Multi-focus acoustic metamaterial lens based on complex amplitude coding and design method thereof

Abstract

The invention relates to the technical field of acoustic super-structure lenses, and discloses a multi-focus acoustic super-structure lens based on complex amplitude coding and a design method thereof, and the design method comprises the steps: employing a photosensitive resin material to prepare the multi-focus acoustic super-structure lens, and enabling the focus number, position, relative intensity and phase of the multi-focus acoustic super-structure lens to be adjustable, according to the invention, a complex amplitude coding method based on a binary complementary matrix is adopted, a super-structure lens transmission coefficient matrix for realizing complex amplitude superposition of a plurality of incident focusing sound fields is constructed, the amplitudes and phases of the sound fields are regulated and controlled at the same time, and a multi-point focusing sound field is formed. And by adjusting the positions of the focuses on the preset straight line or any closed track, the multiple focuses are synthesized into a sound needle or a sound vortex. According to the invention, a complex amplitude coding method based on superpixels is adopted, a phase-type super-structure lens is constructed, flexible regulation and control of positions, intensity and phases of a plurality of focuses in a space are realized, a sound needle and a sound vortex are synthesized, and the application value is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of acoustic meta-lens, and in particular to a multi-focus acoustic meta-lens based on complex amplitude coding and a design method thereof. Background Art

[0002] The convergence of ultrasonic energy, namely ultrasonic focusing, has a wide range of applications in many fields (such as medical treatment, drug delivery, particle capture, and nondestructive testing). Traditional ultrasonic focusing can be achieved through transducer arrays or curved lenses, which are bulky, costly, and time-consuming to produce. Metalenses use subwavelength structural units to precisely control the phase of transmitted ultrasound waves, and their thin and light characteristics have attracted widespread attention from researchers. At present, the research on metalenses that achieve single-point ultrasonic focusing is relatively mature. Metalenses that achieve single-point ultrasonic focusing can achieve high-precision acoustic energy focusing, and related technologies have been promoted in practice; metalenses that achieve multi-point ultrasonic focusing can achieve acoustic energy focusing at multiple spatial positions at the same time, expanding the application range of ultrasonic technology. However, achieving acoustic energy focusing at different spatial positions requires the design of more complex metalens structures, which increases the difficulty of design and manufacturing. Therefore, metalenses that achieve acoustic energy focusing at different spatial positions need further research. Summary of the invention

[0003] The purpose of the present invention is to provide a multi-focal acoustic metalens based on complex amplitude coding and a design method thereof, and to construct a phase-type metalens by adopting a complex amplitude coding method based on superpixels, so as to realize flexible control of the position, intensity and phase of multiple focal points in space, synthesize multi-focal points of acoustic needles or acoustic vortices, and improve the application value.

[0004] To achieve the above object, the present invention provides the following solutions:

[0005] The present invention provides a design method for a multi-focal acoustic meta-lens based on complex amplitude coding, wherein a photosensitive resin material is used to prepare the multi-focal acoustic meta-lens, wherein the number, position, relative intensity and phase of the focus of the multi-focal acoustic meta-lens are adjustable, and a multi-focal point synthetic acoustic needle or acoustic vortex is realized, and the method specifically comprises the following steps:

[0006] A modulation phase formula is constructed based on the focal coordinates, focal length and wave number in the background medium, and the phase of the incident sound wave is modulated using the modulation phase formula to achieve single-point focusing;

[0007] Based on the modulation phase formula, a complex amplitude coding method based on a binary complementary matrix is ​​used to construct a meta-lens transmission coefficient matrix that realizes the superposition of complex amplitudes of multiple incident focused sound fields. Based on the transmission coefficient matrix, the wavefront modulation of the incident sound wave passing through the multi-focus acoustic meta-lens is realized.

[0008] Based on the modulation of the transmission coefficient matrix, the amplitude and phase of the sound field are simultaneously controlled to form a multi-point focused sound field. By adjusting the position of the focus on a preset straight line or an arbitrary closed trajectory, multiple focuses are synthesized into a sound needle or a sound vortex.

[0009] Furthermore, the modulation phase formula is constructed based on the focal coordinates, focal length and wave number in the background medium, specifically including:

[0010]

[0011] Where P i (x,y) represents the additional phase required to focus the i-th focus, k is the wave number in the background medium, (x i ,y i ) is the focal coordinate, f i is the focal length, C i is a constant, and its specific value does not change the focal coordinates and focal length. (x, y) represents Cartesian coordinates.

[0012] Furthermore, the complex amplitude coding method based on a binary complementary matrix is ​​used to construct a meta-lens transmission coefficient matrix for realizing the superposition of complex amplitudes of multiple incident focused sound fields, specifically including:

[0013] The initial transmission coefficient matrix t(x,y) is constructed as a superposition form of two pure phase distributions to obtain two pure phase distributions;

[0014] Use superpixels to construct a binary complementary matrix M 1 With M 2 , where M 1 +M 2 =1;

[0015] Using the binary complementary matrix M 1 With M 2 , superimpose the two pure phase distributions to obtain the superimposed phase

[0016] Determine the amplitude A(x,y) of the transmission coefficient matrix, where A(x,y) can be any positive number;

[0017] Based on superposition phase and amplitude A(x,y), and obtain the meta-lens transmission coefficient matrix that realizes the superposition of complex amplitudes of multiple incident focused sound fields

[0018] Furthermore, the initial transmission coefficient matrix t(x, y) is constructed as a superposition form of two pure phase distributions to obtain two pure phase distributions, specifically including:

[0019] Based on the multi-point focusing of the outgoing sound wave in space and the modulation phase of a single point, the initial expression of the transmission coefficient matrix t(x,y) is constructed as follows:

[0020]

[0021] Among them, A i represents the amplitude of the i-th incident focused sound field, j represents the imaginary unit, P i (x,y) represents the additional phase required to focus on the i-th focus.

[0022] The transmission coefficient matrix t(x,y) is constructed as the superposition of two pure phase distributions:

[0023]

[0024] In the formula, It represents the maximum value of the amplitude of the incident sound field focused at any multiple points;

[0025] Among them, the two pure phase distribution expressions are as follows:

[0026]

[0027] Furthermore, the binary complementary matrix M is constructed by using superpixels 1 With M 2 , specifically including:

[0028] Take 4 consecutive basic pixel elements as a 2×2 super pixel unit, use the numbers 1, 2, 3, and 4 to distinguish these 4 basic pixel elements, and divide the entire phase plane into multiple super pixel units. The positions of the digital sequences in each super pixel unit are randomly arranged, and the corresponding positions of odd numbers 1 and 3 are filled with "0", and the corresponding positions of even numbers 2 and 4 are filled with "1", and the first binary matrix M1 is obtained; conversely, the corresponding positions of odd numbers 1 and 3 are filled with "1", and the corresponding positions of even numbers 2 and 4 are filled with "0", and the second binary matrix M2 is obtained. The two binary matrices M1 and M2 are complementary matrices.

[0029] Furthermore, the use of the binary complementary matrix M 1 With M 2 , superimpose the two pure phase distributions to obtain the superimposed phase The specific expressions are as follows:

[0030]

[0031] Furthermore, the method of adjusting the position of the focus on the preset straight line to synthesize the sound needle with multiple focuses specifically includes:

[0032] After selecting the needle-shaped focus center, half of the half-length and width of the Airy spot focused at the needle-shaped focus center is used as the starting basis for constructing the intervals between each focus of the needle-shaped focus;

[0033] A genetic algorithm is used within the focusing range to adjust the position of each focus, and as the focusing range is reduced, a needle beam with an intensity fluctuation of less than 10% is obtained within the focusing range.

[0034] Furthermore, the method of adjusting the position of the focus on any closed track to synthesize the acoustic vortex with multiple focuses specifically includes:

[0035] The acoustic vortex can be synthesized by selecting the focus position on any closed trajectory and assigning a periodic spiral phase to each focus in turn in a clockwise or counterclockwise direction.

[0036] Furthermore, the method of preparing the multi-focal acoustic meta-lens using a photosensitive resin material further includes:

[0037] Assume that when a vertically incident sound wave passes through a photosensitive resin with a height of h, the phase shift produced is expressed as follows:

[0038]

[0039] Where n represents the refractive index of the longitudinal wave of the sound wave in the photosensitive resin material, λ 0 represents the wavelength of the incident sound wave;

[0040] Based on formula (8), the height h distribution of the photosensitive resin material on the surface of the multi-focal acoustic metalens is determined.

[0041] The present invention also provides a multi-focus acoustic meta-lens based on complex amplitude coding designed by the above-mentioned design method.

[0042] According to the specific embodiments provided by the present invention, the multi-focal acoustic metalens based on complex amplitude coding and the design method thereof provided by the present invention disclose the following technical effects: a multi-focal acoustic metalens is prepared by using a photosensitive resin material, and a complex amplitude coding method based on superpixels is used to construct a phase-type metalens to achieve flexible control of the position, intensity and phase of multiple focal points in space; based on the modulation of the transmission coefficient matrix, the amplitude and phase of the sound field are simultaneously controlled to form a multi-point focused sound field, and by adjusting the position of the focus on a preset straight line or an arbitrary closed trajectory, multiple focal points synthesize a sound needle or a sound vortex.

[0043] Among them, acoustic needle-shaped focusing, i.e., acoustic needle, has potential applications in scenarios such as tumor ablation and photoacoustic imaging. The present invention adopts a method of synthesizing acoustic needles with multiple focuses to construct a needle-shaped focused beam whose length, quantity, and direction can be controlled. Acoustic vortex is a special waveform with a topological phase. The present invention adopts a method of synthesizing vortices with multiple focuses to construct an acoustic vortex whose spatial position, geometric shape, and topological charge can be adjusted. The meta-lens based on the design method described in the present invention can realize various forms of acoustic needles or acoustic vortices, and is expected to obtain important applications in the fields of acoustic detection, imaging, and medical treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0045] Figure 1 A schematic diagram of constructing a multi-focus sound field based on complex amplitude coding provided by an embodiment of the present invention, wherein (a) is an initial amplitude and phase distribution diagram of the superposition of any multiple incident focused sound fields; (b) is a schematic diagram of superimposed phase construction; (c) is an amplitude and phase distribution diagram of the meta-lens transmission coefficient matrix;

[0046] Figure 2 The normalized intensity distribution diagram of the multi-focus synthetic acoustic needle with the same center position and different lengths in the numerical simulation of the embodiment of the present invention, where (a) is a 16-focus construction with a half-length width (FLHM) of 12.4λ 0 Normalized intensity distribution of the acoustic needle, (b) is the half length width (FLHM) of 25 focal points is 13.6λ 0 Normalized intensity distribution of the acoustic needle, (c) 36 focus construction half length width (FLHM) is 18.7λ 0 Normalized intensity distribution diagram of the acoustic needle;

[0047] Figure 3 The normalized sound intensity distribution diagram of the multi-focus synthetic tilted sound needle and the parallel double sound needle of the embodiment of the present invention is numerically simulated, wherein (a) is the normalized sound intensity distribution diagram of the multi-focus synthetic tilted sound needle, and (b) is the normalized sound intensity distribution diagram of the parallel double needle;

[0048] Figure 4 The normalized sound intensity and phase distribution diagram of the multi-focus synthetic circularly symmetric acoustic vortex numerically simulated in the embodiment of the present invention, wherein (a) is the normalized sound intensity distribution diagram of the xz plane with different topological charges, and (b) is the normalized sound intensity and phase distribution diagram of the xy plane with different topological charges;

[0049] Figure 5 The normalized sound intensity and phase distribution diagrams of the triangular symmetric and square symmetric acoustic vortices synthesized by numerical simulation of the multi-focus according to the embodiment of the present invention are shown in Figure 1, wherein (a) is the normalized sound intensity and phase distribution diagram of the triangular symmetric acoustic vortex with different topological charges on the xy plane, and (b) is the normalized sound intensity and phase distribution diagram of the square symmetric acoustic vortex with different topological charges on the xy plane. DETAILED DESCRIPTION

[0050] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0051] The purpose of the present invention is to provide a multi-focus acoustic meta-lens based on complex amplitude coding and a design method thereof. The proposed acoustic meta-lens is made of photosensitive resin material, and the number, position, intensity and phase of the focus are adjustable. The meta-lens based on the design method can realize various forms of acoustic needles or acoustic vortices, and is expected to be used in the fields of acoustic detection, imaging, and medical treatment.

[0052] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0053] The design method of the multi-focal acoustic meta-lens based on complex amplitude coding provided by the present invention adopts a photosensitive resin material to prepare the multi-focal acoustic meta-lens, the number, position, relative intensity and phase of the focus of the multi-focal acoustic meta-lens are adjustable, and a multi-focal point synthetic acoustic needle or acoustic vortex is realized, which specifically comprises the following steps:

[0054] Step 1: construct a modulation phase formula based on the focal coordinates, focal length and wave number in the background medium, and use the modulation phase formula to modulate the phase of the incident sound wave to achieve single-point focusing; the modulation phase formula constructed in this step can achieve single-point focusing by only modulating the phase of the incident sound wave;

[0055] The corresponding modulation phase can be expressed by formula (1):

[0056]

[0057] Where P i (x,y) represents the additional phase required to focus the i-th focus, k is the wave number in the background medium, (x i ,y i ) is the focal coordinate, the multi-focal acoustic metalens is located in the z = 0 plane, so f i is the focal length, Ci is a constant whose specific value does not change the focal coordinates and focal length and is generally zero. (x, y) represents Cartesian coordinates.

[0058] Step 2: Based on the modulation phase formula, a complex amplitude coding method based on a binary complementary matrix is ​​used to construct a transmission coefficient matrix of the complex amplitude superposition of multiple incident focused sound fields. Based on the transmission coefficient matrix, the wavefront modulation of the incident sound wave passing through the multi-focus acoustic meta-lens is realized;

[0059] When the incident plane acoustic wave passes through the metalens, the modulation of the wavefront by the metalens can be represented by the transmission coefficient matrix t(x, y). In order to achieve multi-point focusing of the outgoing acoustic wave in space, t(x, y) can be expressed as formula (2).

[0060]

[0061] In the formula, A i represents the amplitude of the i-th incident focused sound field, j represents the imaginary unit, P i (x,y) represents the additional phase required to focus on the i-th focus.

[0062] From the above formula, we can know that t(x,y) is the superposition of the complex amplitudes of multiple incident focused sound fields. Figure 1 (a) shows the initial amplitude A of an incident sound field focused at any multiple points. 0 (x,y) and phase Since it is difficult for the superstructure unit to achieve simultaneous control of the amplitude and phase of the sound field, the present invention proposes a complex amplitude coding method using a binary complementary matrix, which is as follows:

[0063] Mathematically, the transmission coefficient matrix t(x,y) can be written as the superposition of two pure phase distributions, as shown in equation (3).

[0064]

[0065] In the formula, It represents the maximum value of the amplitude of the incident sound field focused at any multiple points;

[0066] The two pure phase distributions can be written as:

[0067]

[0068] In the formula, and like Figure 1 Obviously, in order to obtain the final phase It is necessary to re-superimpose the two phase diagrams. To this end, the present invention designs a binary complementary matrix M 1 With M 2To achieve the above functions, the binary complementary matrix is ​​constructed using superpixels. Four consecutive basic pixel elements are used as a 2×2 superpixel unit. The four basic pixel elements are distinguished by the numbers 1, 2, 3, and 4. The entire phase plane is divided into multiple superpixel units. The positions of the digital sequences in each superpixel unit are randomly arranged, and the corresponding positions of odd numbers 1 and 3 are filled with "0", and the corresponding positions of even numbers 2 and 4 are filled with "1", and the first binary matrix M1 is obtained; conversely, the corresponding positions of odd numbers 1 and 3 are filled with "1", and the corresponding positions of even numbers 2 and 4 are filled with "0", and the second binary matrix M2 is obtained. The two binary matrices M1 and M2 are complementary matrices.

[0069] Therefore, the final superposition phase It can be expressed as formula (5).

[0070]

[0071] Finally, the amplitude A(x,y) of the transmission coefficient matrix of the metalens can take any positive number, and the phase distribution is like Figure 1 The present invention forms a multi-point focused sound field according to the above method, and uses angular spectrum diffraction calculation to verify the theory. When calculating, the incident plane sound field frequency is set to 500kHz, the sound speed is 1500m / s, and the sound field radius is 16λ 0 ,λ 0 Indicates the wavelength of the incident sound wave, with a pixel interval of 1mm.

[0072] Step 3: Based on the modulation of the transmission coefficient matrix, the amplitude and phase of the sound field are simultaneously controlled to form a multi-point focused sound field. By adjusting the position of the focus on a preset straight line or an arbitrary closed trajectory, multiple focuses are synthesized into a sound needle or a sound vortex.

[0073] Taking the construction of needle-shaped focus as an example, after selecting the needle-shaped focus center, half of the half-length width of the Airy spot focused at that position is used as the starting basis for constructing the intervals between each focus of the needle-shaped focus. A genetic algorithm is used within the focus range to adjust the position of each focus. As the focus range is reduced, a needle-shaped beam with an intensity fluctuation of less than 10% within the focus range can be obtained.

[0074] like Figure 2 As shown, by adjusting the position of the focus on the preset straight line, the focus is focused on the meta-lens 30λ 0 The needle beams with 16, 25 and 36 focal points are successfully constructed by the present invention. Figure 3 As shown, thanks to the flexibility of the binary complementary matrix for complex amplitude encoding, oblique and symmetrical needle-shaped focusing are also successfully constructed by the present invention.

[0075] Numerical simulations show that under diffraction limited conditions, the half-width of the needle-shaped focus is affected by its center position and the geometric length of the metasurface. The closer the center position is, the larger the metasurface is, the smaller the half-width of the focus center is, and the focus length can increase with the increase in the number of focal points.

[0076] The above method also shows great application value in generating acoustic vortices. The present invention selects the position of the focus on any closed trajectory and assigns a periodic spiral phase to each focus in turn in a clockwise or counterclockwise direction. In order to better present the acoustic vortex, the calculation pixel interval is selected as 0.1λ 0 ,like Figure 4 as well as Figure 5 As shown, the circular acoustic vortex and the regular polygonal acoustic vortex are successfully constructed by the present invention. In practical applications, the above method can also be used to realize the construction of a three-dimensional acoustic vortex as required.

[0077] Taking the construction of a circular acoustic vortex as an example, the coordinates x of each focus are i ,y i And additional phase C i It can be written as.

[0078]

[0079] Where R is the radius, l is the topological charge, N is the number of foci, and θ n Represents the polar angle of the nth focus. As the number of focuses on the trajectory increases, the intensity distribution becomes continuous, forming an intensity ring, and then each focus is given a periodic spiral phase in turn in the clockwise or counterclockwise direction. As the topological charge l increases, the sound intensity distribution along the circular trajectory becomes sharp, the sound field phase changes periodically, and the acoustic vortex is constructed. The radius of the constructed circular acoustic vortex ring and the topological charge are freely and flexibly adjustable. In order to better present the acoustic vortex, the calculation pixel interval is selected as 0.1λ 0 ,like Figure 4 As shown in (a), the normalized sound intensity distribution of the acoustic vortex xz plane for l = 1, -1, 2, -2, the acoustic vortex is focused at the preset 15λ 0 Department, Figure 4 (b) shows the normalized sound intensity and phase distribution of the acoustic vortex in the xy plane for l=1, -1, 2, -2.

[0080] The same is true for regular polygon vortexes. The coordinates of each point on a plane regular polygon can be written as:

[0081]

[0082] Where q represents the number of sides of the polygon and p represents the curvature of the side. n represents the polar angle of the nth focus, such as Figure 5As shown in (a), when q=3, p=10, the plane equilateral triangle acoustic vortex with l=1, -1, 2, -2 is successfully constructed by the present invention. Figure 5 As shown in (b), when q=4, p=15, a planar regular quadrilateral acoustic vortex of l=1, -1, 2, -2 is successfully constructed by the present invention.

[0083] Similarly, three-dimensional acoustic vortices can also be constructed using the same method. Non-circularly symmetric perfect acoustic vortices can provide more degrees of freedom. At the same time, they can also realize the simultaneous generation and superposition of multiple acoustic vortices, which has important application value in particle manipulation and acoustic communication.

[0084] The meta-lens is prepared using a photosensitive resin material. According to the design method of the present invention, in the air, a Helmholtz resonant cavity or a spatial curled structure can be used as a meta-unit to make a meta-lens; in addition, an underwater super-lens can be prepared, using photosensitive resins (PR resin for short) of different heights to construct a super-lens, and photosensitive resin blocks of different heights are used as phase-controlled meta-units to meet the needle-shaped focusing phase shift requirements. In addition to PR resin, other conventional 3D printing materials can also be used for photosensitive resins.

[0085] When a vertically incident plane wave passes through a PR resin with a height of h, a certain phase shift will occur, which can be expressed as.

[0086]

[0087] Where n represents the refractive index of the longitudinal wave of the sound wave in the photosensitive resin material used to prepare the superlens, is the phase shift caused by a vertically incident plane wave passing through a photosensitive resin (PR resin) with a height of h, λ 0 Represents the wavelength of the incident sound wave.

[0088] For example, the mass density ρ, Young's modulus E and Poisson's ratio σ of PR resin are 1130 kg / m 3 , 2.3GPa and 0.45. According to the formula in Fundamentals of Acoustics, 3rd edition, P337-340.

[0089]

[0090] It can be concluded that the propagation speed of sound waves in PR resin is c L is 2785m / s, where λ and μ are Lamm constants. The background medium is a mass density of ρ w =1000kg / m 3 , the speed of sound is c w = 1500m / s of water. Furthermore, according to n = c w / cL The refractive index of the longitudinal wave in the PR resin can be calculated to be 0.54. According to equation (8), the height distribution required for the metasurface can be obtained.

[0091] The present invention also provides a multi-focus acoustic meta-lens based on complex amplitude coding designed by the above-mentioned design method.

[0092] In summary, the present invention proposes an acoustic meta-lens that realizes multi-point focusing in space, including design principles, numerical calculations and fabrication. The proposed acoustic meta-lens is made of photosensitive resin material, and the number, position, relative intensity and phase of the focus can be adjusted, realizing multi-focus synthetic acoustic needles or acoustic vortices, and is expected to be used in acoustic detection, imaging, medical treatment and other fields.

[0093] The remaining technical features in the above embodiments can be flexibly selected by those skilled in the art according to actual conditions to meet different specific practical needs. However, it is obvious to those skilled in the art that it is not necessary to adopt these specific details to implement the present invention. In other examples, in order to avoid confusing the present invention, the well-known components, structures or parts are not specifically described, which are all within the technical protection scope defined by the technical solution claimed for protection in the claims of the present invention.

[0094] Modifications and changes made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the scope of protection of the claims attached to the present invention. In the above description, a large number of specific details are set forth in order to provide a thorough understanding of the present invention. However, it is obvious to those of ordinary skill in the art that these specific details are not necessary to practice the present invention. In other examples, in order to avoid confusing the present invention, well-known technologies, such as specific construction details, operating conditions and other technical conditions, are not specifically described.

[0095] The principles and implementation methods of the present invention are described in this article using specific examples. The description of the above embodiments is only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A design method for a multi-focus acoustic meta-lens based on complex amplitude coding, characterized in that: A multi-focal acoustic meta-lens is prepared by using a photosensitive resin material, wherein the number, position, relative intensity and phase of the focus of the multi-focal acoustic meta-lens are adjustable, so as to realize a multi-focal synthetic acoustic needle or an acoustic vortex, and specifically comprises the following steps: A modulation phase formula is constructed based on the focal coordinates, focal length and wave number in the background medium, and the phase of the incident sound wave is modulated using the modulation phase formula to achieve single-point focusing; Based on the modulation phase formula, a complex amplitude coding method based on a binary complementary matrix is ​​used to construct a meta-lens transmission coefficient matrix that realizes the superposition of complex amplitudes of multiple incident focused sound fields. Based on the transmission coefficient matrix, the wavefront modulation of the incident sound wave passing through the multi-focus acoustic meta-lens is realized. Based on the modulation of the transmission coefficient matrix, the amplitude and phase of the sound field are simultaneously controlled to form a multi-point focused sound field. By adjusting the position of the focus on a preset straight line or an arbitrary closed trajectory, multiple focuses are synthesized into a sound needle or a sound vortex.

2. The design method of a multi-focal acoustic meta-lens based on complex amplitude coding according to claim 1, characterized in that: The modulation phase formula is constructed based on the focal coordinates, focal length and wave number in the background medium, specifically including: Where P i (x,y) represents the additional phase required to focus the i-th focus, k is the wave number in the background medium, (x i ,y i ) is the focal coordinate, f i is the focal length, C i is a constant, and its specific value does not change the focal coordinates and focal length. (x, y) represents Cartesian coordinates.

3. The design method of a multi-focal acoustic meta-lens based on complex amplitude coding according to claim 2, characterized in that: The complex amplitude coding method based on a binary complementary matrix is ​​used to construct a meta-lens transmission coefficient matrix for realizing complex amplitude superposition of multiple incident focused sound fields, specifically including: The initial transmission coefficient matrix t(x,y) is constructed as a superposition form of two pure phase distributions to obtain two pure phase distributions; Superpixels are used to construct binary complementary matrices M1 and M2, where M1+M2=1; Using the binary complementary matrices M1 and M2, the two pure phase distributions are superimposed to obtain the superimposed phase Determine the amplitude A(x,y) of the transmission coefficient matrix, where A(x,y) can be any positive number; Based on superposition phase and amplitude A(x,y), and obtain the meta-lens transmission coefficient matrix that realizes the superposition of complex amplitudes of multiple incident focused acoustic fields 4. The design method of a multi-focal acoustic meta-lens based on complex amplitude coding according to claim 3, characterized in that: The initial transmission coefficient matrix t(x, y) is constructed as a superposition form of two pure phase distributions to obtain two pure phase distributions, specifically including: Based on the multi-point focusing of the outgoing sound wave in space and the modulation phase of a single point, the initial expression of the transmission coefficient matrix t(x,y) is constructed as follows: Among them, A i represents the amplitude of the i-th incident focused sound field, j represents the imaginary unit, A0(x,y), They represent the amplitude and phase of the incident sound field focused at any multiple points respectively; The transmission coefficient matrix t(x,y) is constructed as the superposition of two pure phase distributions: In the formula, It represents the maximum value of the amplitude of the incident sound field focused at any multiple points; Among them, the two pure phase distribution expressions are as follows:

5. The design method of a multi-focal acoustic meta-lens based on complex amplitude coding according to claim 4, characterized in that: The use of superpixels to construct binary complementary matrices M1 and M2 specifically includes: Take 4 consecutive basic pixel elements as a 2×2 super pixel unit, use the numbers 1, 2, 3, and 4 to distinguish these 4 basic pixel elements, and divide the entire phase plane into multiple super pixel units. The positions of the digital sequences in each super pixel unit are randomly arranged, and the corresponding positions of odd numbers 1 and 3 are filled with "0", and the corresponding positions of even numbers 2 and 4 are filled with "1", so as to obtain the first binary matrix M1; conversely, the corresponding positions of odd numbers 1 and 3 are filled with "1", and the corresponding positions of even numbers 2 and 4 are filled with "0", so as to obtain the second binary matrix M2. The two binary matrices M1 and M2 are complementary matrices.

6. The design method of a multi-focal acoustic meta-lens based on complex amplitude coding according to claim 5, characterized in that: The two pure phase distributions are superimposed using the binary complementary matrices M1 and M2 to obtain the superimposed phase The specific expressions are as follows:

7. The design method of a multi-focal acoustic meta-lens based on complex amplitude coding according to claim 1, characterized in that: The method of synthesizing a sound needle by adjusting the position of the focus on a preset straight line includes: After selecting the needle-shaped focus center, half of the half-length and width of the Airy spot focused at the needle-shaped focus center is used as the starting basis for constructing the intervals between each focus of the needle-shaped focus; A genetic algorithm is used within the focusing range to adjust the position of each focus, and as the focusing range is reduced, a needle beam with an intensity fluctuation of less than 10% is obtained within the focusing range.

8. The design method of a multi-focal acoustic meta-lens based on complex amplitude coding according to claim 1, characterized in that: The method of synthesizing an acoustic vortex with multiple focuses by adjusting the position of the focus on any closed track specifically includes: The acoustic vortex can be synthesized by selecting the focus position on any closed trajectory and assigning a periodic spiral phase to each focus in turn in a clockwise or counterclockwise direction.

9. The design method of a multi-focal acoustic meta-lens based on complex amplitude coding according to claim 1, characterized in that: The method of using a photosensitive resin material to prepare a multi-focal acoustic meta-lens also includes: Assume that when a vertically incident sound wave passes through a photosensitive resin with a height of h, the phase shift produced is expressed as follows: Where n represents the refractive index of the longitudinal wave of the sound wave in the photosensitive resin material, and λ0 represents the wavelength of the incident sound wave; Based on formula (8), the height h distribution of the photosensitive resin material on the surface of the multi-focal acoustic metalens is determined.

10. A multi-focal acoustic meta-lens based on complex amplitude coding designed by the design method according to any one of claims 1 to 9.

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