新型二维材料碳化钛纳米片光限幅特性研究

作为一类新型二维纳米材料,金属碳/氮化物纳米片（MXene）具有高的比表面积和电导率,以及组分、层数与厚度灵活可控的优点,在储能、催化、传感和光学等领域具有潜在应用价值。研究了一种MXene材料碳化钛（Ti3C2TX）纳米片分散液的非线性光学效应及其响应机制。发现Ti3C2TX纳米片分散液在532、1 064 nm纳秒脉冲激光作用下表现出优异的光限幅性能,其限幅阈值分别为0.14 J/cm2和0.12 J/cm2;通过测量非线性散射光强度随入射光功率密度的依赖关系,发现该材料光限幅响应机制主要起源于非线性光学散射效应。与传统的光限幅材料C60比较,该材料具有光限幅阈值低、响应波长范围宽等优点。     

All-optical switching of infrared optical radiation using isotropicliquid crystal

Experimental results of the transmission to total-internal reflection switching of a step-on CO₂ laser beam by a dielectric-clad, isotropic liquid crystal film is presented. The use of the isotropic phase allows for polarization independence of the processes, while maintaining a moderate thermal nonlinearity. Dependence of switching speed on incident angle and energy density is evaluated. Experimental observations of the nonlinear switching effect with microsecond CO₂ laser pulses are also presented     

Dispersion of Nonlinear Elements as a Source of Electromagnetic Shock Structure (Letters)

Electromagnetic shock structure in nonlinear capacitance transmission lines can be resolved, and the energy losses associated with shock propagation explained, by including a resistance in series with the nonlinear capacitance. This resistance is inevitably present as the circuit representation of the nonvanishing relaxation time for the establishment of polarization in the nonlinear dielectric. Karbowiak and Freeman have dismissed this viewpoint as "not tenable!" This is a rebuttal of that statement.     

光纤探针探测水下激光爆炸

针对小范围内水下激光爆炸所产生的等离子体冲击波检测问题，对单模石英光纤探针探测方法进行了初步实验研究。采用1064nm调Q激光脉冲聚焦蒸馏水产生冲击波，利用自制光纤探测器进行了检测。结果发现除探测到冲击波信号外，爆炸所产生的等离子体闪光信号也进入光纤，两者混合叠加。同时由于水下激光爆炸所产生的射流对周围固壁面的破坏作用，光纤探针在实际检测过程中不能过分靠近爆炸源，对300mJ激光能量，该光纤探针安全范围应大于0．6mm。     

Orientational photorefractive effects in nematic liquid crystalfilms

We present a detailed theoretical discussion of the fundamental mechanisms for the recently observed nonlinear electro-optical effects in nematic liquid crystal films. Detailed expressions for the three principal mechanisms for space charge fields formation and various dielectric, conductivity and flow induced torques that are responsible for the liquid crystal director axis reorientation process are derived. We have observed strong optical wave mixing effects such as two-beam energy coupling and self-diffraction; the observed effects, their magnitude and dynamics, and dependencies on various physical parameters are found to be in good agreement with theories     

Theory of electrooptic shock radiation in nonlinear optical media

A general theory is presented for electrooptic shock (Cerenkov) radiation in transparent crystals having a linear electrooptic effect. Significant amounts of shock radiation require subpicosecond pulses of suitably focused laser radiation to produce a moving pulse of polarization in the medium. The theory obtains expressions for the radiated electric field, power spectrum, and total energy. Both 2D and 3D geometries are discussed as well as the proper focusing conditions. Numerical examples are given based on the nonlinear material LiTaO3.     

Resolution of the Paradox Concerning Energy Flow on Nonlinear Transmission Lines (Short Papers)

The energy relations and the constitutive relations pertaining to a nonlinear transmission line are examined in detail. It is concluded that mathematical models which have been used in certain studies and which lead to the paradox concerning energy dissipation in the shock front are inadmissible. Correct models are free from such paradoxes. The work leads to the formulation of the hypothesis of realization: it is impossible to realize a continuous loss-free transmission medium which would be characterized by nonlinear distributed inductance L(I) and capacitance C(V), unless the medium is dispersive.     

Asymmetric Trilayer All-Polymer Dielectric Composites with Simultaneous High Efficiency and High Energy Density: A Novel Design Targeting Advanced Energy Storage Capacitors

Polymeric dielectrics have attracted intensive attention worldwide because of their huge potential for advanced energy storage capacitors. Thus far, various effective strategies have been developed to improve the inherent low energy densities of polymer dielectrics. However, enhanced energy density is always accompanied by suppressed discharge efficiency, which is detrimental to practical applications and deserves considerable concern. Targeting at achieving simultaneous high energy density and high discharge efficiency, the unique design of asymmetric all-polymer trilayer composite consisting of a transition layer sandwiched by a linear dielectric layer and a nonlinear dielectric layer is herein reported. It is demonstrated that the nonlinear dielectric layer offers high energy density, while the linear dielectric layer provides high discharge efficiency. Especially, the transition layer can effectively homogenize the electric field distribution, resulting in greatly elevated breakdown strength and improved energy density. In particular, a high efficiency of 89.9% along with a high energy density of 12.15 J cm⁻³ are concurrently obtained. The asymmetric trilayer all-polymer design strategy represents a new way to achieve high-performance dielectric energy storage materials.     

激光冲击金属板料成形的残余应力分布研究

为了研究激光冲击成形后残余应力对板料性能的影响,采用高功率Nd:glass激光器对金属薄板激光冲击成形,使用X射线衍射法对激光冲击成形后的金属薄板的表面残余应力分布分析研究。薄板成形后的凹面分布存在较大的残余压应力,而凸面边缘存在较小的残余拉应力;残余应力随着激光能量的增加而增加,但当到达一定值时,凹面残余压应力达到最大值;在冲击凹面中心点处的残余应力值最大,周围处的残余应力以中心点为圆心向外呈非线性递减。在不同的凹模孔径下采用8mm光斑、30J激光能量单次冲击,发现存在一个凹模孔径阈值,小于阈值时,残余应力随着孔径的增大而增大;大于阈值时,随着孔径的增大而减小,阈值大小大约为20mm。结果表明,该研究对进一步研究残余应力的控制有重要作用,且对材料的抗疲劳性能、抗应力腐蚀、表面性能研究有很重要的理论和工程价值。     

An exposure model for electron-sensitive resists

We present a mathematical model for the exposure of electron-sensitive resists where an electron beam is incident normal to a substrate coated with a thin layer of resist. We include both the scattering of the incident electrons as they penetrate the resist and the electrons backscattered from within the resist and from the substrate. The calculations yield contours of equal absorbed energy density, and these are interpreted as the contours which bound the resist after development. The absorbed energy density is found as the sum, for all electrons, of the product of the energy absorbed per unit length of trajectory and the flux density of electrons at the point in question. We first calculate the absorbed energy density for an electron beam of vanishingly small cross section (an incident delta function) and then convolve that result with a beam of Gaussian current-density distribution to obtain the reSult for a single beam location. For poly(methyl methacrylate) resist, we study the achievable dot resolution, as a function of the incident charge, for various incident energies-and substrates. Since our main interest is in computer-controlled resist exposures in which patterns are generated as a succession of dots, we calculate the absorbed energy density contours for a line generated in that manner. Detailed comparison is made with the experimental results of Wolf et al., by fitting a single point on one contour at one beam energy to account for the unknown developer sensitivity. The resulting contours predict the undercutting effect experimentally observed for the 5-20-keV beam energies studied. The developed shape and linewidth are found to be nonlinear functions of the incident charge per unit length. Experimental data for the linewidth at 20 keV are presented and compared with theory.     

Three-dimensional computer model of electric fields in internaldefibrillation

The automatic internal defibrillator delivers a low-energy shock directly to the heart. Optimal strategies for these shock deliveries are determined by studying a three-dimensional computer model of the electric fields produced by initial defibillation electrodes. A finite-element analysis technique is used to calculate energy and current density distributions in three commonly used electrode configurations: (1) patch-patch (PP), (2) catheter-patch (CP), and (3) catheter-catheter (CC). analysis of these simulations indicates that : (1) the PP and CP configurations are more effective at channeling energy to the myocardium than the CC configuration; (2) small electrodes and the edges of the electrodes give rise to high local current densities which might cause damage to the myocardium: (3) energy delivered to the myocardium is not significantly altered for different electrode placements tested; (4) electrode size influences current density distribution, especially near the electrodes; and (5) energy distribution is sensitive to the relative conductances of the myocardial tissue and blood     

CO2激光诱导液滴射流等离子体的实验研究

为了研究激光诱导射流等离子体特性,了解激光诱导液滴等离子体的发展过程,基于脉冲激光-液滴同步作用系统,采用阴影法,观测了激光作用液滴的阴影图像,取得了液滴在CO2脉冲激光作用下的演化过程数据。对图像进行处理获得了激光诱导液滴等离子体冲击波膨胀范围随时间的变化,并估算出了产生冲击波的激光能量。结果表明,空气冲击波的膨胀半径在当前观测时间范围内线性膨胀,约32%的激光能量用于产生冲击波。空气冲击波的变化规律对激光诱导液态燃料点火的研究提供了一定的参考依据。     

激光热冲击引起PZT压电薄膜铁电性能的变化

应用高能量单脉冲激光作用在锆钛酸铅 (PZT)压电薄膜上 ,研究脉冲激光的热冲击对PZT薄膜性能产生的影响。发现在激光未烧熔薄膜的能量密度下 ,经过激光作用后 ,PZT薄膜的铁电性能发生变化 :在外加电压为 6V时 ,剩余极化强度值Pr 从 32 6 99μC/cm2 变到 2 6 316 μC/cm2 ;矫顽电场保持为 38 396kV/cm不变 ;疲劳性能变稳定 ,在循环 1 75× 10 9次时 Pr 衰变率由 4 4 3%变为 34 7%。最后讨论分析了产生这种现象的微观机理。     

Interface states in SiSiO2 interfaces

Interface state parameters were studied in MOS capacitors over a wide range of energy by conductance and capacitance measurements at various temperatures from room temperature to liquid nitrogen temperature. A new technique was developed for analysis of the data which allows to obtain the density of states, the capture cross section, the surface potential and the dispersion parameter from the conductance and capacitance vs. frequency curves. The density of interface states as well as the electron capture cross section were found to be a function of energy only and to be independent of temperature. Maxima in the density of states have not been found.     

激光功率密度对板料激光冲击成形性能的影响

为了研究激光功率密度对铝合金板激光冲击成形性能的影响,采用数值仿真的方法,选用不同激光功率密度对铝合金板进行激光冲击成形数值模拟,获得不同激光功率密度与冲击次数下的板料成形极限,并对不同激光功率密度冲击下板料的成形深度、厚度及应力分布进行了分析。结果表明,板料在中心处容易发生破裂;板料的成形极限与激光功率密度大小成反比,而与激光冲击次数成正比;通过合理选择激光功率密度和冲击次数可提高铝合金板的成形性能。     

Electrodynamics of anisotropic media with space and time dispersion

The average stress tensor, power-flow density, momentum density, and energy density for electromagnetic waves in a linear stationary anisotropic medium with space and time dispersion are clarified with the help of the second-order nonlinear terms in the equation of motion for polarization. The wave fields are connected with the medium-mass motion through the nonlinear terms; the net force between the fields and the medium mass plays a crucial role in interpreting the stress tensor and the wave momentum. Asymmetry in the space-space part of the energy-momentum tensor, which is quadratic in the field fluctuations, is ascribed to anisotropic restoring forces of the polarizations; asymmetry in the space-time part is attributed to nonvanishing average forces between the fields and the medium-mass motion.     

不同初始分布向列相液晶电容特性的相场法研究

液晶盒内的液晶分子在不同的外加电场作用下重新排布,其极化指向发生改变,从而影响液晶盒整体的电学特性。本文考虑液晶材料的挠曲电效应,分析液晶盒的整体自由能表达式,包含弹性形变自由能密度、介电自由能密度和挠曲电自由能密度。基于液晶弹性理论和相场方法,构建描述液晶核指向矢分布及演化的相场方程,并运用离散方法推导出液晶盒约化电容的表达式。讨论边界强锚定的混合液晶盒(HAN)和平行液晶盒(PAN)表面电荷密度对液晶核指向矢偏转角的影响。进一步分析了挠曲电系数、表面电荷密度以及液晶盒厚度对两种不同初始指向分布液晶盒电容性能的影响。数值分析结果表明,表面电荷密度越大、挠曲电系数越小,液晶核指向矢偏角越大。在混合液晶盒中,约化电容随着表面电荷或液晶厚度的增加而增加,最终趋于饱和;而在平行液晶盒中,当表面电荷密度或液晶盒厚度达到某个临界值时,液晶核的指向矢才发生改变,液晶的约化电容才有明显的变化。随着表面电荷密度增加,液晶核指向矢发生偏转的临界厚度减小。     

Nonlinear gain effects due to carrier heating and spectralholeburning in strained-quantum-well lasers

The authors present numerical results for the nonlinear gain effects due to carrier heating and spectral holeburning in 50 Å strained In_xGa_1-xAs/Al_0.3Ga_0.7 As quantum-well lasers. Calculations are performed on the basis of a 4×4 matrix system consisting of the usual Kohn-Luttinger Hamiltonian and a strain Hamiltonian for the valence band structure. In addition, the authors perform a small-signal analysis based on four dynamic equations for the photon density, carrier density, and two supplementary equations for the electron and hole energy densities to obtain information about nonlinear gain coefficients. The results indicate that the nonlinear gain is enhanced with the strain mainly due to the rapid increase of the carrier heating effect as the carrier density at the lasing threshold decreases, and that carrier heating is about five times as important compared to spectral holeburning     

Calculation of linear and nonlinear intersubband optical absorptions in a quantum well model with an applied electric field

Analytic forms of the linear and the third-order nonlinear optical intersubband absorption coefficients are obtained for general asymmetric quantum well systems using the density matrix formalism, taking into account the intrasubband relaxation. Based on this model, we calculate the electric field dependence of the linear and the third-order nonlinear intersubband optical absorption coefficients of a semiconductor quantum well. The energy of the peak optical intersubband absorption is around 100 meV (wavelength is 12.4 μm). Thus, electrooptical modulators and photodetectors in the infrared regime can be built based on the physical mechanisms discussed here. The contributors to the nonlinear absorption coefficient due to the electric field include 1) the matrix element variation and 2) the energy shifts. Numerical results are illustrated.     

铝合金板激光冲击变形实验及有限元模拟

为了研究金属板料在脉冲激光辐照下的响应、激光冲击下板料的变形特性、激光脉冲能量对金属板料变形量的影响以及脉冲激光光斑内冲击波压力的分布情况,采用高功率钕玻璃激光系统对LD31板进行了单次冲击变形实验,同时利用有限元软件ABAQUS对板料变形过程进行了模拟。结果表明,激光冲击条件下板料变形时呈现粘塑性性质;激光脉冲能量是影响板料变形量的主要因素,且板料变形大小随脉冲能量的增加呈非线性增大;激光冲击时激光光斑作用区域内冲击波压力并不是均匀分布,而是沿径向减小。