建筑物对不同体制电磁脉冲时域响应数值分析

 针对一般建筑物内空间电场强度时域变化特点,采用时域有限差分方法,数值模拟单极性高斯脉冲、双极性微分高斯脉冲以及窄带调制方波脉冲平面波在建筑物内传播、反射及透射过程;并对比分析不同脉冲参数对空间场强最大值及分布的影响。在一定脉冲宽度的微分高斯脉冲激励下,建筑物空间内最大场强变化范围值较高,为3．０~5.5 dB,其空间场强增强分布区域较大,并随脉宽增加而减小,但脉冲的最小重复频率相对要求最高。相应脉宽的高斯脉冲造成空间最大场强及分布区域范围最小,而脉冲的最小重复频率所需最低。当载频为房间谐振频率的窄带脉冲入射下,空间最大场强变化范围值相对最高,空间场强增强区域总体趋势随脉冲载频频率单调增加。     

球面电磁波斜入射建筑物内场的数值分析

为了分析窄带电磁脉冲源近、远场辐照大型建筑物内部空间电磁场分布特点,采用时域有限差分方法,对比分析平面波、球面波场源正面斜入射多层多单元建筑物的计算结果,并较为全面地分析了各房间各水平面中心电场强度幅值、各层各水平面电场强度最大值分布情况。建筑物各层相对应高度面上场强分布近似,在电磁波传播方向上窗户房间内部区域场强较强,两种波场源入射结果基本一致;在其余区域场强较弱,特别是在较大空间内的区域,球面波场源入射下场强相对更弱。其中通过与待模拟建筑物外形相紧凑性匹配的球面波场源构建,模拟仿真了窄带球面电磁波在真空空间的辐射传播,计算结果验证所构建球面波近场源准确可行。     

高频脉冲电场中水滴运动规律研究

基于水滴在脉冲电场中的受力分析,建立了高频脉冲电场中液滴运动模型及轨迹方程。采用高速显微摄像系统研究了油中水滴的运动规律,分析了脉冲电场形式、强度及频率对水滴运动轨迹和速度的影响规律。模拟结果表明,在直流脉冲电场中,场强主要影响单液滴运动的速度,频率主要影响单液滴的加速及减速时间。试验表明,单液滴运动平均速度随频率的升高而加快;双液滴相对运动速度随场强增加,在直流脉冲电场中减小,在方波电场和交流脉冲电场中变大;双液滴在脉冲电场中的相对运动速度由高至低依次为:交流脉冲电场、方波电场和直流脉冲电场。     

FDTD结合DFT分析宽频高功率微波大气传播

联立麦克斯韦方程与电子流体方程,用时域有限差分法(FDTD)模拟高斯型和阻尼正弦型等宽频高功率微波(HPM)的大气传播.在每个时间网格上,根据窄带脉冲的电子速度,通过离散傅立叶变换(DFT)方法求解出宽频脉冲的等效电场,将等效电场和压强代入电离参数公式,使电离参数随空间网格不断更新,提高计算准确性.结果表明,宽频HPM脉冲幅值、脉宽以及海拔高度等参数对大气击穿有明显的影响;大气击穿导致尾蚀效应;随着传播距离的增加,宽频HPM脉冲的尾部衰减加剧,脉宽缩短,引起宽频脉冲的频谱出现展宽、分裂及中心频率移动等现象.     

返波体对混响室内场均匀性的影响

以带有返波结构的混响室为基础,研究了在复杂腔体内部返波结构的改变对场分布的影响效果。根据微波混沌理论分析了不规则腔内的场分布特征;结合统计电磁学,确立了在高功率微波短脉冲激励下的混响室中进行场均匀性和各向同性验证的指标;对复杂腔体中返波体的大小、数量及分布范围等因素进行了研究。数值模拟结果显示:凸起结构的尺寸越大,其对腔内电场的影响效果越为明显,尤其是腔内电场分布的统计均匀性和各向同性结果,都能得到很好的改善。同时凸起结构越多,分布的范围越广,腔内电场的统计均匀性和能流的统计均衡性就越好。     

源搅拌对混响室场均匀性的改善

 研究了在短脉冲波激励下,源搅拌方法对混响室内场分布的影响效果。分析了混响室内场分布特征的影响因素,得到了影响源搅拌的相关参量。研究改变激励源的位置对腔体内场分布的影响效果,主要对比了电场的最大值、分布标准差等电场统计特征。结果表明：通过连续地移动激励源对场分布进行搅拌,混响室内的电场最大值可以达到约6.7 kV/m,而且场值的空间分布标准差降至3 dB以下,能量分布也更加均衡。因此,采用源搅拌方法可以有效地改善腔内的场分布,提高场分布的均匀性,有利于构造均匀的电磁场环境。     

高功率微波对建筑物内计算机作用效果评估方法

提出了一种评估高功率微波对建筑物内计算机作用效果的方法。建立建筑物的电磁计算模型,利用时域有限差分法对高功率微波进入建筑物的过程进行数值模拟,求出计算机所在高度面上的时域最大场强,定义大于效应阈值区域的面积与建筑物总面积的比例为有效面积比,高功率微波对建筑物内计算机的作用比例即等于有效面积比。并开展实验对此方法的有效性进行验证,估计的计算机效应比例和实验结果相差20%以内。改变入射场强,得到计算机效应比例与入射场强的关系,与理论分析结果一致。     

Tesla变压器次级线圈电位分布

 为研究Tesla变压器锥形次级线圈的电位和电场分布问题,建立了非均匀参数双传输线模型,采用差分方法获得波动方程的数值解。计算结果表明：脉冲形成线充电过程中,最高场强点从次级线圈小半径端逐渐向大半径端移动;脉冲形成线快速放电后,次级线圈场强最大点位于大半径端部;脉冲形成线不能正常放电时,次级线圈最大场强点也位于大半径端部,此时场强达到最大值,约为平均场强的1.5倍。     

9.33 GHz高功率脉冲微波对IAR20鼠肝细胞和L-02人肝细胞增殖的影响

研究了9.33 GHz高功率脉冲微波对IAR20鼠肝细胞和L-02人肝细胞增殖的影响,利用噻唑蓝比色法测量细胞增殖并对实验数据进行拟合,得到脉冲个数、场强和脉宽与细胞增殖之间的关系。当脉冲微波场强与脉宽保持不变,脉冲微波细胞效应随脉冲个数呈现非线性的指数递增规律。当脉冲微波的脉冲个数、脉宽一定时,场强越大,细胞增殖被抑制的程度越大;当脉冲个数、场强不变,脉宽越大,细胞增殖受到抑制的作用越明显,即脉冲微波细胞效应与场强和脉宽成正比。相同脉冲微波参量对不同种类细胞增殖的影响是不同的,对IAR20鼠肝细胞的影响比对L-02人肝细胞的影响略大。     

基于神光Ⅲ原型装置的冲击点火分解实验北大核心CSCD

冲击点火是一种新型点火方式,介绍了国内进行的冲击点火分解实验。实验结果表明:相比于方波脉冲,在冲击峰整形脉冲作用下激光与等离子体相互作用明显增强,背向散射光的份额增加,散射光谱来自于不同密度的等离子体区域。实验中也观察到了方波条件下冲击波在CH样品中的传播过程,与模拟计算结果较为符合。     

Determination of the mid-IR femtosecond surface-damage threshold of germanium

To assess the potential uses of germanium as a nonlinear material in the mid IR we have measured the surface-damage threshold of germanium optical windows using femtosecond pulses at a wavelength of 3.9 μm. By working with a wavelength corresponding to a photon energy of less than half the band-gap energy, free-carrier generation due to one- and two-photon absorption was eliminated. The laser pulses had an energy of 5.5 μJ, a duration of 255 fs, and were focused to a waist size of approximately 100 μm. The multi-shot damage threshold of the germanium windows was estimated to be approximately 94 GW/cm². The data should be of immediate benefit to industrial laser researchers and applications engineers working in the mid-IR region.     

Manipulation of multi-transparency windows and fast-slow light transitions in a hybrid cavity optomechanical system

We study the generation of quadruple-transparency windows and the implementation of a conversion between slow and fast light in a hybrid optomechanical system. By demonstrating the generation of these transparency windows one by one, we analyze the physical mechanism through which each transparency window forms in detail. Additionally, we discuss how the system parameters affect the formation of transparency windows and conclude that the location, width, and absorption of each transparency window can be arbitrarily manipulated by varying the appropriate parameters. Moreover, when the pump field is changed from red to blue detuning, conversions between slow and fast light occur in the output field. These interesting properties of the output field can be applied to achieve the coherent control and manipulation of light pulses using cavity optomechanical system.     

Effect of magnetic field on a multi window ladder type electromagnetically induced transparency with 87Rb atoms in vapour cell

In this work, we experimentally study the effect of externally applied magnetic field on a ladder type EIT in a vapour cell consisting of ⁸⁷Rb atoms. The introduction of magnetic field causes the Zeeman splitting of the hyperfine levels of ⁸⁷Rb atoms and hence the number of available windows of transparency increases. We report the observation of nine such windows. Such multi window EIT systems are capable of storing pulses at the different frequencies, corresponding to these windows hence paving the way for realization of multi frequency quantum memories. Also, the total bandwidth of storage is 218.4 MHz which is two orders of magnitude higher than that typically obtained in single window EIT based storage systems. These systems have tremendous applications in the field of speedy transmission of data over a long distance quantum communication channel.     

基于指数再生窗Gabor框架的窄脉冲欠Nyquist采样与重构

基于Gabor框架的窄脉冲信号采样及重构效果已经得到验证, 其解决了有限新息率(finite rate of innovation, FRI)采样方法无法在波形未知的情况下重构出脉冲波形的问题.但是目前的Gabor框架采样系统的窗函数构造复杂且难以物理实现.本文将指数再生窗函数引入Gabor框架, 将窗函数序列调制部分简化为一阶巴特沃斯模拟滤波器, 构造了Gabor系数重构所需要的压缩感知(compressed sensing, CS)测量矩阵.为了使得测量矩阵满足信号精确重构所需的约束等距特性(restricted isometry property, RIP), 根据高阶指数样条函数能量聚集特性, 选择了最优的窗函数支撑宽度, 推导了信号重构所需的约束条件, 还对其鲁棒性进行了分析.本文通过仿真实验对上述分析进行了有效验证, 该系统可应用于测试仪器、状态监测、雷达及通信领域等多种背景下的窄脉冲信号采样与重构.     

Large four-wave mixing of spatially extended excitonic states in thin GaAs layers

We study the size dependence of the nonlinear response of weakly confined excitons for the size region beyond the long wavelength approximation regime. The observed degenerate-four-wave mixing signal of GaAs thin layers exhibits an anomalous size dependence, where the signal is resonantly enhanced at a particular thickness region. The theoretical analysis elucidates that this enhancement is due to the size-resonant enhancement of the internal field with a spatial structure relevant to the nondipole-type excitonic state. These results establish the formerly proposed new type of size dependence of nonlinear response due to the nonlocality induced double resonance.     

Interaction of counterpropagating few cycle light pulses in nonlinear dielectric media and radiation generation at combination frequencies in this process

The regularities of the interaction of counterpropagating optical pulses of only a few oscillations of the light field in nonlinear dielectric media are studied theoretically. The temporal structure of the pulse field, due to its interaction with the counterpropagating pulse, is shown to be varied such that the field zeros are the most shifted in time and its extreme values are not shifted. The center of mass of the spectral density is shifted to the short-wavelength region. The efficiency of these phenomena is determined by the energy of the counterpropagating pulse and does not depend on its duration and spectral composition. The possibility of wave generation at tripled and combination frequencies is demonstrated for the media whose size is less than the interaction region. The efficiency of this generation with respect to the phase mutual modulation is shown to be increasing with a decrease in the number of oscillations in colliding pulses.     

Ar原子团簇与飞秒强激光相互作用产生的高能离子计算

利用改进的“等离子体球”模型模拟了Ar原子团簇与飞秒强激光相互作用的物理过程.改进后的模型弱化了原模型在共振吸收附近团簇内部屏蔽电场的异常增强行为，从而使其更为合理，计算得到的Ar离子平均动能与以往的实验结果符合.还定量地研究了Ar离子平均动能及其平均电荷态与团簇尺寸以及激光参数之间的变化关系. 关键词： 团簇 飞秒激光 高能离子 共振吸收     

Concentric toroids as a wideband and efficient substrate for surface-enhanced Raman spectroscopy applications

Designing a structure having high local field enhancement, wideband resonance, and large hot spot area is the key element to obtain a large enhancement factor for surface-enhanced Raman spectroscopy applications. Here, the concentric toroid structures in dimer configuration is proposed, which shows a large local field intensity in a wide spectral range and the region that leads to a high-surface-enhanced Raman scattering signal intensity. Calculations show that the average surface-enhanced Raman scattering enhancement is up to 60 times more compared to the conventional dimer toroid structures with similar size.     

双金属纳米天线在少周期激光中的宽带超快电磁响应

研究了耦合的核壳双金属纳米粒子天线的光场增强与超快时域响应. 在少周期激光的应用中, 如表面等离激元金属纳米器件中的超快响应探测元件, 这种双金属天线展示了超宽带共振频谱及控制局域表面等离激元增强的能力. 研究了二聚体、三聚体以及七聚体, 并且发现三聚体Ag/Au核壳结构显现了极高的场增强能力, 其场振幅增强因子超过了120, 并在5 fs的时间内场振幅迅速衰减为30, 保持了超快的光学响应特性. 利用该结构的表面等离激元增强效应产生高次谐波, 对于产生超快阿秒脉冲有着潜在的重要应用价值. 关键词： 纳米光学天线 双金属球壳结构 少周期激光脉冲 表面等离激元     

Effect of metallic nanoparticle sizes on the local field near their surface

We have used numerical calculations based on Mie theory to analyze the near field distribution patterns for 4–150 nm spherical silver nanoparticles (nanospheres). We have shown that as the nanoparticle sizes increase, the region where “hot spots” are concentrated is shifted to the forward hemisphere. We have observed a nonmonotonic dependence of the maximum attainable local field enhancement factor on the size of the silver nanospheres. We have determined a correlation between the optimal nanosphere size for the maximum attainable local field enhancement factor and the optical absorption efficiency factor. We have established a nonmonotonic dependence of the optimal size of the nanoparticles and the maximum attainable local field enhancement factor on the refractive index of the surrounding medium. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 6, pp. 831–837, November–December, 2008.