layer

We create planar-periodic alignment in nematic liquid crystal (LC) cell by using a command layer of azo-dye molecules directly deposited on the cell substrates and exposed with two interfering laser beams of opposite circular polarizations. Permanent high efficiency polarization gratings are thus created. The diffraction efficiency of those gratings is controlled by a uniform electric field applied across the cell. The electro-optical properties of such polarization gratings are studied. Obtained gratings can be used for electrically controlled discrimination and detection of polarized components of light.

Recording of polarization holograms in photodarkened amorphous chalcogenide films

Polarization gratings were recorded in amorphous As2S3 chalcogenide thin films in three orthogonal recording configurations. The analysis of polarization discrimination properties of such vector gratings revealed a poor performance. It was shown that the main reason for this is the presence of nondesired scalar modulation of the same period. An exception is (s+p) recording configuration, where a pure polarization grating may be obtained even for relatively high photoexposures.

The role of van der Waals bonding in photosensitivity of chalcogenide glasses

We report on a comparative study of photosensitivity (scalar and vectoral photoinduced effects and photoexpansion) in Ga-La-S and conventional, As 2 S 3 , chalcogenide glasses. Both scalar (photorefraction and photoexpansion) and vectoral (photoinduced anisotropy) effects are 2-3 orders of magnitude weaker in Ga-La-S glasses, such as Ga 1.4 La 0.6 S 3 , than in As 2 S 3 glass. We relate the suppression of photosensitivity in Ga-La-S glass to the lack of van der Waals bonding in its structure, while this bonding accounts for photosensitivity in conventional chalcogenide glasses, such as As 2 S 3 . We propose that photosensitivity in chalcogenide glasses is accounted for by photoinduced change in polarizability of van der Waals type hyperpolarizable bonds. The presence of van der Waals bonding in the glass structure may affect quantitative predictions of the theory of glass-network rigidity/constraints, which relates glass stability/fragility to the average co-ordination number of constituent atoms that is entirely due to covalent bonding.

Electrically variable liquid crystal lens based on the dielectric dividing principle

Theoretical modeling is performed for a liquid crystal (LC) lens that uses a combination of two dielectric lenses and voltage dividing principle to shape the electric field in space. Electric field, LC reorientation, and optical phase retardation profiles are obtained by numerical simulations. The obtained results are compared with experimental ones, and good agreement is obtained validating the proposed two-dimensional model that uses a limited number of dielectric and geometrical control parameters for this type of lens.

Photoinduced volume changes related to photoinduced anisotropy in chalcogenide glasses

We report metastable photoinduced volume expansion (hyperexpansion) of up to 10 µm in height and several millimetres in diameter for the chalcogenide glass As 2 S 3 . This is an order of magnitude larger than reported to date in any material, including chalcogenide glasses. We suggest a model for the effect and propose that its origin is similar to the origin of photoinduced anisotropy in glassy chalcogenides proposed earlier. The model is based on the anisotropic chemical bonding found in most chalcogenide glasses; the anisotropy arises from the fact that chalcogen atoms exhibit both strong covalent bonding and weak van der Waals bonding. The origin of the photoinduced volume changes (expansion and contraction) is in a thermally induced diffusion, or athermal tunnelling, of photoexcited chalcogen atoms, with possible subsequent sublimation from the sample surface. The primary role of hyperpolarizable van der Waals bonds is supported by the absence of photoinduced volume changes in the chalcogenide glass Ga 1.4 La 0.6 S 3, in which chalcogen atoms do not bond via van der Waals bonds.

Photoinduced birefringence in tellurite glasses

We report on photoinduced birefringence in TeO2-based glasses on exposure to linearly polarized subbandgap light. It is shown that the influence of glass-network modifiers, such as ZnO, Na2O, BaO, and of dopant Er2O3, on the photoinduced birefringence is not substantial indicating a dominating role of the TeO2 component. Photoinduced birefringence is optically reversible and its mechanism is proposed to be due to laser-induced alignment of intrinsic anisotropic defects of tellurite glass, such as distorted TeO4 bipyramids.

Strong relief grating fabrication in amorphous chalcogenide glasses by light polarization modulation

We report on the possibility of strong relief grating fabrication in amorphous chalcogenide As2S3 glass using polarization modulated near bandgap light illumination. Such gratings are created using low intensity and spatially uniform light illumination. The obtained relief structures are strong, very smooth and do not require post-exposure development procedures. They are stable to the heat treatment and the uniform photo-exposure. The possible mechanism of such relief structure formation is also briefly discussed.

Photoinduced dichroism in amorphous As2S3 thin film

We have performed an experiment in a chalcogenide thin film that is questionning some aspects of the currently accepted phenomenological model for the photoinduced anisotropy in such material. By analyzing the kinetics of the photoinduced linear dichroism, we were able to explain our experimental results by revisiting the current model which is based on anisotropic microstructures. This model is modified to include disk-shaped polarizability tensor. The approach we present is a useful tool to extract information on the optical properties of microscopic structures based on a macroscopic phenomenon.

Recording of polarization holograms in photodarkened amorphous As2S3 thin films

The inscription of polarization gratings in amorphous As2S3 thin films is shown to be accompanied by a scalar modulation of the same period. An exception is the (s + p) recording configuration where a pure polarization grating may be obtained even for relatively high photoexposures.