Ultrasound effect on neutron Bragg diffraction in a perfect and deformed silicon single crystal

The features of neutron Bragg diffraction in a perfect and bent (deformed) silicon single crystal have been studied under ultrasound excitation. In contrast to a perfect crystal where an increase in the diffraction intensity has been observed with an increase in the ultrasound wave’s amplitude, in a deformed silicon crystal the intensity sharply decreases (two times) already at small voltages on a piezoelectric transducer. The depth and position of minima of the total intensity depend on the ultrasound wave’s amplitude and the bending radius of a crystal. To explain the observed effects the modified Penning-Polder-Kato model has been successfully used, in which the role of ultrasound is reduced to the formation of resonance transitions of imaging points between different sheets of dispersion surface. Experimental proofs of validity of the used model are presented. Estimations of the probabilities of one- and multiphonon scattering processes obtained from experimental data have a good agreement with theoretical ones.     

Characterization and on‐line adjustment of the sagittal‐bent Laue crystal profile

The sagittal‐bent Laue monochromator can provide an ideal way to focus high‐energy X‐ray beams. However, the anticlastic curvature induced by sagittal bending has a great influence on the crystal performance. Thus, characterizing the bent‐crystal shape is very important for predicting the performance of the bent‐crystal monochromator. In this paper the crystal profile is measured by off‐line optical metrology and on‐line X‐ray experiments. The off‐line results showed that the bent‐crystal surface could be well fitted to a saddle surface apart from a redundant cubic term which was related to the different couples applied on the crystal. On‐line characterization of the meridional and the sagittal radius of the bent crystal includes double‐crystal topography and ray‐tracing measurement. In addition, the double‐crystal topography experiment could be used as a quick diagnostic method for the bending condition adjustment. The sagittal radius of the bent crystal was characterized through a ray‐tracing experiment by using a particularly designed tungsten mask. Moreover, rocking curves under different bending conditions were measured as well. The results were highly consistent with analytical results derived from the elastic theory. Furthermore, radii along different vertical positions under various bending conditions were measured and showed a quadratic relationship between the vertical positions and the meridional radii.     

同步辐射聚焦镜压弯机构设计与面形误差分析

基于同步辐射聚焦镜的原理，对压弯机构进行设计，对压弯理论进行介绍, 并推导了压弯镜理论面形的计算过程；根据设计理论建立了基于柔性铰链的压弯机构的模型，并对压弯机构的参数进行计算，有效长度为84.9 mm，最大力矩为259.8×10-3 N·m。设计并校核了压弯机构，满足了压弯机构在材料力学上的要求；最后针对压弯镜存在的理论面型误差，通过对压弯镜进行有限元仿真得到模拟值，与理论值进行对比，分析了不同的因素对面型误差的影响。     

Wormlike chain theory and bending of short DNA

The probability distributions for bending angles in double helical DNA obtained in all-atom molecular dynamics simulations are compared with theoretical predictions. The computed distributions remarkably agree with the wormlike chain theory and qualitatively differ from predictions of the subelastic chain model. The computed data exhibit only small anomalies in the apparent flexibility of short DNA and cannot account for the recently reported AFM data. It is possible that the current atomistic DNA models miss some essential mechanisms of DNA bending on intermediate length scales. Analysis of bent DNA structures reveal, however, that the bending motion is structurally heterogeneous and directionally anisotropic on the length scales where the experimental anomalies were detected. These effects are essential for interpretation of the experimental data and they also can be responsible for the apparent discrepancy.     

Influences of flexible defect on the interplay of supercoiling and knotting of circular DNA

Knots are discovered in biophysical systems, such as DNA and proteins. Knotted portions in knotted DNA are significantly bent and their corresponding bending angles are comparable with or larger than the sharp bending angle resulting in flexible defects. The role of flexible defects in the interplay of supercoiling and knotting of circular DNA were predicted by a Monte Carlo simulation. In knotted DNA with a particular knot type, a flexible defect noticeably enhances the supercoiling of the knotted DNA and the decreasing excitation energy makes the knotted portion more compact. A reduction in twist rigidity and unwinding of flexible defects are incorporated into the numerical simulations, so that interplay of supercoiling and knotting of circular DNA is studied under torsional conditions. Increasing unwinding not only results in a wider linking number distribution, but also leads to a drift of the distribution to lower values. A flexible defect has obvious effects on knotting probability. The summation of equilibrium distribution probability for nontrivial knotted DNA with different contour length does not change with excitation energy monotonically and has a maximum at an intermediate value of excitation energy around 5 kBT. In the phase space of knot length and gyration radius of knotted DNA, knot length does not anticorrelate with its gyration radius, which is attributed to the flexible defect in the knotted portion, which leads to the release of bending energy and inhibited the competition between entropy and bending energy.     

Bending and springback theory of metal-polymer sandwich laminates

In this report an analysis is made of the behavior of sandwich beams in which the core polymer is laminated on both sides with surface metal sheets, each of which has a different thickness and mechanical properties when they are loaded with a uniform bending moment which is then released resulting in springback of the bent sandwich beam. It is assumed that the polymer behaves elastically because the bending strain in the core is small and its elastic limit is much larger than that of metals. Sandwich beams have various elastoplstic stess distributions when bent depending on the mutual relationships between their dimensions, the mechanical moduli, and the applied bending moment. Further, residual curvatures, shifting position of neutral axis, and residual stress distributions in sandwich beams variously elastoplastically stressed initially on the decrease of applied bending moment are analyzed.     

Proton spin relaxation and self-diffusion in some ‘isotropic’ nematogens

Angular distributions of reactive scattering arising from a long-lived collision complex dissociating via a transition state that approximates to a linear rotator are predicted for H atom displacement occurring over a distribution of bending angles β with respect to the rotator axis. A conical angular distribution may be generated when only a narrow range of bending angles contributes to the reactive scattering. However, the angular distribution may take on quite different character as the distribution of bending angles becomes broader. Thus, an isotropic angular distribution is generated by a bending angle distribution which follows the spherical polar weighting P(β) ∝ sin β. Forward and backward scattering is generated by bending angle distributions favouring less strongly bent displacement, while sideways scattering is generated by a bending angle distribution favouring more strongly bent displacement. The experimental angular distribution for OH + CO reactive scattering which shows mild forward and backward peaking, corresponds to a range of bending angles for H atom displacement from the H-OCO transition state which is broadened about the nominal preferred direction β = 45° but avoids strongly bent directions with β ≈ 90°.     

Piezoelectric energy nanoharvester based on an array of ZnO whisker nanocrystals and a flat copper electrode

A piezoelectric low-power nanogenerator based on an array of ZnO whisker nanocrystals and a movable flat top electrode has been developed. It has been shown that this element generates ac current under the effect of mechanical vibrations with a frequency up to 100 Hz, which makes it possible to accumulate the capacitor charge. The investigation of the sample on a vibration stand under the acceleration of 9.8 m/s² has demonstrated that the stable variable signal from 50 mV to 1.5 V appears at a frequency of 20–120 Hz. The experimental dependence of the generated voltage on the weight of the load applied to the top electrode is obtained. The appearance of the ac signal is explained in the context of the classical piezoelectric effect, namely, the ZnO whisker nanocrystals in the starting state are already bent, and the different-sign charges are generated under the effect of mechanical vibrations during bending and unbending.     

Stimulated four-photon mixing in bent few-mode optical fibres

A divided-pump four-photon mixing process is used to investigate the behaviour of modes on a bent few-mode optical fibre. The second-order mode set becomes non-degenerate, even for slight bending; the two modes propagate along the fibre with different group delays and preferred axes of symmetry. Our experimental results are compared with a theoretical analysis.     

内窥镜的红外激光传输用柔性空芯光纤的研制

通过理论计算确立了多功能介质-金属结构空芯光纤的结构参量,优化了液相镀膜法的有关条件,明确了具体制作参量。制作了以聚碳酸酯毛细管为基管的,高柔韧性的,可同时低损耗传输红外目标波长激光和可见导航光红外的空芯光纤。对光纤传输性能进行了测试。在2.94μm波长的Er∶YAG和0.63μm波长的红色半导体激光器的直线损耗分别为0.4 dB/m和3 dB/m。组装在医疗内窥镜中的柔性空芯光纤,在先端以0.9 cm半径135°角弯曲时,对Er∶YAG激光仍有近70%的传输效率;绿色导航光在内窥镜中的的损耗值为11 dB,绿色指示点在内窥镜的视窗中清晰可见。结果表明此种光纤在内窥镜的激光传输方面有重要的应用价值。     

Automatic fringe analysis of the induced anisotropy of bent optical fibres

A new theoretical model considering the refraction of the incident light beam by the fibre is suggested to determine the refractive index profile of bent optical fibres. This new model (slabs model) considering the cross section of the bent optical fibre consists of large number of slabs. The slabs model bases on the refraction of the incident beam by the fibre. The refractive index profile of the optical fibre cladding before bending obtained using the automated Fizeau interferometer with the aid of suggested model is compared with other models such as, the homogenous model and the multilayer model to verify the ability of this slab model. The refractive index profile of the bent optical fibre cladding is investigated using this suggested model. In addition, the new model is used to obtain the induced birefringence and the guiding parameters. The bending radius is recommended to be greater than 7.1 mm for the used optical fibre. The consideration of the refraction increases the accuracy of the results.     

Effect of a rippling mode on resonances of carbon nanotubes

A recent study determined the Young's modulus of carbon nanotubes by measuring resonance frequency and using the modulus-frequency relation resulting from the linear vibration theory. It leads to the report that the Young's modulus decreases sharply, from about 1 to 0.1 TPa with the diameter D increasing from 8 to 40 nanometers, and the investigators attributed this decrease to the emergence of an unusual bending mode during the measurement that corresponds to rippling on the inner arc of the bent nanotubes. The nonlinear analysis presented in this paper that captures the rippling mode suggests that the effective Young's modulus can indeed decrease substantially with increasing diameter, and that the results from the classical linear theory may be invalid in such measurements.     

Characteristic Analysis of Bending Coupling Between Two Optical Waveguides

By means of the coupled mode theory and transfer matrix technique, a method is presented for analyzing the bending coupling between straight and bent rectangular waveguides, or between two bent rectangular waveguides. Calculations show that an effective coupling region exists around the central coupling point, in which the bending coupling appears, beyond which the bending coupling is very weak, and the propagation power in each waveguide becomes constant. When the bending radius becomes larger or the central coupling gap becomes smaller, this effective coupling region becomes wider, in which the coupling behavior becomes stronger.     

弯曲波导研究进展及其应用

本文主要分析了弯曲波导损耗机理,包括传输损耗、辐射损耗、模式转换损耗。重点综述了设计低损耗弯曲波导的方法,包括波导材料、弯曲波导的曲线形状、波导种类、脊型波导的宽度、脊高、弯曲半径、模场分布、弯曲波导曲线形状和其他新型波导结构等。简要概括了近年来设计和制备低损耗弯曲波导的代表性工作。介绍了弯曲波导在集成光学中的应用。通过对弯曲波导的损耗及耦合机制理论的不断完善,实现光在较小弯曲半径的低损耗传输,从而提高集成光学的集成度是弯曲波导今后的发展趋势。     

一种用于碳纳米管模拟的分子结构力学方法

提出一种基于分子力学的分子结构力学方法,该方法用分子力学中的力场势能函数表述系统的势能,从能量原理出发,在小变形假设的基础上建立系统方程．基于此方法,模拟了单壁碳纳米管的拉伸特性,得到碳纳米管弹性模量的尺度依赖性关系．在碳纳米管的弯曲分析中,将计算结果和材料力学中的理论结果进行了比较,发现随着碳纳米管半径的增大,计算结果与理论值趋于一致．     

Multifiber optical catheter with bending control of distal end: Applications of Raman biospectroscopy

This work demonstrates a multifiber optical catheter with bending control of the distal end, whose main use is to collect Raman signal for biospectroscopy. Also, a method for fiber background subtraction is presented, allowing removal of fiber emission peaks from gross Raman spectrum. The fiber optic catheter with seven fiber optics wrapped up in a resin and a biocompatible flexible teflon tube has been modified to incorporate a mechanical device with gauntlet, which allows bending of the distal extremity to access a desired location obeying the anatomy of the organ. The mechanical device (gauntlet) was built halfway from the catheter proximal excitation tip, allowing the bending of the distal end of about 38 mm diameter. Dispersive Raman spectra at 785 nm excitation wavelength from samples were measured without and with bending (from 120 mm decreasing to 14 mm) of the distal end of the catheter. An optical catheter without the gauntlet was used as a reference. Results demonstrated that both catheters (with bending mechanism and standard one) had about the same Raman signal when the device was bent semicircularly to 38 mm diameter (bending limit for the mechanism), with negligible implication to the Raman signal-to-noise relation (SNR) from sample. When both catheters were bent in a full turn, the Raman signal decreased to about 40% when bending reached 14 mm diameter, mainly due to losses introduced by changes in the fiber total internal reflection. The background removal was efficient, with minimum residual from fiber peaks. This optical catherer could be very useful in laser diagnosis and clinical applications, introducing a way to control the fiber tip position and angle on the tissue or organ. Published in Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 97–102, January–February, 2007.     

弯曲氧化锌微米线微腔中的回音壁模

本文用微区共聚焦荧光光学显微镜在垂直于氧化锌微米线c轴的方向测量微米线上不同位置的光谱,通过对比直的和弯曲的微米线上TE和TM偏振的光谱,观察到了弯曲氧化锌微米线中不同位置的回音壁模式的移动,其机理是在弯曲应力作用下激子能级发生了移动,带边附近的介电常数也随之变化,导致微腔中的回音壁模式发生移动.     

Electrical properties and switching mechanisms of flexible organic-inorganic bistable devices

The electrical properties of flexible organic-inorganic bistable devices fabricated with aluminum (Al) sandwiched between tris-(8-hydroxyquinoline) aluminum (Alq₃) and zinc selenide (ZnSe) layers were investigated. Current–voltage (I–V) measurements were conducted under conditions before, during, and after bending while changing the bending distance of the base plastic polyethylene terephthalate (PET) systematically. The maximum ON/OFF current ratios of the flexible bistable devices at flat and bent conditions were approximately 2.7×10⁴ and 1.9×10⁴, respectively. The conduction mechanisms in both ON and OFF states were analyzed by a theoretical model.     

基于全矢量模式匹配法的三维弯曲波导本征模式计算

采用精确、严格的全矢量模式匹配法，计算了三维脊型弯曲波导和条形弯曲波导的本征模式，分析了模式折射率和辐射损耗随弯曲半径的变化关系.计算结果与已报道的有限差分法的结果符合很好.对文献中已报道但没有深入分析的现象给出了合理的解释.半径为50和12 μm的条形波导的计算结果表明：当波导的弯曲半径减小时，发生在分界面处的TE-TM模耦合会显著增强，这将导致弯曲波导中的模式不再是传统意义上的准TE模和准TM模，而是混合模.随着半径的减小，混合模的横向分量之间的差异会减小，当两个横向分量达到近似相等的强度时，波导的偏振相关性将会大大减小. 关键词： 导波光学 全矢量模式匹配法 微环谐振腔 光波导理论     

Reliable Polymer-Stabilized Ferroelectric Liquid Crystal Device Using Thin Plastic Substrates

We developed a rollable polymer-stabilized ferroelectric liquid crystal (FLC) display device using thin plastic substrates. In a device using 200-μm-thick substrates which are fastened by polymer walls and networks made by photopolymerization-induced phase separation, disorder of the FLC alignment was caused by exfoliation of these walls in curvature radii of under 30 mm. Otherwise, the uniformity of the FLC alignment was maintained even after a device using 100-μm-thick substrates was bent at a radius of 7 mm. The enhanced bending tolerance does not depend on the FLC alignment direction, and the device could be bent both convexly and concavely without any FLC alignment change. A rollable device with a size of 100 × 100 mm exhibited uniform grayscale display operation between crossed polarizers when bent with a radius of 15 mm.