Single-molecule spectroscopy and detection are powerful techniques for the study of single fluorescent particles and their interaction with their environment. We present a low-cost system for simultaneous real-time acquisition, storage of inter-photon arrival times and the calculation and display of the fluorescence time trace, autocorrelation function and distribution of delays histogram for single-molecule experiments. From a hardware perspective, in addition to a multi-core computer, only a standard low-cost counting board is required as processing is software-based. Software is written in a parallel programming environment with time crucial operations coded in ANSI-C. Crucial to system performance is a simple and efficient real-time autocorrelation algorithm (acf) optimized for the count rates (approximately 10(4) cps) encountered in single-molecule experiments. The algorithm's time complexity is independent of temporal resolution, which is maintained at all time delays. The system and algorithm's performance was validated by duplicating the signal from the photon detector and sending it to both the ordinary counter board and a commercial correlator simultaneously. The data acquisition system's robustness under typical single-molecule experimental conditions was tested by observing the diffusion of Rhodamine 6G molecules in deionized water.

Analysis of the electrostatic characteristics and the gate capacitance of typical nanostructured carbon nanotube field effect transistors (CNTFETs) were performed numerically. A previously developed parallelized electrostatic Poisson's equation solver (PPES) is employed, coupled with a parallel adaptive mesh refinement (PAMR) to improve the numerical accuracy near the region where variation of potentials are significant. CNTFETs with four typical configurations of the gate electrode, the bottom gate (BG), the double gate (DG), the top gate (TG), and the surrounding gate (SG) were simulated. Effects of the nanotube arrangement and the gate length on the gate capacitance are presented and discussed. The simulation results show that SG-CNTFET possesses the largest gate capacitance among various structures. However, TG-CNTFET is recommended for practical applications by taking into account both the device performance and the difficulty of fabrication. According to the simulated gate capacitance, estimation of the on-state current of CNTFETs is possible.

The field-emission characteristics of a single silicon nano-emitter were investigated by means of experiments and simulation models. The emitter array was fabricated by dry etching using an inductively coupled plasma (ICP) through a three-step process. A novel experimental technique was developed to precisely measure the field-emission current from a single silicon emitter. Accompanying these measurements, a parallelized three-dimensional particle-in-cell (PIC) code, in which the Fowler–Nordheim emission law was implemented at the emitter surface, was employed to simulate the emission current both with and without taking into account the space charge effect. Results show that the simulated I–V relationship when considering the space–charge effect was in excellent agreement with the actual measurements. Noticeably, the predicted turn-on voltage was found to be within only 9.3% of the experimental data. In addition, both the predicted and experimental data demonstrated a consistent single linear slope in the FN plot, which indicates that the field emission from a single silicon tip is a barrier-tunnelling, quantum mechanical process.

A gated single carbon nanotube field emitter with magnetic focusing is proposed and simulated using a parallelized Poisson’s equation solver, coupled with the ray tracing of electrons, on an unstructured tetrahedral adaptive mesh. The magnetic focusing for the electrons can be achieved by a vertically downward magnetic focusing field (−Bz) through the use of either external solenoids or permanent magnets around the field-emission array. The simulation results, assuming uniform magnetic field inside a field-emission unit, are compared with those conventional tetrode-type field emitters using an electrostatic focusing structure. The results reveal that the magnetic focusing design can promise much higher emission current, while a much smaller spot size results at the anode. In addition, the magnitude of the applied gate voltage in the range of 60–120V shows little influence on the electron-beam diameter at the anode. The proposed magnetic focusing method can also possibly reduce the complexity of the fabrication without the electrostatic focusing structure. Noticeably, a distribution, similar to the Airy function, is obtained that shows the dependence of the spot size at the anode on the magnetic flux intensity. Thus, under suitable magnetic focusing conditions, it is possible to produce well-defined microelectron sources for many field-emission applications, such as novel parallel electron-beam lithography or field-emission displays.

A parallelized three-dimensional self-consistent electrostatic particle-in-cell (PIC) code using unstructured tetrahedral mesh is proposed. Parallel implementation of the current unstructured PIC-FEM code is realized on distributed-memory PC-cluster system utilizing dynamic domain decomposition. Completed code is verified by simulating a quasi-1D RF argon gas discharge with results comparable to previous experimental observations and simulations. Parallel performance with dynamic domain decomposition of the PIC code is tested using a 3D RF argon gas discharge on a PC-cluster system. Results show that parallel efficiency can achieve 83% at 32 processors with dynamic domain decomposition. Some possible improvement of the code performance is demonstrated. Completed code is then applied to predict field emission without and with space-charge effect, and to simulate the RF magnetron argon plasma to demonstrate its capability in handling practical problems.

Summary form only given. In the previous ICOPS meeting, we have presented a parallel 3-D PIC code using the finite-element method with an unstructured tetrahedral mesh for the flexibility of modeling objects with complex geometry. In addition, the dynamic domain decomposition using the graph-partitioning technique is employed for a better load balancing among the processors. Parallel efficiency of this code, implemented on HP clusters could be as high as 82% with 32 processors (40 particles per cell, ~30,000 nodes). However, one of the major drawbacks of this code is the relatively poor runtime performance as compared to the previous PIC codes using the finite-difference method with a structured mesh. In this paper, we will present some improvements, including the Poisson's equation solver and the particle tracing technique, to greatly enhance the code performance. First, we have replaced the original parallel conjugate gradient method by either a sparse direct matrix solver (MUMPS) for fewer processors (<10) or a preconditioned (geometrical additive-Schwartz method) for more processors (>10). With the MUMPS for fewer processors, the assembled coefficient matrix is factorized into the L and U matrices once initially and they are stored for further use at each time step. At each time, only the source term (charge density) changes while the L and U matrices remain unchanged, which makes solving the matrix equation very fast. Second, a tetrahedral mesh is replaced by a multi-block hybrid structured-unstructured mesh to both maintain the flexibility of dealing with complicated geometry and the maximal efficiency of particle tracing. In the structured-mesh block with the pure hexahedral cells, the particle tracing takes advantage of the simple relation between mesh coordinate and mesh index, which is very fast. While in the unstructured-mesh block with the mixed tetrahedral and pyramid cells, the similar technique is adopted. A RF capacitive discharge between t- o circular electrodes in a hexahedral metal chamber is used to demonstrate the performance improvement and preliminary results show reduction of the runtime up to five times can be achieved in the test example

A parallel electrostatic Poisson's equation solver coupled with parallel adaptive mesh refinement (PAMR) is developed in this paper. The three-dimensional Poisson's equation is discretized using the Galerkin finite element method using a tetrahedral mesh. The resulting matrix equation is then solved through the parallel conjugate gradient method using the non-overlapping subdomain-by-subdomain scheme. A PAMR module is coupled with this parallel Poisson's equation solver to adaptively refine the mesh where the variation of potentials is large. The parallel performance of the parallel Poisson's equation is studied by simulating the potential distribution of a CNT-based triode-type field emitter. Results with similar to 100 000 nodes show that a parallel efficiency of 84.2% is achieved in 32 processors of a PC-cluster system. The field emission properties of a single CNT triode- and tetrode-type field emitter in a periodic cell are computed to demonstrate their potential application in field emission prediction. (c) 2006 Elsevier B.V. All rights reserved.

The development of a parallel three-dimensional (3-D) adaptive mesh refinement (PAMR) scheme for an unstructured tetrahedral mesh using dynamic domain decomposition on a memory-distributed machine is presented in detail. A memory-saving cell-based data structure is designed such that the resulting mesh information can be readily utilized in both node- or cell-based numerical methods. The general procedures include isotropic refinement from one parent cell into eight child cells and then followed by anisotropic refinement which effectively removes hanging nodes. A simple but effective mesh-quality control mechanism is employed to preserve the mesh quality. The resulting parallel performance of this PAMR is found to scale approximately as N1.5 for Nproc⩽32. Two test cases, including a particle method (parallel DSMC solver for rarefied gas dynamics) and an equation-based method (parallel Poisson–Boltzmann equation solver for electrostatic field), are used to demonstrate the generality of the PAMR module. It is argued that this PAMR scheme can be applied in any numerical method if the unstructured tetrahedral mesh is adopted.

A parallel three-dimensional electrostatic particle-in-cell Monte Carlo Collision (PIC-MCC) code using an unstructured tetrahedral mesh is developed and validated in this paper. Poisson's equation is discretized by a nodal finite element method using a linear interpolation function. Charged-particle motion is traced cell by Cell using a leap-frog method and the Boris scheme. Code is validated using quasi-one-dimensional radiofrequency gas discharge by observing the bi-Maxwellian electron energy probability distribution function at low pressure. Parallel performance of the PIC code is tested using a three-dimensional radiofrequency gas discharge on a PC-cluster system. Results show that parallel efficiency of 83% can be achieved at 32 processors with dynamic domain decomposition.

Summary form only given. In the past, prediction of field-emission properties has generally ignored the space-charged effects caused by the emitted electrons near the emitting surface. Normal procedures of predicting the field-emission properties are, first, to solve the Poisson's equation once and for all and, then, move the emitted electrons following the predicted unchanged electric field. However, it has been shown that the resulting emitted current is strongly dominated by the space-charged effect as well as the local fields near the emitting surface. Thus, it is necessary to solve the electrical field self-consistently by taking into account the movement of emitted electrons in the cell. In this paper, field-emission properties of a coaxial-type carbon-nanotube emitter are simulated using a parallel 3D PIC code utilizing unstructured tetrahedral mesh, which is presented in another paper in this meeting. Use of multiple gates not only enhances the local electrical fields near the emitting cathode, but also it helps to focus and control the electron flow onto the anode that is important in designing the field emission display. Computational cells near the emitting surface are refined adaptively according to the improved solution using the parallel adaptive mesh refinement module (PAMR). Results of corresponding I-V characteristics due to effects of geometry of the field emitter, location of the gates, distance between the anode and cathode, magnitude of the applied voltage on the gates are discussed in the presentation

Summary form only given. In this report, a parallel three-dimensional electrostatic particle-in-cell (PPIC) method using unstructured tetrahedral mesh is proposed and tested. Poisson's equation for electrostatic distribution is formulated using the Galerkin nodal finite element method (FEM), while motion of charged particles are traced using cell-by-cell particle tracking scheme, under the influence of self-consistent electric field. Linear shape function for electrostatic potential is utilized in the FE formulation. Only nonzero entries of the coefficient matrix of the Poisson's equation are stored using either compressed sparse row (CSR) scheme for smaller problem or "randomly packed" scheme for larger problem. Resulting sparse matrix equation is then solved by preconditioned conjugate gradient method. Charged-particle motion is traced cell-by-cell using leap-frog integration method and Boris scheme, by taking advantage of the cell-neighboring information that is derived from the element connectivity. Parallel implementation of the current unstructured PIC method is realized on memory-distributed machine utilizing domain decomposition via multi-level graph-partitioning technique. A parallel adaptive mesh refinement (PAMR) module for 3D unstructured tetrahedral mesh can also be coupled to this PPIC for better resolution near locations having large field change. Completed code is then tested on a 24-node PC-cluster system by simulating the field-emission properties of a single carbon nanotube under the application of the external electrical field

An enhanced motion estimation scheme is incorporated into the interframe wavelet coding architecture in this paper. Interframe wavelet coding has the advantage of SNR, temporal, and spatial scalability and is a potential candidate for the on-going MPEG-21 scalable video coding standard. Motion-compensated temporal filtering (MCTF) is one of its essential components. Therefore, motion estimation plays an important role in deciding the coding performance. In this paper, we modified the motion estimation syntax/scheme originally specified in the MPEG Advanced Video Coding (AVC) and use it in the interframe wavelet structure. Besides, the techniques of l-block, bidirectional motion estimation, /spl lambda/-value adjustment and motion information partitioning are employed. Simulation results show very promising performance particularly on subjective quality.

We present here the crucial effects of material anisotropy on optical field induced pattern formation in the one-feedback-mirror arrangement which utilizes the nematic liquid crystal film as the nonlinear medium. By using the quasi-static electric-field-biased planar-aligned homogeneous nematic liquid crystal (NLC) films, we observe both the hexagon and the roll patterns which can be switched optically due to the intrinsic anisotropic distribution of the threshold intensity. The anisotropy comes from the anisotropic nonlinear response of the NLC film and is the crucial factor for such a one-feedback-mirror system to form both the roll and hexagon patterns. The observed phenomena can be explained from the linear stability analysis of the governing diffusion-like equation. The experimental results indicate that the stable roll patterns are formed at low input light power and the stable hexagon patterns formed at high input power.

We present here the pattern formation in the transverse profile of a continuous-wave laser beam passing through a parallel planar-aligned nematic liquid crystal (NLC) film biased by a quasistatic electric field and then reflected back to the sample cell by a single planar mirror. The effect of the biasing voltage is studied through the ability to change the nonlinearity by modulating the orientation of the NLC molecules electrically. By suitably modulating the quasistatic electric field the optical pattern transition from the hexagon to the roll is achieved. The pattern transition can be explained by the linear stability analysis for the nonlinear liquid crystal film with optical feedback.

We investigate the influence of the Frank elastic constant anisotropy on optical pattern formation using nematic liquid crystal (NLC) films in one-feedback-mirror systems. The governing diffusion-like equation for the parallel planar-aligned and the vertical-aligned NLC films with an externally biasing quasi-static electric field under the hard boundary assumption is derived. The intrinsic Frank elastic constant anisotropy induces the anisotropy of the diffusion length, which results in the anisotropic distribution of the threshold intensity for optical pattern formation. The effects of the elastic constant anisotropy on optical pattern formation are studied for the first time and the obtained numerical results can reasonably explain the optical patterns that are formed.

A general evaporation model of a single liquid fuel droplet under various ambient turbulence conditions at intermediate droplet Reynolds numbers is proposed combining the current and previous experimental results. Extensive evaporation experiments of single fuel droplets are conducted at room temperature in quasi-laminar and turbulent environments at Reynolds numbers in the range of 72-333. Tested hydrocarbon fuels include by pentane, hexane, heptane, octane and decane. Flow fields, behind a perforated plate or a circular disk in a vertical low-speed wind tunnel, provide the required ambient turbulent environments for droplet evaporation experiments. The ambient turbulence intensities vary in the range of 1-60% and the integral length scales in the range of 0.5-20 times of the initial droplet diameter. Results show that normalized evaporation rate (K/K-L) decreases with increasing effective Damkohler number (0.0001 < Da(v) < 0.1) and is approximately equal to unity with further increasing Damkohler number (0.1 < Da(v) < 1). (C) 2003 Elsevier Ltd. All rights reserved.

Different flow channel designs and heaters made from different materials were tested for improving the performances of silicon-based thermal conductivity detectors. One of the designs involved an electric heater sandwiched between two identical flow channels for high heat transfer rates. The heater of the other design was suspended over a slot to reduce heat losses. The flow channels were etched in silicon wafers and nickel heating elements were deposited on Pyrex glass, polyimide, and silicon nitride membranes. The transient behaviors of the heaters and the wafer temperatures were measured and analyzed for different voltages. The effects of flow channel design and membrane material on the heat transfer characteristics and sensitivities of the detectors were examined. Simple heat transfer models were developed to aid in understanding and diagnosing detector behaviors and performances. The polyimide heater had the best signal conditions. The warm-up times of the TCDs were found to be primarily dependent upon the package dimensions and properties. The double-channel TCD exhibited 20% higher heat transfer rate compared to the single-channel design, but the sensitivities of these two designs differed only slightly.