Ultrasonic Lens Research Articles

Generation of bessel-like beam by a binary ultrasonic lens

Bessel beams have already been used in acoustic tweezers, ultrasonic medical imaging, and Doppler velocity estimation due to their non-diffractive and self-healing properties. We proposed a binary ultrasonic lens, with ultra-thin planar periodic structures for efficient conversion of the plane wave into a Bessel-like beam. The effectiveness of long-axis focusing has been demonstrated through simulations and experiments with FWHM of 8.5 mm and 9.5 mm around 10 mm. By varying the operating frequency of the incident wave from 1.9 MHz to 2.2 MHz, the position of the long-axis focusing point can be adjusted experimentally from 23.05 mm to 28.95 mm. Additionally, the beam generated by the binary ultrasonic lens can form a focused sound field when passing through multi-layered tissues and the self-healing capability of the Bessel-like beam has been numerically validated, showing strong robustness. This binary ultrasonic lens for a Bessel-like beam would conform better to ergonomic and miniaturization requirements, and expand versatile applications in clinical diagnosis and therapy.

Focus control of a concave–convex ultrasonic gel lens in the radial direction

Optical image stabilization (OIS) systems maintain the three-dimensional focal position of a lens through mechanical actuation systems. This paper examines an optical lens for OIS that utilizes ultrasound vibration to alter the focal position, not only in the depth direction but also in the radial direction. The lens has a simple structure with no mechanical moving parts and consists of an ultrasound transducer divided into four pieces, a glass disk, and a transparent viscoelastic gel film that functions as a lens. The acoustic radiation force generated by the resonant flexural vibration of the glass disk can alter the surface profile of the gel film, allowing for a variable-focus function. The concave and convex lenses can be interchanged using two resonant vibration modes: the standing-wave mode, in which the vibration loop appears at the center, and the traveling-wave mode, in which the vibration node appears at the center. The positions of ultrasound vibrations on the lens can be controlled in a two-dimensional plane by adjusting the driving amplitudes of each channel, thereby achieving focus control in the radial direction. The focusing characteristics of the lens are evaluated through ray-tracing simulation.

Nonlocal inverse design of an ultrasonic lens for underwater manipulation of orbital angular momentum

Nonlocal inverse design of an ultrasonic lens for underwater manipulation of orbital angular momentum

Methods to design and evaluate transcranial ultrasonic lenses using acoustic holography

Methods to design and evaluate transcranial ultrasonic lenses using acoustic holography

How to fix an ultrasonic variable-focus liquid crystal lens for substrate-mountable applications

This paper proposed a board-mounted ultrasonic variable-focus liquid crystal (LC) lens. The lens controls the focus using the acoustic radiation force generated by the resonant flexural vibration mode. The LC lens was fixed to an aluminum substrate with a hole whose aperture corresponded to the inner diameter of the transducer. The part of the LC lens attached to the substrates functioned as a fixed condition, and the flexural vibration mode was successfully generated. The fixed lens exhibited a gradual focal change with current, confirming that fixing the condition affected the rate of focal change.

Flexible acoustic lens-based surface acoustic wave device for manipulation and directional transport of micro-particles

Microfluidics is an emerging technology that is playing increasingly important roles in biomedical and pharmaceutical research and development. Surface acoustic waves (SAWs) have been combined with microfluidics technology to establish a SAW-based microfluidics technology that uses the unique interaction between the two techniques to manipulate substances effectively in fluids on the surface of a substrate. This paper reports a method to generate SAWs using conventional planar ultrasonic transducers and acoustic lenses. Additionally, this method is introduced to manipulate particles effectively on a substrate surface. It is demonstrated that the particle positions can be manipulated precisely in any direction on the substrate surface, thus enabling high-precision particle manipulation. We also proposed the generation of nonplanar SAWs via appropriate design of the acoustic lens and realized directional particle transport. In addition, structures to enhance forward-propagating acoustic beams are proposed. The proposed method has potential for use in microfluidics and biomedical applications, allowing tasks such as flexible cell manipulation on a chip to be performed without complex design or micromachining.

点状散乱体配置最適化経頭蓋音響レンズの収束および集束特性の頭蓋骨形状依存性

We have modeled the skull by a circular array of point-like scatterers, and optimized the arrangement of point-like scatterers constructing a transcranial ultrasonic lens by the condition that the sound field intensity within the skull has a peak at a focal point and vanishes at any other points, and shown that an ultrasonic lens with sufficient function is realized. In this paper, we analyze the case of an elliptical skull, and show that even for this case as well, the proposed optimization yields a arrangement of scatterers with equivalent performance to the case of a circular skull.

2E2-4 Board-mounted ultrasonic variable-focus liquid crystal lens

2E2-4 Board-mounted ultrasonic variable-focus liquid crystal lens

Study on Synthetic Aperture Focusing Technique (SAFT)

The Synthetic Aperture Focussing Technique (SAFT) restores ultrasonic images obtained from either B or C scans having focusing distortion. With the help of SAFT, improvement in image resolution was obtained without the use of traditional ultrasonic lenses. The images from both the B and the C scans have a lateral resolution limited by the focus of ultrasonic transducers. The techniques commonly used to improve the lateral resolution include using additional lenses or by using synthetic focus. The use of lenses makes the beams more collimated but introduces a considerable loss in transmitted signals. The use of the SAFT is efficient since it does not introduce any additional losses, but it can only be used in a digital signal acquisition system. The use of SAFT brings significant benefits to mechanized ultrasonic testing. In addition to the more precise defect localization and the better separation of neighbouring defect indications, the improvement of the structure-related signal-to-noise ratio (SNR) is an important reason for the use of SAFT. It can be used for defect analysis and offers a high degree of robustness when configured correctly. Sufficient computing power is required to carry out the SAFT analysis. In order to reduce the computing effort, the data from an ultrasonic test can be evaluated separately for each layer using 2D-SAFT. Although these results are volume related and can be displayed in three dimensions (and are therefore sometimes misleadingly referred to as 3D SAFT), some of the benefits of SAFT remain untapped

Focusing Characteristics of a Transcranial Ultrasonic Lens of a Point-like Scatterer Array Partially Covering the Head

This lens can focus an incident plane wave to the diffraction limit in the direction perpendicular to the incident direction, but the peak width in the propagation direction is more than five times that. This phenomenon is a characteristic of Fresnel lenses, and this lens, which is a kind of lens, is also not avoided. It can be improved by increasing the width of the lens, whereas it is not practical. In this work, as an alternative method, we propose to arrange a point-like scatterer so as to partially cover the head. It is shown that the proposed method can suppress the spread of the focal spot at the focal point in the propagation direction.

Precise micro-particle and bubble manipulation by tunable ultrasonic bottle beams

This paper reports a method to generate tunable bottle beams using an ultrasonic lens, by which the bottle position can be precisely adjusted with the change of the acoustic frequency. Therefore, the position of a single particle or bubble in liquid can be manipulated without using phased array which is costly and huge with complex circuits. Furthermore, we introduced this method to multiple bubble manipulation using acoustic holography. The bottle properties against frequency are theoretically and experimentally analyzed. It is shown that the bottle position depends almost linearly on the operating frequency, which provides a basis for the precise manipulation of bubbles and particles. In addition, the relationship between the acoustic radiation force and the drag force under different incident acoustic pressures is considered, establishing a limit on the moving velocity of the trapped particles. The ultrasonic field observation is further demonstrated by Schlieren imaging system. The proposed method has potential biomedical applications, such as more flexible cell manipulation and targeted drug delivery in vivo, as well as potential applications in the study of chemical reactions between micro objects.

Fiber optic Fabry\u2013Perot sensor that can amplify ultrasonic wave for an enhanced partial discharge detection

Ultrasonic wave is a powerful tool for many applications, such as structural health monitoring, medical diagnosis and partial discharges (PDs) detection. The fiber optic extrinsic Fabry–Perot interferometric (EFPI) sensor has become an ideal candidate for detecting weak ultrasonic signals due to its inherent advantages, and each time with a performance enhancement, it can bring great application potential in broadened fields. Herein, an EFPI ultrasonic sensor for PDs detection is proposed. The sensing diaphragm uses a 5-μm-thickness and beam-supported structure to improve the responsive sensitivity of the sensor at the resonant frequency. Furthermore, the ability of the sensor to detect characteristic ultrasonic signal of PDs is further enhanced by assembling a Fresnel-zone-plate (FZP)-based ultrasonic lens with the sensing probe to amplify the ultrasonic wave before it excites the sensing diaphragm. The final testing results show that the originally developed sensor owns the sensitivity of − 19.8 dB re. 1 V/Pa at resonant frequency. While, when the FZP is assembled with the probe, the sensitivity reaches to − 12.4 dB re. 1 V/Pa, and leads to a narrower frequency band, which indicates that the proposed method has a great potential to enhance the detection ability of sensor to characteristic ultrasonic wave of PDs.

Mechanically tunable ultrasonic metamaterial lens with a subwavelength resolution at long working distances for bioimaging

In this study, a novel acoustic doublet meta-material lens has been designed and tested to demonstrate both a far-field focal point and ultra-long collimation characteristics past the Fresnel zone. The switching of the two behaviors can be adjusted by a simple linear mechanical translation of one of the lens units. The doublet lens can focus the sound wave beyond 38λ away from the experiment’s lens, which is farther than any existing ultrasonic transducer or meta-lenses lenses. In terms of collimating behavior, the doublet lens is a unique metamaterial lens that experimentally demonstrates a long and narrow collimating beam over 70λ. Besides the design and characterization, the meta-lens have been used to detect real objects, including inorganic and organic matter. A subwavelength spatial resolution has been demonstrated. The detection limit was 0.26λ in the monostatic setup and 0.62λ in a bistatic experimental setup. This lens demonstrates super-resolution detection capabilities at distances of 42λ and can enable ultrasonic diagnostics deep within a material or a biological tissue. The experimental performance of the doublet meta-material lens illustrated its potential to apply acoustic metamaterial elements in a practical imaging application, including the detection of biological tissues

Cylindrical 3D printed configurable ultrasonic lens for subwavelength focusing enhancement

In this study, we report the characteristics of acoustic jets obtained through a mesoscale (radius less than 5 wavelengths) ABS cylinder made with a 3D printer. We have analyzed the influence of cylinder size on the characteristic parameters of an acoustic jet, such as maximum acoustic intensity at focus, Full Width at Half Maximum and length of Acoustic Jet. FWHM below 0.5 wavelength in AJ was experimentally obtained. It has been observed that there are two operating regimes depending on the cylinder radius: the resonant and the non-resonant. In the resonant regime, the excitation of Whispering Gallery Modes results in optimal parameter values of the acoustic jet. However, as it is a resonant regime, any minimal variation in cylinder size, working frequency or refractive index would make resonance disappear. In non-resonant mode, a phononic crystal has been embedded inside the cylinder and the characteristic parameters of the acoustic jet have been studied. These have been observed to improve. Finally, we have shown that curved acoustic jets can be obtained with the ABS cylinder with a phononic crystal embedded inside.

Path tracing estimators for refractive radiative transfer

Rendering radiative transfer through media with a heterogeneous refractive index is challenging because the continuous refractive index variations result in light traveling along curved paths. Existing algorithms are based on photon mapping techniques, and thus are biased and result in strong artifacts. On the other hand, existing unbiased methods such as path tracing and bidirectional path tracing cannot be used in their current form to simulate media with a heterogeneous refractive index. We change this state of affairs by deriving unbiased path tracing estimators for this problem. Starting from the refractive radiative transfer equation (RRTE), we derive a path-integral formulation, which we use to generalize path tracing with next-event estimation and bidirectional path tracing to the heterogeneous refractive index setting. We then develop an optimization approach based on fast analytic derivative computations to produce the point-to-point connections required by these path tracing algorithms. We propose several acceleration techniques to handle complex scenes (surfaces and volumes) that include participating media with heterogeneous refractive fields. We use our algorithms to simulate a variety of scenes combining heterogeneous refraction and scattering, as well as tissue imaging techniques based on ultrasonic virtual waveguides and lenses. Our algorithms and publicly-available implementation can be used to characterize imaging systems such as refractive index microscopy, schlieren imaging, and acousto-optic imaging, and can facilitate the development of inverse rendering techniques for related applications.

Sub-wavelength lateral detection of tissue-approximating masses using an ultrasonic metamaterial lens

Practically applied techniques for ultrasonic biomedical imaging employ delay-and-sum (DAS) beamforming which can resolve two objects down to 2.1λ within the acoustic Fresnel zone. Here, we demonstrate a phononic metamaterial lens (ML) for detection of laterally subwavelength object features in tissue-like phantoms beyond the phononic crystal evanescent zone and Fresnel zone of the emitter. The ML produces metamaterial collimation that spreads 8x less than the emitting transducer. Utilizing collimation, 3.6x greater lateral resolution beyond the Fresnel zone limit was achieved. Both hard objects and tissue approximating masses were examined in gelatin tissue phantoms near the Fresnel zone limit. Lateral dimensions and separation were resolved down to 0.50λ for hard objects, with tissue approximating masses slightly higher at 0.73λ. The work represents the application of a metamaterial for spatial characterization, and subwavelength resolution in a biosystem beyond the Fresnel zone limit.

Directional Ultrasound Source for Solid Materials Inspection: Diffraction Management in a Metallic Phononic Crystal.

In this work, we numerically investigate the diffraction management of longitudinal elastic waves propagating in a two-dimensional metallic phononic crystal. We demonstrate that this structure acts as an “ultrasonic lens”, providing self-collimation or focusing effect at a certain distance from the crystal output. We implement this directional propagation in the design of a coupling device capable to control the directivity or focusing of ultrasonic waves propagation inside a target object. These effects are robust over a broad frequency band and are preserved in the propagation through a coupling gel between the “ultrasonic lens” and the solid target. These results may find interesting industrial and medical applications, where the localization of the ultrasonic waves may be required at certain positions embedded in the object under study. An application example for non-destructive testing with improved results, after using the ultrasonic lens, is discussed as a proof of concept for the novelty and applicability of our numerical simulation study.

Acoustic trapping of particles using a Chinese taiji lens

Focused acoustic vortex (FAV) beams can trap particles in a contactless and non-destructive way and the method has thereby attracted much attention. In contrast to the traditional complex and expensive transducer array, we propose a fast and cheap method to generate FAV beams in water using an ultrasonic holographic lens engraved with the Chinese taiji pattern. This method can obtain high transmission efficiency, and hence the strong trapping force makes the particles trapped stably in a straight line. The formation of the FAV beams derives from a superposition of the spiral phase of a Laguerre Gaussian beam and the focusing phase. Because of the phase singularity of this beam, the intensity of the ultrasonic field on the beam axis is zero, thereby forming a strong gradient surround the beam axis. The trapping and manipulation of polystyrene particles with a radius of 150 μm is realized in the gradient field of the FAV beam. The proposed single beam acoustic trapping method does not depend on the reflector, making it more suitable for the manipulation of cells in vivo.

Theoretical and Experimental Study of Ultrasonic Vibration-Assisted Laser Polishing 304 Stainless Steel

With the increasing requirements for the surface quality of the workpiece, many kinds of hybrid manufacturing have developed recently. Ultrasonic vibration-assisted laser polishing (UVLP) is a new hybrid manufacturing method. Different from the traditional method of applying ultrasonic vibration on the workpiece, this article applies ultrasonic vibration on the lens. Besides, the thermal mechanisms of the 304 stainless steel polished by traditional laser polishing (TLP) and UVLP were analyzed to reveal the influence of ultrasonic vibration on the polishing effect. The ultrasonic vibration lens transforms the laser polishing into an intermittent polishing process, which affects the laser energy density and the quality of the polished surface. This research was focused on the experimental analysis of the surface morphology and surface roughness of the 304 stainless steel that was processed by UVLP, and compared with TLP. And the effects of laser power, scanning speed, focus offset, and amplitude on the quality of the polished surface were obtained. The preferred process parameters of UVLP 304 stainless steel were obtained by the range analysis and variance analysis of the orthogonal experiment. Experimental results have shown that UVLP can reduce the surface roughness of 304 stainless steel from $2.777~\mu \text{m}$ to $0.512~\mu \text{m}$ at most. Thermal mechanism analysis results were verified by experimental results. Choosing appropriate processing parameters can effectively improve the quality of the polished surface, making the processing effect of UVLP significantly better than TLP.

Ultra-compact ultrasonic metalens for underwater focusing

There are significant challenges to design acoustic metasurface in the ultrasonic regime where the wavelength is small. Moreover, the routine design rule based on effective medium approximation is invalid for underwater ultrasound. Our recent development of metasurface provides a great opportunity to solve above-mentioned challenges and the results demonstrate the great potential to apply this metasurface in the field of ultrasonic focusing. Here, we presented a focusing metasurface lens with extreme simplicity (slot structure) and ultra-compact size (deep-subwavelength spacing and thickness). Instead of tuning the acoustic path length individually, we exploited the strong wave couplings between the deep-subwavelength-spaced units for the phase modulation. A microscopic coupled-wave theory was used to predict the phase profile, based on which the metasurface lens for ultrasonic focusing was optimized. Particularly, broadband focusing was also exhibited due to the non-resonant arrangement of the proposed metasurface lens. The numerical and experimental results agreed well with each other, effectively validating the feasibility of the proposed metasurface lens. The proposed approach provides a feasible avenue for the design of simple and ultra-compact ultrasonic lenses that would be suitable in various fields of ultrasonics.