Anisotropy In Chalcogenide Glasses Research Articles

Permanent photoalignment of liquid crystals on nanostructured chalcogenide glassy thin films

Photoalignment of nematic liquid crystals is obtained on the chalcogenide glassy thin film of As2S3 using irradiation with polarized blue light. A uniform homogeneously aligned device is obtained with high contrast and strong anchoring. The device alignment quality is permanent as checked by following its functionality over a period of few months. The origin of the observed photoalignment is attributed to the photoinduced anisotropy in chalcogenide glasses. No differences between the different As2S3 film thicknesses observed, thus supporting the proposition that some orientational order is photoinduced on the surface of the glass and responsible for the photoalignment.

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.

A model of photoinduced anisotropy in chalcogenide glasses

An electronic model of photoinduced optical anisotropy (PA) is suggested. Within the framework of this model, PA is attributed to photogeneration and trapping of oriented geminate electron–hole pairs. This attribution is possible in a disordered material with appropriately correlated traps, illuminated by polarized light that can generate aligned geminate pairs. Since creation of stable oriented geminate pairs occurs in the dispersive transport regime, the PA build-up can be fitted with a stretched-exponential time dependence.

An electronic model of photoinduced optical anisotropy in chalcogenide glasses

A purely electronic model of photoinduced optical anisotropy in glassy semiconductors is formulated. The model is based on the concept of geminate electron-hole pairs captured by correlated deep traps. The kinetics of photoinduced anisotropy are controlled by the photogeneration, trapping and recombination of geminate pairs. Predictions of the model are shown to be in agreement with experimentally observed basic characteristic features of photoinduced anisotropy.

The question of temperature-dependent intrinsic dipoles in chalcogenide glasses

The claim that the kinetics of photoinduced anisotropy in chalcogenide glasses prove the presence of a temperature-dependent concentration of electric dipoles is re-examined. It is concluded that neither a temperature dependence of electric dipoles nor even the presence of such dipoles is necessary to explain the experimental observations.

Mechanism and kinetics of photoinduced anisotropy in chalcogenide glasses

Optical anisotropies, caused by the illumination of chalcogenide glasses with polarized light, are summarized and discussed. In particular, the time and temperature dependencies of the photoinduced dichroism in a-AsSe films are used for an analysis of the effects in terms of macroscopic and microscopic models that have been proposed. A microscopic model, based on the intrinsic charged defects of the chalcogenides, is shown to explain adequately the orientational, kinetic and temperature aspects of the phenomenon.

Photoinduced anisotropy in chalcogenide glasses in a wide spectral range studied by reflectance difference spectroscopy

We present the results of an investigation of photoinduced anisotropy in films and bulk samples of chalcogenide glasses As 50Se 50 and Ge 20As 20S 60 using reflectance difference spectroscopy. Anisotropy in a wide spectral range (1.5–5.5 eV) can be generated and reorientated many times by linearly polarized light. We conclude that possibility of orientation and reorientation with the linearly polarized light not only of defects and scattering centers but also of the interatomic covalent bonds is characteristic for different chalcogenide glasses. We have found that doping of a chalcogenide glass Ge 20As 20S 60 by rare earths does not change the parameters and characteristics of photoinduced anisotropy.

Temperature dependence of the photoinduced anisotropy in chalcogenide glasses: Activation energies and their interpretation

Three distinct regions have been observed in the measured temperature dependence of photoinduced anisotropy in chalcogenide glasses (Se and AsSe). At high temperatures the relaxation of the induced anisotropy is governed by an Arrhenius law with a change of activation energy at the glass-transition temperature ${\mathrm{T}}_{\mathrm{g}}$ (thus suggesting a method for detection of ${\mathrm{T}}_{\mathrm{g}}$ in glasses). Conversely, the low-temperature relaxation is not thermally activated. These observations are explained by introducing a triple-well configurational diagram for the valence-alternation pairs (VAP's) and their environments, which are believed to be responsible. A transient neutral state connects two (initial and final) charged states of the VAP's. A symmetry reversal of the pyramid centered at an overcoordinated chalcogen atom is suggested as a microscopic mechanism for the photoinduced anisotropy.

Photoinduced generation and reorientation of linear dichroism in AsSe glassy films.

The kinetics of linear dichroism generation and reorientation in glassy AsSe films induced by linearly polarized He-Ne laser light of different intensities is studied by means of a polarization-modulation technique. Two distinct processes are identified: creation of photoinduced defects and their photostimulated orientation and reorientation. The first process can be carried out by both unpolarized and polarized light and has a much larger time constant than the second one. The annealing temperatures of the two effects are also shown to be different. Comparison of restoration of the initial dichroism-generation kinetics after annealing with the photodarkened AsSe film thermobleaching leads to a conclusion about certain identity of the defects responsible for the photodarkening and photoinduced anisotropy phenomena. Tikhomirov and Elliott's model of photoinduced anisotropy in chalcogenide glasses is further developed.

Metastable optical anisotropy in chalcogenide glasses induced by unpolarized light.

Semiconducting chalcogenide glasses become optically anisotropic solids with the optical axis parallel to the direction of the inducing light beam after exposure to unpolarized light. This photoinduced anisotropy (PA) can be detected in bulk samples by means of a probing beam directed perpendicularly to an unpolarized inducing beam. In chalcogenide films, PA induced by an unpolarized glancing-angle beam can be detected by means of a probing beam that is incident normal to the films.

Mechanism of laser-induced optical anisotropy in chalcogenide glasses

Mechanism of laser-induced optical anisotropy in chalcogenide glasses

Photodarkening and light induced anisotropy in chalcogenide glasses

A unified description of photodarkening and light induced anisotropy in chalcogenide glasses is presented in terms of bistable atomic configurations