Temperature tunability of cavity-semiconducting waveguide coupling in a two-dimensional photonic crystal

We study the coupling efficiency between a cavity resonator and semiconducting waveguide in a two-dimensional photonic crystal by varying the temperature. We used the revised plane wave expansion and finite difference time domain methods to evaluate the coupling efficiency. The photonic crystal waveguide is composed of a row of InSb semiconducting materials, and the efficiency was calculated at various temperatures. The findings indicate that the temperature can be used as a useful efficiency controller.     

Growth and properties of Li,Ta modified (K,Na)NbO3 lead‐free piezoelectric single crystals

Li, Ta modified (K,Na)NbO₃ single crystals with the size of 18 mm × 18 mm × 10 mm were successfully grown by top‐seeded solution growth method, with orthorhombic–tetra‐gonal phase transition temperature ～79 °C and Curie temperature ～276 °C. The electromechanical coupling factors k₃₃ and k_t were found to be ～88% and ～65%, respectively. The piezoelectric coefficient d₃₃ for the [001]_c poled crystals reached 255 pC/N. In addition, the electromechanical coupling factor exhibited high stability over the temperature range of –50 °C to 70 °C, making these lead free crystals good candidates for electromechanical applications. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Convergence properties of the supercell approach in the study of local defects in solids

The “supercell” scheme is applied to the study of local defects in MgO (Ca substitution, cation and anion vacancies) and bulk silicon (carbon substitution). The trend of the quantities of interest (defect formation energy, geometrical relaxation, charge distribution around the defect) as a function of the supercell size is explored; when neutral defects are considered, supercells containing 50 to 100 atoms are large enough to allow for most of the nuclear and electronic relaxation and to produce a negligible interaction between defects in different cells. These conclusions apply both to ionic and covalent host crystals. Present day ab initio quantum mechanical periodic computer programs can handle cells of such a size at a relatively low cost and high numerical accuracy.

When charged defects are considered (vacancies in MgO), the supercell scheme must be modified in order to avoid Coulomb divergencies, but the usually adopted correction, which consists in introducing a compensating uniform background of charge, generates spurious higher order electrostatic interactions, which are far from being negligible. The resulting defect formation energies show very slow, if any, convergence trends and “a posteriori” semiclassical corrections proposed in the literature do not represent a general solution to the problem. On the other hand, other properties, such as atom relaxation and charge distribution, show a much faster convergence than energy with respect to the cell size.     

X-ray studies of the smectic B phase of the 4-bromobenzylidene-4′-alkoxyanilines

The results of X-ray studies of the nine members (4?÷?12) of the 4-bromobenzylidene-4′-alkyloxyaniline homologous series (nBBAA) in smectic B (SmB) and smectic A (SmA) phases are presented. Interestingly, no SmC phase was identified. The thickness of smectic layers in the SmA and SmB phases was estimated showing unexpectedly that in SmA it is bigger than in the SmB. For the SmB phase, orthorhombic unit cell parameters were determined as a function of temperature. The obtained data are discussed in relation to rotational dynamics.     

Slow molecular processes in a nematic liquid crystal confined to random porous network formed by aerosil particles

Frequency dispersion measurements of proton spin–lattice relaxation rates (R ₁) of liquid crystal 4-propyl-4′-pentylazoxybenzene in bulk and confined samples (in random porous network of aerosil nano-particles) are reported in isotropic and nematic phases. Significant low-frequency increase in R ₁ in confined samples indicates slow molecular reorientations mediated by translational displacements near the adsorbing porous surface. The resulting dispersion behavior of R ₁ (～ω ^{? p} ) reflects the nature of the random surface (p?=?0.5 for equi-partition of the diffusive modes). The observed temperature-independent exponent in the isotropic phase (p?=?0.34) indicates the abundance of low-wavelength surface modes. Its temperature-dependent higher values in the nematic phase (from 0.59 to 0.65 on cooling), and increased spin–lattice coupling via this mechanism, show progressive onset of longer wavelength modes. A detailed analysis shows the effect of confinement on the order director fluctuations, molecular reorientations, and translational diffusion of the molecules.     

夹持方式和加工误差对大口径薄型KDP晶体附加面形的影响

惯性约束聚变频率转换系统中,大口径薄型KDP晶体的面形质量是影响频率转换效率能否达到设计要求的关键因素之一。针对45放置状态下口径为400 mm400 mm的三倍频KDP晶体,采用ANSYS有限元分析软件,建立了不同夹持方式和具有不同加工误差的KDP晶体模型和夹具模型,分析了加工误差对不同夹持方式下KDP晶体附加面形的影响,给出了不同加工误差和不同夹持情况下,KDP晶体附加面形的P-V值和RMS值。研究结果表明,夹持方式和加工误差是引起KDP晶体附加面形变化的重要因素,正面压条夹持方式即使在晶体和夹具存在加工误差时也可以较好地控制晶体的附加面形。     

采用紫外光提高双掺杂铌酸锂晶体中全息记录的灵敏度和光栅强度

提出了一种在双掺杂铌酸锂晶体中用调制的双紫外光进行非挥发全息记录的方法。与通常的用紫外光敏化的非挥发全息记录相比,这种方法可以大幅度地提高光栅强度和记录灵敏度。联立双中心物质方程和双光束耦合波方程,数值分析了光栅强度和衍射效率随时间的变化并讨论了掺杂浓度和记录光强对紫外光非挥发全息记录机制下光折变效应的影响。研究发现,紫外光记录得到的深浅中心的光栅具有相同的相位,总的光栅（深浅中心光栅的叠加）强度为两光栅强度之和,固定过程中深中心的光栅得到增强;增大深浅中心掺杂的浓度可以提高光栅强度,增大记录紫外光的光强可以增加光栅的强度和记录灵敏度。理论模拟可以证实并预测实验结果。     

新型全固态激光器作用过程中的激光参数分析

对影响LD泵浦的Nd∶GdVO4,Nd∶YVO4和Nd∶YAG激光器输出特性的增益介质长度、腔内损耗、输出镜透过率、泵浦光和振荡光的光斑大小进行了分析和数值模拟。当腔内固有损耗为2%,输出镜透过率为10%时,Nd∶Gd-VO4,Nd∶YVO4和Nd∶YAG晶体的振荡阈值分别为1.77W、0.55W和3.34W。在25W泵浦功率下,利用1mm长的Nd∶GdVO4,Nd∶YVO4和Nd∶YAG晶体可分别得到13.17W,13.26W和8.43W的输出功率,相应的输出镜最佳透过率分别为24%,44%和16%。     

A phenomenological theory of the isotropic to chiral smectic-C phase transition

In this paper we discuss the direct isotropic to chiral smectic-C phase transition on the basis of a phenomenological theory. The model free energy is written in terms of the coupled order parameters including the spontaneous polarization. We present a detailed analysis of the different phases that can occur and analyze the question under which conditions a direct isotropic to chiral smectic-C phase transition is possible when compared to other phase transitions. On the basis of this model the isotropic-smectic-C^* transition is always of first order. The theoretical predictions are compared with the available experimental results.-1     

Mach–Zehnder interferometers in photonic crystals

Photonic crystal technology allows the creation of optical waveguides with low sharp-bending losses as well as ultra-low group velocity. This last property is particularly interesting to develop highly-compact optical devices based on the controlled modification of the optical phase of the signals traveling through the waveguides. Among these devices, the Mach–Zehnder interferometer acquires fundamental importance because it can be used as a building block of more complex optical devices and functionalities such as optical filters, wavelength demultiplexers, channels interleavers, intensity modulators, switches and optical gates. In this paper, the performance of a Mach–Zehnder interferometer consisting of two coupled-cavity waveguides with different lengths created in a two-dimensional photonic crystal is theoretically analyzed. We also provide simulation results using a finite-difference time-domain code that confirm the theoretical analysis. The main limitations in the performance of the structure are addressed and discussed.