Large-area Raman enhancement substrates using spontaneous dewetting of gold films and silver nanoparticles deposition

We present a cost-effective method for making large-area surface enhanced Raman scattering (SERS) substrates by using spontaneous dewetting of ultrathin gold film. The dewetting of 5-nm-thick gold film formed high-density gold islands ranged from 40 nm to 80 nm. The measured SERS signal was 5 times stronger than synthesised gold nanoparticles. The SERS signals can be further increased by depositing small silver nanoparticles on the dewetted film. The experimental results showed 5-nm-thick silver coating increased SERS signals up to 10 times. The calculations by finite-difference time-domain method verified such SERS enhancement originated from enhanced electric fields between gold islands and silver nanoparticles. In the application, we demonstrated an all-optical measurement of pH values in microfluidic devices by using SERS signals of para-mercaptobenzoic acid.     

Rigorous interference and diffraction analysis of diffractive optic elements using the finite-difference time-domain method

The Finite-Difference Time-Domain (FDTD) method has proven to be a useful tool to analyze electromagnetic scattering phenomena. In this work, the FDTD method is applied at optical wavelengths. More precisely, we present the results obtained using the FDTD algorithm to simulate the performance of optical devices such as volume diffraction gratings. The Perfectly Matched Layers (PML), Total-Field Scattered-Field formulation (TF/SF) and Near-Field to Far-Field transformation (NF/FF) are some add-ons included in order to correctly calculate the far field distribution obtained from the numerical near-field values computed in the simulation region. These values in the near-field region are computed by illuminating the grating with of a plane wave at the Bragg angle of incidence. In addition, we compare the results obtained by the FDTD method to those obtained using the Rigorous Coupled Wave Theory (RCWT) applied to diffraction gratings. As will be seen in this paper there is good agreement between the two approaches, thus validating our FDTD implementation.     

Implementation of substrates for surface-enhanced Raman spectroscopy for continuous analysis in an optofluidic device

The fabrication of substrates for surface-enhanced Raman spectroscopy (SERS), which offer high enhancement factors as well as spatially homogeneous distribution of the enhancement, plays an important role for expanding the surface-enhanced Raman spectroscopy to a powerful quantitative and non-invasive measurement technique. In this paper, a method for the fabrication of capable SERS-active substrates by laser treatment of gold films supported on glass with single 351?nm UV-laser pulses is presented. Resulting nanometer scaled structures show enhancement factors of up to 10⁶ with very high spatial reproducibility for a monolayer of benzenethiol. A method for integration of these substrates into PDMS microchannels is shown. A technique for the generation of a simple mold master for PDMS replication is presented. Rhodamine 6G is used as model system to demonstrate continuous measurements on a solid SERS-active substrate in a microchannel. The label-free detection of the biological molecule albumin is improved by an order of magnitude.     

多层纳米银／壳聚糖膜用于芘的表面增强拉曼测定

多环芳烃类物质的高疏水性质使得其在金属表面吸附能力差,从而拉曼增强信号弱。该文针对此问题,制备了以壳聚糖为骨架,纳米银颗粒为热点的多层纳米银／壳聚糖复合膜表面拉曼增强基底,成功用于芘的表面增强拉曼检测。由于壳聚糖本身的富集作用,目标分子被吸附、富集于银纳米颗粒形成拉曼热点,可检测0．01μmol／L的芘。     

Overlapping computation and communication of three-dimensional FDTD on a GPU cluster

Large-scale electromagnetic field simulations using the FDTD (finite-difference time-domain) method require the use of GPU (graphics processing unit) clusters. However, the communication overhead caused by slow interconnections becomes a major performance bottleneck. In this paper, as a way to remove the bottleneck, we propose the ‘kernel-split method’ and the ‘host-buffer method’ which overlap computation and communication for the FDTD simulation on the GPU cluster. The host-buffer method in particular enables overlapping without any modifications to the update-kernels that are already in use. We also present theoretical formulas to predict the overlap threshold and the total throughput for each method. By using our overlap methods with 6 GPU nodes, we demonstrate that the total performance of 3D FDTD reaches 92% of a six-fold increase, which is the upper limit that would be reached if there were no communication overhead.     

High Spatial Order Energy Stable FDTD Methods for Maxwell’s Equations in Nonlinear Optical Media in One Dimension

In this paper, we consider electromagnetic (EM) wave propagation in nonlinear optical media in one spatial dimension. We model the EM wave propagation by the time-dependent Maxwell’s equations coupled with a system of nonlinear ordinary differential equations (ODEs) for the response of the medium to the EM waves. The nonlinearity in the ODEs describes the instantaneous electronic Kerr response and the residual Raman molecular vibrational response. The ODEs also include the single resonance linear Lorentz dispersion. For such model, we will design and analyze fully discrete finite difference time domain (FDTD) methods that have arbitrary (even) order in space and second order in time. It is challenging to achieve provable stability for fully discrete methods, and this depends on the choices of temporal discretizations of the nonlinear terms. In Bokil et al. (J Comput Phys 350:420–452, 2017), we proposed novel modifications of second-order leap-frog and trapezoidal temporal schemes in the context of discontinuous Galerkin methods to discretize the nonlinear terms in this Maxwell model. Here, we continue this work by developing similar time discretizations within the framework of FDTD methods. More specifically, we design fully discrete modified leap-frog FDTD methods which are proved to be stable under appropriate CFL conditions. These method can be viewed as an extension of the Yee-FDTD scheme to this nonlinear Maxwell model. We also design fully discrete trapezoidal FDTD methods which are proved to be unconditionally stable. The performance of the fully discrete FDTD methods are demonstrated through numerical experiments involving kink, antikink waves and third harmonic generation in soliton propagation.     

Applications of anisotropic one‐step leapfrog HIE‐FDTD method in microwave circuit and antenna

To verify the effect of artificial anisotropy parameters in one‐step leapfrog hybrid implicit‐explicit finite‐difference time‐domain (FDTD) method, we calculated several microwave components with different characteristics. Introduced auxiliary field variable can reduce the program difficulty and improve the computational efficiency without additional computational time and memory cost. Analyses of the numerical results are proved that the calculation time is reduced to about one‐sixth compared to the traditional FDTD method for the same example simulated. The memory cost and relative error are remained at a good level. The numerical experiments for microwave circuit and antenna have been well demonstrated the method available.     

Performance analysis and optimization of three-dimensional FDTD on GPU using roofline model

The Finite-Difference Time-Domain (FDTD) method is commonly used for electromagnetic field simulations. Recently, successful hardware-accelerations using Graphics Processing Unit (GPU) have been reported for the large-scale FDTD simulations. In this paper, we present a performance analysis of the three-dimensional (3D) FDTD on GPU using the roofline model. We find that theoretical predictions on maximum performance agrees well with the experimental results. We also suggest the suitable optimization methods for the best performance of FDTD on GPU. In particular, the optimized 3D FDTD program on GPU (NVIDIA Geforce GTX 480) is shown to be 64 times faster than the naively implemented program on CPU (Intel Core i7 2600).     

Topology optimization of metal nanostructures for localized surface plasmon resonances

This note presents an inverse design methodology of metal nanostructures for localized surface plasmon resonances, based on the topology optimization approach. Using the proposed method, determination of the metal distribution in nanostructures is implemented for surface enhanced Raman spectroscopy to maximize the enhancement factor. The obtained results demonstrate that the outlined approach can be used to design metal nanostructure with resonant peak and significant enhancement factor at specified incident wavelength, and to control the shift of the resonant peak by topologically optimizing the nanostructure.     

Flexible bistable chiral nematic liquid crystal display with enhanced memory characteristic by surface treatment

We proposed a flexible bistable chiral splay nematic liquid crystal display with the enhanced memory characteristics by the surface treatment with reactive mesogen (RM). With the polymerized RM structure on alignment layers adopted, the energy barrier between splay and π‐twisted states in the bistable chiral splay nematic liquid crystal mode is increased because of the enhancement of the azimuthal anchoring energy, and thus, the spontaneous relaxation from the π‐twisted state to the initial splay state is remarkably impeded. As a result, the memory retention time became twice as long as that of the conventional cell without the polymerized RM structure, and the stable memory characteristics were maintained against the external deformation.     

Plasmonic enhanced fluorescence spectroscopy using side-polished microstructured optical fiber

We report excitation of surface plasmon in a gold-coated side-polished D-shape microstructure optical fiber (MOF). As the leaky evanescent field from the fiber core becomes highly localized by the plasmon wave, its intensity also gets amplified significantly. Here we demonstrate an efficient use of this intensified field as excitation in fluorescence spectroscopy. The so-called plasmonic enhanced fluorescence emission from Rhodamine B has been investigated experimentally. First, plasmonic effect alone was found to provide an immediate fluorescence enhancement factor of two. Second, experimental results show a good agreement with theoretical modeling. Strong evanescent field generation and surface enhancement with simple metallic coating makes this fiber based device a good candidate for compact fluorescence spectroscopy.     

三维FDTD众核算法的设计与实现

在电磁学中,时域有限差分算法(FDTD)能够精确地模拟空间中电磁场的变化,在电介质器件设计领域得到了广泛的应用。众核(many-core)处理器片上计算资源丰富,对于计算密集型课题有较好的适应性。通过对麦克斯韦方程FDTD仿真算法的分析,并根据众核处理器的特性,实现了FDTD算法的众核并行。实验结果表明,FDTD算法在众核处理器平台上具有较好的计算效率,能够很好地发挥众核结构的优势。     

Au纳米粒子修饰纳米片状结构衬底的SERS研究

通过电化学共沉积和化学脱合金处理在金属W片上制备了纳米片状基底,将Au纳米粒子通过等离子溅射到纳米片状基底上得到表面增强拉曼散射(SERS)衬底。采用扫描电子显微镜(FE-SEM)、能谱仪(EDX)对复合纳米衬底进行表征,罗丹明6G作为探测分子对SERS衬底的拉曼表面增强效果进行检测。通过实验发现:三维空间结构的纳米片状结构具有拉曼表面增强效应,溅射Au纳米颗粒得到的Au纳米片衬底信号增强效果显著。     

钢板表面低对比度微小缺陷图像增强和分割

目的 环境干扰及光学元件不稳定等因素往往会造成钢板表面图像照度不均,钢板表面的微小缺陷具有图像灰度不均、对比度低、形态微小等特点,给后续图像分析和缺陷识别带来因难。为此,提出一种钢板表面低对比度微小缺陷图像增强和分割算法,以消除照度不均并突出缺陷信息,从而有效分割缺陷目标。方法 采用小波-同态滤波算法进行图像增强处理,即先利用小波变换对图像进行分解,再基于同态滤波对小波低频系数进行图像灰度修正,同时对高频系数进行高通滤波,然后将处理后的小波低频系数和高频系数进行重构得到增强的图像,从而达到消除照度不均、增强缺陷细节信息的目的。最后利用最大类间方差法（Otsu法）确定自适应阈值提供给Canny算子进行边缘检测。结果 采用本文算法对钢板表面多类型低对比度表面微小缺陷进行研究,有效消除了光照不均;单一的Otsu阈值分割和Canny算子难以有效检测这些缺陷,而本文Otsu-Canny算法的正确检测率达96%。结论 采用小波-同态滤波进行图像增强处理后,再利用Otsu-Canny算法对钢板表面多类型、低对比度的微小缺陷进行边缘检测取得了良好效果。     

基于Kirchhoff积分的瞬态声场计算方法

传统时域有限差分(FDTD)法计算远场瞬态声场特性时,通常需要将外推边界上各个时间步的场值都存储下来,占用了大量的内存,影响了计算效率.针对此问题,提出一种新的外推方法,它将柯西霍夫(Kirchhoff)公式应用于FDTD数值计算中,将积分面上每一步的近场都叠加到它有贡献的远场时刻上去,近远场变换可以与FDTD的场分量迭代同步进行,不需要将外推边界上每一时间步的场值都存储下来,减少了存储空间.针对绝对软和绝对硬的圆柱目标进行仿真计算,并与标准FDTD外推结果进行比较,结果表明:与标准FDTD的计算结果基本吻合,在节省内存量的同时不影响其计算精度.     

Implementation and optimization of GPU‐based parallel one‐step leapfrog ADI‐FDTD for far‐field scattering problems

The one‐step leapfrog alternative‐direction‐implicit finite‐difference time‐domain (ADI‐FDTD), free from the Courant‐Friedrichs‐Lewy (CFL) stability condition and sub‐step computations, is efficient when dealing with fine grid problems. However, solution of the numerous tridiagonal systems still imposes a great computational burden and makes the method hard to execute in parallel. In this paper, we proposed an efficient graphic processing unit (GPU)‐based parallel implementation of the one‐step leapfrog ADI‐FDTD for the far‐field EM scattering simulation of objects, in which we present and analyze the manners of calculation area division and thread allocation and a data layout transformation of z components is proposed to achieve better memory access mode, which is a key factor affecting GPU execution efficiency. The simulation experiment is carried out to verify the accuracy and efficiency of the GPU‐based implementation. The simulation results show that there is a good agreement between the proposed one‐step leapfrog ADI‐FDTD method and Yee's FDTD in solving the far‐field scattering problem and huge benefits in performance were encountered when the method was accelerated using GPU technology.     

A CMOS compatible process for monolithic integration of high-aspect-ratio bulk silicon microstructures

We report a CMOS compatible bulk micromachining method for the integration of high-aspect- ratio single crystal silicon MEMS （micro electromechanical systems） devices and signal conditioning circuit on a standard silicon wafer. The trench refilling and residual silicon removing techniques are used to acquire a proper electrical insulation between the different actuation and sensing elements situated on either fixed or movable parts of an MEMS device. To demonstrate the compatibility of the process, an integrated MEMS accelerometer was implemented. Test results show that the resistance between different elements of the device is larger than 1012 Ω. The electrical properties of the transistors that experienced MEMS fabrication agree well with those without ]VIEMS process, indicting the CMOS compatibility of the process.     

交流电沉积制备纳米枝晶拉曼基底

使用交流电沉积方法在预定的微区域内制备了纳米枝晶表面增强拉曼散射基底。通过调节交流和偏置直流电压可以控制基底的形貌、位置以及生长方向。从电化学过程、液电耦合场2个角度对拉曼基底的生长机理进行了阐述。激光共焦显微拉曼实验进一步表明：使用四巯基吡啶作为指针分子,该基底具有良好的拉曼活性。     

Ultra-sensitive, label-free probing of the conformational characteristics of amyloid beta aggregates with a SERS active nanofluidic device

One of the primary pathological hallmarks of Alzheimer’s disease is the formation of neutric plaques in the brain. The aggregation of amyloid beta peptide (Aβ) is central to the formation of these plaques and thus trace detection and characterization of these aggregates can have significant implications for understanding and diagnosing diseases. Here we have demonstrated a label-free surface enhanced Raman scattering technique combined with nanofluidics that is able to sensitively detect Aβ aggregates and to characterize their structural and surface properties at concentrations that are much lower than the limit of detection of existing instrumentation. With our device we have successfully detected Aβ aggregates formed at a very low concentration range of 10?fM to 1?μM and shown that the extent of protein aggregation and its resulting conformational characteristics are dependent on the initial Aβ concentration. The ability to observe the early stages of the aggregation process with analytical techniques, like that demonstrated here, could help to develop a better understanding of the conditions which lead to conformational disease, such as neurodegenerative diseases.     

A novel hybrid algorithm based on FDTD and WCS‐FDTD methods

In this article, a hybrid algorithm based on traditional finite‐difference time‐domain (FDTD) and weakly conditionally stable finite‐difference time‐domain (WCS‐FDTD) algorithm is proposed. In this algorithm, the calculation domain is divided into fine‐grid region and coarse‐grid region. The traditional FDTD method is used to calculate the field value in the coarse‐grid region, while the WCS‐FDTD method is used in the fine‐grid region. The spatial interpolation scheme is applied to the interface of the coarse grid region and fine grid region to insure the stability and precision of the presented hybrid algorithm. As a result, a relatively large time step size, which is only determined by the spatial cell sizes in the coarse grid region, is applied to the entire calculation domain. This scheme yields a significant reduction both of computation time and memory requirement in comparison with the conventional FDTD method and WCS‐FDTD method, which are validated by using numerical results.