Multi-beam Technique Research Articles

Formation and applications of periodic structures in transparent materials induced by single fs laser beam

In this paper, we present our recent research development on single fs laser beam induced periodic structures, i.e. nanovoid array, tilted grating structures, nanograting etc, and discuss the mechanisms and promising applications of the observed periodic structures. Periodic structures can be used as grating and lens etc. Usually they are prepared by point-by-point or line by line laser scanning and using multi-beam laser interference technique. However, periodic nanovoid array was observed in transparent materials along the propagation direction of single fs laser beam. (1) By adopting a physical model which combines the nonlinear propagation of fs laser pulses with the spherical aberration (SA) effect at the interface of two mediums of different refractive indices, reasonable agreements between the simulation results and the experimental results were obtained. By comparing the fluence distributions of the case with both SA and nonlinear effects included and the case with only consideration of SA, we suggest that SA, which results from the refractive index mismatch between air and fused silica glass, is the main reason for the formation of the self-organized void array. (2) We observed self-organized microgratings in the bulk SrTiO3 crystal by scanning the laser focus in the direction perpendicular to the laser propagation axis. The groove orientations of those gratings could be controlled by changing the irradiation pulse number per unit scanning length, which could be implemented either through adjusting the scanning velocity at a fixed pulse repetition rate or through varying the pulse repetition rate at a fixed scanning velocity. (3)

Internal stress evolution during field-induced crystallization of anodic tantalum oxide

Amorphous, anodically grown tantalum pentoxide serves as a dielectric layer in capacitors using anodes constructed from tantalum. These capacitors are extensively used in demanding high-reliability applications such as military, aerospace and medical systems. One of the degradation mechanisms in this oxide layer is crystallization induced by applied electric fields. Previous studies have described many attributes of this process. However, the role of stress and strain in the crystallization process has not been explicitly studied to date. The purpose of the present study is to directly monitor the stress evolution and understand its role in the field-induced crystallization of anodic tantalum oxide (ATO). To this end, tantalum was deposited onto quartz substrates by electron beam evaporation and subsequently anodized in acidic electrolytes. A multi-beam optical technique was then used for the in situ measurement of stress evolution in the anodic thin films during the application of electric fields whose strengths are on the order of 5.5×106V/cm. The salient features of field-driven crystallization in acidic electrolytes were additionally tracked using microscopy. The results indicate that field-induced crystallization of ATO is associated with compressive stresses. The methods used in the present study offer a promising new approach for a detailed mechanistic understanding of the stresses associated with the field crystallization process in ATO.

Application of Multi-Beam Technique in Microwave Tubes

The applications of multi-beam technique in microwave tubes including klystrons and traveling wave tubes have been analyzed. A type of 5-beam traveling wave tube slow-wave structure was designed, and dispersion characteristics and coupling impedance characteristics were simulated. According to the simulated results, it can be concluded that dispersion of multi-beam traveling wave tube is satisfactory and the coupling impedance is high, and multi-beam technique can be widely used in microwave tubes because it can improve the plus and output power of microwave tubes.

X-ray Dose Reduction through Adaptive Exposure in Fluoroscopic Imaging

X-ray fluoroscopy is widely used for image guidance during cardiac intervention. However, radiation dose in these procedures can be high, and this is a significant concern, particularly in pediatric applications. Pediatrics procedures are in general much more complex than those performed on adults and thus are on average four to eight times longer1. Furthermore, children can undergo up to 10 fluoroscopic procedures by the age of 10, and have been shown to have a three-fold higher risk of developing fatal cancer throughout their life than the general population2,3. We have shown that radiation dose can be significantly reduced in adult cardiac procedures by using our scanning beam digital x-ray (SBDX) system4-- a fluoroscopic imaging system that employs an inverse imaging geometry5,6 (Figure 1, Movie 1 and Figure 2). Instead of a single focal spot and an extended detector as used in conventional systems, our approach utilizes an extended X-ray source with multiple focal spots focused on a small detector. Our X-ray source consists of a scanning electron beam sequentially illuminating up to 9,000 focal spot positions. Each focal spot projects a small portion of the imaging volume onto the detector. In contrast to a conventional system where the final image is directly projected onto the detector, the SBDX uses a dedicated algorithm to reconstruct the final image from the 9,000 detector images. For pediatric applications, dose savings with the SBDX system are expected to be smaller than in adult procedures. However, the SBDX system allows for additional dose savings by implementing an electronic adaptive exposure technique. Key to this method is the multi-beam scanning technique of the SBDX system: rather than exposing every part of the image with the same radiation dose, we can dynamically vary the exposure depending on the opacity of the region exposed. Therefore, we can significantly reduce exposure in radiolucent areas and maintain exposure in more opaque regions. In our current implementation, the adaptive exposure requires user interaction (Figure 3). However, in the future, the adaptive exposure will be real time and fully automatic. We have performed experiments with an anthropomorphic phantom and compared measured radiation dose with and without adaptive exposure using a dose area product (DAP) meter. In the experiment presented here, we find a dose reduction of 30%.

X-ray Dose Reduction through Adaptive Exposure in Fluoroscopic Imaging

X-ray fluoroscopy is widely used for image guidance during cardiac intervention. However, radiation dose in these procedures can be high, and this is a significant concern, particularly in pediatric applications. Pediatrics procedures are in general much more complex than those performed on adults and thus are on average four to eight times longer1. Furthermore, children can undergo up to 10 fluoroscopic procedures by the age of 10, and have been shown to have a three-fold higher risk of developing fatal cancer throughout their life than the general population2,3.We have shown that radiation dose can be significantly reduced in adult cardiac procedures by using our scanning beam digital x-ray (SBDX) system4-- a fluoroscopic imaging system that employs an inverse imaging geometry5,6 (Figure 1, Movie 1 and Figure 2). Instead of a single focal spot and an extended detector as used in conventional systems, our approach utilizes an extended X-ray source with multiple focal spots focused on a small detector. Our X-ray source consists of a scanning electron beam sequentially illuminating up to 9,000 focal spot positions. Each focal spot projects a small portion of the imaging volume onto the detector. In contrast to a conventional system where the final image is directly projected onto the detector, the SBDX uses a dedicated algorithm to reconstruct the final image from the 9,000 detector images.For pediatric applications, dose savings with the SBDX system are expected to be smaller than in adult procedures. However, the SBDX system allows for additional dose savings by implementing an electronic adaptive exposure technique. Key to this method is the multi-beam scanning technique of the SBDX system: rather than exposing every part of the image with the same radiation dose, we can dynamically vary the exposure depending on the opacity of the region exposed. Therefore, we can significantly reduce exposure in radiolucent areas and maintain exposure in more opaque regions. In our current implementation, the adaptive exposure requires user interaction (Figure 3). However, in the future, the adaptive exposure will be real time and fully automatic.We have performed experiments with an anthropomorphic phantom and compared measured radiation dose with and without adaptive exposure using a dose area product (DAP) meter. In the experiment presented here, we find a dose reduction of 30%.

Multi-instrument observations of soft electron precipitation and its association with magnetospheric flows

[1] We present a multi-instrument study on the variations of optical auroras and ionospheric electron densities during an interval of a series of fast earthward flows in the magnetotail on 3 March 2009. The flow-related auroral signatures include intermittent higher-latitude (>68° magnetic latitude) intensifications manifested in green and blue line auroras and more latitudinally extended red line auroral intensifications and expansions. During the same interval the Poker Flat incoherent scatter radar (PFISR) detected F region ionospheric electron density enhancements which, together with the red line auroral intensifications, give evidence for soft electron (<1 keV) precipitation associated with fast magnetospheric flow activity. We demonstrated the southward motion of ionospheric electron density patches in correspondence to individual earthward flow bursts and auroral activations. By virtue of the multibeam technique of PFISR we construct the altitudinal profile of the density patches and estimate that the characteristic energies of the precipitating electrons were on order of a few hundred eV, comparable to the observed electron temperature in the near-Earth central plasma sheet (CPS). We propose that the fast flows give rise to enhanced ELF wave activity, which causes strong pitch angle diffusion of the soft electron population in the CPS via wave-particle interactions. The precipitation may be further aided with a moderate field-aligned potential drop comparable to or smaller than the CPS electron temperature. When the flows penetrate into the inner plasma sheet, the adiabatic drift motion of soft electrons may lead to a decreasing trend of electron energy with decreasing radial distance, which is manifested in PFISR observations as an ascending trend of the altitude range of the density patches toward the equatorward auroral border.

A Random Multi-beam Multiplexing Technique for the Downlink of Multiuser MIMO System

该文提出了一种随机多波束多用户复用技术，充分利用多用户分集以及基站多天线的空间自由度来提高系统吞吐量。不同于传统的随机波束形成技术，该技术首先在给定预编码码本内随机选取一个码字，然后调度多个空分复用用户以及其余预编码矩阵。该文采用了一种逐次调度的方式，第一次训练调度一个主发送用户并确定一个次发送预编码矩阵，通过第二次训练选择次发送用户，这种方式能以很小的反馈开销有效控制复用用户之间的相互干扰。同时，该文所提技术能进一步推广到用户具有不同天线配置的异构情形。仿真结果表明，该文技术在具有不同相关性的信道环境下都能获得较优的系统吞吐量。

Neutron Visual Sensing Techniques Using the High Intensity Neutron Sources

In this manuscript, the fundamentals and merits of neutron visual sensing techniques, high-speed consecutive visual sensing technique, high-speed-scanning dynamic 3D visualization technique, multi-beam 3D velocimetry technique and energy-selective neutron imaging technique are summarized. These techniques have been developed mainly for the R&D of the nuclear energy system using the high-flux research reactors, JRR-3 and JRR-4, and a high-intensity pulsed neutron source, J-PARC. Very unique visualization and measurement, e.g. dynamic 3D velosimetry in JRR-4 were successfully carried out by these neutron visual sensing techniques. An energy-selective neutron imaging, e.g. the neutron resonance absorption imaging in J-PARC is extremely unique and newly developed visualization and measurement technique. Analytical investigation for the feasibility of multiphase flow experiment in J-PARC was also reported using a Monte Carlo neutron simulation code. It was confirmed from the analytical work that the measurement error of the void fraction of two-phase flow can be reduced by the energy-selective neutron imaging technique.

Imaging and MTI processing based on dual-frequencies dual-apertures spaceborne SAR

Based on dual-frequencies dual-apertures spaceborne SAR (Synthetic Aperture Radar), a new SAR system with four receiving channels and two operation modes is presented in this paper. SAR imaging and Moving Target Indication (MTI) are studied in this system. High resolution imaging with wide swath is implemented by the Mode I, and MTI is completed by the Mode II. High azimuth resolution is achieved by the Displaced Phase Center (DPC) multibeam technique. And the Coherent Accumulation (CA) method, which combines dual channels data of different carrier frequency, is used to enhance the range resolution. For the data of different carrier frequency, the two aperture interferometric processing is executed to implement clutter cancellation, respectively. And the couple of clutter suppressed data are employed to implement Dual Carrier Frequency Conjugate Processing (DCFCP), then both slow and fast moving targets detection can be completed, followed by moving target imaging. The simulation results show the validity of the signal processing method of this new SAR system.

Reconstruct azimuth signal and suppress interbeam ambiguities of SPCMB SAR with hybrid filterbank

Conventional Synthetic Aperture Radar (SAR) systems cannot obtain high-resolution and wide-swath illumination area due to the well-known minimum antenna area constraint. Single Phase Center MultiBeam (SPCMB) technique can overcome this limitation by adding spatial sampling through multiple receivers in azimuth direction. Unfortunately, this approach will lead to an increase of azimuth ambiguities (interbeam ambiguities), because each receive beam’s mainlobe overlaps with the other ones’ sidelobes. This paper proves that the front part of SPCMB SAR systems can be considered to be a hybrid filterbank. Therefore, the azimuth signal can be reconstructed and the interbeam ambiguities can be effectively suppressed by a well-designed hybrid filterbank.

Holographic Patterning of Composite Polymeric Materials for Photonic Applications

We report the results of an extended investigation on holographic patterning of novel liquid crystal-polymer materials, developed with the aim of producing one- and two-dimensional periodic structures to be used in optical devices for data storage and processing. Characterization of reflection gratings recorded at 405 nm resulted in values of diffraction efficiency (> 60%), sensitivity (up to 104 cm/J) and refractive index modulation (Δn ∼ 0.01) that make these materials interesting for optical storage applications. Large-area two-dimensional periodic structures have been also patterned using a multibeam photolitigraphic technique. These planar photonic crystals have many possible uses, both in fundamental and applied physics.

Flare imaging with multibeam systems: Data processing for bubble detection at seeps

Multibeam sonar surveys have been conducted since their invention in the 1970s; however, mainly reflections from the seafloor were considered so far. More recently, water column imaging with multibeam is becoming of increasing interest for fisheries, buoy, mooring, or gas detection in the water column. Using ELAC SEABEAM 1000 data, we propose a technique to detect gas bubbles (flares) although this system is originally not designed to record water column data. The described data processing represents a case study and can be easily adapted to other multibeam systems. Multibeam data sets from the Black Sea and the North Sea show reflections of gas bubbles that form flares in the water column. At least for reasonably intense gas escape the detection of bubbles is feasible. The multibeam technique yields exact determination of the source position and information about the dimension of the gas cloud in the water. Compared to conventional flare imaging by single‐beam echo sounders, the wide swath angle of multibeam systems allows the mapping of large areas in much shorter time.

Electrochemical Processing of Ultrathin Metallic Oxides Featuring in-situ Monitoring of Growth Stress Transitions

A multi-beam curvature measurement technique was applied, in combination with classical chronopotentiometry, for investigating in-situ the growth stresses developing during galvanostatic anodisation of titanium thin films. The titanium electrodes were anodised in acidic electrolytes (HNO3, H2SO4 and H3PO4) with current densities in the range of 0.25 to 1 mA/cm². Two distinct stages were observed in the evolution of the cell voltage (V) with time, characterised by a different value of the slope dV/dt. In terms of growth stresses, these two stages were observed to correspond to two distinct regimes as well, associated with the development of, respectively, compressive and tensile stresses. The origin of the growth stress evolution in the second stage was rationalised in terms of an extended oxide growth model, taking into account the creation of free-volume at the metal/oxide interface.

Bunched electron beam properties measurement by means of single-shot multibeam cross-correlation technique

A new single-shot multibeam cross-correlation technique is proposed for the measurement of a bunched electron beam temporal structure. As an optical laser pulse modulated by the Coulomb field of the electron bunch in an electro-optical crystal, it is proposed to use a non-uniform pulse modulated both in time and in space. In the proposed technique, measurement of the time profile is realized during a single laser shot. In this case, as a result of electro-optical modulation of a non-uniform femtosecond pulse, the information on the time profile of the Coulomb field is contained in only the spatially shifted spectral components of the modulated pulse.

Modeling Laser-Spallation Rock Drilling

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 95746, "Modeling of Laser Spallation Drilling of Rocks for Gas- and Oilwell Drilling," by Z. Xu, SPE, Y. Yamashita, and C.B. Reed, Argonne Natl. Laboratory, prepared for the 2005 SPE Annual Technical Conference and Exhibition, Dallas, 9–12 October. High-power lasers can weaken, spall, melt, and vaporize Earth materials, with thermal spallation being the most energy-efficient rock-removal mechanism. The most interesting focus of recent laser rock-drilling research is on developing a laser rock-spallation technique to drill large and deep holes in rocks. Laser spallation has a rock-removal rate higher than conventional rotary drilling and flame-jet spallation. Research also is focused on the use of laser rock spallation to make perforation channels with improved permeability of the perforated rocks. Introduction Laser rock spallation is a rock-removal process that uses laser-induced thermal stress to fracture the rock into small fragments before it melts. High-intensity-laser energy, focused on a rock that has very low thermal conductivity, causes the local rock temperature to increase instantaneously. This results in a local thermal stress that spalls the rock. Previous test data show that laser rock spallation is the most energy efficient among all laser rock-removal mechanisms. Recent research and development concentrating on use of advanced high-power lasers to drill and complete oil and gas wells has focused on two fronts. The first was to develop a multibeam laser rock-spallation technique to drill large and deep holes in rocks with a rock-removal rate higher than that of conventional rotary drilling and flame-jet spallation. In this approach, each laser beam spalls a hole as big as the beam spot and half the beam diameter deep. Multiple beams are overlapped to remove a layer of rock. Layer by layer, a large and deep hole is drilled. The second focus was to develop a laser rock-perforation technique for oil- and gas-well completion applications. Perforating oil and gas wells requires creating a hole through a composite structure of steel casing, cement, and formation rock. Current explosive-charge perforation methods, while capable of creating the holes, significantly reduce the rock permeability. Laboratory tests demonstrate that laser beams not only cut rocks efficiently, but also increase the permeability of spalling-drilled rock significantly. An innovative laser perforating system will allow the oil and gas industry to improve injection and production rates.

Switchable quasi-crystal structures with five-, seven-, and ninefold symmetries

We report on the design, fabrication, and characterization of switchable quasi-crystal structures in holographic polymer-dispersed liquid-crystal materials using a multibeam hololithography exposure technique. By interfering multiple coherent laser beams on a liquid crystal-polymer mixture, one can create quasi-crystal morphologies on a mesoscale. The quasi-crystal structures with five-, seven-, and ninefold symmetries are confirmed by mapping of scanning electron microscope images to the calculated isointensity profiles and comparison of their diffraction patterns to the Fourier transforms of the calculated isointensity profiles. Diffraction properties and electro-optic switching parameters of the quasi-crystal samples are presented, and their refractive index modulation is estimated to be 3×10−3 using coupled-wave theory.

P-133: Color Separation Element with Concentric Symmetry for Display Application

We report on the design, fabrication, and potential application of electrically switchable quasi-crystal diffractive elements using a multi-beam holo-lithgraphy exposure technique. These quasi-crystal gratings diffract light in the radial direction with concentric symmetry, serving as efficient color separation elements for potential use in display applications. In addition, we report the fabrication of quasi-crystals using a holographic method and their electro-optical properties.

Fabrication of two- and three-dimensional periodic structures by multi-exposure of two-beam interference technique

A simple and efficient optical interference method for fabricating high quality two- and three-dimensional (2D and 3D) periodic structures is demonstrated. Employing multi-exposure of two-beam interference technique, different types of periodic structures are created depending on the number of exposure and the rotation angle of the sample for each exposure. Square and hexagonal 2D structures are fabricated by a multi-exposure of two-beam interference pattern with a rotation angle of 90 masculine and 60 masculine between two different exposures, respectively. Three-exposure, in particular, results in different kinds of 3D structures, with close lattice constants in transverse and longitudinal directions, which is difficult to be obtained by the commonly used multi-beam interference technique. The experimental results obtained with SU-8 photoresist are well in agreement with the theoretical predictions. Multi-exposure of two-beam interference technique should be very useful for fabrication of photonic crystals.

Periodic Structures of Organic-Titania Hybrid Materials Recorded by Multi-Beam Laser Interference Technique

Photosensitive organic-titania hybrid materials have been prepared from metal alkoxides and various organic ester compounds with \(C{=}C\) double bonds. The refractive index of the film increases with the decrease of the concentration of the organic ester compound, and the highest refractive index of 1.62 is obtained when 2-hydroxymethyl acrylate (HOA) is used as the organic ester and the molar ratio of HOA to Ti is 0.5. The material with the highest refractive index is exposed to femtosecond pulse using the multi-beam laser interference technique. After laser irradiation, the irradiated parts of the material are photopolymerized and periodic structures can be obtained by development of the unirradiated parts. In the case of laser irradiation of 120 μ J total energy for 5 min, the periodic structure obtained corresponds to 2D photonic crystal structure which is composed of two parts; the material with the highest refractive index and the air.

Revealing all types of threading dislocations in GaN with improved contrast in a single plan view image

In this letter, we demonstrate a transmission electron microscope based technique which reveals all types of threading dislocations (TDs) in GaN with high contrast over a relatively large area, even if the specimen is bent. This method uses a bright-field image with the crystal oriented at the ⟨1–21–3⟩ zone axis, taken using multi-beam diffraction conditions. Such an image reveals all screw, edge, and mixed types of threading dislocations. The multi-beam imaging technique described here for GaN is more generally applicable to counting the total dislocation density in a wide range of materials and structures.