Optimization of wide-band linear arrays

An optimization method is proposed for linear arrays to be used in ultrasound systems under wide-band operation. A fast algorithm, the threshold accepting, has been utilized to determine the element positions and weight coefficients of a linear array that generates a desired beam pattern. To reduce the computational burden in the optimization procedure, an efficient numerical routine for the beam pattern evaluation has been implemented. We address the optimization problem of both dense and sparse wideband arrays. In the first case, the goal is to minimize the side-lobe energy by varying the element weights; we compare the optimized beam pattern with that obtained with classical shading functions, showing that better results can be achieved with a wide-band optimization. We also consider the optimization of the layout (positions and weights) of a sparse linear array to achieve a desired beam pattern with a fixed or minimum number of array elements. The comparison of the proposed method with a narrow-band optimization algorithm is presented, showing that better performances (about -7 dB further reduction of the side-lobe level) can be achieved with a wide-band sparse array optimization. Further numerical simulations are given, showing that the proposed method yields better results than wideband sparse random arrays and periodic arrays with the same aperture width     

Delta-doped electron-multiplied CCD with absolute quantum efficiency over 50% in the near to far ultraviolet range for single photon counting applications

We have used molecular beam epitaxy (MBE) based delta-doping technology to demonstrate nearly 100% internal quantum efficiency (QE) on silicon electron-multiplied charge-coupled devices (EMCCDs) for single photon counting detection applications. We used atomic layer deposition (ALD) for antireflection (AR) coatings and achieved atomic-scale control over the interfaces and thin film materials parameters. By combining the precision control of MBE and ALD, we have demonstrated more than 50% external QE in the far and near ultraviolet in megapixel arrays. We have demonstrated that other important device performance parameters such as dark current are unchanged after these processes. In this paper, we briefly review ultraviolet detection, report on these results, and briefly discuss the techniques and processes employed.     

A Simple Top‐Down/Bottom‐Up Approach to Sectored,Ordered Arrays of Nanoscopic Elements Using Block Copolymers

A top‐down/bottom‐up approach is demonstrated by combining electron‐beam (e‐beam) lithography and a solvent annealing process. Micellar arrays of polystyrene‐block‐poly(4‐vinylpyridine) (PS‐b‐P4VP) with a high degree of lateral order can be produced on a surface where sectoring is defined by e‐beam patterning. The e‐beam is used to crosslink the block copolymer (BCP) film immediately after spin‐coating when the BCP is disordered or in a highly ordered solvent‐annealed film. Any patterns can be written into the BCP by crosslinking. Upon exposure to a preferential solvent for the minor component block followed by drying, cylindrical nanopores are generated within the nonexposed areas by a surface reconstruction process, while, in the exposed areas, the films remain unchanged. Nickel nanodot arrays can be placed over selected areas on a surface by thermal evaporation and lift‐off process.     

Microlens-array-based exit-pupil expander for full-color displays

Two-dimensional arrays of microlenses can be used in wearable display applications as numerical aperture expanders or exit-pupil expanders (EPEs) to increase the size of the display exit pupil. A novel EPE approach that uses two microlens arrays (MLAs) is presented. The approach is based on cascading two identical microlens arrays spaced precisely at one focal-length distance with submicrometer registration tolerances relative to each other. The ideal MLA for this application requires a 100% fillfactor, sharp seams between microlenses, and a perfect spherical profile. We demonstrate a dual-MLA-based EPE that produces excellent exit-pupil uniformity and better than 90% diffraction efficiency for all three wavelengths in a color-display system. Two-MLA registration is performed with submicrometer precision by use of far-field alignment techniques. Fourier optics theory is used to derive the analytical formulas, and physical optics beam propagation is used for numerical computations. Three MLA fabrication technologies, including gray-scale lithography, photoresist reflow, and isotropic etching, are evaluated and compared for an EPE application.     

Design of wide-band arrays for low side-lobe level beam patterns by simulated annealing

We address the window design problem of linear arrays to be used in wide-band beamforming systems. During last decades, stochastic optimization algorithms have proven to be very efficient in optimal beamforming synthesis, but most research interest has focused on linear and two-dimensional arrays under narrow-band excitation. In the present paper, we introduce a general approach, based on the simulated annealing optimization algorithm, to design uniformly spaced wide-band arrays that generate low side-lobe level beam patterns. The effectiveness of different beam pattern formulations is evaluated within the proposed framework; we also consider three different methods to control the side-lobe level, yielding different array performances. Moreover, we investigate the influence of the frequency bandwidth on the optimized beam patterns.     

Design aspects of focal beams from high-intensity arrays

As the applications of ultrasonic thermal therapies expand, the design of the high-intensity array must address both the energy delivery of the main beam and the character and relevance of off-target beam energy. We simulate the acoustic field performance of a selected set of circular arrays organized by array format, including flat versus curved arrays, periodic versus random arrays, and center void diameter variations. Performance metrics are based on the -3-dB focal main lobe (FML) positioning range, axial grating lobe (AGL) temperatures, and side lobe levels. Using finite-element analysis, we evaluate the relative heating of the FML and the AGLs. All arrays have a maximum diameter of 100λ, with element count ranging from 64 to 1024 and continuous wave frequency of 1.5 MHz. First, we show that a 50% spherical annulus produces focus beam side lobes which decay as a function of lateral distance at nearly 87% of the exponential rate of a full aperture. Second, for the arrays studied, the efficiency of power delivery over the -3-dB focus positioning range for spherical arrays is at least 2-fold greater than for flat arrays; the 256-element case shows a 5-fold advantage for the spherical array. Third, AGL heating can be significant as the focal target is moved to its distal half-intensity depth from the natural focus. Increasing the element count of a randomized array to 256 elements decreases the AGL-to-FML heating ratio to 0.12 at the distal half-intensity depth. Further increases in element count yield modest improvements. A 49% improvement in the AGL-to-peak heating ratio is predicted by using the Sumanaweera spiral element pattern with randomization.     

一种新颖的超大点阵分束实现方法

首次提出了一种实现超大卢、阵光束分束的新方法，它是利用位相迭加原理和ＶＬＳＩ技术将光栅分束器和位相型菲涅耳微透镜列阵集成为一体，构成了一种新型的光束分束列阵器件—一菲涅耳计算机源生全息分来器（ＣＧＦＢＳ），以实现１２８×１２８，甚至更高卢、阵数的光点分束。     

X-ray imaging: a generalized approach using phase-space tomography

We discuss the role of coherence in x-ray imaging and consider how phase-space tomography can be used to extract information about partial coherence. We describe the application of phase-space tomography to x-ray imaging and recover the spatial coherence properties of a one-dimensional soft (1.5 keV) x-ray beam from a synchrotron undulator source. We present phase-space information from a Young's experiment and observe negative regions in the quasi-probability distribution. We show that, given knowledge of the coherence of the beam, we can use partially coherent diffraction data to recover fully coherent information, and we present some simple experimental demonstrations of this capability.     

Elastic Fields in Immersed Isotropic Solids from Phased Arrays: The Time Harmonic Case

Elastic fields in immersed isotropic solids from phased arrays are investigated theoretically in both far- and near-field regions for a variety of beams. The angular spectrum approach (ASA) is used here as a tool to evaluate both far-field beam patterns and near-fields in immersed solids with planar interfaces. Based on the angular spectra of the arrays in a fluid and the transmission coefficients of longitudinal and shear waves at the fluid / solid interface, the angular spectra in the immersed solid are obtained and used for analyzing far-field beam patterns and for constructing near-fields in the immersed solid. The numerical calculations are optimally implemented. Several examples are presented and analyzed of nonsteered and steered, nonfocused and focused, and longitudinal and transverse beams in copper submerged in water. Emphasis is placed on elastic fields from the oblique incidence of beams steered by arrays. One method of focusing a steered beam is investigated that, in phasing, takes into account refraction at the interface, so that the beam is directed at the desired angle in the solid. Analysis of far-field beam patterns in immersed solids obtained from the ASA is helpful for the design and development of arrays and for the adequate use of existing arrays in the immersion measurements in nondestructive evaluation.     

Focused ion beam lithography and anodization combined nanopore patterning

In this study, focused ion beam lithography and anodization are combined to create different nanopore patterns. Uniform-, alternating-, and gradient-sized shallow nanopore arrays are first made on high purity aluminum by focused ion beam lithography. These shallow pore arrays are then used as pore initiation sites during anodization by different electrolytes. Depending on the nature of the anodization electrolyte, the nanopore patterns by focused ion beam lithography play different roles in further pore development during anodization. The pore-to-pore distance by focused ion beam lithography should match with that by anodization for guided pore development to be effective. Ordered and heterogeneous nanopore arrays are obtained by the focused ion beam lithography and anodization combined approach.     

Partially coherent analysis of imaging and interferometric phased arrays: noise, correlations, and fluctuations

Phased arrays are of considerable importance for far-infrared, submillimeter-wave, and microwave astronomy; they are also being developed for areas as diverse as optical switching, radar, and radio communications. We present a discretized, modal theory of imaging and interferometric phased arrays. It is shown that the average powers, field correlations, power fluctuations, and correlations between power fluctuations at the output ports of an imaging, or interferometric, phased array can be determined for a source in any state of spatial coherence and polarization, once the synthesized beam patterns are known. It is not necessary to know anything about the internal construction of the beam-forming networks; indeed, the beam patterns can be taken from experimental data. The synthesized beams can be nonorthogonal and even linearly dependent. Our theory leads to many conceptual insights and opens the way to a range of new design and simulation techniques.     

模拟退火算法设计阵元缺失基阵方向图的方法

基阵方向图设计是声纳设计中的核心和重要部分,由于声纳系统的使用环境比较复杂,为了快速、有效地在阵元缺失的情况下对基阵方向图进行设计,引入了模拟退火算法对这类问题进行处理。模拟退火算法是近年来出现的全局优化算法,易于编程、有良好的收敛性。计算机仿真结果表明,模拟退火算法可以很好地完成在阵元缺失情况下对基阵方向图的补偿设计。     

Element shape design of 2-D CMUT arrays for reducing grating lobes

Fully populated two-dimensional (2-D) arrays are needed to produce high quality ultrasonic volumetric images for real-time applications, but they present many challenges for their physical realization because of the large number of elements. In fact, lambda/2 and lambda minimum spacing between elements is required, respectively, for pyramidal and rectilinear scanning in order to avoid unwanted grating lobes (GLs). However, in past years, capacitive micromachined ultrasonic transducer (CMUT) technology has made possible the production of arrays with large flexibility in element shape and size. In this paper, this property is analyzed, and a new element shape, based on the concept of spatial interpenetration of adjacent elements, is proposed in order to design fully populated 2-D CMUT arrays with a low number of elements, whose beam characteristics are valid for volumetric imaging. Through the use of simulations, it is demonstrated that arrays with pitch larger than lambda (up to 3lambda) used for rectilinear scanning, have notably lower GLs than the equivalent standard arrays designed according to the classical squared element shape. As consequence, the proposed geometry has the advantage of reducing the number of elements (up to a factor of 9) and of enlarging the element size, implying an increase of the SNR relative to the single element. When beam steering is required, arrays can be designed with pitch equal to lambda, reducing the number of elements by a factor of 4 if the maximum steering angle is limited to +/-15 degrees .     

Close-packed noncircular nanodevice pattern generation by self-limiting ion-mill process

We describe a self-limiting, low-energy argon-ion-milling process that enables noncircular device patterns, such as squares or hexagons, to be formed using precursor arrays of uniform circular openings in poly(methyl methacrylate) defined using electron beam lithography. The proposed patterning technique is of particular interest for bit-patterned magnetic recording medium fabrication, where square magnetic bits result in improved recording system performance. Bit-patterned magnetic medium is among the primary candidates for the next generation magnetic recording technologies and is expected to extend the areal bit density limits far beyond 1 Tbit/in(2). The proposed patterning technology can be applied either for direct medium prototyping or for manufacturing of nanoimprint lithography templates or ion beam lithography stencil masks that can be utilized in mass production.     

Continuous phase plate for laser beam smoothing

We discuss the beam smoothing principle of a continuous phase plate (CPP) while the input light is varying. The analysis model of the process in which the laser beam with random phase noise propagates through a CPP has been established. With this model the beam smoothing mechanism of the CPP for the laser beam with a different phase aberrations can be described. A method to optimize the smoothing result is introduced.     

Nanosynthesis of tunable composite materials by room-temperature pulsed focused electron beam induced chemical vapour deposition

Hydrocarbons inherently present in standard high-vacuum scanning electron microscopes can be favorably used for co-deposition with functional molecules injected into the chamber. By varying the beam exposure pulse time the carbon content incorporated into the deposit can be tuned. In the particular case when the hydrocarbons are provided by surface diffusion, the composition depends also on the size of the final deposits. This dependency can be used as an additional parameter, besides the beam pulse time, in order to tune the metal/matrix ratio and to obtain new nanoscale materials with tailored physical properties. We present and discuss experimental results on composition tunability by pulsed electron-beam deposition for the two-adsorbate system Co2(CO)8/hydrocarbon and their use in fabricating Hall nanosensors of cobalt-carbon nanocomposite material with enhanced magnetic sensitivity and high magnetic spatial resolution.     

Ultrasonic heating with waveguide interstitial applicator array

Ultrasound interstitial applicators can be used for heating tumors near air and bone interfaces, where use of noninvasive ultrasound methods becomes difficult. We describe in this paper an ultrasonic waveguide three-applicator array for interstitial thermotherapy. We first discuss the temperature distribution and the pattern of heat deposition using the effective thermal conductivity equation. This equation is used then for finite element analysis of temperature modeling. We discuss heating in porcine brain tissue and present results with the array in a large volume tissue phantom. Simulation results agreed well with the experiment. The average difference between measured and simulation temperatures was <0.4°C, less than the precision of temperature determination. Further modeling of temperature distribution in the human brain was based on the above simulations. We show that the arrays can be useful for thermotherapy in tissues with an appropriate choice of physical parameters of the applicators     

Propagation of coherently combined truncated laser beam arrays with beam distortions in non-Kolmogorov turbulence

The propagation properties of coherently combined truncated laser beam arrays with beam distortions through non-Kolmogorov turbulence are studied in detail both analytically and numerically. The analytical expressions for the average intensity and the beam width of coherently combined truncated laser beam arrays with beam distortions propagating through turbulence are derived based on the combination of statistical optics methods and the extended Huygens-Fresnel principle. The effect of beam distortions, such as amplitude modulation and phase fluctuation, is studied by numerical examples. The numerical results reveal that phase fluctuations have significant influence on the spreading of coherently combined truncated laser beam arrays in non-Kolmogorov turbulence, and the effects of the phase fluctuations can be negligible as long as the phase fluctuations are controlled under a certain level, i.e., a>0.05 for the situation considered in the paper. Furthermore, large phase fluctuations can convert the beam distribution rapidly to a Gaussian form, vary the spreading, weaken the optimum truncation effects, and suppress the dependence of spreading on the parameters of the non-Kolmogorov turbulence.     

The growth of freestanding single carbon nanotube arrays

Regular arrays of freestanding single carbon nanotubes (CNTs) were prepared on Ni dot arrays by dc plasma-enhanced chemical vapour deposition. The size of the Ni dot was reduced for single CNT growth by means of conventional photolithography and a lateral wet-etch process. The vertical alignment of a single CNT was directly dependent on the location of the catalyst metals. Using this method, well-separated and well-defined regular arrays of freestanding CNTs can be fabricated and the process can be scaled up at a lower cost than electron beam lithography, which is encouraging for applications in field emitters and nanoelectrodes.     

Modeling diffraction in free-space optical interconnects by the mode expansion method

Free-space optical interconnects (FSOIs), made up of dense arrays of vertical-cavity surface-emitting lasers, photodetectors and microlenses can be used for implementing high-speed and high-density communication links, and hence replace the inferior electrical interconnects. A major concern in the design of FSOIs is minimization of the optical channel cross talk arising from laser beam diffraction. In this article we introduce modifications to the mode expansion method of Tanaka et al. [IEEE Trans. Microwave Theory Tech. MTT-20, 749 (1972)] to make it an efficient tool for modelling and design of FSOIs in the presence of diffraction. We demonstrate that our modified mode expansion method has accuracy similar to the exact solution of the Huygens-Kirchhoff diffraction integral in cases of both weak and strong beam clipping, and that it is much more accurate than the existing approximations. The strength of the method is twofold: first, it is applicable in the region of pronounced diffraction (strong beam clipping) where all other approximations fail and, second, unlike the exact-solution method, it can be efficiently used for modelling diffraction on multiple apertures. These features make the mode expansion method useful for design and optimization of free-space architectures containing multiple optical elements inclusive of optical interconnects and optical clock distribution systems.