E. O. Gavrish

Effect of dispersed CdSe/ZnS quantum dots on optical and electrical characteristics of nematic liquid crystal cells

We have studied the electrical and optical characteristics of cells filled with nematic liquid crystal (NLC) based on cyanobiphenyls with positive dielectric anisotropy, containing dispersed 3.5-nm-sized CdSe/ZnS composite semiconductor nanoparticles (quantum dots, QDs) with a concentration of 0.1–0.2 wt %. In addition to a decrease in the threshold voltage of the electrooptical splay effect, the doping with QDs leads also to a decrease in the phase delay of light and the effective dielectric permittivity of NLC cells. These characteristics are reduced by half during the storage of NLC cells containing about 0.2 wt % QDs, which is related to the self-organization of QDs.

Study of dynamics and relaxation optical response of nematic liquid crystals doped with CdSe/ZnS quantum dots

This study is devoted to the dynamics of the electro-optical response and relaxation time of LC cells with a nematic liquid crystal doped with CdSe/ZnS semiconductor quantum dots (QDs) and homogeneously oriented by rubbed polyimide. The influence of the nanoparticle concentration as well as the parameters of the alternating electric field has been studied. Experimental dependence of the response and relaxation time on the concentration of QDs was obtained. The increase of concentration of QDs in the range of 0.5–1.5 mg/ml results in a reduction of the optical response time. The threshold voltage reduced twice with increasing QD concentration up to 1.5 mg/ml as compared with the pure LC. For the first time it was shown that applying of unipolar square wave electric field to the LC cell reduces the optical response time even twice regardless to the frequency of the alternating electric field compared to bipolar square wave voltage.

Optical response from dual-frequency hybrid-aligned nematic liquid crystal cells

Dual-frequency hybrid-aligned nematic liquid crystal cells and the influence of the parameters of a control electric field on their optical response are studied. It is found that the harmonic oscillations of the optical transmission in such cells are observed in the interval between low frequency-to-high frequency voltage switchings unlike in conventional twisted nematic cells. A V-shaped bistable optical response is obtained by successively applying sinusoidal electric fields with frequencies of 1 and 30 kHz to a twisted nematic cell. For a liquid crystal layer 8 μm thick and an applied voltage of 50 V, the response time is 10 ms. In a hybrid-aligned twisted-nematic cell with a large initial tilt angle of the director (about 70°), the V-shaped optical response is observed when the inclined homeotropic state is switched to the twisted state by applying a 30-kHz field. The initial structure of the layer recovers as a result of natural elastic relaxation, and the response time increases roughly fourfold.

The doping of the polyimide alignment layer by semiconductor quantum dots

We investigated the electro-optic properties of nematic liquid crystal cells oriented by polyimide (PI) layer doped with 3.5 nm semiconductor quantum dots (QDs) CdSe/ZnS at concentrations of 0.05 and 0.1 wt. %. It is shown that doping PI orienting layer by QDs reduces the permittivity and the phase delay, as well as increases the electrical resistance of the cells. Also we observed deceleration of liquid crystal (LC) optical response caused by the screening effect of the orienting layer.

Influence of the alignment layer and the liquid crystal layer thickness on the characteristics of electrically controlled optical modulators

The screening effect of the amorphous hydrogenated carbon (a-C:H) alignment layer and its dependence on the thickness of a dual-frequency nematic liquid crystal (NLC) layer have been studied. Optimization of the a-C:H layer thickness allows a threshold voltage for the optical S-effect to be reduced and the characteristic switching time and relaxation time of 0.5 and 2.5 ms, respectively, to be obtained for a phase retardation of 2π at a wavelength of 0.86 μm.

Investigation of orientational order in a nematic liquid crystal with semiconductor CdSe/Zns QDs by IR spectroscopy

The dichroic ratio and orientational-order parameter of fundamental bands have been studied in the IR spectrum for a nematic liquid crystal based on alkylcyanobiphenyls and its suspensions with semiconductor CdSe/ZnS QDs with a core size of 5 nm. Analyses have been performed by the method of multiple frustrated total internal reflection. It is shown that an increase in the nanoparticle concentration from 0.5 to 1.3 wt % improves the orientation uniformity of the liquid crystal near the interface with the orienting surface. The orientational order parameter in a suspension with 1.3 wt % QDs has increased in comparison with the initial liquid crystal by more than 10% for the fundamental bands in alkylcyanobiphenyl molecules, the stretching vibrations of which correspond to the direction of their long axes.

Features of the Optical Response and Relaxation of the Nematic LC Doped with CdSe/ZnS Quantum Dots

Dynamics of the electro-optical response and relaxation time of nematic liquid crystal (NLC) cells doped with CdSe/ZnS semiconductor quantum dots (QDs) and homogeneously oriented by rubbed polyimide were investigated. The influence of nanoparticles’ concentration as well as parameters of alternating electric field were shown. The optical response time decreased 1.5 time by increasing concentration of QDs up to 1.5 mg/mL as it was compared with the pure LC. Applying unipolar square wave electric field to the LC cell reduces optical response time twice comparing to bipolar meander. However, we observed slowdown of the relaxation process both in case of doping NLC by nanoparticles and in case of applying unipolar square wave pulses.

Screening Effect of a-C:H Alignment Layer and its Influence on Characteristics of LC Cells

The influence of the hydrogenated carbon alignment layer electrostatic screening effect on characteristics of dual frequency nematic liquid crystal was investigated. The screening effect was declined as the result of the threshold voltage reduction from 6 V to 1 V connected with the fourfold decrease in alignment layer thickness. The pretilt angle rising and the phase retardation reduction were observed at the same time. It is shown that hydrogenated carbon alignment layer application is able to change electrical properties of the cells as well as to accelerate the switching time at the wavelength of 1.55 μm.

Screening effect of a-C:H alignment layer in nematic liquid crystal cell

The electrical and optical characteristics of nematic liquid crystal (NLC) cells and their dependence on the thickness of an amorphous hydrogenated carbon (a-C:H) alignment layer have been studied. An increase in the a-C:H layer thickness favors enhancement of the screening of a bias voltage applied to this layer, which is manifested by an increase in the threshold voltage of the electrooptical splay effect. This is accompanied by a decrease in the initial (pretilt) director angle, which is evidence for an increase in the anchoring energy due to the field of space charge localized at the interface.

Comparison the Properties of LC Cells with CdSe/ZnS QDs Embedded Into Nematic LC Matrix and the Polyimide Alignment Layer

We compared electro-optical characteristics of LC cells with 0.05 and 0.1 wt.% semiconductor quantum dots (QDs) CdSe/ZnS embedded into nematic liquid crystal and polyimide layers. The phase delay of the LC cells increased with addition of the QDs into the liquid crystal layer. Also we observed decreasing the phase delay of the LC cells with QDs embedded into the polyimide layer. The threshold voltage reduced in case of doping LC with QDs, in contrast to the LC cells with QDs in the polyimide layers. The optical response times of LC doped with QDs were less than in case of adding the same into the polyimide layer.