A Three-Dimensional Ray-Tracing Study of R-X Mode Waves during High Geomagnetic Activity

We further present a three-dimensional (3D) ray-tracing study on the propagation characteristic of the superluminous R-X mode waves during high geomagnetic activity following our recent two-dimensional results [J. Geophys. Res. 112(2007)A10214]. We perform numerical calculations for this mode which originates at specific altitude r=2.0R_E in the source cavity along a 70° night geomagnetic field line. We demonstrate that the ray path of the R-X mode is essentially governed by the azimuthal angle of the wave vector k. Ray paths starting with azimuthal angle 180° (or in the meridian plane) can reach the lowest latitude, but stay at relatively higher latitudes with the azimuthal angles other than 180° (or off the meridian plane). The results further supports the previous finding that the R-X mode may be physicallypresent in the radiation belts under appropriate conditions.     

Interhemispheric Comparison of Dipole Tilt Angle Effects on Latitude of Mid-Altitude Cusp

A statistical study of interhemispheric comparison of dipole tilt angle effect on the latitude of the mid-altitude cusp is preformed by a data set of the Cluster cusp crossings over a 5-year period. The result shows that the dipole tilt angle has a clear control of the cusp latitudinal location. When the dipole tilts sunwards, the cusp is shifted poleward. The northern cusp moves 1° ILAT for every 15.4° increase in the dipole tilt angle, while the southern cusp moves 1° ILAT for every 20.8° increase in the dipole tilt angle. This suggests that an interhemispheric difference appears in the dependence of cusp latitudinal location on the dipole tilt angle.     

高超声速条件下7°直圆锥边界层转捩实验研究

在Ma=6低噪声风洞中开展了半锥角7?的直圆锥边界层转捩相关实验研究.利用响应频率达到MHz量级的高频压力传感器对圆锥壁面脉动压力进行了测量,研究了高超声速圆锥边界层中扰动波的发展过程.结果表明:高超声速圆锥边界层中第二模态扰动波产生的位置以及扰动波特征频率和波长等参数受雷诺数影响较大,当单位雷诺数从2×106m~(-1)增加到8×106m~(-1)时,第二模态波的特征频率从55 k Hz增加到226 k Hz;随着单位雷诺数增加,边界层中扰动增长速度加快,第二模态波出现在圆锥表面更靠近上游的位置;相同单位雷诺数条件下,随着第二模态波的向下游传播,其特征频率逐渐减小.通过对比发现自由来流湍流度对边界层中扰动波的发展同样有较大影响,自由来流湍流度降低,边界层中的第二模态波的特征频率明显减小.利用互相关分析得出第二模态扰动波在边界层中的传播速度大约为当地主流速度的0.8—0.9倍.在1?小攻角条件下,圆锥迎风面和背风面边界层发展呈现出明显的差异,背风面边界层中扰动发展提前,第二模态波出现在更靠近上游的位置,而迎风面中扰动发展受到抑制,第二模态波特征频率更大.     

Accurate method for determining tilt bias angles in thin films of nematic liquid crystals

We have developed a new method for measuring tilt bias angles in spatially uniform and nonuniform thin films of nematic liquid crystals. The method employs modulation ellipsometry, based on the use of an exponentially decaying light wave to probe the boundary layer. Oscillations of the director of the liquid crystal, which are induced by the flexoelectric torque, are excited with an external periodic field. A periodic variation of the ellipticity of the light wave reflected from the interface is detected at both the first and second harmonics of the exciting electric field. When these two Fourier components of the electrooptic response are known, it is possible to calculate both the tilt bias angle θ₀ of the director and the dynamic deviation δ₀ of the tilt bias angle. The angles θ₀ and δ₀ measured by this method on the surface of an electrode (ITO) and on the surface of a ferroelectric film (a copolymer of vinylidene fluoride and trifluoroethylene), oriented in a corona discharge, were equal to θ ₀=5.1°, δθ=0.5° and θ₀=89°, δθ=0.06°, respectively. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 6, 387–392 (25 March 1998)     

Wavelet Cross-Spectrum Analysis of Multi-Scale Disturbance Instability and Transition on Sharp Cone Hypersonic Boundary Layer

Experimental measurement of hypersonic boundary layer stability and transition on a sharp cone with a half angle of 5° is carried out at free-coming stream Mach number 6 in a hypersonic wind tunnel. Mean andfluctuation surface-thermal-flux characteristics of the hypersonic boundary layer flow are measured by Pt-thin-film thermocouple temperature sensors installed at 28 stations on the cone surface along longitudinal direction. At hypersonic speeds, the dominant flow instabilities demonstrate that the growth rate of the second mode tends to exceed that of the low-frequency mode. Wavelet-based cross-spectrum technique is introduced to obtain the multi-scale cross-spectral characteristics of the fluctuating signals in thefrequency range of the second mode. Nonlinear interactions both of the second mode disturbance and the first mode disturbance are demonstrated to be dominant instabilities in the initial stage of laminar-turbulence transition for hypersonic shear flow.     

Microstructures of grain boundaries in (001)-oriented strontium bismuth tantalate thin films grown by pulsed laser deposition

(001)-oriented strontium bismuth tantalate thin films have been grown on Pt/TiO₂/SiO₂/Si (100) substrates by pulsed laser deposition. The room-temperature current–electric field dependence of the films has been investigated, which revealed a space-charge-limited conduction mechanism. The microstructures of grain boundaries and structural defects in these films were also examined by transmission electron microscopy and high-resolution transmission electron microscopy, respectively. The grains of the films deposited at 550 °C exhibited polyhedral morphologies, and the average grain size was about 50 nm in length and 35 nm in width. At a small misorientation angle (8.2°) tilt boundary, a regular array of edge dislocations with about 3-nm periodic distance was observed, and localized strain contrast near the dislocation cores was also observed. The Burgers vector b of the edge dislocation was determined to be [110]. At a high misorientation angle (39.0°) tilt grain boundary lattice strain contrast associated with the distortion of lattice planes was observed, and the mismatching lattice images occurred at about 2 nm along the boundary. The relationship between microstructural defects at grain boundaries and leakage currents of these films is also discussed. Received: 8 September 2000 / Accepted: 18 December 2000 / Published online: 28 February 2001     

圆锥透镜对偏轴球面波光束的衍射模式

球面波入射圆锥透镜产生的出射光束中心光斑直径较小且焦深长，适合通讯领域长距离测量和准直。针对实际操作中球面波光束入射时很可能与圆锥透镜的中心轴线发生偏移或倾斜，使衍射光场发生变化，着重研究圆锥透镜对偏轴球面波光束的衍射模式，并通过衍射理论和稳相法分析，研究其径向衍射光场的分布特性。使用半径为15mm，底角为1°的圆锥透镜进行计算机仿真和实验，最后将实验结果与理论分析和仿真结果相比较，证明球面波的曲率半径和出射光束的传输距离越小，偏轴对衍射模式的影响越小。该研究结果表明：在激光通讯等长距离通信中，使用曲率半径小的球面波光束代替平面波入射圆锥透镜，不仅可以扩展焦深，还可以降低偏轴对测量的影响。     

非对称条形纳磁体的铁磁共振频率和自旋波模式

通过建立微波激励下的非对称条形多铁纳磁体的微磁模型,研究了倾斜角和缺陷角对该形纳磁体的铁磁共振谱和自旋波模式的影响.通过对微磁仿真得到的动态磁化数据进行分析发现,非对称条形纳磁体倾斜角度增加,铁磁共振频率随之增加,而这一现象与纳磁体的缺陷角度无关.倾斜角不变,非对称条形纳磁体的铁磁共振频率与缺陷角度呈单调递增关系,并且不同缺陷角度纳磁体的自旋波模式显示出极大的差异.非对称条形纳磁体与矩形纳磁体相比,它的自旋波模式局部化,具体为非对称条形纳磁体的自旋波模式不对称且高进动区域存在于边缘,表现为非对称边缘模式.倾斜角改变导致纳磁体内部退磁场变化,引起纳磁体边缘模式的移动,而中心模式对倾斜角的变化并不敏感.最后,对建立的模型在高频微波磁场激励下的磁损耗进行了分析,验证了模型的可靠性.这些结论说明缺陷角和倾斜角可用于纳磁体自旋波模式和铁磁共振频率的调谐,所得结果为可调纳磁微波器件的设计提供了重要的理论依据和思路.     

Effect of thickness on the stability of transparent conducting impurity‐doped ZnO thin films in a high humidity environment

The resistivity of transparent conducting Al‐ and Ga‐doped ZnO (AZO and GZO) thin films prepared with a thickness in the range from 20 to 200 nm on glass substrates at a temperature below 200 °C was found to increase with exposure time when tested in a high humidity environment (air at 90% relative humidity and 60 °C). The resistivity stability (resistivity increase) was considerably affected by the thin film thickness. In particular, thin films with a thickness below about 50 nm were very unstable. The increase in resistivity is interpreted as carrier transport being dominated by grain boundary scattering resulting from the trapping of free electrons due to oxygen adsorption on the grain boundary surface. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Lattice dynamics studies of grain boundary structures: I. Method and application to stability

The method of lattice dynamics has been applied to an atomistic model of a grain boundary with emphasis on its utility in examining structural instability. A 36.9° [001] coincidence symmetrical tilt boundary was used with a Morse pairwise potential. Several structures derived by a static relaxation procedure were studied. Stable structures are characterized by a vibrational spectrum consisting of real frequencies only, while one of the statically relaxed structures yielded a pair of imaginary frequencies and therefore was unstable. A comparison of the lattice dynamics method with other methods for testing stability of a defect configuration is given.     

Stability characteristic of hypersonic flow over a blunt wedge under freestream pulse wave

To investigate the stability characteristic of hypersonic flow under the action of a freestream pulse wave, a high-order finite difference method was employed to do direction numerical simulation (DNS) of hypersonic unsteady flow over an 8° half-wedge-angle blunt wedge with freestream slow acoustic wave. The evolution of disturbance wave modes in the boundary layer under a pulse wave and a continuous wave are compared, and the wall temperature effect on the hypersonic boundary layer stability for a pulse wave disturbance is discussed. Results show that, both for a pulse wave and a continuous wave in freestream, the disturbance waves inside the nose boundary layer are mainly a fundamental mode; the Fourier amplitude of pressure disturbance mode in the boundary layer for a pulse wave is far less than that for a continuous wave, and the band frequency of the former is wider than that of the latter. All disturbance modes decay rapidly along the streamwise in the nose boundary layer. In the non-nose boundary layer, the dominant mode is transferred from fundamental mode into second harmonic. The transformation of dominant mode for a pulse wave appears much earlier than that for a continuous wave. Different frequency disturbance modes present different changes along streamline in the boundary layer, and the frequency band narrows around the second harmonic mode along the streamwise. Keen competition and the transformation of energy exist among different modes in the boundary layer. Wall temperature modifies the stability characteristic of the hypersonic boundary layer, which presents little effect on the development of fundamental modes and cooling wall could accelerates the growth of the high frequency mode as well as the dominant mode transformation.     

Investigation of the Circular Waveguide Radiating Slotted Structures Using Polarization Degeneration of the Main Mode Wave

The unusual paired slotted radiating structure of a circular waveguide is investigated when the wave of main mode H₁₁ is propagating in it. This structure is constructed in such a way that the phenomenon of the polarization degeneration of wave with this mode is taken into account. In the waveguide cross-section, the centers of the paired half-wave transverse slots are spaced by an angle γ over both sides of the waveguide diametric line along the direction of polarization of the H₁₁ mode. The radiation patterns at different spaced angles γ equal to 45°, 65°, and 72° as well as at the two-tiered location of such paired slots are investigated. The second tier is located on the distance equal to the half of the waveguide wavelength along the waveguide longitudinal axis. The pair slots of both tiers have the same γ angle but they are located at the opposite sides of the waveguide. The possibility is shown of synthesizing an omnidirectional radiation pattern by combination of the two-tiered structure with the opposite frontal slots and the two-tiered structure with the oppositely paired slots.     

可压剪切层线性稳定性特征直接数值模拟

基于伪随机数生成技术促白噪声扰动,以高精度迎风／对称紧致混合差分算法求解二维／三维非定常可压Navier－Stokes方程,揭示了可压自由剪切层初始剪切过程中扰动的线性演化特征,以及该过程对扰动波数和方向的内在选择性,验证了所用算法的有效性,表明线性理论同数值模拟相结合是可压剪切层研究的合理途径之一。     

空间光混频器分光比调整与90°相位差补偿方法

对空间光混频器的90°相位差补偿的几种方法进行了比较分析,并在此基础之上提出了两种新方法.第一种方法采取旋转本振光支路的1/4波片来补偿相位差,旋转信号光支路的第一个1/2波片来调整I路与Q路分光比;第二种方法通过旋转本振光支路的1/2波片和1/4波片到计算出来的角度来实现预定的相位差和分光比.对两种方法进行仿真分析和系统实验.采用第一种方法时,1/4波片快轴与x轴的夹角在-10°~10°变化时,相位差补偿范围为-14°~29°,分光比在0.7~1.4范围内变化;当1/2波片的快轴与y轴的夹角在35°~55°变化时,分光比在0.47~2.1范围内变化.采用第二种方法求解出I/Q路相位差分别为80°、85°、90°、95°、100°,I/Q路分光比分别为0.5、0.75、1、1.5、2时,1/2波片的快轴和1/4波片的快轴的位置.采用这两种方法均可以简单而精确地实现设定的相位差和分光比,有利于光锁相环的相位锁定以及解调出的信号强度的提高.     

Development of tilted fibre Bragg gratings using highly coherent 255 nm radiation

This paper reports the study on development of tilted fibre Bragg gratings using highly coherent 255 nm radiation, obtained from the second harmonic generation (SHG) of copper vapour laser (CVL). The transmission and reflection spectra of the tilted fibre Bragg gratings (TFBG) were studied for the tilt angles of 0° (normal FBG), 1°, 3° and 4° between the fibre axis and the interference fringe plane. It was observed that as the angle of fibre axis and phase mask increased, the main Bragg peak shifted towards the higher wavelength and transmission dip decreased. The transmission dip of the cladding mode first increased and then decreased after reaching a maximum with the increase in the tilt angle.     

合成冷/热射流控制超声速边界层流动稳定性

为了探究超声速边界层流动稳定性及其转捩控制机理,提出基于合成冷/热射流的边界层速度-温度耦合控制方法,并通过数值模拟研究了Ma=4.5超声速平板边界层不稳定波的传播,采用线性稳定性理论中的时间模式分析了壁面吹吸、射流温度、扰动频率、扰动振幅等对不稳定波控制效果的影响.结果表明:无射流控制时,边界层内同时存在不稳定的第一模态扰动波和第二模态扰动波,且二维波形式的第二模态占主导地位;壁面吹吸作用下,仅出现更加不稳定的第二模态,第一模态被抑制;速度-温度耦合控制下,射流温度对扰动模态的不稳定区域大小及扰动增长率影响显著,射流温度与来流温度不同时,温度的脉动使得流动转捩为湍流的速度加快,边界层速度型更加饱满,抗干扰能力增强,流动稳定性提高;高频的吹吸扰动对流场的控制效果优于低频扰动,扰动频率超过400 Hz时,第二模态扰动波时间增长率降低,扰动分量对边界层速度剖面和温度剖面的修正加快,第二模态更加稳定;扰动振幅减小为主流速度的1%时,仅出现时间增长率较小的第二模态,控制效果较好,进一步减小时,第一模态重新出现,并且波数范围与第二模态先重合后分离,对应的时间增长率先增加后减小.研究结果为边界层转捩控制技术提供了新的思路.     

Generation and Modulation of Phase Difference of Output Intensities in a Feedback Nd:YAG Laser with an Extracavity Waveplate Rotated

External anisotropic feedback effects on the phase difference behaviour of output intensities in a microchip Nd：YAG laser are presented. By rotating a quarter wave plate placed in the external cavity, the angle between laser initial polarization direction and o-axis of the wave plate is tuned from -45°to 45°, which results in variable extra-cavity birefringence along two orthogonal detection directions. With only one optical path and one wave plate, laser intensities of the two orthogonal directions, both modulated by the external cavity length, are output with a tunable phase difference, which can be continuously changed from zero to twice as large as that of the waveplate. Experimental results as well as a theoretical analysis based on Fabry-Perot cavity equivalent model and the refractive index ellipsoid, are presented. The potential applications of this phenomenon are also discussed.     

基于二维结构薄膜的偏振选择相位光栅的研究

利用二维结构薄膜构建了具有偏振选择特性的新型相位光栅,借助严格耦合波分析(RCWA)方法计算了光栅各级衍射强度随入射光波长及入射角的变化,发现在垂直入射情况下,波长600—640 nm范围内,相位光栅对横向电学(TE)模主要产生0级衍射,而对横向磁学(TM)模产生±1级衍射,在波长633nm处,0级衍射光的偏振消光比为109.8,±1级衍射光的偏振消光比为334.6.利用时域有限差分方法对这种相位光栅的偏振分束现象进行了模拟,偏振分离角在玻璃基板内可以达到10°左右,最后模拟了入射角为23°时光栅对不同偏     

激光聚焦扰动作用下高超声速边界层稳定性实验研究

在马赫数6、单位雷诺数3.1×10⁶/m的条件下对半锥角7°直圆锥边界层稳定性开展了实验研究.以激光聚焦于流场中局部空间而产生的膨胀冲击波作为人工添加的小扰动,分析了该扰动对高超声速圆锥边界层流动稳定性的影响.实验中利用响应频率达到兆赫兹量级的高频压力传感器对圆锥壁面脉动压力进行测量,通过对压力数据进行短时傅里叶分析和功率谱分析发现,相比于不添加激光聚焦扰动的结果,添加激光聚焦扰动使边界层中第二模态波的出现位置提前,且扰动波的幅值大幅度地增加,在相同的流向范围内,激光聚焦扰动将边界层中的扰动波从线性发展阶段推进到非线性发展阶段,其对边界层中扰动波发展的促进效果明显.同时,激光聚焦位置的不同对边界层中扰动波的发展也具有不同的影响.当激光直接聚焦于圆锥壁面X=100 mm位置时,边界层中频率为90 kHz的扰动波幅值增长最快,在X=500 mm的位置处其幅值放大倍数为3.81,相比而言当激光聚焦位置位于圆锥前方自由来流中时,边界层幅值增长最快的扰动波频率大幅减小为73 kHz,相同范围内,其幅值放大倍数为4.51倍.由此可见,当激光聚焦位置位于圆锥上游的自由来流中时,其对边界层中扰动波的影响更为显著.     

偏斜椭圆激光驻波场作用下中性原子沉积纳米光栅结构特性分析

分析了椭圆激光驻波场的偏斜对中性原子运动过程和沉积过程的影响,对不同偏斜角度椭圆激光驻波场作用下中性铬原子沉积纳米光栅结构的特性进行了仿真研究,由仿真结果可以看出,随着偏斜椭圆形激光束偏斜角的增加,对应于不同y平面,激光驻波场汇聚中性原子所形成纳米光栅条纹的对比度不断减小、半高宽不断增大.当椭圆长短轴之比为2:1条件下,椭圆激光驻波场的偏斜角为0°时,纳米光栅的条纹半高宽为3.2 nm,条纹对比度为36:1,而当偏斜角为15°时,激光驻波场中心位置处的沉积条纹的半高宽为6.5 nm,条纹对比度为24:1,而当椭圆激光驻波场偏斜角度达到30°时,沉积条纹的单峰结构将会产生分裂,形成了双峰结构,且随着偏斜角的增加,沉积条纹的分裂越严重,纳米光栅的沉积质量越差.对于其他长短轴比例条件下的激光场亦可根据比例关系获得相应的纳米光栅沉积特性. 关键词： 原子光刻 偏斜椭圆激光驻波场 纳米光栅