Standing Wave Field Research Articles

Prospects for the expansion of standing wave ambient pressure photoemission spectroscopy to reactions at elevated temperatures

Standing wave ambient pressure photoemission spectroscopy (SWAPPS) is a promising method to investigate chemical and potential gradients across solid-vapor and solid-liquid interfaces under close-to-realistic environmental conditions, far away from high vacuum. Until now, these investigations have been performed only near room temperature, but for a wide range of interfacial processes, chief among them being heterogeneous catalysis, measurements at elevated temperatures are required. One concern in these investigations is the temperature stability of the multilayer mirrors, which generate the standing wave field. At elevated temperatures, degradation of the multilayer mirror due to, for example, interdiffusion between the adjacent layers, decreases the modulation of the standing wave field, thus rendering SWAPPS experiments much harder to perform. Here, we show that multilayer mirrors consisting of alternate B4C and W layers are stable at temperatures exceeding 600 °C and are, thus, promising candidates for future studies of surface and subsurface species in heterogeneous catalytic reactions using SWAPPS.

Acoustic characteristics of looped-tube thermoacoustic refrigerators with external and in-built acoustic drivers: A comparative study.

Thermoacoustic refrigerators (TARs) are in the first place acoustic systems. It is indispensable to conduct an acoustic analysis of the entire system before a thermodynamic analysis of the thermoacoustic core. This study focuses on the acoustic characteristics of looped-tube TARs integrated with single (or dual) external (or in-built) acoustic driver(s). System-level acoustic models are established for the looped-tube TARs, and their acoustic performances are discussed and compared. Results show that looped-tube TARs with a single acoustic driver have a standing-wave acoustic field no matter whether the configuration is symmetric or not. The eigenmodes of the TAR are not influenced by the location of the external acoustic driver but are affected by the location of the in-built acoustic driver. New pairs of resonance and anti-resonance frequencies show up for TARs with an asymmetric configuration. Compared with looped-tube TARs with a single external (or in-built) acoustic driver, looped-tube TARs with dual external (or in-built) acoustic drivers have the same (or different) eigenmodes. A standing-wave acoustic field is present inside the loop only when the two acoustic drivers work in in-phase and anti-phase modes. At other phase differences between the two acoustic drivers, the acoustic field inside the loop becomes hybrid, containing both standing- and traveling-wave components. The theoretical methodology and analytical results in this study are valuable in comprehending the acoustic behavior of electrically-driven looped-tube TARs, providing useful guidelines for the active control of acoustic field and heat transport in the thermoacoustic core.

S1‐Guideline Cutaneous Angiosarcomas – Update 2021

S1‐Guideline Cutaneous Angiosarcomas – Update 2021

A 0.35-THz Extended Interaction Oscillator Based on Overmoded and Bi-Periodic Structure

An improved topology of Extended interaction oscillators (EIO) is presented with enhanced efficiency at 0.35 THz. A bi-periodic ladder-type structure is used for optimizing the performance of the TM $_{{31}}- 2\pi $ mode operation, based on bi-periodic interaction gaps composed of staggered short and long slots. The bi-periodic interaction mechanism permits to operate the circuit with a standing-wave field between the $\pi $ and $2\pi $ modes, thus providing the potential to combine the advantages of both $\pi $ and $2\pi $ modes in terms of both coupling and output performance. The resulting transverse TM31 operating mechanism exhibits a good agreement with the bi-periodic structure in terms of coupling performance, with the short slot length positively correlated with the value of effective characteristic impedance ${M}^{\,{2}}{R}/{Q}$ . The circuit demonstrates attractive coupling and output characteristics by optimization of ${M}^{\,{2}}{R}/{Q}$ and ${Q}_{e}$ . A state-of-the-art value ${M}^{\,{2}}{R}/{Q}$ of 59.69 $\Omega $ and a moderate ${Q}_{e}$ of 2523.52 are achieved for a proposed eight-period ladder-type EIO. An RF power of 540 W is obtained with a 42.5 kV, 0.6 A sheet beam, with 2.1% electronic efficiency at 0.35 THz.

Acoustofluidic Patterning inside Capillary Tubes Using Standing Surface Acoustic Waves

Acoustofluidic platforms have great potentials to integrate capillary tubes for controlling and manipulating microparticles and biological cells in both non-flowing and continuous-flow settings. In order to effectively manipulate microparticles/cells inside capillary tubes, it is essential to fully understand and control the patterns generated inside the capillary tubes with different cross-sections, and to investigate the influences of configuration and position arrangement of electrodes along with the capillary tubes. This paper aims to systematically investigate the patterning and alignment of microparticles inside glass capillary tubes using thin film surface acoustic wave (SAW) devices. Through both experimental studies and numerical modelling, effects of various cross-section geometries of the capillary tubes and their positioning with respect to the direction of interdigital transducers (IDTs) of the SAW device in both a stationary fluid and a continuous flow fluid were studied. Results showed that for the rectangular glass capillary tubes, the patterned lines of particles are parallel to the tube's side walls, irrelevant to the tube positions along with the IDTs, which is mainly caused by the standing wave field generated inside the rectangular glass tube. Whereas for the circular glass capillary tubes, alignment patterns of particles are quite different along the tube's height. At the bottom of the circular tube, particles are patterned into lines parallel to the tube direction, because the acoustic waves propagate into the water and form a standing wave along the direction of the circular tube. Whereas at the middle height of the tube, the particles are patterned into lines perpendicular to the tube direction, because the formed standing waves also propagate around the circular cross-section of the tube and are perpendicular to the tube direction. For the cases with a continuous liquid flow, under the agitation of acoustic waves, particles are patterned in lines parallel to the flow directions for both the rectangular and circular glass tubes, and the fluid flow enhances and smoothens the patterned lines of the particles.

Development of the microwave-surface-wave-sustained low-pressure discharge in standing wave field

Propagation of the low-pressure surface-wave-sustained microwave discharge becomes unstable in the final stage of the discharge development. The development of such instability is associated with the field perturbations in the region near the discharge front. The discharge development becomes stochastic that manifests itself in partial decays, jumps and stops of the front propagation. By means of setting the configuration of the field created by the discharge along its propagation path, it becomes possible to control the discharge development. The discharge propagation was studied in the standing wave field between two metal mirrors. It was found that between the mirrors, the discharge propagates irregularly in the form of successive plasma bunches with the λ/2 length.

Three-dimensional atom localization with high-precision and high-resolution via a microwave field in an atomic system

A scheme for three-dimensional (3D) atom localization with high-precision and high-resolution is proposed and its control is studied in a four-level atomic system driven by an additional microwave field. Because of the spatial dependent interaction between atoms and standing wave fields, the atoms can be localized in a domain with widths as small as 0.014λ by appropriately adjusting the system parameters. The probe absorption spectrum of the atomic system gives the position information of the atoms and different evolution patterns of the localization structures in this study. The population distribution and its 3D localization structures can be easily adjusted by parameters of the microwave field, which can achieve a 100% probability of observing the atom in a specific space and the accuracy with the value of 0.014λ. Compared with 1D or 2D localizations, 3D localization can further improve the precision of position measurement and develop its spatial resolution, which may produce a wider variety of practical applications with high precision requirement.

Development of a water-filled four-sensor impedance tube

Four-sensor impedance tubes are readily utilized for characterization of the acoustic properties of materials in a gaseous host medium, as described in the standard ASTM E2611-19. Similar acoustic characterization is desired for materials submersed in water. Previous studies have documented the challenges of performing measurements in water-filled impedance tubes due to interaction with the elastic tube wall and difficulty in obtaining a suitable end-termination condition. Building upon previous demonstration of a two-sensor water-filled impedance tube [J. Acoust. Soc. Am. 113, 3245 (2003)], this paper discusses the development and demonstration of a four-sensor water-filled impedance tube with an anechoic termination. The technique utilizes two sensors on each side of the test section to simultaneously characterize both the incident and transmitted standing wave field and extract the complex impedance and wavenumber from the test sample. Discussion will focus on the challenges of performing such measurements in a water-filled tube along with methods for overcoming these challenges. Validation measurements of homogeneous materials in the 1–10 kHz band will be presented to demonstrate that both the complex impedance and wavenumber of a material submersed in water can be extracted using this approach. [Work supported by ONR.]

Acoustic patterning of endothelial cells in vivo to produce microvascular networks

Previous work from our lab developed an ultrasound standing wave field (USWF) technology to noninvasively pattern endothelial cells within collagen hydrogels to fabricate vascularized engineered tissue constructs. Here, we translate this technology towards acoustic patterning and microvascular network formation in vivo. To enable acoustic patterning in vivo, we developed a two-transducer ultrasound system with intersecting beams to produce an USWF at the intersection of the crossed beams. Acoustic radiation forces associated with the USWF patterned cells into planar bands located at nodal planes. Distance between planar bands was controlled through design of acoustic frequency and angle between ultrasound beams. To demonstrate acoustic patterning in vivo, solutions of collagen and human umbilical vein endothelial cells (HUVEC) were injected subcutaneously into flanks of mice and then exposed to an USWF. High-frequency ultrasound imaging was employed to visualize initial acoustic patterning in tissues. Ten days after USWF-mediated cell patterning, tissues were excised, and vessels derived from USWF-patterned cells were identified using an antibody specific to human von Willebrand factor (hVWF). Microvessels expressing hVWF were present in USWF-exposed tissues and not in sham-exposed controls. Furthermore, blood vessels expressing hVWF contained red blood cells, indicating anastomosis of ultrasound-fabricated microvessels with host vasculature and tissue perfusion.

Time-domain dynamic electric field distribution of low-oscillation sculptured structure dispersive mirrors

The dispersion oscillation and damage threshold of a dispersive mirror (DM) are the key influencing factors for high-quality pulses and outputting high power density. Here, we studied the femtosecond damage behavior of low-oscillation DMs(LODM) with SiO2 sculptured layer. The monochromatic standing wave field(SWF) distribution widely used in ultrafast laser damage analysis is sufficient in Bragg mirror. In more complex systems, the actual electric field intensity(typically smaller) will dynamically change due to the broad bandwidth and the time difference of pulse propagation. Time-domain dynamic electric field intensity displays that the spatio-temporal evolution of the pulse within the stacks averages the electric field intensity, especially the electric field in the deeper layers. The dynamic effect of few-cycle pulses or attosecond pulses damage will be more meaningful. The time-domain dynamic electric field model can elucidate the ultrafast laser damage behavior. Introducing the time-domain dynamic electric model into the merit function can help us design optical coatings with high damage thresholds.

Tunable optical vortex array in a two-dimensional electromagnetically induced atomic lattice.

Optical vortex arrays (OVAs) containing multiple vortices have been in demand for multi-channel optical communications and multiple-particle trapping. In this Letter, an OVA with tunable intensity and spatial distribution was implemented all-optically in a two-dimensional (2D) electromagnetically induced atomic lattice (EIL). Such a square lattice is constructed by two orthogonal standing-wave fields in 85Rb vapor, resulting in the periodically modulated susceptibility of the probe beam based on electromagnetically induced transparency (EIT). An OVA with dark-hollow intensity distribution based on 2D EIL was observed in the experiment first. This work thus studied the nonlinear 2D EIL process both theoretically and experimentally, presenting, to the best of our knowledge, a novel method of dynamically obtaining and controlling an OVA and further promoting the construction of all-optical networks with atomic ensembles.

Critical-dimension scanning electron microscope characterization of smoothly varying wave structures with a Monte Carlo simulation

Scanning electron microscopy (SEM) characterization of a smoothly varying nanograting structure (a Pt-coated Cr grid on a Si substrate) with a sinusoidal waveform has been carried out by a Monte Carlo (MC) simulation technique. Previous studies with critical-dimension (CD) SEM (CD-SEM) have mostly concerned line structures with sharp edges so that there is an obvious edge bloom in the linescan profile of secondary electrons. In contrast, the present grating structures prepared by a laser-focused atomic deposition technique have a smoothly varying waveform in the cross-sectional profile, which pose greater difficulty for quantitative structural characterization by SEM. The grating structure, with a fixed pitch of λ/2 as the period of the standing-wave laser light field, where λ is the wavelength of the corresponding laser light, can be used as an ideal nanoscale metrological length tool; therefore, it is important to characterize its structural features over a large deposited area by SEM imaging for quality control, with the aim of mass reproduction. The present work extends the CD characterization of MC simulation methods to more complex structures. Taking into account different experimental factors, i.e. primary electron beam parameters, geometrical parameters and material properties, the unknown geometrical parameters (i.e. base height, peak height, linewidth shrinkage and peak tilt angle) of the grating lines have been successfully extracted from the experimental linescan profiles of SEM images.

Flume Experiments on Energy Conversion Behavior for Oscillating Buoy Devices Interacting with Different Wave Types

Oscillating buoy device, also known as point absorber, is an important wave energy converter (WEC) for wave energy development and utilization. The previous work primarily focused on the optimization of mechanical design, buoy’s array configuration and the site selection with larger wave energy density in order to improve the wave energy generation performance. In this work, enlightened by the potential availability of Bragg reflection induced by multiple submerged breakwaters in nearshore areas, we investigate the energy conversion behavior of oscillating buoy devices under different wave types (traveling waves, partial and fully standing waves) by flume experiments. The localized partial standing wave field is generated by the Bragg resonance at the incident side of rippled bottoms. Furthermore, the fully standing wave field is generated by the wave reflection of vertical baffle installed in flume. Then the wave power generation performance is discussed under the conditions with the same wave height but different wave types. The experimental measurements show that the energy conversion performance of the oscillating buoy WEC could be improved under the condition of standing waves when compared with traveling waves. This work provides the experimental comparison evidence of wave energy conversion response of oscillating buoy devices between travelling waves and standing (fully or partial) wave conditions.

Measurement of the acoustic streaming pattern in a standing surface acoustic wave field

The application of standing surface acoustic waves (sSAW) has enabled the development of many flexible and easily scalable concepts for the fractionation of particle solutions in the field of microfluidic lab-ona-chip devices. In this context, the acoustic radiation force (ARF) is often employed for the targeted manipulation of particle trajectories, whereas acoustically induced flows complicate efficient fractionation in many systems [Sehgal and Kirby (2017)]. Therefore, a characterization of the superimposed fluid motion is essential for the design of such devices. The present work focuses on a structural analysis of the acousticallyexcited flow, both in the center and in the outer regions of the standing wave field. For this, experimental flow measurements were conducted using astigmatism particle tracking velocimetry (APTV) [Cierpka et al. (2010)]. Through multiple approaches, we address the specific challenges for reliable velocity measurements in sSAW due to limited optical access, the influence of the ARF on particle motion, and regions of particle depletion caused by multiple pressure nodes along the channel width and height. Variations in frequency, channel geometry, and electrical power allow for conclusions to be drawn on the formation of a complex, three-dimensional vortex structure at the beginning and end of the sSAW.

Resonant Stimulated Photorefractive Scattering.

We present the first observations, and a complete theoretical explanation, of stimulated photorefractive scattering in a high- Q crystalline cavity. The standing-wave light field in the cavity induces an ultranarrow and long-lived Bragg grating through the photorefractive effect. The spatial phase of the grating is automatically matched to that of the standing wave. The scattering from the grating strengthens the standing wave, which then further reinforces the grating itself. Eventually, the mode is seen to split into a doublet, thereby disrupting the usual strict periodicity of the mode spectrum.

Electron Round Lenses with Negative Spherical Aberration by a Tightly Focused Cylindrically Polarized Light Beam

Free electrons moving in an optical standing wave field feel the ponderomotive potential, acting as a refractive-index medium in electron optics. Emerging technologies involving this potential have been proposed and realized in electron microscopy, such as electron phase-contrast imaging using a laser standing wave in an optical enhancement cavity. However, the interaction between electrons with a cylindrically distributed optical field has not been investigated although its suitability for electron-optical imaging systems. In this study, we theoretically show that the divergence and convergence forces are provided by tightly focused cylindrically polarized light beams. The radially and azimuthally polarized beams with an annular profile are focused using a high numerical aperture optical lens. The intensity distributions at the focus function are concave and convex electron round lenses, respectively. The convex lens formed by the azimuthally polarized beam possesses negative (opposite sign) spherical aberration compared with conventional electron round lenses created by electrodes and magnetic coils. This remarkable result will contribute to the innovative design of electron-optical imaging systems and bring new capabilities into matter-wave optics.

Population density gratings creation and control in resonant medium by half-cycle terahertz pulses

Electromagnetically induced gratings (EIG) are created by standing-wave laser field in resonant media. Such gratings can be also created by few-cycle electromagnetic pulses counter-propagating in the medium via coherent Rabi oscillations of atomic inversion. In this case, instantaneous cross-section of the pulses in the medium is not necessary for grating formation. In this paper, we revise our recent results in study of such grating formation and their control by few-cycle pulses coherently propagating in a resonant medium. We demonstrate the grating formation and their control in three-level medium excited by three subcycle THz pulses.

Mathematical modeling and experimental study on solid–liquid suspension separation in ultrasonic standing wave field

A new method of removing waste chips is proposed by focusing on the key factors affecting the processing quality and efficiency of high energy beams. Firstly, a mathematical model has been established to provide the theoretical basis for the separation of solid–liquid suspension under ultrasonic standing wave. Secondly, the distribution of sound field with and without droplet has been simulated. Thirdly, the deformation and movement of droplets are simulated and tested. It is found that the sound pressure around the droplet is greater than the sound pressure in the droplet, which can promote the separation of droplets and provide theoretical support for the ultrasonic suspension separation of droplet; under the interaction of acoustic radiation force, surface tension, adhesion, and static pressure, the droplet is deformed so that the gas fluid around the droplet is concentrated in the center to achieve droplet separation, and the droplet just as a flat ball with a central sag is stably suspended in the acoustic wave node.

Shape- and Element-Sensitive Reconstruction of Periodic Nanostructures with Grazing Incidence X-ray Fluorescence Analysis and Machine Learning.

The characterization of nanostructured surfaces with sensitivity in the sub-nm range is of high importance for the development of current and next-generation integrated electronic circuits. Modern transistor architectures for, e.g., FinFETs are realized by lithographic fabrication of complex, well-ordered nanostructures. Recently, a novel characterization technique based on X-ray fluorescence measurements in grazing incidence geometry was proposed for such applications. This technique uses the X-ray standing wave field, arising from an interference between incident and the reflected radiation, as a nanoscale sensor for the dimensional and compositional parameters of the nanostructure. The element sensitivity of the X-ray fluorescence technique allows for a reconstruction of the spatial element distribution using a finite element method. Due to a high computational time, intelligent optimization methods employing machine learning algorithms are essential for timely provision of results. Here, a sampling of the probability distributions by Bayesian optimization is not only fast, but it also provides an initial estimate of the parameter uncertainties and sensitivities. The high sensitivity of the method requires a precise knowledge of the material parameters in the modeling of the dimensional shape provided that some physical properties of the material are known or determined beforehand. The unknown optical constants were extracted from an unstructured but otherwise identical layer system by means of soft X-ray reflectometry. The spatial distribution profiles of the different elements contained in the grating structure were compared to scanning electron and atomic force microscopy and the influence of carbon surface contamination on the modeling results were discussed. This novel approach enables the element sensitive and destruction-free characterization of nanostructures made of silicon nitride and silicon oxide with sub-nm resolution.

Noncontact manipulation of particles in air by switching pulse drive of ultrasonic speakers arrayed on a semicylinder

Noncontact micromanipulation techniques are necessary for micromachine technology, biotechnology, materials processing, and so on. By using a bolted Langevin transducer and a reflector, one can trap small objects in air at nodes of sound pressure in a standing wave field, but such a technique requires high-voltage drive and careful tuning to achieve resonance. In this study, a large number of small ultrasonic speakers were placed on the inner surface of a semicylinder to form a strong sound field in the focus line. We showed that the pressure node can be moved, and thus the small object, along the focus line simply by switching the driving ultrasonic speakers without using complex electronics. We also showed that the object can be trapped and manipulated even at a duty ratio of 50% in tone burst waves. The above results are discussed by showing the experimental and calculated sound pressure distributions.