渐变螺距螺旋线慢波系统高频特性的仿真分析

 为实现渐变螺距螺旋线慢波结构高频参数的高精度估计，基于MAFIA仿真平台，研究了螺距变化对超宽带螺旋线慢波系统(4~18 GHz)的色散、互作用阻抗与衰减常数的影响，获得了各高频参数随螺距变化的规律。研究表明：相速几乎随螺距增大而线性变大，互作用阻抗与螺距是非线性的关系，衰减常数随螺距增大而非线性地减小；由于在维持电子注与电磁波速度同步的限制下，螺距变化的幅度不可能很大，因此可近似按线性关系来处理互作用阻抗及衰减常数与螺距之间的关系。由此提出了一种可精确计算渐变螺距螺旋线慢波结构高频参数的方法－线性插值法     

Reflection spectrum of a cholesteric liquid crystal with structural defects

An expression is obtained for the reflection coefficient of circularly polarized light normally incident on the film of a cholesteric liquid crystal with a variable helix pitch. It is shown that, in the case of a single defect (local change in the helix pitch), the spectrum of light reflection from a cholesteric acquires a dip corresponding to the defect mode. New qualitative features appear in the reflection spectrum of a cholesteric with two defects as the distance between them varies.     

铁电液晶螺旋结构的理论近似研究

由铁电液晶（FLC）手性近晶C相下螺旋结构的理论近似计算得出FLC一个螺距内平均折射率的表达式,根据表达式可得出一个螺距内的FLC分子作为一个整体可以看成一个向列相液晶分子模型的结论.当FLC沿螺旋轴方向的厚度等于FLC螺距的整数倍时,液晶盒内垂直取向的FLC分子可看作向列相液晶模型组成的集合.ZLI-3654型FLC与5CB型向列相液晶的实验结果验证了上述结论,理论结果和实验结果一致.这一理论可为畸变螺旋FLC和垂直排列畸变螺旋FLC器件的制备以及FLC的应用提供理论指导和更深的认识. 关键词： 铁电液晶 螺距 平均折射率 向列相液晶     

Surface plasmon polariton coupling between a metal nanowire and a metal helix

The transmittance and steady-state electrical field distribution of a silver nanowire–helix system are investigated using the finite-difference time-domain method. In the nanowire–helix system, surface plasmon polaritons are coupled into the helix or squeezed into the space between the nanowire and helix. The transmittance strongly depends on the topologic shapes of the helix, especially the pitch height. Thus, the nanowire–helix system enables the detection of the displacement associated with helical deformation.     

Network of nano-droplets by a tri-block polymer

Mixtures of oil in water nano-droplets with two molecular weights of a tri-block polymer was studied by quasi elastic light scattering and small angle X-ray scattering. The results showed that the size and interaction of droplets didn’t change with increase of the tri-block polymer length but the order parameters increased. The increase of length of the tri-block biopolymer changed the dynamics of the droplets. A network formation is resulted with increase of the amount of tri-block polymer in the microemulsions.     

Estimation of the relaxation parameters of a cholesteric liquid by the parameters of acoustic streaming

The possibility of determining the relaxation time of cholesteric liquid crystals with a large helix pitch from the parameters of acoustic streaming is demonstrated experimentally.     

Nonlinear time-dependent simulation of helix traveling wave tubes

A one-dimensional nonlinear time-dependent theory for helix traveling wave tubes is studied. A generalized electromagnetic field is applied to the expression of the radio frequency field. To simulate the variations of the high frequency structure, such as the pitch taper and the effect of harmonics, the spatial average over a wavelength is substituted by a time average over a wave period in the equation of the radio frequency field. Under this assumption, the space charge field of the electron beam can be treated by a space charge wave model along with the space charge coefficient. The effects of the radio frequency and the space charge fields on the electrons are presented by the equations of the electron energy and the electron phase. The time-dependent simulation is compared with the frequency-domain simulation for a helix TWT, which validates the availability of this theory.     

Compression of a suspension of helical Yukawa rods

ABSTRACT

We present a Monte Carlo simulation study of the compression of helical Yukawa rods under isobaric conditions. The model is a chiral liquid crystal mesogen, which mimics, for instance, cellulose nanocrystals. It has several parameters (e.g. surface charge and internal pitch), which influence the compression route and the final structure. The strongest dependence of the structure of the condensed phase is found for the internal pitch of the charge helix: decreasing the internal pitch reduces the maximum density under otherwise equal conditions.     

From selective to wide-band light reflection: a simple thermal diffusion in a glassy cholesteric liquid crystal

Due to their helicoidal structure, cholesteric liquid crystals exhibit remarkable optical properties. Selective light reflection occurs when the pitch (repeat distance) is of the order of the wavelength of incident light propagating along the helix axis. The wavelength bandwidth, due to the optical anisotropy, is typically limited to 50 nm which is insufficient for some applications (full-colors displays, for example). By introducing a pitch gradient in the helix during a novel two-step process in a cholesteric glass, we show that reflection may occur over a wavelength bandwidth greater than 300 nm. First, the reflection bandwidth is adjusted by thermal annealing. Then, the optical properties are permanently stored by quenching the viscous material to a glass at room temperature. The two steps, pitch gradient establishment and film hardening, are independently controlled. The present process exhibits some reversibility and properties intrinsic to the glassy state are gained: laser-writing high resolution full-color images on solid films for image recording or high-density optical data-storage are indeed conceivable. Received 17 December 1998     

Four-wave mixing and optical-field-induced helical structure in liquid crystalline materials

Four-wave mixing with two circularly polarized counter-propagating waves inducing a helical birefringence structure can be used to measure a number of material parameters of a liquid crystalline medium. Application to a cholesteric substance is particularly interesting. A resonant effect is expected when the induced helix has a pitch coinciding with the natural helical pitch of the medium towards the isotropic → cholesteric transition. Field-induced shift in the transition temperature is also expected.     

Optimization Design of Helix Pitch for Efficiency Enhancement in the Helix Travelling Wave Tubes

The output section of a helix travelling wave tube usually contains a helix pitch taper for high rf electron efficiency. By keeping the rf field as synchronous as possible with the decelerating electron beam bunches, the rf field can extract much more energy from the beam, and thus the maximum electron efficiency can be realized. Recently, a global simulated annealing algorithm has been employed to design the helix pitch profile so as to improve the electron efficiency as much as possible. From the numerical results, it is concluded that the electron efficiency can be enhanced by about 4%-8%.     

On mechanisms of the helix pitch variation in a thin cholesteric layer confined between two surfaces

Two possible mechanisms of the temperature-induced variation and jump of the helix pitch in a spatially bounded planar layer of a cholesteric liquid crystal (LC) was considered within the framework of the continuum theory of elasticity. These mechanisms are related to the existence of two configuration curves of the system free energy. The states with local free energy minima on each of the configuration curves correspond to topologically equivalent configurations of the LC director distribution and are quasi-equivalent in this sense. The transitions between such quasi-equivalent states are especially important in the first mechanism of the helix pitch jump proceeding without participation of defects. The second mechanism is related to transitions between the ground states of different configuration curves corresponding to topologically nonequivalent configurations. This mechanism requires either participation of disclination lines or the formation of defects.     

Nonlinear “brightening” of a film of nonabsorbing chiral nematic under selective reflection conditions

A strong decrease in the reflection coefficient of a film of a nonabsorbing chiral nematic (cholesteric) is observed experimentally in the region of selective reflection under the action of a powerful beam of circularly polarized light. The independence of the effect from the average power density (and its dependence only on the peak power density) allow it to be attributed to an increase in the pitch of the cholesteric helix to such a degree that it is completely unwound, an effect previously observed only in static and low-frequency electric and magnetic fields, in the strong field of the light wave. These are the first experiments in which, on account of the specially chosen irradiation conditions, the changes produced in the pitch of the helix by the field of the light wave can accumulate over time, so that a nonthermal mechanism can be invoked to explain the nonlinear brightening of a mirror made of a chiral nematic. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 6, 403–407 (25 March 1996)     

Method for producing nanomaterials in the plasma of a low-pressure pulsed arc discharge

A new method for the production of nanomaterials in the plasma of a low-pressure arc discharge is developed and experimentally studied. This method can be used to synthesize nanoparticles 5–10 nm in size with a narrow size distribution. In this method, a low-pressure arc discharge is used to melt a material, to disperse the molten material, to deliver liquid material droplets to the plasma, to cool the liquid nanoparticles forming in the plasma up to their solidification, and to deposit the solidified nanoparticles onto a substrate.     

Attenuation of solar radiation by a water mist from the ultraviolet to the infrared range

A model is developed for the hemispherical transmittance of direct and scattered solar radiation from a cloudless atmosphere by a mist layer of water droplets in order to investigate the potential of water misting systems to serve as a protection from solar irradiation with particular emphasis on harmful UV radiation. The proposed model is based on published spectral experimental data for solar irradiation, Mie theory for interaction of the radiation with single spherical droplets, and radiative transfer theory. Known limiting solutions are employed to simplify the Mie calculations. The modified two-flux approximation is used to account for both direct and diffuse irradiation in lieu of a numerical solution for the full radiative transfer equation in anisotropically scattering media. The role of the governing parameters of a disperse water curtain of water droplets, water content, and droplet size for sample conditions is studied in some detail, particularly in the near-ultraviolet part of the spectrum where radiation can result in human tissue damage.     

Dynamics of jumpwise temperature pitch variations in planar cholesteric layers for a finite strength of surface anchoring

The dynamics of pitch jumps in cholesteric layers with a finite surface anchoring strength under variations in temperature is investigated theoretically. General expressions are presented that connect the dynamics of pitch jumps with the parameters that determine the process, such as the viscosity, the specific form of the anchoring potential, and the dimensionless parameter S _d = K₂₂/Wd, where W is the depth of the anchoring potential, K₂₂ is the twist elastic modulus, and d is the layer thickness. It is found that the shape of the anchoring potential significantly influences the temporal behavior of the cholesteric helix in the process of a pitch jump. To illustrate this revealed dependence of the pitch jump dynamics on the shape and strength of the anchoring potential, the problem was investigated for two different models of the surface anchoring potential for a jump mechanism in connection with the director at the surface slipping over the barrier of the anchoring potential. Calculations for the unwinding (winding) of the helix in the process of the jump were performed to investigate the case of infinitely strong anchoring on one surface and finite anchoring on the other, which is important in applications. The results show that an experimental investigation of the dynamics of the pitch jumps will make it possible to distinguish different shapes of the finite strength anchoring potential and, in particular, it will provide a means for determining whether the well-known Rapini-Papoular anchoring potential is the best suited potential relevant to the dynamics of pitch jumps in cholesteric layers with a finite surface anchoring strength. The optimal conditions for experimental observation of these phenomena are briefly considered.     

Investigation of the frustration between ferroelectricity and antiferroelectricity in smectics

A molecular-statistical theory that describes the smectic phases with frustration between the ferroelectric and antiferroelectric ones has been developed. The molecular model is used to estimate the mean-field parameters responsible for the formation of complex smectic phases with long periods. A general phase diagram is constructed and the helix pitch is estimated in all phases. The results obtained are compared with the experimental ones.     

Propagation of hypersound and acoustic relaxation in the isotropic phase of cholesteryl miristate

The features of the sound propagation at a frequency of 1.1 GHz in the isotropic phase of cholesteryl miristate are studied. The analysis of experimental data in terms of the theory of interacting modes shows that the peculiarities of the temperature behavior of sound velocity and absorption coefficient are determined by the interaction between the hypersonic wave and the fluctuations of the cholesteric helix pitch.     

Helical ordering of hydrogen bonds between pairs of DNA bases

The interaction between hydrogen bonds and conformational elastic degrees of freedom has been investigated using the simplest model of a double-strand DNA molecule. The hydrogen bonds are described in terms of two-level quantum systems. After excluding conformational degrees of freedom, one has effective interaction among two-level systems. In the ground state of an ideal double helix, hydrogen bonds in a DNA molecule also have a helical order induced by conformational degrees of freedom. The pitch of the hydrogen-bond helix (and even its sign under certain conditions) is different from that of the basic helix pitch and, generally speaking, is incommensurate with the latter. This effect can, possibly, lead to an inversion of the sign of the circular dichroism in spectral bands, which was detected in some experiments. Zh. éksp. Teor. Fiz. 115, 940–950 (March 1999)