H. L. Margaryan

Influence of Longitudinal Electric Field on Time Characteristics of Freedericksz Transition

Electrooptic properties of liquid crystal cells with semiconductor substrate are studied. A DC electric field has been applied along the semiconductor substrate. It is shown, that presence of such field results both in reduction of Freedericksz threshold and in essential reduction of characteristic reorientation and relaxation times. For more understanding of the mechanism of current flow through the liquid crystal – semiconductor interface, the Volt-Ampere (V-I) and spectral characteristics of photocurrent for such system have been investigated.

Conformations of DNA in the presence of nanomole concentrations of Co+2 ions and mezo-tetra(4-N-oxyethylpiridil) porphyrin

Conformation changes of calf thymus DNA induced by nanomole concentrations of Co+2 and TOEPyP4 and influence of this ion on porphyrin binding mode with DNA duplex were studied at two NaCl concentrations (10 and 100 mM) using circular dichroism spectroscopy and absorption data. It was shown that addition of 85 nM Co+2 into TOEPyP4/DNAb.p. = .015 solution caused an immediate change of the intercalative binding mode of porphyrin to external binding modes at low ionic strengths (10 mM NaCl). Increase of Co+2 concentration from 165 to 650 nM caused a cooperative transition of DNA from B form to C-like conformation. Similar changes in binding mode and DNA conformation were observed at 100 mM NaCl, but they took place at higher concentrations of Co+2 ions. Incubation of TOEPyP4/DNAb.p. = .015 in the presence of nanomole concentrations of Co+2 ions at 36.6 °C showed a transition of DNA-B form into Z-like conformation. Maximal percent of Z-like DNA was observed when the incubation time was from 6 to 8 h. Longer incubation times of [DNA-TOEPyP4]+Co+2 complexes caused a considerable recovery of CD spectra in UV region and a strong decrease in absorption at 260 nm. Here, we discuss some mechanisms of transition from B-DNA to C- and Z-like conformations.

Influence of the Voltage Applied to the Semiconductor Substrate on Birefringence of Nematic Liquid Crystal

The influence of longitudinal voltage, applied along semiconductor substrate of planar-oriented liquid-crystal cell, on the birefringence of nematic liquid crystal (NLC) has been studied. It is shown that with the fixed value of transverse voltage the application of longitudinal voltage creates an additional possibility to control the electro-optically induced phase shift of light passing through NLC. The dependence of phase shift vs. control voltages is obtained. The results of simulation of the longitudinal voltage influence on the process of NLC reorientation are given. The obtained results can be used for development and fabrication of liquid-crystal phase modulators.

Interface effects on semiconductor-liquid crystal structure

We study orientational processes due to externally controlled interaction between surface atoms of a semiconductor and the interfacial molecules of liquid crystals(LC). Two different types of LC reorientation is observed: one due to an electric field acting along the interface between the semiconductor and the LC; and the conventional volume reorientation caused by the electric field acting between the LC substrates. The time characteristic and the threshold value of one type of reorientation depend on the degree of development of the other type of orientation.

Some Peculiarities of the Recording of Polarization Diffraction Gratings by Doubling Method

We investigated the dependence of angular dispersion of replica on its distance from the master polarization diffraction grating (PDG). The distribution of field diffracted on master PDG was calculated using the method of finite-difference time-domain. According to the numerical model, within the transition region, conditioned by interference of secondary waves with the same polarization state, the angular dispersion of replica changes depending on the distance between master and replica. Outside the transition region, the angular dispersions of replica and master completely coincide. The experimental investigation was also carried out, the results of which are in good agreement with the calculated data.

Recording of geometric phase elements based on liquid crystal polymers

A technique is developed for recording the centrally-symmetric liquid crystal phase plates with the anisotropic orientation of molecules providing a smooth change of the optical axis in a thin film of liquid crystal polymer. The technique enables one to record the elements with the cylindrically-symmetric and planar-symmetric distributions. Such structures can be used to develop the optical elements with the new functional possibilities. Using the developed techniques, the polarization-sensitive Fresnel lens is realized which is functioning either as a collecting or as a scattering lens for the light beams with the orthogonal circular polarizations.

Radiation control in the mid IR wavelength range using the liquid crystal phase grating

The numerical finite difference time domain (FDTD) model of liquid crystal diffraction phase grating is developed and the possibility to use this grating for the spatial-time control of radiation in the mid IR wavelength range is shown.

Electrically Controlled Phase Gratings for Terahertz Radiation Based on Nematic Liquid Crystal

A mathematical model of a new type of liquid crystal (LC) based diffraction grating for the terahertz frequency range is proposed. Numerical time-integration by the finite-difference time-domain (FDTD) method of Maxwell-equation systems, describing the proposed structure, has been performed. The partial differential equation, describing the electro-optical induced orientation of the LC molecule in the external electric field, is calculated by the method of lines (MOL). The dependence of induced birefringence vs. external control voltage is obtained for 6CB nematic liquid crystal (NLC).

Some Peculiarities of Transient Processes in Planar Periodic–Planar Polarization Diffraction Gratings

The transient processes in planar periodic–planar polarization diffraction gratings under influence short bipolar electric pulses are investigated. It is shown that the application of bipolar pulses with certain amplitude and frequency leads to significant increase of diffraction efficiency in non-zero diffraction orders as compared to steady-state diffraction. The dependence of diffraction efficiency of each diffraction order on both the amplitude and frequency of applied pulses is studied. It is shown that the maximum diffraction efficiency of necessary order can be controlled by choosing frequency and amplitude of external pulses. This phenomenon can be used for creation of adaptive polarization diffraction gratings functioning in none-steady state mode.

Improvement of turn-on time of liquid-crystal phase retarder by using an additional control voltage

We investigate theoretically and experimentally the process of turn-on time control of a nematic liquid crystal (NLC) based phase retarder by means of an additional control voltage. We propose a theoretical model of reorientation of NLC molecules in crossed electric fields (conventional transverse field and additional longitudinal field). The dynamics of reorientation of NLC molecules is considered depending on the ratio of amplitudes of control voltages. We show the possibility to reduce the turn-on time in NLC-based phase-retarder management by means of two control voltages.