N. H. Hakobyan

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.

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).

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.

Investigation of characteristics of electrically-controlled liquid-crystal retarders

We present results of studies of main characteristics of electrically-controlled retarders fabricated on the basis of planar- and twist-oriented liquid crystals (LC). The investigated characteristics are time of switching and the calibration curve of controllable phase incursion. Computer simulation and numerical calculations of the processes of LC director reorientation in an external electric field have been performed. Based on LabView software a special program is constructed for computer control of the shape, frequency, and amplitude of the external voltage applied to LC cell. An experiment has been implemented on realization, with use of this program, of the prescribed phase retardation of an optical beam. Calculated results agree with experimental data.

Influence of additional electric field on the birefringence of a nematic liquid crystal

Influence of the longitudinal voltage applied along the semiconductor substrate of a planar-oriented liquid crystal cell on the birefringence of a nematic liquid crystal (NLC) was investigated. It is shown that for a fixed magnitude of the transverse field the application of a longitudinal voltage creates an additional possibility to control the electro-optically induced phase incursion of the radiation passing through NLC. Dependence of the phase incursion on the control voltages is obtained. The results of simulation of the longitudinal voltage effect on the NLC reorientation are described. The obtained results can be used in development and designing of NLC phase modulators.

Investigation of Transient Processes in Nematic Liquid Crystal Cells with Semiconductor Substrate, Caused by the Electric Field Applied to Semiconductor

In the present work the results of detailed researches on characteristics of transient processes, appearing by application of DC field to semiconductor substrate of liquid crystal cell have been carried out. It is shown, that oscillations, caused by longitudinal DC field applied to the semiconductor differ from the oscillations, caused by the Freedericksz transitions. The dependences of character and number of these oscillations on the applied DC field value, the type and thickness of nematic liquid crystal are investigated. The probable mechanisms explaining the observable phenomena are discussed.