Controlled thin-film quasi-waveguide dye-laser in a liquid-crystal matrix

Laser generation in a thin oriented liquid crystal-dye layer is investigated experimentally. By inserting an electro-tunable planar-oriented liquid crystal cell into the resonator, tuning of the generation wavelength was realized. Studies were performed in the quasi-waveguide generation regime.     

Photoelectric properties and photoinduced changes in ann-GaAs-glass-metal structure

A study is made of the photoelectric properties of metal-insulator-semiconductor (MIS) structures ofn-GaAs with a V₂O₅−B₂O₃−CaO insulating layer before and after illumination with subthreshold radiation. It is suggested that these properties are formed on account of the combined action of the barrier photo-emf and the Dember photo-emf dominating at low and high frequencies. The changes discovered in the photoelectric and noise properties of the structures are explained by the occurrence of photoinduced transformations in the ensemble of semiconductor defects. This is confirmed by a metallographic investigation. It is noted that the characteristics of photoinduced changes in crystalline and amorphous systems have common features. State Academy of Control and Radioelectronic Systems, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 6–12, August, 1996.     

Influence of the structure of the director field on the optical properties of a nematic liquid-crystal droplet

The changes in the optical properties of a spherical, bipolar, liquid-crystal droplet due to the change in structure of its director field under the influence of an applied external field are investigated on the basis of calculations using the nested-ellipsoids model in the discretedipole approximation. Graphs are plotted for the attenuation efficiency factor and the angular structure of the scattered radiation for the two typical cases of strong and weak dependence of the optical properties of the droplet on the change in structure of its director field. Zh. Tekh. Fiz. 69, 86–90 (November 1999)     

Role of molecular shape on bent-core liquid-crystal structures

We show how frustrations in molecular packing generated by molecular substitutions and electrostatic potentials necessitate a local C1 symmetry and lead to spontaneous polarization splay and layer bending in bent-core liquid crystals. By estimation of the entropic changes due to curvature, we calculated the elastic constants that drive the spontaneous layer bend and polarization splay and obtained length scales of these deformations. Finally, we propose a structural phase diagram which describes how the experimentally observed helical superstructures, layer undulation, polarization splay, and spontaneous fiber formation depend on the direction and magnitude of the spontaneous polarization.     

Localized optical states in a liquid-crystal structure adjacent to a metal

The spectrum of light transmission through a structure consisting of a metallic layer and a cholesteric liquid crystal with an induced planar defect is calculated. The possibility of existence of localized states at the metal–dielectric interface of this system is demonstrated. The transmission spectra at different positions of the defect in the structure are studied.     

Formation of uniaxial molecular films by liquid-crystal imprinting in a magnetic field

Scanning tunneling microscopy was used to study molecular order in monolayer organic films formed by solution-phase growth from thermotropic liquid crystal solvents. The films develop macroscopically uniaxial alignment, with adlayer orientation controlled by an external magnetic field through interactions mediated by the liquid crystal. Results are presented for two films deposited from nematic and smectic- A solvents, along with a discussion of the alignment mechanism.     

Space-domain lock-in amplifier based on a liquid-crystal spatial light modulator

We present a two-dimensional (2D) spatial lock-in amplifier that provides a contrast ratio of more than 10,000:1 for transmitted and blocked intensity patterns using a conventional liquid-crystal spatial light modulator. The device is based on spatial-domain modulation-demodulation of intensity patterns under coherent imaging conditions. The operation of the 2D lock-in amplifier is illustrated by implementing Young's double-slit arrangement for measurements of the mutual coherence between individual emitters of a 2D phase-coupled array of vertical cavity surface emitting lasers.     

Multidimensional control of femtosecond pulses by use of a programmable liquid-crystal matrix

We report the automated generation of high-fidelity spatiotemporally shaped femtosecond pulses by use of an optically addressed, two-dimensional liquid-crystal spatial light modulator. A single input pulse was divided into many independent regions, and each region was shaped temporally. By changing the imaging geometry we accomplished either real-space or wave-vector shaping.     

Phase-shift error as a result of molecular alignment distortions in a liquid-crystal point-diffraction interferometer

We report the observation of nematic director distortions around the diffracting element of a liquid-crystal point-diffraction interferometer. The observed director field distortions are similar to those reported in the literature for other liquid-crystal guest-host systems. We show how the alignment distortion changes as a function of the voltage applied to the liquid-crystal cell, leading to an observed phase-shift error. Tailoring of surface anchoring conditions and judicious choice of phase-shift algorithm can improve device accuracy.     

Fluid motion and molecular reorientation in a homeotropically orientated nematic liquid-crystal cell

The linearized Ericksen-Leslie differential equations, which couple fluid motion and director reorientation to each other, are reduced to a set of time varying differential equations for two pulsed optical waves incident at an angle upon a homeotropically orientated liquid-crystal cell. The differential equations are solved by a numerical method. The fluid velocity and the director angle are plotted as a function of space and time. It is shown that the reaction of fluid motion upon director reorientation is small.     

Magnetic properties of molecular compounds based on a new polydentate ligand

We have obtained a new Cu-based ligand using diaminobenzidine which has four coordination sites, opening new possibilities to synthesize extended structures with magnetic interactions. Two metal complexes, Cu₂Mn₂(bopba) and Cu₂Co₂(bopba), where Cu₂(bopba) is a new polydentate ligand, were successfully obtained and characterized by elemental analysis, IR-spectroscopy, X-ray diffraction and magnetic measurements. The results indicate a ferrimagnetic character of the Mn₂Cu₂ compound, with a high saturation moment at temperatures above liquid helium and a weak ferromagnetic behavior of the Co₂Cu₂ compound.     

Photoelectric properties of nInSb-nPbTe-nCdTe isotype structure prepared by vacuum pulsed-laser deposition technique

With use of the vacuum pulsed-laser deposition technique an isotype nInSb-nPbTe-nCdTe structure is fabricated. Spectral dependence of photoresponse of the obtained structure is studied at the temperature 120 K. In the curve of spectral dependence a change in the sign of photoresponse is observed for both zero bias and external voltage. Qualitative explanation of this phenomenon is given.     

IR spectroscopic analysis of the structure of a biospecific sorbent based on a polyethylene matrix

We have used IR spectroscopy to study the structure of a biospecific sorbent based on a polyethylene matrix, modified by radiation grafting of acrylic acid with L-tryptophan methyl ester as the ligand. We show that grafted polyacrylic acid forms on polyethylene beads. We have established that chemical addition of the ligand to the modified polyethylene matrix occurs with participation of COOH groups of the polyacrylic acid and NH2 groups of the L-tryptophan methyl ester. We have observed that in the end product, strong hydrogen bonds are formed between the carbonyl groups of the ligand and the unreacted carboxyl groups of the grafted polyacrylic acid.     

Visualization of the langmuir-blodgett film structure in liquid-crystal cells

The use of liquid-crystal (LC) cells for the visualization of the Langmuir-Blodgett (LB) film structure is described. Images appearing in LC cells upon LB film application are presented. The images are analyzed in relation to the LB film structure (application conditions) and to the relief formed by indium tin oxide (ITO) electrodes.     

Accurate color predictability based on a spectral retardance model of a twisted-nematic liquid-crystal display

In this work we present the application of a simple physical model to accurately predict the broadband spectral transmittance and colorimetric properties of a twisted-nematic liquid crystal display (TNLCD). We spectroscopically calibrate the retardance parameters to evaluate the spectrum of the light transmitted by a TNLCD sandwiched between two linear polarizers. When the TNLCD is illuminated with a broadband light source, the full spectrum can be predicted as a function of the addressed gray level for any arbitrary orientation of the polarizers. Thus, the color of the transmitted light can be also be estimated with very good accuracy. As an example, a polarizers' configuration is shown that yields, without using color filters, a relatively large color gamut compared to the standard configuration. Experimental results confirming the validity of such predictions are presented, both on the measured spectral responses as well as on the trajectories at different chromatic diagrams.     

基于滑模变结构控制及预测跟踪的光电吊舱控制方法北大核心CSCD

光电吊舱在稳像和目标跟踪过程中存在摩擦力矩、不平衡力矩等干扰力矩,从而影响速度环响应精度;另一方面,吊舱视频跟踪器图像传输和处理造成的延迟也会造成跟踪滞后,因此必须进行延时补偿。提出基于预测跟踪的滑模变结构控制方法,采用微分预测跟踪器实现对视频跟踪器的延迟补偿,采用预测跟踪器估计出的目标运动角速率构成自适应补偿参数,以调整滑模变结构控制量,改进的滑模控制算法在补偿干扰力矩的同时抑制了抖振现象。仿真及实验结果表明:改进的控制方法能够有效补偿干扰力矩和跟踪器延迟造成的误差,相对于传统PID控制,其跟踪误差减小为原来的1/3,并且该算法已经在相关系统上得到应用。