Focal Width Research Articles

Liquid microlens compound structure for enhanced sub-micron resolution imaging and accelerated reconfigurable deformation manipulation

The optical observation of sub-micron structures encounters significant challenges due to the inadequate spatial shape matching of optical devices and the limitations imposed by diffraction. These factors result in suboptimal imaging resolution and the introduction of defocus distortion. One approach to mitigating these limitations involves utilizing a liquid microlens (LML) assisted microscope, which offers real-time and localized control capabilities. However, the dynamic tuning of LML is subject to volatility and instability, adversely affecting sub-micron resolution imaging. To address this issue, a contour-following coating inspired by the natural cornea is applied to the droplet's surface, resulting in the formation of liquid microlens compound structures (LMLCS). Firstly, photoresist microholes are fabricated on the optical disc. Then, liquid self-assembly technology is used to fill glycerol in the microholes. At last, a conformal spreading method of UV-curable adhesive precursor film is developed to ensure the combination of the microlens bilayer structure. Through the application of optical information transmission theory, finite difference time domain (FDTD) method, and the principle of Abbe microscopic imaging, the imaging magnification process and sub-micron resolution characteristic of the LMLCS are revealed. Remarkably, the obtained focal width at half maximum (FWHM) of 1.54 μm is smaller than the theoretical value, enabling reconfigurable sub-micron resolution observation and 1.63 times imaging magnification of 226 nm grating lines under the optical microscope. Compared with unencapsulated LML, this compound structure exhibits better sub-micron resolution capability, greater imaging magnification, and faster adaptive dynamic response characteristics. Consequently, the LMLCS introduces exciting prospects for exploring optical sub-micron measurement and sensing devices with exceptional performance.

Differential time series analysis shows deceleration in melanoma mortality prior to the widespread use of highly effective therapies

Although the recent drop in melanoma mortality has been attributed to the introduction of newer therapies, the impact of ongoing public efforts remains unknown. Characterize and model melanoma mortality trends before the era of molecular and immune therapies (1969-2014) in the U.S. and Australia. Differential time series analysis based on population-ascertained melanoma mortality rates from the U.S. and Australia. Mortality rates were modeled and compared to the trajectories of ten other cancers. Melanoma mortality rates have been significantly decelerating since the 1970s in both the U.S. (P<.0001) and Australia (P=.0021). Zero acceleration occurred around 2001 (95% CI: 1996, 2008) for the U.S. and 2004 (95% CI: 1999, 2011) for Australia. Male mortality rates decelerated 3x-4x faster than females in both countries. Melanoma mortality followed a similar quadratic function (R2>0.9) to 10 other cancers, albeit with a later inflection point (1986 vs 2001) and broader focal width. Absolute mortality data used without further stratification or considering cancer incidence or covariates. Melanoma deaths have been decelerating for the past 5decades, reaching an inflection point around 2001, suggesting that mitigating campaigns were already afoot in both the U.S. and Australia before the advent of modern therapies.

Quantitative Measurement and Evaluation of High-Resolution Ultrasonic Sound Fields using a Novel Automated Ultrasonic Immersion Scanner

Ultrasonic probes are an integral part of the automated ultrasonic non-destructive testing and evaluation (NDT&amp;E) systems to detect and size the defects in a wide range of structural materials in production lines. A complete quantitative assessment of the performance characteristics of an application-specific UT probe is needed to be evaluated on the basis of the international standard DIN EN ISO 22232-2. This requirement of full assessment not only improves the quality assurance of the manufactured probes by evaluating the acceptance criteria but also provides the useful technical information to the end-user to optimize the automated ultrasonic testing on-site. In addition, the evaluation of probe characteristics should be carried periodically throughout their service life. In this work, we developed a novel high-precision ultrasonic immersion scanner to evaluate the full ultrasonic probe characteristics which include the squint angle measurement in three different reference planes (XY, XZ, and YZ), RF-signal and its frequency spectrum at water-steel interface, focal distance, focal range, focal width, focal length and also the angle of beam divergence in different planes of the sound field distribution. In the newly developed UT immersion scanner, for the first time, we successfully integrated the high-precision Hexapod with six axes to achieve a few micrometer (~15 µm) spatial-resolution in the measured sound field patterns and a good angular resolution (~0.05°) in the squint angle measurement. In addition, we used a verified UT instrument in accordance with the standard DIN EN ISO 22232-1 to obtain more accurate amplitude values (±0.5 dB) while scanning a test block. The automated scanning, data acquisition, evaluation, visualization and the automated test report generation are carried based on the standard DIN EN ISO 22232-2. The measured sound beam characteristics of both focused and non-focused application-specific ultrasonic probes with center frequencies ranging from 0.2 - 15 MHz using the pulse-echo technique on a 3 mm half-ball steel reflector are presented. In addition, the measured sound beams of an application-specific 5 MHz multi-element transmit-receive (TR) probe using the 3 mm flat-bottomed-hole (FBH) reflector in a steel reference plate are discussed. Finally, the quantitative analysis of the measured sound field results and the acceptance criteria in accordance with DIN EN ISO 22232-2 are discussed.

Microstrip array antenna for precise near-field focusing

ABSTRACT An array of microstrip antennas is introduced to achieve precise near-field focusing. A precise focusing approach of synthesising weights for array excitations is employed in the proposed near-field-focused (NFF) array, consisting of 3 × 3 subarrays. The usage of this approach allows the beam angle of every subarray to point to the focal spot precisely at the working frequency of 10 GHz. By introducing this working mechanism, a good trade-off has been achieved in the performance. The design can form a small focal width, about 6.6 mm, with a small location error of the focal spot of 8%. Meanwhile, the sidelobe level and the aperture remain small. To establish the effectiveness of this approach, we have designed, simulated, and measured this work.

A Manufacturability‐Driven Method for Designing Metamaterial Focusing Acoustical Lenses

Herein, a method for designing and fabricating acoustic metamaterial focusing lenses for imaging applications such as nondestructive testing is proposed. First, a refractive index distribution is designed using the time delay method for a target focal length. Second, unit cell structures that can achieve the required refractive indices are found using the effective property retrieval method. At this point, factors such as fabrication cost and manufacturing limitations are considered. Finally, the pressure field from the designed lens is calculated using finite element (FE) modelling and iterated until the required focusing performance is achieved. To increase manufacturability, the unit cells necessarily become larger than required for the homogeneous regime. As an example, a lens of focal length 200 ± 10 mm at a frequency of 40 kHz is designed using cross‐shaped unit cells and fabricated using three‐dimensional printing. The design methodology leads to 2.4 mm (wavelength/3.6) unit cells which operate near the cut‐off frequency of the first mode. The lens is experimentally tested, achieving a focal length of 208 mm and a focal width of 12 mm (1.4 × wavelength), and showing good agreement with an FE model. The focusing ability of the lens is demonstrated by measuring the size of various holes in thin plates.

PMMA based compound kinoform lenses for sub-150 nm hard x-rays focusing

It is the first time to fabricate the polymethyl methacrylate (PMMA) based compound kinoform lenses (CKLs) for nano-focusing of hard x-rays. The PMMA CKLs with a physical aperture of 160 µm and a focal length of 20 mm are designed and fabricated by x-ray LIGA technology at the Beijing Synchrotron Radiation Facility. To improve the stiffness of the PMMA lines, PMMA sheets with high molecular weight are fabricated, and the results revealed that the PMMA lines with higher molecular weight were much stronger. After optimization, CKLs with the narrowest lines of 4 µm width can achieve 110 µm height and 2.99 nm surface roughness without deformation. The CKLs were tested at the Diamond Light Source, which provided a focal width of 122 nm for 12 keV x rays. Though the PMMA has a poor radiation tolerance, it has a better process tolerance, and it can lay a good foundation for the fabrication of SU8 based CKLs by LIGA technology, which has a better radiation tolerance. Compared to the Si CKLs fabricated by dry etching, the PMMA CKLs are higher in depth, smoother at the sidewall, and have better nanofocusing.

Statistical spatial properties of a light field on a target plane

Studying the light field characteristics of a target plane is critical in controlling the laser-plasma instability (LPI), which is necessary to increase laser energy utilization and compression symmetry in high-power laser facilities. In this study, a statistical method is used to analyze the transmission light characteristics of a target plane for single and multiple beams. We reconstructed the light transmission model on the target plane and analyzed the relationships between the focal spot width and incidence angle, and speckle width and incidence angle using the autocorrelation function. In addition, the relationship between the interference pattern and incident beams is obtained by deriving the beam superposition theory. The results show that the speckle and focal spot widths are stretched in the direction in which the incident plane projects. The direction of the interference structure generated by multiple-laser beams is perpendicular to the line joining of the sub-beams, and the period is related to both wavelength and incidence angle. Experimental results are consistent with the theoretical analysis. The influence of incident beam number on the focal spot uniformity is also studied. The results are of great significance for regulating the sub-beam incidence direction, understanding light properties to further improve focal spot uniformity and suppress the LPI.

Novel diffraction limit implied geometric lens imaging model based on a finite number of pixels for integral imaging display

We propose a novel diffraction limit implied geometric lens imaging (DLI-GLI) model considering diffraction effect on a finite number of pixels in elemental image (EI)and 3D depth priority-oriented integral imaging (DPO-II) showing variations of the lateral resolution (LR) within enhanced depth of focus (DoF) range graphically. We assigned 3 representative gaps between lens array and display panel including gap at conventional DPII to find the optimized DoF and enhanced LR. We show that the number of pixels in an elemental lens (EL), and focal length and width of the lens determine practical LR and DoF at the focal plane, and the numerically induced DOP-II with two specified DPO-gaps which are larger than focal length have the enhanced LR and DoF comparing with those of the conventional DPII presenting the optimized LR and DoF near to two DPO-gaps. We show that one of the DPO-gaps has largest DoF and another has largest resolution within some part of DoF in the on-axial DPO-spot size diagram (DPO-SD). Though this DPO-SD, it is expected that the various 3D geometric imaging configurations according to the application fields of 3D imaging could be designed as well. Finally, we verified that the numerically induced 3 geometrical structures based on the proposed model show the similar results with measurements of the optical experiments on the built-up DPO-asymmetric II (DPO-AII) system.

A Multi-Spark Electrohydraulic Shock Wave Generator with Adjustable Pressure Field Distribution and Beam Steering Capability.

All clinical shock wave lithotripters produce an axisymmetric acoustic field without accounting for the anatomic features of the kidney or respiratory motion of the patient. This work presents a steerable and adjustable focusing electrohydraulic (SAFE) shock wave generator design with variable beam size and shape. 90 electrohydraulic transducers are mounted concentrically on a spherical basin with adjustable connection to individual transducers. Each transducer consists of 45 3D-printed titanium microelectrodes embedded in epoxy with a tip diameter of 0.3 mm. All the transducers are arranged in 5 concentric rings and sub-divided into 6 sectors. By changing the connections of individual transducers, the focused pressure field produced by the transducer array can be either axisymmetric with a -6 dB focal width of 14.8 mm in diameter, or non-axisymmetric with a long axis of 22.7 mm and a short axis of 15.1 mm. The elongated beam produces a peak positive pressure of 33.7±4.1 MPa and comminution efficiency of 42.2±3.5%, compared to 36.2±0.7 MPa and 28.6±6.1% for axisymmetric beam after 150 pulses at 20 kV. We have demonstrated that the SAFE shock wave generator can produce an elongated non-axisymmetric pressure field with higher stone comminution efficiency. The SAFE shock wave generator may provide a flexible and versatile design to achieve accurate, stable, and safe lithotripsy for kidney stone treatment.

Time reversal of long coda waves: Experiments in the nonlinear regime.

A problem that arises when the time-reversal process is applied in a nonlinear regime is related to the generation of harmonics: conventional piezoelectric transducers cannot work properly at the frequency of the second harmonic. Then, the time-reversed wave cannot be completely emitted. Few works provide a solution to this issue. Thus, we study the alternative of performing a cross correlation of the wavefield. In a linear regime, this procedure is an accurate method for estimating real time-reversal properties. To study both procedures in the nonlinear regime in detail, we measure the wavefield of a wave that (1) traverses a multiple scattering medium, composed by a random set of parallel copper rods and (2) propagates inside a reverberant cavity, consisting of an aluminum case immersed in water. Cross correlation yields a virtually focused wavefield, where the focal width at the frequency of the first, second, and third harmonics can be measured. We compare these values with those obtained in a real time-reversal experiment. Results suggest that both time-reversal procedures are equivalent. In addition, we discuss the possibility of amplitude estimation at the focal spot and the limits of this work based on a theoretical model.

Simultaneous multi-target ultrasound neuromodulation in freely-moving mice based on a single-element ultrasound transducer

Objective. Ultrasound neuromodulation has become an emerging method for the therapy of neurodegenerative and psychiatric diseases. The phased array ultrasonic transducer enables multi-target ultrasound neuromodulation in small animals, but the relatively large size and mass and the thick cables of the array limit the free movement of small animals. Furthermore, spatial interference may occur during multi-target ultrasound brain stimulation with multiple micro transducers. Approach. In this study, we developed a miniature power ultrasound transducer and used the virtual source time inversion method and 3D printing technology to design, optimize, and manufacture the acoustic holographic lens to construct a multi-target ultrasound neuromodulation system for free-moving mice. The feasibility of the system was verified by in vitro transcranial ultrasound field measurements, in vivo dual-target blood-brain barrier (BBB) opening experiments, and in vivo dual-target ultrasound neuromodulation experiments. Main results. The developed miniature transducer had a diameter of 4.0 mm, a center frequency of 1.1 MHz, and a weight of 1.25 g. The developed miniature acoustic holographic lens had a weight of 0.019 g to generate dual-focus transcranial ultrasound. The ultrasonic field measurements’ results showed that the bifocal’s horizontal distance was 3.0 mm, the −6 dB focal spot width in the x-direction was 2.5 and 2.25 mm, and 2.12 and 2.24 mm in the y-direction. Finally, the in vivo experimental results showed that the system could achieve dual-target BBB opening and ultrasound neuromodulation in freely-moving mice. Significance. The ultrasonic neuromodulation system based on a miniature single-element transducer and the miniature acoustic holographic lens could achieve dual-target neuromodulation in awake small animals, which is expected to be applied to the research of non-invasive dual-target ultrasonic treatment of brain diseases in awake small animals.

Market dynamism and sustainability in heritage tourism destinations: The enhancing role of bridging capital

PurposeIn recent years, interest in analysing the conceptualisation, measurement, determinants and consequences of sustainability orientation (SO) has intensified. The authors respond to a growing demand for research that delves into external and relational factors of SO in the context of hospitality and tourism industry. This paper aims to analyse how market dynamism (MD) influences the SO of firms in heritage tourism destinations (HTDs) and how bridging capital affects this relationship.Design/methodology/approachThe authors empirically analysed a sample of 238 companies in the UNESCO World Heritage Cities of Peru. The proposed hypotheses have been tested using partial least squares structural equation modelling.FindingsThe results show a curvilinear relationship between MD and SO. Furthermore, bridging capital enhances the effects of MD, reducing the focal width of the U-shaped curve. Specifically, when hospitality and tourism firms (HTFs) in tourist destinations operate in environments with low-medium levels of dynamism, the diverse links generated slow down the development of an SO. However, the more dynamic the environment, the greater is the SO of firms with large amounts of bridging capital.Practical implicationsThe findings highlight the importance of conducting an analysis based on a contingent approach that considers the coherence between external elements of the environment and internal factors. Such an approach helps to understand how HTFs in developing countries establish their SO. Managers should be aware of changes in demand and seek coherence between the level of MD and their inter-organisational relationships, and so be able to assess potential opportunities through developing an SO.Originality/valueThis study contributes to a better understanding of the sustainability commitment of HTFs in the sustainability of HTDs. This study connects the population ecology and social capital approaches, explaining this relationship from a disaggregated perspective – social and environmental – in the context of a developing country.

An endoluminal cylindrical sectored-ring ultrasound phased-array applicator for minimally-invasive therapeutic ultrasound.

The size of catheter-based ultrasound devices for delivering ultrasound energy to deep-seated tumors is constrained by the access pathway which limits their therapeutic capabilities. To devise and investigate a deployable applicator suitable for minimally-invasive delivery of therapeutic ultrasound, consisting of a 2D cylindrical sectored-ring ultrasound phased array, integrated within an expandable paraboloid-shaped balloon-based reflector. The balloon can be collapsed for compact delivery and expanded close to the target position to mimic a larger-diameter concentric-ring sector-vortex array for enhanced dynamic control of focal depth and volume. Acoustic and biothermal simulations were employed in 3D generalized homogeneous and patient-specific heterogeneous models, for three-phased array transducers with 32, 64, and 128 elements, composed of sectored 4, 8, and 16 tubular ring transducers, respectively. The applicator performance was characterized as a function of array configuration, focal depth, phasing modes, and balloon reflector geometry. A 16-element proof-of-concept phased array applicator assembly, consisting of four tubular transducers each divided into four sectors, was fabricated, and characterized with hydrophone measurements along and across the axis, and ablations in ex vivo tissue. Simulation results indicated that transducer arrays (1.5 MHz, 9mm OD × 20mm long), balloon sizes (41-50 mm expanded diameter, 20-60 mm focal depth), phasing mode (0-4) and sonication duration (30s) can produce spatially localized acoustic intensity focal patterns (focal length: 3-22 mm, focal width: 0.7-8.7 mm) and ablative thermal lesions (width: 2.7-16 mm, length: 6-46 mm) in pancreatic tissue across a 10-90 mm focal depth range. Patient-specific studies indicated that 0.1, 0.46, and 1.2cm3 volume of tumor can be ablated in the body of the pancreas for 120s sonications using a single axial focus (Mode 0), or four, and eight simultaneous foci in a toroidal pattern (Mode 2 and 4, respectively). Hydrophone measurements demonstrated good agreement with simulation. Experiments in which chicken meat was thermally ablated indicated that volumetric ablation can be produced using single or multiple foci. The results of this study demonstrated the feasibility of a novel compact ultrasound applicator design capable of focusing, deep penetration, electronic steering, and volumetric thermal ablation. The proposed applicator can be used for compact endoluminal or laparoscopic delivery of localized ultrasound energy to deep-seated targets.

Silicon-photonics focused ultrasound detector for minimally invasive optoacoustic imaging.

One of the main challenges in miniaturizing optoacoustic technology is the low sensitivity of sub-millimeter piezoelectric ultrasound transducers, which is often insufficient for detecting weak optoacoustic signals. Optical detectors of ultrasound can achieve significantly higher sensitivities than their piezoelectric counterparts for a given sensing area but generally lack acoustic focusing, which is essential in many minimally invasive imaging configurations. In this work, we develop a focused sub-millimeter ultrasound detector composed of a silicon-photonics optical resonator and a micro-machined acoustic lens. The acoustic lens provides acoustic focusing, which, in addition to increasing the lateral resolution, also enhances the signal. The developed detector has a wide bandwidth of 84 MHz, a focal width smaller than 50 µm, and noise-equivalent pressure of 37 mPa/Hz1/2 - an order of magnitude improvement over conventional intravascular ultrasound. We show the feasibility of the approach and the detector's imaging capabilities by performing high-resolution optoacoustic microscopy of optical phantoms with complex geometries.

Kinetic phenomena of helical plasma waves with orbital angular momentum

An accurate description of plasma waves is fundamental for the understanding of many plasma phenomena. It is possible to twist plasma waves such that, in addition to having longitudinal motion, they can possess a quantized orbital angular momentum. One such type of plasma wave is the Laguerre–Gaussian mode. Three-dimensional numerical particle-in-cell simulations demonstrate the existence of stable long-lived plasma waves with orbital angular momentum. These waves can be shown to create large amplitude static magnetic fields with unique twisted longitudinal structures. In this paper, we review the recent progress in studies of helical plasma waves and present a new analytical description of a standing Laguerre–Gaussian plasma wave mode along with 3D particle-in-cell simulation results. The Landau damping of twisted plasma waves shows important differences compared to standard longitudinal plasma wave Landau damping. These effects include an increased damping rate, which is affected by both the focal width and the orbital number of the plasma wave. This increase in the damping rate is of the same order as the thermal correction. Moreover, the direction of momentum picked up by resonant particles from the twisted plasma wave can be significantly altered. By contrast, the radial electric field has a subtle effect on the trajectories of resonant electrons.

Optimization of concentration performance at focal plane considering mirror refraction in parabolic trough concentrator

ABSTRACT Parabolic trough solar thermal technology is the most cost-effective approach for concentrated solar energy applications. The concentrator thickness significantly affects the performance of the parabolic trough collector. To examine the effect of mirror refraction in the concentrator, the focal plane concentration for different concentrator thicknesses was studied using theoretical calculations, simulations, and experimental verification. A longitudinal offset of the focal plane is proposed to optimize the focal plane concentration. The deviation of the reflected sunlight depending on the concentrator thickness was found to strongly affect the concentration in the focal plane. When the concentrator thickness was increased by a factor of 5, the focal plane was 5 times as wide, the maximum energy flux density in the focal plane decreased by a factor of 5, and the optical efficiency decreased by 0.24%. The longitudinal offset of the absorber plane position was introduced to avoid local overheating of the absorber and low collector temperature by maintaining the focal plane width and maximum energy flux density in the focal plane within appropriate ranges. The experimental results showed that with optimization, the maximum energy flux density, uniformity, and optical efficiency increased by 5.75%, 3.35%, and 2.43%, respectively.

Heat management of a compact x-ray source for microbeam radiotherapy and FLASH treatments.

Microbeam and x-ray FLASH radiation therapy are innovative concepts that promise reduced normal tissue toxicity in radiation oncology without compromising tumor control. However, currently only large third-generation synchrotrons deliver acceptable x-ray beam qualities and there is a need for compact, hospital-based radiation sources to facilitate clinical translation of these novel treatment strategies. We are currently setting up the first prototype of a line-focus x-ray tube (LFxT), a promising technology that may deliver ultra-high dose rates (UHDRs) of more than 100 Gy/s from a table-top source. The operation of the source in the heat capacity limit allows very high dose rates with micrometer-sized focal spot widths. Here, we investigate concepts of effective heat management for the LFxT, a prerequisite for the performance of the source. For different focal spot widths, we investigated the temperature increase numerically with Monte Carlo simulations and finite element analysis (FEA). We benchmarked the temperature and thermal stresses at the focal spot against a commercial x-ray tube with similar power characteristics. We assessed thermal loads at the vacuum chamber housing caused by scattering electrons in Monte Carlo simulations and FEA. Further, we discuss active cooling strategies and present a design of the rotating target. Conventional focal spot widths led to a temperature increase dominated by heat conduction, while very narrow focal spots led to a temperature increase dominated by the heat capacity of the target material. Due to operation in the heat capacity limit, the temperature increase at the focal spot was lower than for the investigated commercial x-ray tube. Hence, the thermal stress at the focal spot of the LFxT was considered uncritical. The target shaft and the vacuum chamber housing require active cooling to withstand the high heat loads. The heat capacity limit allows very high power densities at the focal spot of the LFxT and thus facilitates very high dose rates. Numerical simulations demonstrated that the heat load imparted by scattering electrons requires active cooling.

Laser-generated focused ultrasound transducer using a perforated photoacoustic lens for tissue characterization.

We demonstrate a laser-generated focused ultrasound (LGFU) transducer using a perforated-photoacoustic (PA) lens and a piezoelectric probe hydrophone suitable for high-frequency ultrasound tissue characterization. The perforated-PA lens employed a centrally located hydrophone to achieve a maximum directional response at 0° from the axial direction of the lens. Under pulsed laser irradiation, the lens produced LGFU pulses with a frequency bandwidth of 6-30 MHz and high-peak pressure amplitudes of up to 46.5 MPa at a 70-µm lateral focal width. Since the hydrophone capable of covering the transmitter frequency range (∼20 MHz) was integrated with the lens, this hybrid transducer differentiated tissue elasticity by generating and detecting high-frequency ultrasound signals. Backscattered (BS) waves from excised tissues (bone, skin, muscle, and fat) were measured and also confirmed by laser-flash shadowgraphy. We characterized the LGFU-BS signals in terms of mean frequency and spectral energy in the frequency domain, enabling to clearly differentiate tissue types. Tissue characterization was also performed with respect to the LGFU penetration depth (from the surface, 1-, and 2-mm depth). Despite acoustic attenuation over the penetration depth, LGFU-BS characterization shows consistent results that can differentiate the elastic properties of tissues. We expect that the proposed transducer can be utilized for other tissue types and also for non-destructive evaluation based on the elasticity of unknown materials.

Tight focusing properties and focal field tailoring of cylindrical vector beams generated from a linearly polarized coherent beam array.

We investigate the focusing properties of cylindrical vector beams (CVBs) generated from the combination of an array of beams, each with sub-apertures and controllable polarization. The analytical expression of the tight focusing field of the combined CVBs has been derived based on the Richard-Wolf vector diffraction integral. To obtain a desired focal spot size which includes efficient sidelobe suppression, the required parameters, such as the exit sub-aperture, numerical aperture and truncation parameter, have been studied in detail. The result shows that the combined CVB distribution has a good match with the theoretical ideal CVB distribution. However, compared with the ideal CVBs, the focal spot width produced by the combined radially polarized beams is smaller. With the increase of initial polarization rotation of sub-aperture, the focal spot width increases, and the focal shape shifts from Gaussian-like to a flat-topped distribution and then to an annular distribution. Furthermore, flexible focal field tailoring can also be realized by adjusting the initial polarization rotation of each sub-aperture. These results might provide a valuable reference for material processing, microlithography and multi-particle manipulation.

Effective Focal Area Dimension Optimization of Shear Horizontal Point-Focusing EMAT Using Orthogonal Test Method

To overcome the shortcomings of low energy conversion efficiency of electromagnetic acoustic transducers (EMATs), point-focusing shear horizontal (PFSH) wave EMAT is used to focus the wave energy into a specific area. Many factors will affect the capability of the focusing transducer, and in addition to considering the signal intensity, the detection accuracy is also required to be investigated. Specifically, to simplify the test process, we use the orthogonal test method to study the effect of different influence parameters on signal intensity and focal area dimensions. Seven factors are selected, and three results are determined in the test. Range analysis shows that for signal amplitude $M$ , the top three impact factors are the coil width $w$ , coil turns $n$ , and focal length lF (equal to bandwidth factor $\alpha $ ). Moreover, magnet number $m$ and frequency $f_{c}$ dominate the effective focal length $l_{\text {fd}}$ , and aperture angle $\theta $ determines the effective focal width $w_{\text {fd}}$ . To enable higher signal intensity and smaller focal area dimensions, it is necessary to consider various factors on the PFSH-EMAT focusing performance. The test’s signal intensity with optimized parameters’ combination at the focal point is nearly 144.42% higher than the average of all the tests, $l_{\text {fd}}$ decreased by 37.84%, and $w_{\text {fd}}$ decreased by 50.59%. The experiment also verified that focusing EMAT with optimized parameters has a better focusing performance.