High Homogeneity of Magnesium Doped LiNbO3 Crystals Grown by Bridgman Method

A series of LiNbO3 crystals doped with various MgO concentrations (0, 3%, and 5 mol%) was simultaneously grown in one furnace by the modified vertical Bridgman method. The wet chemistry method was used to prepare the polycrystalline powders, and the growth conditions were optimized. The full width at half maximum of high-resolution X-ray rocking curves for (001) reflection of 5 mol% Mg doped lithium niobate (LN) crystal was about 8″, which meant it possessed high crystalline quality. The OH− absorption spectra shifted to 3534.7 cm−1, and the UV absorption edge violet shift indicated that 5 mol% MgO successfully doped in LN and exceeded the threshold. The extraordinary refractive index gradient of 5 mol% Mg doped LN crystal was as small as 2.5 × 10−5/cm, which exhibited high optical homogeneity.

Influence of temperature diffusion on molecular reorientation in nematic liquid crystals

In this paper, the theoretical analyses of optically induced molecular reorientation are presented in a nematic liquid crystal cell. Due to the absorption of the light beam, the temperature of the cell increases, which induces change of anisotropy and the Frank elastic coefficients. In our analyses, a simple model is used. It is based on the Frank–Oseen elastic theory to describe molecular reorientation and on a heat diffusion equation to model thermal effects. Molecular reorientation, its width and maximum value, and extraordinary refractive index are analyzed versus various values of absorption coefficient, rate of change of elastic coefficient, and thermal conductance. It appears that thermal effects do not influence the width of the molecular reorientation. The nonlocality is nearly unaffected by the increase of temperature. However, the maximum molecular reorientation might increase with temperature, in most cases, the extraordinary refractive index decreases with temperature. Only for unlikely high rates of change of elastic coefficients, the refractive index increases with temperature. Moreover, the transition between liquid crystal and isotropic phases is also discussed. The range of input beam width for which the material lasts in a liquid crystal state is also presented. Simulations are performed for parameters corresponding to two liquid crystals: low birefringent 1550 and 6CHBT of a medium birefringence.

FP-LMTO study of structural, electronic, thermodynamic and optical properties of MgxCd1−x Se alloys

Structural, electronic and optical properties of MgxCd1−x Se (0 ≤ x ≤ 1) are calculated for the first time using density functional theory. Our results show that these properties are strongly dependent on molar fraction of particular components — x. The bond between Cd and Se is partially covalent and the covalent nature of the bond decreases as the concentration of Mg increases from 0 % to 100 %. It is found that MgxCd1−x Se has a direct band gap in the entire range of x and the band gap of the alloy increases from 0.43 to 2.46 eV with the increase in Mg concentration. Frequency dependent dielectric constants ɛ1(ω), ɛ2(ω) refractive index n(ω) are also calculated and discussed in detail. The peak value of refractive indices shifts to higher energy regions with the increase in Mg. The larger value of the extraordinary refractive index confirms that the material is a positive birefringence crystal. The present comprehensive theoretical study of the optoelectronic properties of the material predicts that it can be effectively used in optoelectronic applications in the wide range of spectra: IR, visible and UV. In addition, we have also predicted the heat capacities (CV), the entropy (S), the internal energy (U) and the Helmholtz free energy (F) of MgxCd1−x Se ternary alloys.

Beam Pointing Precision Control by Using a Liquid Crystal Optical Phased Array

For laser beam steering, a pure optical-electric system without mechanical or inertial parts is used. The new device is a Liquid Crystal Optical Phased Array (LCOPA). Supplying electric field to liquid crystal cell with electrodes, the extraordinary refractive index changes, which results in phase shift that modulates the direction of incident beam. The advantages of this device are small size, less energy consuming, programmable and addressable control. A one dimensional device is designed and tested. The resolution of the steering angle is 20 μrad, the range is ±2o. The effect of flyback region and phase valley on diffraction efficiency is also discussed briefly.