E. A. Konshina

Ellipsometric study of a-C:H films

Abstract The optical constants, porosity and thickness of amorphous hydrogenated carbon (a-C: H) films were determined by multiple-angle ellipsometry at an optical wavelength of 632.8 nm. The films were produced by a plasma-activated CVD process in a dc glow discharge of acetylene, toluene and octane. The results indicate that the refractive index of the a-C: H films can be changed over the interval 2.35–1.55 by increasing the deposition rate and an appropriate choice of hydrocarbon precursor. A reduction of the refractive index correlates with a decrease in the extinction index in the range 0.3–0.01. The influence of the chemical nature of the hydrocarbon precursor on optical constants in the visible region is found from the results of the ellipsometric measurements. The variations of the experimental data are reviewed in view of the a-C: H structure model proposed previously. The porosity of the a-C: H films has been determined by a new technique based on the ellipsometric measurements.

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.

Ionic impurities in nematic liquid crystal doped with quantum dots CdSe/ZnS

ABSTRACT We investigated the dielectric losses and the ionic currents in the nematic liquid crystal (NLC) doped with semiconductor quantum dots (QDs) of CdSe/ZnS core – shell type and covered with trioctylphosphine oxide (TOPO) molecules. The dielectric loss tangent of the NLC composites increased with increasing the QDs concentration from 0.1 to 0.3 wt%. The density of mobile ions in the composites increased linearly and the average values of ions mobility in the composites decreased with increasing the QDs concentration. The fast ions with the mobility of about 10–10 m2/V·s and the slow ions with the mobility of about 10–11 m2/V·s were detected in the NLC composites. The growth of the content of slow ions took place with increasing the QDs concentrations. Increasing the dielectric loss tangent was observed with increasing the duration of sonication time of the NLC composites to prepare homogeneous suspensions. The fragmentation of the CdS/ZnS shell as a result of the sonication may lead to the appearance of the slow ions in the NLC composites. Graphical Abstract

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.

Determining the director tilt and phase lag of liquid-crystal cells by optical methods

This paper presents the results of calculations and optically determined experimental values of the initial director tilt, the layer thickness, and the phase lag for cells with a nematic liquid-crystal (LC). Layers of germanium monoxide (GeO), amorphous hydrogenated carbon (a-C:H), and a combination of them were used in the LC cells to vary the director tilt. Known methods for determining the director tilt are compared with a method proposed by the authors. The proposed method has an advantage in the case of oblique orientation of the slope angle of the LC for angles greater than 15° and is of interest for studying LC devices of adaptive optics, since, along with the main parameter—the maximum phase increment—it makes it possible to estimate the director tilt.

Ways of increasing the response rate of electrically controlled optical devices based on nematic liquid crystals

This review is devoted to the problem of increasing the response rate of liquid-crystal (LC) devices based on nematic liquid crystals (NLCs), such as optical switches, modulators, adjustable attenuators, and stabilizers for the near-IR region. Along with well-known methods such as the regime in which devices operate with displacement and dual-frequency addressing of NLCs with sign inversion of the dielectric anisotropy, which makes it possible to control the relaxation process, the possibility of accelerating the operation by using polymer-dispersed and polymer-stabilized LC mixtures is considered. It is shown to be necessary to optimize the supply regimes of the LC devices. The dynamic responses of devices that use the electrooptic S and twist effects are compared. The prospects of using NLC devices as active components of optical-fiber networks are considered.

Optical modulators based on a dual-frequency nematic liquid crystal

This paper experimentally investigates the dynamic characteristics of electrically controlled modulators based on liquid crystals (LCs) in the visible and near-IR regions and the effect of the electric-field parameters on them, along with the variation of the conditions of the interphase interaction of the LCs with the orienting surface. It is shown that it is effective to use a dual-frequency liquid crystal for phase and amplitude modulation of radiation with wavelength 1.55μm. In an LC modulator operating on the S effect, a 2π phase lag is obtained in a time of 2ms. The switching times can be reduced to the microsecond range when the twist effect is used and the LC layer is about 7μm thick by increasing the voltage from 30to50V.

Absorption and the optical gap of a-C:H films produced from acetylene plasmas

Films of amorphous hydrogenized carbon (a-C:H) are prepared by chemical vapor deposition using a dc glow discharge. The absorption is studied over wavelengths of 400–2400 nm. Two Gaussian-like bands are identified with peaks at ∼600 and ∼800 nm. The ratio of the integrated intensities of these bands (I600/I800) is found to decrease as the voltage is raised. The changes in the ratio I600/I800 are compared with the variation in the fundamental Raman scattering bands in order to clarify the nature of the visible absorption in the a-C:H films. Two simple schemes for transparent insulating films with ET⩾1.5 eV and for “black” conducting a-C:H films with ET⩽1 eV are proposed for explaining the relationship between the structure of the absorption edge and the width of the Tauc gap (ET).

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.

Impact of titanium dioxide nanoparticles on purification and contamination of nematic liquid crystals

We have investigated the impact of titanium dioxide nanoparticles on the ionic contamination of liquid crystals. Nematic liquid crystals with high and low initial ionic contamination have been examined. It has been shown that titanium dioxide nanoparticles reduced the ion density of liquid crystals with high initial ionic contamination from 134.5 × 1012 cm−3 to 63.2 × 1012 cm−3. In the case of liquid crystals with low initial ionic contamination, the nanoparticles led to an insignificant increase of ion density from 19.8 × 1012 cm−3 to 25.7 × 1012 cm−3.