A design and fabrication method of an acoustic metasurface structure for ultrasonic penetration of a solid aberration layer

Abstract

This invention discloses a method for designing and fabricating an acoustic metasurface structure that allows ultrasound to penetrate solid aberration layers. The invention designs an acoustic metasurface composed of 2-bit three-dimensional units arranged in a specific pattern. This invention combines a genetic algorithm and a reverse optimization strategy to accurately solve the topological configuration of high-transmittance units with a phase difference of π / 2. The metasurface is supported by a substrate conformally to the curved aberration layer. Experiments show that attaching this single-layer metasurface to the aberration layer not only enables sound waves emitted in air to penetrate solid barriers and enter water with extremely high efficiency, but also allows for phase modulation of the transmitted waves. Specific experiments on three-dimensional axial focusing of curved solid aberration layers demonstrate that a 25 dB amplitude enhancement can be achieved at the focal point. This invention has the advantages of simple structure and high control efficiency, providing a solution for non-contact detection technologies in fields such as transcranial medical ultrasound and industrial pipeline inspection.

Description

Technical Field

[0001] This invention belongs to the technical field of acoustic signal transmission, specifically relating to a method for designing and fabricating an acoustic metasurface structure that allows ultrasound to penetrate a solid aberration layer. Background Technology

[0002] In fields such as non-destructive testing of pipelines and transcranial ultrasound in medicine, ultrasound waves need to be transmitted from a fluid medium across a solid medium. However, due to the significant difference in acoustic impedance between air and water and solid structures (such as equipment casings and biological skeletons), sound waves are strongly reflected when crossing interfaces, resulting in extremely low transmitted energy. Furthermore, modulating the wavefront using an ultrasonic phased array is expensive. In addition, curved solid layers often exist in actual detection environments, making it difficult for traditional acoustic equipment to target specific curved interfaces.

[0003] Therefore, how to simultaneously break through the huge acoustic impedance barrier and provide free wavefront modulation to achieve efficient enhanced transmission and precise focusing of sound waves from air through curved solids into water is a key technical problem that urgently needs to be solved in the field of acoustic engineering. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing a method for designing and fabricating an acoustic metasurface structure that allows ultrasound to penetrate solid aberration layers. For existing curved solid aberration layers, the method optimizes the transmission efficiency and phase of sound waves in an air-solid-water transmission scenario. A two-dimensional metasurface unit with a specific phase and perfect transmission effect is designed using topology optimization. By stretching the two-dimensional metasurface, four three-dimensional units with a π / 2 phase difference and high transmission efficiency are obtained. For the curved solid layer, a suitable substrate is designed, and the lower side of the substrate perfectly fits the curved solid. The four three-dimensional units are arranged on the upper plane of the substrate according to phase coding theory to form the final acoustic metasurface. By perfectly fitting the designed acoustic metasurface onto the surface of the original solid aberration layer, efficient sound wave transmission and three-dimensional focusing across the curved solid aberration layer can be achieved. Experimentally, a 25 dB increase in focal amplitude was observed.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A method for designing and fabricating an acoustic metasurface structure that allows ultrasound to penetrate a solid aberration layer includes the following steps: Step (1): Determine the frequency of the sound wave to be emitted in the air. and according to Calculate the frequency of the sound wave Lower sound wave wavelength Determine the base thickness and solid aberration layer thickness and its material properties; Step (2): Based on the determined sound wave wavelength and the thickness of the solid barrier layer Determine the dimensions of the acoustic metasurface unit structure, which is rectangular in shape and has a width of [missing value]. The height is The interior of the structure is evenly divided into sections with sides of length [missing information]. If the number of pixels is s, then the number of pixels in the width direction is m = a / s, and the number of pixels in the width direction is n = b / s. We can take s = 0.7 mm, m = 22, and n = 12. Step (3): Using topology optimization, the transmission efficiency and transmission phase obtained by sampling the sound wave in the transmission field are used as optimization targets. Different material parameters are assigned to each pixel point, and then the transmittance and transmission phase of a single acoustic metasurface unit are calculated as the sound wave propagates from one side of the air through the solid barrier layer to the other side of the water under periodic boundary conditions. Step (4): Using a computer program, repeat step (3) to continuously adjust the material parameter distribution of several pixels in the acoustic metasurface unit cell structure. Iterate continuously to make the transmittance of the calculated sound wave from one side of the air through the solid aberration layer to the other side of the water close to 100% and have a specified transmission phase, thereby obtaining the optimal material parameter distribution, which is the acoustic metasurface two-dimensional unit structure. Step (5): Extrude the obtained two-dimensional unit topology. And in the stretching direction, there is a width of 2 between the units. Air gaps are added to reduce the coupling between elements. In the simulation, the transmission peak, peak frequency and transmission phase of the three-dimensional elements are calculated. Due to the presence of gaps, the calculation results of different configurations will be offset to different degrees. If the desired requirements cannot be met, the corresponding two-dimensional topology configuration is re-optimized. This process continues until four three-dimensional metasurface elements with the same peak frequency and a phase difference of π / 2 between them are obtained, and they are encoded as "00", "01", "10" and "11". Step (6): For the curved solid aberration layer, obtain its surface geometry equation, design a complementary substrate to fit it perfectly on the aberration layer, arrange the encoded three-dimensional units on the upper side of the substrate according to the phase encoding theory based on Fermat's principle, and finally print the complete acoustic metasurface in one piece.

[0006] Furthermore, the transmission efficiency calculated by sampling is obtained from the complex sound pressure levels sampled from both sides of the sound field, with complex sound pressure levels collected in both the incident and transmission fields. and The transmission efficiency is then expressed as The transmission phase is represented as During the optimization process, the unit configuration is continuously adjusted to achieve the set target transmission phase and a transmission efficiency close to 100%.

[0007] Furthermore, the material parameters selected are three materials: metasurface unit cell, solid barrier layer, and gas component. The metasurface unit cell and solid barrier layer can be epoxy resin, and the gas material can be air. However, the selection is not limited to the above two material parameters. Different materials are selected for the metasurface unit cell and solid barrier layer according to the target requirements for topology optimization design.

[0008] Furthermore, during the optimization of metasurface units, some constraints are set to facilitate subsequent processing and ensure the effectiveness of the actual results. For example, the outermost row of pixels on both sides of the unit is set to be air to reduce the mutual coupling between units. The metasurface unit cell structure is required to be symmetrical from left to right and the internal right-angle structure is rounded. However, these constraints are not limited to the above-mentioned constraints.

[0009] Furthermore, in the material parameter distribution, the metasurface unit structure, substrate structure, and solid aberration layer structure corresponding to the solid material need to be considered as elastic bodies during the topology optimization process. At the same time, the interaction between airborne sound and the structure needs to be considered. The vibration of the structure plays an important role in realizing the efficient transmission and phase shift of sound waves across the solid aberration layer.

[0010] Furthermore, to achieve the desired function in the transmission region, it is necessary to reverse-engineer the arrangement of the encoded 2-bit three-dimensional metasurface units on the upper side of the substrate, and provide the corresponding phase through the encoding units at specific positions; theoretically, since there are air gaps around the three-dimensional units, the weak coupling between them can achieve the shaping of the target transmission wavefront.

[0011] One of the above technical solutions has the following beneficial effects: This invention applies the concept of acoustic metasurfaces to address the problems of severe reflection and difficulty in wavefield shaping in transcranial ultrasound in the field of non-contact medical ultrasound. With the optimization objectives of sound wave transmission efficiency and transmission phase across solid aberration layers from air into water, an acoustic metasurface is designed in reverse using topology optimization. This acoustic metasurface consists of a substrate structure that perfectly conforms to a curved solid aberration layer and four three-dimensional units with perfect transmission performance and a transmission phase difference of π / 2 between them. By placing the designed acoustic metasurface tightly against the surface of the curved solid aberration layer, efficient sound wave transmission across multiple gas-solid-liquid media can be achieved simultaneously. The acoustic metasurface structure and its design and assembly method provided by this invention are simple to design. The structure can be fabricated using 3D printing. The metasurface unit structure and materials can be flexibly adjusted for the target solid aberration layer, greatly simplifying the design process and fabrication method. The shape of the substrate can be flexibly adjusted to perfectly fit a specific curved aberration layer, which can effectively handle curved structures. The arrangement of 2-bit three-dimensional units on the upper side of the substrate can be flexibly adjusted for the three-dimensional wave field function of different transmission regions, which greatly reduces the manufacturing cost of phased array probes. It has important application value in fields such as non-destructive testing and medical detection. Attached Figure Description

[0012] The features, advantages and technical effects of exemplary embodiments of the present invention will now be described with reference to the accompanying drawings.

[0013] Figure 1 The present invention provides a functional schematic diagram and structural design process diagram of an acoustic metasurface that enables enhanced and focused transmission of sound waves from air to water through a solid aberration layer.

[0014] Figure 2 The four two-dimensional unit structures with perfect transmission and a transmission phase difference of π / 2 between each other, as designed in Embodiment 1 of the present invention, are shown in the corresponding frequency response curves and the transmission sound pressure field diagram at 21 kHz.

[0015] Figure 3 This image shows the metasurface sample and the solid aberration layer sample to be penetrated in the two-dimensional focusing experiment of Embodiment 1 of the present invention.

[0016] Figure 4 This is a schematic diagram of two-dimensional axial focusing in Embodiment 1 of the present invention, and a diagram of the sound field transmission effect when the distance between different sound sources and metasurfaces is different in experiments and simulations.

[0017] Figure 5 The diagram shows four three-dimensional unit structures with high-efficiency transmission and a transmission phase difference of π / 2 between each other, as designed in Embodiment 2 of the present invention, and their corresponding frequency response curves.

[0018] Figure 6This is an image of the metasurface sample and the solid aberration layer sample to be penetrated in the three-dimensional focusing experiment of Embodiment 2 of the present invention.

[0019] Figure 7 This is a schematic diagram of the three-dimensional axial focusing used in Example 2 and a sound field transmission effect diagram of different cross sections of the experiment.

[0020] Figure 8 This is a transmission energy distribution diagram of the metasurface in Example 3 facing solid phase difference layers of different shapes. Detailed Implementation

[0021] If certain terms are used in the specification and claims to refer to specific components, those skilled in the art will understand that hardware manufacturers may use different names to refer to the same component. This specification and claims do not distinguish components based on differences in name, but rather on differences in function. The term "comprising" as used throughout the specification and claims is an open-ended term and should be interpreted as "comprising but not limited to." "Approximately" means that within an acceptable margin of error, those skilled in the art can solve the technical problem and substantially achieve the technical effect within a certain margin of error.

[0022] The present invention will be further described in detail below with reference to the accompanying drawings, but this is not intended to limit the scope of the invention.

[0023] like Figure 1 The diagram shows the design principle of an acoustic metasurface structure for efficient sound wave transmission across a solid barrier layer, as provided by this invention. It describes a method for designing and fabricating an acoustic metasurface structure for efficient sound wave transmission and phase modulation across a curved solid aberration layer, from air to water. The acoustic metasurface includes a substrate structure conformally to the curved solid aberration layer and discrete 2-bit encoded metasurface units. The lower side of the substrate is constructed using geometric equations extracted from the shape of the curved solid aberration layer to conform to it. The upper side of the substrate is a plane for arranging the metasurface units, and the lower side is a curved surface that perfectly fits the curved structure. Each 2-bit metasurface unit has enhanced transmission and a transmission phase difference of Π / 2 between them, arranged according to a specific encoding sequence on the upper plane of the substrate.

[0024] The metasurface unit employs topology optimization, with the transmission efficiency of sound waves during transmission across solid media as the optimization objective. Different material parameters are assigned to each pixel, and the transmittance and transmission phase of a single acoustic metasurface unit under periodic boundary conditions are calculated as the sound wave propagates from air on one side through a solid aberration layer to water on the other. Using a computer program, this step is repeated, continuously adjusting the material parameter distribution of several pixels in the acoustic metasurface unit cell structure. This iterative process ensures that the calculated transmittance of sound waves propagating from air on one side through a solid barrier layer to water on the other side is close to 100%, and the transmission phase is close to a preset value. By changing the preset transmission phase, the optimal material parameter distribution of four units with perfect transmission and a transmission phase difference of π / 2 is obtained. These units are then encoded and arranged on a substrate to finally obtain the acoustic metasurface.

[0025] Example 1

[0026] This embodiment provides an acoustic metasurface for achieving efficient transmission and two-dimensional focusing of sound waves through a planar solid aberration layer, from air into water, as well as its preparation and testing methods. Addressing the problem of significant acoustic impedance mismatch between air, solid, and water, the transmission coefficient of sound waves across the medium is used for optimal transmission. and transmission phase The objective function is defined as follows. A finite element numerical model is established, and a topology optimization algorithm is used to reverse-engineer the internal geometry of the two-dimensional metasurface unit. The target operating frequency is set to 21 kHz. The aberration layer to be penetrated is a 3 cm thick planar square plate structure made of epoxy resin. Figure 3 As shown. The width of the acoustic metasurface unit structure is... and height Take them respectively = 15.4 mm, = 9.8 mm, divided into 22 pixels in the width direction and 14 pixels in the height direction, for a total of 308 pixels. The solid part of the material is set as photosensitive resin, and the background fluid is air. A topology optimization program is used to optimize the material parameter distribution of these 512 pixels. After multiple iterations, four two-dimensional topological unit structures with perfect transmittance (T = 1) and a transmission phase difference between them precisely satisfying π / 2 are finally extracted. These correspond to 2-bit phase codes of "00" (0 phase), "01" (π / 2 phase), "10" (π phase), and "11" (3π / 2 phase), respectively. Their structural vibration modes, frequency response curves, and sound pressure distribution at 21 kHz are shown below. Figure 2 As shown, all four units can achieve perfect transmission across gas-solid-liquid media at 21 kHz, and the transmission phases differ from each other by Π / 2, which meets the design requirements.

[0027] For planar solid aberration layers, a substrate with both a planar bottom and top surface and a thickness of 7 mm is designed. The focal length required for two-dimensional focusing is determined based on... Using Fermat's principle to calculate each spatial position above the base Required compensation phase Continuous phase distribution The data is discretized into the four encoded states mentioned above. Based on the discretized phase sequence, the corresponding two-dimensional units are arranged on a planar substrate. The overall structure is integrally formed using photopolymerization 3D printing technology, as shown in the metasurface sample. Figure 3 As shown.

[0028] The prepared metasurface sample was tightly bonded to the incident side (air end) of the planar solid aberration layer to be penetrated. A loudspeaker was placed on the air side as the sound source, and a hydrophone was arranged inside the water tank and connected to a displacement machine to scan the transmitted sound field. Field scanning tests were conducted under different distances between the sound source and the metasurface (e.g., d = 0.98 m, 0.80 m, 0.60 m). Experimental and simulation results are as follows: Figure 4 As shown, the two are highly consistent, and the sound pressure field clearly shows that the sound wave penetrates the solid barrier with extremely high efficiency and achieves high-intensity two-dimensional axial focusing at the predetermined focal position in the water, proving the feasibility of the acoustic metasurface design method provided by this invention for realizing efficient transmission and two-dimensional focusing of sound waves across the solid barrier layer from air to water.

[0029] Example 2

[0030] This embodiment further discloses an acoustic metasurface for achieving efficient enhanced sound wave transmission and precise three-dimensional spatial focusing using a curved solid aberration layer with a specific surface curvature. The two-dimensional unit topology from Embodiment 1 is stretched along the Z-axis, preferably by a stretching length equal to the unit width *a*, forming a three-dimensional unit. To avoid strong vibrational coupling between adjacent three-dimensional units in spatial arrangement, an air gap of 2*s* (2 pixels) is maintained between each unit in the stretching direction. The topology is further reverse-engineered using finite element simulation and a genetic optimization algorithm. Finally, four 2-bit three-dimensional metasurface units with the same peak transmission frequency at the target frequency (20.56 kHz) and a transmission phase difference of π / 2 are obtained. Figure 5 As shown, its frequency response curves exhibit high consistency and excellent transmission performance at the target frequency. The calculated sound pressure distribution shows that the transmission phases of the four units differ from each other by Π / 2.

[0031] The surface geometry equation z = f(x,y) of the curved solid aberration layer is obtained using a CAD model. A complementary substrate is designed to perfectly match the lower surface profile of the substrate with z = f(x,y), enabling seamless and conformal bonding to the solid aberration layer. Simultaneously, the upper surface of the substrate is constrained to be a uniform plane z = z0. Based on the required three-dimensional axial focal point coordinates (x,y...)... f ,y f , z f ), calculate the target phase distribution in a two-dimensional plane on the upper surface of the substrate. The encoded three-dimensional unit array is arranged on the upper surface and integrally fabricated to obtain... Figure 6 The test sample is shown. This single-layer metasurface was perfectly attached to a curved solid aberration layer (incident surface), and a small amount of liquid ultrasonic coupling agent was used to ensure that there were no excess air layers at the contact surface blocking sound wave transmission. The three-dimensional sound field was scanned underwater. Figure 7 As shown in the experimental sound field transmission diagrams at different cross-sections, compared to an exposed curved solid without a metasurface, the metasurface in this embodiment not only breaks the gas-solid-liquid three-phase impedance barrier but also achieves perfect three-dimensional ultrasonic focusing. The sound pressure amplitude at the underwater focal point is significantly enhanced by up to 25 dB, completing the three-dimensional wave field reshaping from air to water.

[0032] Example 3

[0033] This embodiment aims to verify the universality and high customization capability of the aforementioned metasurface structure, design process, and conformal bonding method for various complex and irregularly shaped solid barriers. To comprehensively verify a wide range of application scenarios, this embodiment constructs three extreme cases commonly found in non-destructive testing, cross-media communication, or medical ultrasound: a hump-shaped aberration layer (a solid layer with local Gaussian protrusions on the surface), an asymmetric aberration layer (asymmetrical surface distribution), and a non-uniform thickness aberration layer (the upper and lower surfaces of the solid layer are not parallel).

[0034] For the three aberration layers mentioned above, their specific surface geometric equations are directly extracted through numerical modeling. On one hand, the conformal complementary technique from Example 2 is used to customize the bonding substrate, eliminating shape errors at the incident interface. On the other hand, the internal phase retardation increment caused by variations in solid layer thickness is addressed. As a perturbation term, it is introduced into the phase compensation calculation of the upper 2-bit cell array on the metasurface. By adjusting the spatial arrangement sequence of the 3D cells on the upper side plane of the substrate, the dual phase errors caused by interface shape distortion and thickness inhomogeneity are compensated in one step. Figure 8 As shown in the sound field transmission energy distribution diagram, when facing three types of complex obstacles, namely hump-shaped, asymmetric, and non-uniform thickness, the acoustic metasurface customized by the method of this invention can effectively achieve enhanced transmission and efficient focusing of sound waves across the curved aberration layer from air to water.

[0035] In water-based environments, regardless of the distortion of the intermediate solid layer, the metasurface can consistently create a clear focal point with symmetrical focal spots, extremely low side lobes, and highly concentrated energy at a designated location. This demonstrates that the single-layer metasurface design of this invention possesses excellent free wavefront modulation capabilities and engineering versatility, greatly expanding the application boundaries of ultrasound in complex transmedium environments.

[0036] Based on the disclosure and teachings of the foregoing specification, those skilled in the art can make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments described above, and any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention are within the scope of protection of the present invention. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on the present invention.

Claims

1. A method for designing and fabricating an acoustic metasurface structure that allows ultrasound to penetrate a solid aberration layer, characterized in that, Includes the following steps: Step (1): Determine the frequency of the sound wave to be emitted in the air. and according to Calculate the frequency of the sound wave Lower sound wave wavelength ,in To determine the speed of sound in air, we need to determine the thickness of the solid barrier layer through which the target passes. and its material properties; Step (2): Based on the determined sound wave wavelength and the thickness of the solid barrier layer The dimensions of the acoustic metasurface unit structure were determined. The structure is rectangular in shape and has a width of [missing information]. The height is The interior of the structure is evenly divided into sections with sides of length [missing information]. If the number of pixels is 0, then the number of pixels in the width direction is 0. Number of pixels in the width direction It is acceptable , , ; Step (3): Using topology optimization, with the transmission efficiency of sound waves in the process of propagation across solid media as the optimization target, different material parameters are assigned to each pixel point, and then the transmittance of a single acoustic metasurface unit cell under periodic boundary conditions is calculated from one side of air through the solid barrier layer to the other side of air. Step (4): Using a computer program, repeat step (3) to continuously adjust the material parameter distribution of several pixels in the acoustic metasurface unit cell structure. Iterate continuously to make the transmittance of the calculated sound wave from one side of the air through the solid barrier layer to the other side of the air close to 100%, thereby obtaining the optimal material parameter distribution, which is the acoustic metasurface unit structure.

2. The method for designing and fabricating an acoustic metasurface structure that allows ultrasound to penetrate a solid aberration layer as described in claim 1, characterized in that: In step (2), the side length of the pixel and the number of arrays in both directions are determined according to the determined acoustic wavelength. The final acoustic metasurface structure is scaled proportionally to the corresponding wavelength at different frequencies to achieve efficient transmission of acoustic waves across the solid barrier layer at different frequencies.

3. The method for designing and fabricating an acoustic metasurface structure that allows ultrasound to penetrate a solid aberration layer as described in claim 1, characterized in that: In step (3), the transmission efficiency calculated by sampling is obtained from the complex sound pressure obtained by sampling from both sides of the sound field. Complex sound pressure is collected in the incident field and the transmission field respectively. and The transmission efficiency is then expressed as During the optimization process, the unit configuration was continuously adjusted to make the transmission efficiency close to 100%.

4. The method for designing and fabricating an acoustic metasurface structure that allows ultrasound to penetrate a solid aberration layer as described in claim 1, characterized in that: In step (3), the material parameters are selected from three materials: metasurface unit cell, solid barrier layer and gas part, wherein the metasurface unit cell and solid barrier layer are selected as epoxy resin, and the gas material is selected as air.

5. The method for designing and fabricating an acoustic metasurface structure that allows ultrasound to penetrate a solid aberration layer as described in claim 1, characterized in that: In step (3), constraints are set during the optimization of the metasurface unit cell.

6. The method for designing and fabricating an acoustic metasurface structure that allows ultrasound to penetrate a solid aberration layer as described in any one of claims 1-5, characterized in that: In step (4), the metasurface unit cell structure and solid barrier layer structure corresponding to the solid material in the material parameter distribution are considered as elastic bodies in the topology optimization process, while the interaction between airborne sound and the structure is also considered.

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