Fast spatially localised electrooptical mode in vertically aligned nematic LCs with negative dielectric anisotropy

ABSTRACT

Electro-optical switching and the liquid crystal (LC) director distribution are studied in spatially periodic electric field for vertically aligned LC with negative dielectric anisotropy. Two electro-optical switching modes characterised by different switching times are observed. These modes are well distinguished optically by choosing proper geometry for the polarisers axes orientation. One of these modes is significantly faster as compared to the other. The fast switching is explained in terms of localised near-to-surface director reorientation. The 3D-numerical simulation shows very good agreement with the experiment: it points out the existence of the disclination lines and field-stabilised walls responsible for the localised director field switching and its relaxation. Possibilities of enhancing the fast mode for high-speed light modulators are discussed.     

Universal features of terahertz absorption in disordered materials

Using an analytical theory, experimental terahertz time-domain spectroscopy data, and numerical evidence, we demonstrate that the frequency dependence of the absorption coupling coefficient between far-infrared photons and atomic vibrations in disordered materials has the universal functional form, C(omega)=A+Bomega(2), where the material-specific constants A and B are related to the distributions of fluctuating charges obeying global and local charge neutrality, respectively.     

Director distribution in field-induced undulated structures of cholesteric liquid crystals

Structures with a periodic in-plane liquid crystal director field modulation induced by an electric field are studied in cholesteric liquid crystals (CLCs). A phenomenon of the electric-field-induced instability in a planarly aligned cholesteric cell is used to create these undulated structures. The initial field-off state is planarly aligned with the cholesteric helix axis oriented perpendicular to the cell substrates. The interaction of the CLC with an electric field results in modulation of the refractive index, which is visualised as stripe domains oriented either along or perpendicular to the rubbing direction at cell alignment surfaces. The threshold electric field for the undulation appearance and a period of stripes are measured experimentally for three Grandjean zones (ratio d/p ~ 0.5, 1.0, and 1.5, where d is a cell thickness and p is the natural cholesteric pitch). For the zone with d/p ~ 1.0 using numerical simulations, we describe in detail the director distribution at an applied electric field. It is found that the in-plane undulated structure is characterised by a conical director rotation on moving along the alignment direction. The conical axis is tilted with respect to the alignment axis. The sign of the tilt angle depends on the handedness of CLC.     

Fast Mode in the Electro-Optical Switching of a Liquid Crystal Cell with Interdigitated Electrodes

Journal of Experimental and Theoretical Physics - The conditions for the emergence of a fast electro-optical mode in a liquid crystal cell with a short period (p = 1.8 μm) of interdigitated...     

In-Plane Switching of Chiral Nematic Liquid Crystals with Different Dielectric Anisotropies

The electro-optic response in planar-aligned chiral nematic liquid crystals in an electric field oriented perpendicular to the helical structure axis is studied as a function of the magnitude and sign of the dielectric anisotropy of liquid crystal in order to understand the mechanism of this electro-optic effect. The main contribution to the electro-optic response is shown to be due to the interaction of the electric field with the dielectric anisotropy. The flexoelectric effect also contributes to the electro-optic response, but its magnitude is much smaller.     

Numerical modeling of sound propagation in an inhomogeneous waveguide with absorption

Computation of the acoustic field in vertically and horizontally inhomogeneous waveguides with allowance for absorption is considered. The method of normal waves and the parabolic equation method are used as alternative approaches to the problem. The efficiency of the algorithms suggested is demonstrated by a test example of sound propagation in a wedge with the acoustic velocity depending on the depth.State University, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, Nos. 1–2, pp. 134–138, January–February, 1995.     

New type of charged defect in amorphous chalcogenides

We report on density-functional-based tight-binding simulations of a series of amorphous arsenic sulfide models. In addition to the charged coordination defects previously proposed to exist in chalcogenide glasses, a novel defect pair, [As(4)](-)-[S(3)](+), consisting of a fourfold coordinated arsenic site in a seesaw configuration and a threefold coordinated sulfur site in a near-planar trigonal configuration, was found in several models. The valence-alternation pairs [S(3)](+)-S-1 are converted into [As(4)](-)-[S(3)](+) pairs under HOMO-to-LUMO electronic excitation. This structural transformation is accompanied by a decrease in the size of the HOMO-LUMO band gap, which suggests that such transformations could contribute to photodarkening in these materials.     

Plasmonic Enhancement of Photocurrent in a Hybrid Structure with a Subwavelength Aluminum Grating

Strong anisotropy of photocurrent excitation is observed in a hybrid photovoltaic structure comprising a film of organic-semiconductor blend and a subwavelength metal grating at one of the electrodes. The results are explained in the context of a numerical model demonstrating different characters of localization of optical fields with different polarizations and, correspondingly, the polarization-selective excitation of plasmons.     

Fast Electro-Optical Switching Mode in an LC Cell while Decreasing the Electrode Grating Period

Crystallography Reports - An increase in the electro-optical switching speed with a decrease in the electrode grating period is observed in a cell with a homeotropically oriented nematic...