Temperature profile of the multilayer structure irradiated by pulsed laser

Two programs are developed to calculate the temperature profile, as well as the reflectance, transmittance and absorption of a given multilayer film structure, in order to better understand the laser energy distribution between the reflectance, transmittance and absorption in each film layer. An inorganic Blu-ray recordable disc (BD-R) structure is used as a practical demonstration of the multilayer structure. The reflectance and absorption of the BD-R structure exhibit opposite trends and oscillate repeatedly with varying lower or upper dielectric layer thickness while the rest of the film thickness remains unchanged. The energy absorption in an absorbed layer depends on the thickness of the dielectric layers, its relative position in the structure and the extinction coefficient of its optical constant. The total absorption ratio of its maximum to minimum can be over 3 when changing the lower dielectric layer thickness of the studied structure. The layer thickness acts as an energy valve to control the energy flow into the multilayer structure. The thermal profile of the multilayer film structure irradiated by a pulsed laser is calculated at different positions in the film layers with time. The calculated temperatures in the recording alloy layer exhibit linear relationship with the applied power level. The effect of the laser duration time on the temperature increase in the recording layer is significant in the first few nanoseconds and becomes saturated if the heat balance is established in the structure. The calculated temperature is consistent with the experimental recording result when the structure is recorded at 4-time BD-R recording speed.     

Modelling self-assembling of colloid particles in multilayered structures

Simulations of particle multilayer build-up in the layer by layer (LbL) self-assembling processes have been performed according to the generalized random sequential adsorption (RSA) scheme. The first (precursor) layer having an arbitrary coverage of adsorption centers was generated using the standard RSA scheme pertinent to homogeneous surface. Formation of the consecutive layers (up to 20) was simulated by assuming short-range interaction potentials for two kinds of particles of equal size. Interaction of two particles of different kind resulted in irreversible and localized adsorption upon their contact, whereas particles of the same kind were assumed to interact via the hard potential (no adsorption possible). Using this algorithm theoretical simulations were performed aimed at determining the particle volume fraction as a function of the distance from the interface, as well as the multilayer film roughness and thickness as a function of the number of layers. The simulations revealed that particle concentration distribution in the film was more uniform for low precursor layer density than for higher density, where well-defined layers of closely packed particles appeared. On the other hand, the roughness of the film was the lowest at the highest precursor layer density. It was also predicted theoretically that for low precursor layer density the film thickness increased with the number of layers in a non-linear way. However, for high precursor layer density, the film thickness increased linearly with the number of layers and the average layer thickness was equal to 1.58 of the particle radius, which is close to the closely packed hexagonal layer thickness equal to 1.73. It was concluded by analysing the existing data for colloid particles and polyelectrolytes that the theoretical results can be effectively exploited for interpretation of the LbL processes involving colloid particles and molecular species like polymers or proteins.     

Softening of magnetorotons in a semi-infinite Fibonacci superlattice

Calculations of interlayer and intralayer screening of the Coulomb interaction on the softening of bulk and surface magnetoroton modes are presented for density and position modulations of the two-dimensional (2D) electron gas (EG) layers of a semi-infinite quasiperiodic superlattice. It is shown that the softening of these modes is due to an increase in the screening by all other layers of the effective intralayer Coulomb interaction. Numerical results are obtained for variable thickness of a 2DEG layer, the separation between layers and the distance between the surface layer and the top metal gate. The critical values of the structure parameters, determining the interlayer and intralayer screening of the Coulomb interaction, are obtained and used in constructing the phase diagrams showing the separation between the quantum fluid and charge-density wave phases.     

Enhanced optical transmission through double-layer gold slit arrays

We investigate the relationship between the transmission and the layer distance of double-layer gold slit arrays by using the finite-difference time-domain method.The results show that the transmission properties can be influenced strongly by layer distance.We attribute the two types of resonant modes to surface plasmon resonance and the localised waveguide resonance.We find that the localised waveguide transmission peak redshifts and becomes broader with increasing layer distance D.We also describe and explain the splitting,shift,and degeneration of the surface plasmon resonant transmission peak theoretically.In addition,to clarify the physical mechanism of the transmission behaviours,we analyse the distributions of electric field and total energy for the three transmission peaks with distance D=45 nm for the double-layer system.Light transporting behaviours are mostly concentrated in the region of the slits as well as the interspaces of the two layers,and for different resonant wavelengths the electric field and energy distributions are different.It is expected that the results obtained here will be helpful for designing subwavelength metallic grating devices.     

The effect of surface stress on the propagation of Lamb waves

This work investigates the possibility of the propagation of Lamb waves in thin solid layers with external traction free surfaces, in the presence of surface elasticity, inertia and residual stress. It is demonstrated that such waves do exist and that their characteristics can be quite different from their classical counterparts. The governing equations with non-classical boundary conditions involving the bulk and surface stress are solved exactly in the frequency-wavenumber domain. This solution is utilized to compute the Lamb wave modes for different layer thicknesses. An efficient strategy to capture all the modes of Lamb waves within a given frequency window is outlined. It is shown that the effect of surface elasticity and inertia becomes significant with increasing frequency and decreasing layer thickness, where the number of modes participating within a given frequency window is more than that permitted by the classical theory. Further, it is observed that the nature of the Lamb wave modes (in terms of negative dispersion) in the presence of surface stress is similar to what predicted by the nonlocal theory and microstructure based continuum theory.     

Possible role of dispersion forces in fracture

Dispersion (London) forces act between any two solid bodies. These forces are small except when the bodies are close together. They decrease rapidly as the distance between the bodies increases. Near the tip of a crack one body separates into two, and the two pieces are close together for a short distance. Thus, dispersion forces may play a role in the behaviour of crack tips. They cannot change the intrinsic surface energy, but they can affect the tip's width, and therefore, the time needed (at a given velocity) to complete the separation process. This in turn may affect the time available for plastic deformation; and therefore the extrinsic fracture surface energy. This may be one reason that polarizability affects fracture. Also, it may relate to the correlation between fracture surface roughness and energy.     

The definition and calculation of interfacial energies for thin films

We provide a new definition of the interfacial energy which eliminates three physically extraneous contributions from the conventional definition: (1) the strain or stress energy due to lattice mismatch between film and substrate; (2) the surface energy of the film-vacuum interface; and, (3) the substrate surface energy contribution from substrate layers below the film layers. This new interface energy then quantifies the variation in interactions among film/substrate, film/film and substrate/substrate bonding. Using this new definition, we derive the equations for evaluation of the interfacial energy in terms of the interaction energy for any atom in each layer of the film/substrate, film/film and substrate/substrate systems. With this formulation, it is simple to determine the dependence of the interfacial energy on the film thickness using virtually any interaction potential. Using a corrected effective medium theory, we present results for a few pseudomorphic film systems containing Ni/Cu, Ni/Ag, Cu/Ag and Rh/Ag on (111) and (100) surfaces. These systems cover a wide range of lattice mismatch and alloy formation energies. The results demonstrate that the new definition of interfacial energy converges after only 3–4 film layers, regardless of the degree of lattice mismatch. We also show that the interfacial energies at (100) and (111) interfaces differ and that the interfacial energy for a given pair of materials depends on which of the materials is the film.     

Controlling Surface Interactions with Grafted Polymers

The interactions between surfaces modified with grafted polymers is studied theoretically. The aim of this work is to find polymer surface modifications that will result in localized attractive interactions between the surfaces. The practical motivation of the work is to find means to control the distance between bilayers and solid supports in supported membranes. Two theoretical approaches are used, the analytical treatment of Alexander and a molecular theory. It is found that grafting each end of the polymer to each surface results in an interaction with a well defined minimum. The location of the minima is found to be very close to the thickness of the polymer layer when the chains are grafted to only one of the surfaces. The predictions of the analytical theory are in excellent agreement with the molecular approach in this case. It is found that increasing the surface coverage increases the strength of the interaction. However, increasing the polymer chain length at fixed surface coverage results in a decrease of the free energy cost associated with separating the surfaces from their optimal distance. For the cases in which grafting to both surfaces is not possible, the molecular theory is used to study the effect of functionalizing segments of the chain to achieve an attractive well. It is found that by functionalizing the free end-groups of the polymers with segments attracted to the membrane, the range of the attractive interaction is significantly larger than the thickness of the unperturbed layer. Functionlizing the middle segments of the chains results in a shorter range attraction but of the same strength as in the end-functionalized layers. The optimal polymer modification is found to be such that the functionlized groups are attracted to the bare surface but are not attracted to the grafting surface. The relevance of the results to the design of experimental surface modifiers is discussed.     

Interaction élastique de défauts ponctuels dans un cristal hexagonal semi-infini avec ou sans adsorbat

The energy of elastic interaction between a point defect and the (0001) surface of a hexagonal crystal is calculated, as well as the energy of the elastic interaction between two defects near such a surface. The defects are represented by the superposition of three mutually perpendicular double forces without moment. The calculation is done by means of a Green function method. As for an isotropic medium, the energy of a point defect presents a variation in x₃^?3 with the distance x₃ to the surface. On the other hand, the mutual interaction between two defects depends upon different geometric parameters, and not simply on an image factor. We also study the effect of a thin adlayer on these elastic interactions. This is done by showing that the presence of the adlayer is equivalent to effective boundary conditions at the surface of the substrate. We derive these conditions and then the elastic energies to the first order in the thickness h of the layer. Finally we present the mean square displacements of atoms in the presence of a clean or adsorbed surface.     

基于绝缘体上硅的一种改进的Mach-Zehnder声光调制器

传统的基于绝缘体上硅的Mach-Zehnder(MZ)声光调制器中,叉指换能器位于两臂的同一侧.为实现高的调制效率,声表面波的波峰和波谷分别调制MZ干涉仪的两臂,这要求控制MZ干涉仪两臂之间的距离为奇数倍声波半波长.但实际上由于传播过程中衬底材料的变化,声波波长会变大,这会导致两臂的间距难以准确设置.另一方面,声波在传播过程中经过MZ干涉仪的一臂后会发生衰减,降低了对另一臂的调制效果,影响了整体的调制效率.本文针对这些问题给出了一种解决方案,把叉指换能器放在MZ波导两臂之间,确保MZ干涉仪两臂到叉指电极中心距离相等.采用有限元法,首先对新提出的结构进行分析,然后通过声光互作用原理得到了材料的折射率变化;进而研究了波导类型、波导宽度、氧化锌厚度及叉指对数等因素对声光调制效率的影响,并对声光调制器的结构参数进行了优化以提高其性能.基于COMSOL Multiphysics的仿真结果表明,当条波导宽度为6μm,氧化锌只覆盖有叉指电极的部分且厚度为2.2μm,控制叉指电极数目为50对时,波导有效折射率变化在驱动电压为1 V时可以达到4.08×10~(-4),比传统结构提高了12%.     

Boundary layer theory modified for analysis of wave flow on the surface of a viscous liquid finite-thickness layer resting on a hard bottom

The theory of a boundary layer that is adjacent to the surface of an indefinitely deep viscous liquid and caused by its periodic motion is modified for analysis of finite-amplitude flow motion on the charged surface of a viscous conductive finite-thickness liquid layer resting on a hard bottom (the thickness of the layer is comparable to the wavelength). With the aim of adequately describing the viscous liquid flow, two boundary layers are considered: one at the free surface and the other at the hard bottom. The thicknesses of the boundary layers are estimated for which the difference between an exact solution and a solution to a model problem (stated in terms of the modified theory) may be set with a desired accuracy in the low-viscosity approximation. It is shown that the presence of the lower (bottom) boundary layer should be taken into account (with a relative computational error no more than 0.001) only if the thickness of the viscous layer does not exceed two wavelengths. For thicker layers, the bottom flow may be considered potential. In shallow liquids (with a thickness of two tenths of the wavelength or less), the upper (near-surface) and bottom layers overlap and the eddy flow entirely occupies the liquid volume. As the surface charge approaches a value that is critical for the onset of instability against the electric field negative pressure, the thicknesses of both layers sharply grow.     

石墨烯表面的特征水分子排布及其湿润透明特性的分子动力学模拟

石墨烯因其独特的分子构型、卓越的物理化学性能而受到广泛关注.本文首先利用分子动力学模拟比较了单层石墨烯、铜、二氧化硅三者表面的浸润性,除了接触角的比较,还分析了基底表面的水分子排布,得到石墨烯表面的特征水分子排布为:表面有两层密集的水分子层,其中靠近基底的密集水分子层中O—H键与垂直基底方向夹角集中在90°附近,并且基底表面的氢键几乎都垂直于基底.另一方面,本文研究了石墨烯浸润透明特性,发现在铜和二氧化硅上添加一层石墨烯,对铜的浸润性影响较小,对二氧化硅的浸润性影响很大,不仅使其上接触角显著增大,还使得基底表面的水分子排布呈现出类似单层石墨烯上的规律.本文使用分子动力学模拟方法从微观尺度验证了文献的实验结果,从基底表面水分子排布角度分析了石墨烯独特的浸润透明特性,为进一步开发石墨烯在微结构设计上的应用提供了理论指导.     

掺杂浓度及掺杂层厚度对Si均匀掺杂的GaAs量子阱中电子态结构的影响

本文通过自洽地求解薛定鄂方程及泊松方程计算了在温度T=0, 有效质量近似下, Si均匀掺杂的GaAs/AlGaAs量子阱系统的电子态结构. 研究了掺杂浓度及掺杂层厚度对子带能量, 本征包络函数, 自洽势, 电子密度分布, 及费米能量的影响. 发现在给定掺杂浓度下, 子带能量随掺杂层厚度的增加单调递减, 自洽势的势阱变宽变浅, 电子密度分布变宽, 峰值变低; 在给定掺杂层厚度下, 随掺杂浓度的增加子带能量及费米能级单调递增, 自洽势阱变深变陡变窄, 电子密度分布的峰值变高, 集中在中心. 关键词： 掺杂 量子阱 电子结构 半导体GaAs     

Geometrical condition for observing Talbot effect in plasmonics infinite metallic groove arrays

The plasmonics Talbot effect in metallic layer with infinite periodic grooves is presented in this study. Numerical approach based on the finite element method is employed to verify the derived Talbot carpet on the non-illumination side. The groove depth is less than the metallic layer thickness; however, for specific conditions, surface plasmons polaritons(SPPs)can penetrate through grooves, propagate under the metallic layer, and form Talbot revivals. The geometrical parameters are specified via groove width, gap size, period, and wavelength, and their proper values are determined by introducing two opening ratio parameters. To quantitatively compare different Talbot carpets, we introduce new parameters such as R-square that characterizes the periodicity of Talbot images. The higher the R-square of a carpet, the more coincident with non-paraxial approximation the Talbot distance becomes. We believe that our results can help to understand the nature of SPPs and also contribute to exploring this phenomenon in Talbot-image-based applications, including imaging, optical systems, and measurements.     

High-frequency guided ultrasonic waves for hidden defect detection in multi-layered aircraft structures

Aerospace structures often contain multi-layered metallic components where hidden defects such as fatigue cracks and localized disbonds can develop, necessitating non-destructive testing. Employing standard wedge transducers, high frequency guided ultrasonic waves that penetrate through the complete thickness were generated in a model structure consisting of two adhesively bonded aluminium plates. Interference occurs between the wave modes during propagation along the structure, resulting in a frequency dependent variation of the energy through the thickness with distance. The wave propagation along the specimen was measured experimentally using a laser interferometer. Good agreement with theoretical predictions and two-dimensional finite element simulations was found. Significant propagation distance with a strong, non-dispersive main wave pulse was achieved. The interaction of the high frequency guided ultrasonic waves with small notches in the aluminium layer facing the sealant and on the bottom surface of the multilayer structure was investigated. Standard pulse-echo measurements were conducted to verify the detection sensitivity and the influence of the stand-off distance predicted from the finite element simulations. The results demonstrated the potential of high frequency guided waves for hidden defect detection at critical and difficult to access locations in aerospace structures from a stand-off distance.     

Properties of ultrathin layers of palladium on a tungsten {110} surface

The growth mode, structure, thermal stability and work function of Pd films up to several monolayers in thickness on a W{110} surface are studied. At room temperature, layer-by-layer growth occurs, at least up to 4 layers. Layers in excess of the first monolayer are metastable and agglomerate at substrate temperatures above 700 K. Pd-W and Pd-Pd interactions within the first layer are investigated with thermal desorption techniques. The binding energy for single Pd atoms to the substrate is 3.6 eV/atom, the energy per lateral bond between nearest Pd atoms is about 0.05–0.12 eV/bond dependent upon range of interaction. These data are based on a phase transition occurring during the desorption of the first layer. The desorption of Pd in excess of the first monolayer is similar to the sublimation of bulk Pd.     

锐钛矿型TiO2(101)面原子几何及弛豫结构的第一性原理计算

采用平面波超软赝势方法计算了锐钛矿型TiO₂(101)面的表面能和表面原子弛豫结构.首先对TiO₂(101)面的6种不同的表面原子终止结构的体系总能量进行了计算,结果表明终止原子为两配位的O原子、次层为五配位的Ti原子的表面结构最为稳定.针对该表面研究了表面能和原子弛豫与模型中原子层数和真空厚度的关系,当原子层数为12层,真空厚度为0.4nm时,表面能收敛度小于0.01J/m².研究发现:表面上两配位的O原子向里移动约0.0012nm,五配 关键词： 第一性原理 2')" href="#">TiO₂ 表面结构 弛豫     

Propagation of gaseous detonation across inert layers

In rotating detonation engines and explosion accidents, detonation may propagate in an inhomogeneous mixture with inert layers. This study focuses on detonation propagation in a stoichiometric H₂/O₂/N₂ mixture with multiple inert layers normal to the detonation propagation direction. One- and two-dimensional simulations considering detailed chemistry are conducted. The emphasis is placed on assessing the effects of inert layer on detonation reinitiation/failure, detonation propagation speed, detonation cell structure and cell size. Specifically, the inert layer thickness and the spacing between two consecutive inert layers are varied. Either detonation reinitiation or failure across the inert layers is observed. It is found that successful detonation reinitiation occurs only at relatively small values of the inert layer thickness and spacing. For each given value of the inert layer spacing, there is a critical inert layer thickness above which detonation fails after crossing the inert layers. This critical inert layer thickness is found to decrease as the inert layer spacing increases. The detailed process of detonation reinitiation across the inert layers is analyzed. The interaction between the transverse shock waves is shown to induce local autoignition/explosion and eventually over-driven detonation development in the reactive layer. The averaged detonation propagation speed in the inhomogeneous mixture is compared to the CJ speed and very good agreement is achieved. This indicates that the inert layer does not affect the detonation propagation speed once successful detonation reinitiation happens. Unlike the detonation speed, the detonation cell structure and cell size are greatly affected by the inert layer results. For the first time, large cellular structure with size linearly proportional to the inert layer spacing is observed for detonation propagation across inert layers. Besides, a double cellular structure is observed for relatively large spacing between inert layers. The formation of double cellular structure is interpreted.     

Numerical simulation of metallic powder flow in a coaxial nozzle in laser direct metal deposition

To investigate the influencing rule of deposited layer’s shape on coaxial powder feeding flow field in the metal forming process, gas–solid two-phase flow theory is used to analyze effect of deposited layers on powder concentration distribution and variation of focus distance from nozzle outlet to convergence point (the center of the convergent zone). Different height and width parameters of deposited layers were chosen to calculate the powder concentration distribution, consequently, and also their effect on additive height of single-trace cladding layer was studied by experimental investigations. The numerical results are in good agreement with experimental observations. The results indicated that additive height of cladding layer was non-uniform under uneven wall thickness of parts fabrication condition. Consequently, the surface of deposited layers with uneven thickness is not smooth, and hence affects surface forming quality.     

Gauge vortices on a square lattice

The effective interaction between gauge vortices on a square lattice is revealed to be attractive at short distance and oscillate between repulsion and attraction at long distance. It is analyzed that the strong attraction at short distance makes the gauge vortices cluster at a given flux concentration; while the oscillatory nature in the interaction potential is the consequence of the existence of the electron Fermi surface in the nonuniform flux phases states.