Multi-focal acoustic metasurface based on complex amplitude coding and design method thereof

Abstract

The application relates to the technical field of acoustic superlenses, and discloses a multi-focal acoustic superlens based on complex amplitude coding and a design method thereof, the design method comprising the following steps: preparing a multi-focal acoustic superlens by using a photosensitive resin material; the number, position, relative intensity and phase of the focal points of the multi-focal acoustic superlens are adjustable; and a multi-focal acoustic needle or acoustic vortex is synthesized, wherein the complex amplitude coding method based on a binary complementary matrix is adopted to construct a superlens transmission coefficient matrix for realizing the complex amplitude superposition of multiple incident focused sound fields, the amplitude and phase of the sound field are simultaneously controlled, a multi-point focused sound field is formed, the positions of the focal points on a preset straight line or an arbitrary closed track are adjusted, and a multi-focal acoustic needle or acoustic vortex is synthesized. The complex amplitude coding method based on superpixels is adopted to construct a phase-type superlens, the position, intensity and phase of multiple focal points in space are flexibly controlled, an acoustic needle and an acoustic vortex are synthesized, and the application value is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of acoustic superlens, in particular to a multi-focus acoustic superlens based on complex amplitude coding and a design method thereof. BACKGROUND

[0002] The convergence of ultrasonic energy, i.e. ultrasonic focusing, has wide applications in many fields such as medical treatment, drug delivery, particle trapping and non-destructive testing. Traditional ultrasonic focusing can be achieved by using a transducer array or a curved lens, which is bulky, costly and time-consuming to produce. Superlenses use subwavelength structural units to precisely control the phase of transmitted ultrasonic waves, and their lightness has attracted extensive attention from researchers. At present, the research on superlenses for single-point ultrasonic focusing is relatively mature, and the single-point ultrasonic focusing superlens can achieve high-precision focusing of acoustic energy, and the related technology has been popularized in practice. The multi-point ultrasonic focusing superlens can achieve acoustic energy focusing at multiple spatial positions simultaneously, which expands the application range of ultrasonic technology. However, the design of a more complex superlens structure is required to achieve acoustic energy focusing at different spatial positions, which increases the difficulty of design and manufacturing. Therefore, the superlens for focusing acoustic energy at different spatial positions needs further research. SUMMARY

[0003] The purpose of the present application is to provide a multi-focus acoustic superlens based on complex amplitude coding and a design method thereof. The complex amplitude coding method based on superpixels is used to construct a phase-type superlens, which realizes flexible regulation of the position, intensity and phase of multiple focal points in space, synthesizes a multi-focus acoustic needle or acoustic vortex, and improves the application value.

[0004] To achieve the above-mentioned purpose, the present application provides the following solutions.

[0005] The present application provides a design method of a multi-focus acoustic superlens based on complex amplitude coding. The multi-focus acoustic superlens is prepared by using a photosensitive resin material. The number, position, relative intensity and phase of the focal points of the multi-focus acoustic superlens are adjustable, which realizes the synthesis of a multi-focus acoustic needle or acoustic vortex. The method specifically comprises the following steps:

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

[0007] Based on the modulation phase formula, a complex amplitude coding method based on binary complementary matrix is used to construct a superlens transmission coefficient matrix for realizing the complex amplitude superposition of multiple incident focusing sound fields. Based on the transmission coefficient matrix, the modulation of the wave front of the incident sound wave when passing through the multi-focus acoustic superlens 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 focal point on the preset straight line or arbitrary closed trajectory, multiple focal points are synthesized into a sound needle or sound vortex.

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

[0010]

[0011] In the formula, P i (x,y) represents the additional phase required for focusing the i-th focal point, k is the wave number in the background medium, (x i ,y i ) is the focal point coordinate, f i is the focal length, C i is a constant, the specific value of which does not change the focal point coordinates and focal length, and (x, y) represents the Cartesian coordinates.

[0012] Further, the complex amplitude encoding method based on binary complementary matrix is used to construct the superlens transmission coefficient matrix that realizes the superposition of multiple incident focused sound field complex amplitudes, specifically including:

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

[0014] The binary complementary matrix M1 and M2 are constructed using superpixels, where M1+M2=1;

[0015] The two pure phase distributions are superimposed using the binary complementary matrix M1 and M2 to obtain the superimposed phase

[0016] The amplitude A(x,y) of the transmission coefficient matrix is determined, which can be any positive number;

[0017] Based on the superimposed phase and the amplitude A(x,y), the superlens transmission coefficient matrix that realizes the superposition of multiple incident focused sound field complex amplitudes is obtained

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

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

[0020]

[0021] wherein, 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 the i-th focal point.

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

[0023]

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

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

[0026]

[0027] Further, the binary complementary matrix M1 and M2 are constructed using superpixels, specifically comprising:

[0028] Four consecutive basic pixel elements are taken as a 2x2 superpixel unit, and the four basic pixel elements are distinguished by numbers 1, 2, 3, and 4. The entire phase plane is divided into multiple superpixel units, and the position of the number sequence in each superpixel unit is randomly arranged. Odd numbers 1 and 3 correspond to positions filled with "0", and even numbers 2 and 4 correspond to positions filled with "1", to obtain the first binary matrix M1. Conversely, odd numbers 1 and 3 correspond to positions filled with "1", and even numbers 2 and 4 correspond to positions filled with "0", to obtain the second binary matrix M2. The two binary matrices M1 and M2 are complementary matrices.

[0029] Further, the two pure phase distributions are superimposed using the binary complementary matrices M1 and M2 to obtain the superimposed phase Specifically represented as follows:

[0030]

[0031] Further, the multiple focal points are synthesized into a sound needle by adjusting the positions of the focal points on a preset straight line, specifically comprising:

[0032] After selecting the needle-shaped focusing center, half the size of the Airy spot half-length and half-width focused at the needle-shaped focusing center position is taken as the starting basis for constructing the spacing between the focal points of the needle-shaped focus.

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

[0034] Further, the multiple focal points are synthesized into a sound vortex by adjusting the positions of the focal points on an arbitrary closed trajectory, specifically comprising:

[0035] The positions of the focal points are selected on an arbitrary closed trajectory, and each focal point is sequentially assigned a periodic spiral phase in a clockwise or counterclockwise direction, so that a sound vortex is synthesized.

[0036] Further, the multi-focal acoustic metasurface prepared from the photosensitive resin material further comprises:

[0037] When a vertically incident sound wave passes through the photosensitive resin with a height of h, the phase shift generated is represented as follows:

[0038]

[0039] wherein 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;

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

[0041] The application also provides a multi-focal acoustic metasurface based on complex amplitude coding designed by the design method.

[0042] According to the embodiments of the application, the multi-focal acoustic metasurface based on complex amplitude coding and the design method thereof have the following technical effects: the multi-focal acoustic metasurface is prepared from the photosensitive resin material, the complex amplitude coding method based on superpixels is used to construct the phase-type metasurface, and the positions, intensities and phases of multiple focal points in space are flexibly regulated; the transmission coefficient matrix-based modulation is used to simultaneously regulate the amplitudes and phases of the sound field, a multi-point focusing sound field is formed, the positions of the focal points on the preset straight line or the arbitrary closed trajectory are adjusted, and multiple focal points are synthesized into a sound needle or a sound vortex.

[0043] The acoustic needle-shaped focusing, i.e., the sound needle, has potential applications in tumor ablation, photoacoustic imaging and other scenarios. The application uses the multi-focal sound needle synthesis method to construct a needle-shaped focusing beam with adjustable length, quantity and pointing direction. The sound vortex is a special wave form with a topological phase. The application uses the multi-focal sound vortex synthesis method to construct a sound vortex with adjustable spatial position, geometric shape and topological charge. The metasurface based on the design method of the application can realize multiple forms of sound needles or sound vortices, and is expected to have important applications in the fields of acoustic detection, imaging, medical treatment and the like. BRIEF DESCRIPTION OF DRAWINGS

[0044] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed in the embodiments. Obviously, the drawings in the following description only constitute some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained from these drawings without creative labor.

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

[0046] Figure 2 The normalized intensity distribution diagram of a multi-focus synthetic sound needle with the same center position and different lengths numerically simulated by the embodiment of the present application, wherein (a) is a normalized intensity distribution diagram of a sound needle with a half length-width (FLHM) of 12.4λ0constructed by 16 focal points, (b) is a normalized intensity distribution diagram of a sound needle with a half length-width (FLHM) of 13.6λ0constructed by 25 focal points, and (c) is a normalized intensity distribution diagram of a sound needle with a half length-width (FLHM) of 18.7λ0constructed by 36 focal points;

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

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

[0049] Figure 5 The normalized sound intensity and phase distribution diagram of a multi-focus synthetic triangularly symmetric and square symmetric sound vortex numerically simulated by the embodiment of the present application, wherein (a) is a normalized sound intensity and phase distribution diagram of a triangularly symmetric sound vortex xy plane with different topological charges, and (b) is a normalized sound intensity and phase distribution diagram of a square symmetric sound vortex xy plane with different topological charges. DETAILED DESCRIPTION

[0050] The technical solutions in the embodiments of the present application will be described clearly and completely below with the accompanying drawings of the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, all the other embodiments obtained by a person of ordinary skill in the art without creative effort belong to the scope of the present application.

[0051] The present application aims to provide a multi-focal acoustic metasurface based on complex amplitude coding and a design method thereof. The acoustic metasurface is made of photosensitive resin material, and the number, position, intensity and phase of the focal points are adjustable. The metasurface based on the design method can realize various forms of acoustic needle or acoustic vortex, and is expected to have important applications in the fields of acoustic detection, imaging, medical treatment, etc.

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

[0053] The design method of the multi-focal acoustic metasurface based on complex amplitude coding provided by the present application uses photosensitive resin material to prepare a multi-focal acoustic metasurface, and the number, position, relative intensity and phase of the focal points of the multi-focal acoustic metasurface are adjustable, realizing multi-focal synthetic acoustic needle or acoustic vortex, and specifically comprising the following steps:

[0054] Step 1, constructing a modulation phase formula based on the focal point coordinates, focal length and wave number in the background medium, modulating the phase of the incident sound wave by the modulation phase formula to realize single-point focusing; the modulation phase formula constructed in this step only modulates the phase of the incident sound wave, thereby realizing single-point focusing;

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

[0056]

[0057] In the formula, P i (x,y) represents the additional phase required for focusing the i-th focal point, k is the wave number in the background medium, (x i ,y i ) is the focal point coordinate, the multi-focal acoustic metasurface is located at the z=0 plane, and thus f i is the focal length, C i is a constant, the specific value of which does not change the focal point coordinate and the focal length, and is generally zero, and (x, y) represents the Cartesian coordinate.

[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 is achieved when it passes through the multi-focal acoustic metalens;

[0059] Among them, 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 A0(x,y) and phase of an arbitrary multi-point focused incident sound field. Since it is difficult for a 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] Where, Indicates 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] Where, and like Figure 1 As shown in (b). Obviously, in order to obtain the final phase The two phase maps need to be superimposed again. To this end, the present invention designs binary complementary matrices M1 and M2 to achieve the above functions. The binary complementary matrices are constructed using superpixels. Four consecutive basic pixel elements are used as a 2×2 superpixel unit. These 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. The positions corresponding to odd numbers 1 and 3 are filled with "0", and the positions corresponding to even numbers 2 and 4 are filled with "1", thereby obtaining the first binary matrix M1. Conversely, the positions corresponding to odd numbers 1 and 3 are filled with "1", and the positions corresponding to even numbers 2 and 4 are filled with "0", thereby obtaining the second binary matrix M2. 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 As shown in (c) in the figure. The present invention forms a multi-point focused acoustic field using the above method and theoretically verifies this using angular spectrum diffraction calculations. The calculations are based on an incident plane acoustic field frequency of 500 kHz, a sound velocity of 1500 m / s, an acoustic field radius of 16λ0, where λ0 represents the wavelength of the incident acoustic wave, and a pixel spacing of 1 mm.

[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 a needle-shaped focus as an example, after selecting the needle-shaped focus center, half the half-length width of the Airy spot focused at that location is used as the starting basis for the focal spacing of the needle-shaped focus. A genetic algorithm is used to adjust the position of each focal point within the focusing range. As the focusing range is reduced, a needle-shaped beam with intensity fluctuation within the focusing range of less than 10% is obtained.

[0074] like Figure 2 As shown, by adjusting the position of the focus on the preset straight line, the present invention successfully constructs 16-focus, 25-focus, and 36-focus needle beams focused at 30λ0 of the meta-lens. At the same time, Figure 3 As shown, thanks to the flexibility of the binary complementary matrix for complex amplitude coding, oblique and symmetrical needle-shaped focusing are also successfully constructed by the present invention.

[0075] Numerical simulation shows that in the case of diffraction-limited, the half-width of needle focusing is affected by the central position and the geometric length of the superstructure surface, the closer the central position, the larger the superstructure surface, the smaller the half-width of the focusing center, and the focusing length can be increased with the increase of the number of focal points.

[0076] The above method also shows great application value in generating acoustic vortices. The present application selects the position of the focal point on an arbitrary closed trajectory and assigns each focal point with a periodic spiral phase in the clockwise or counterclockwise direction. To better present the acoustic vortex, the pixel interval is selected as 0.1λ0, as shown in Figure 4 and Figure 5 It is shown that the circular acoustic vortex and the regular polygon acoustic vortex are successfully constructed by the present application. In practical applications, three-dimensional acoustic vortex construction can also be realized according to the needs by using the above method.

[0077] Taking the construction of a circular acoustic vortex as an example, the coordinates x i , y i and the additional phase C i of each focal point can be written as.

[0078]

[0079] where R represents the radius, l represents the topological charge, N represents the number of focal points, and θ n represents the polar angle of the nth focal point. As the number of focal points on the trajectory increases, the intensity distribution becomes continuous, forming an intensity ring. Then each focal point is assigned with a periodic spiral phase in the clockwise or counterclockwise direction. As the topological charge l increases, the acoustic intensity distribution along the circular trajectory becomes sharp, the acoustic field phase changes periodically, and the acoustic vortex is constructed. The constructed circular acoustic vortex ring radius and topological charge are freely adjustable. To better present the acoustic vortex, the pixel interval is selected as 0.1λ0, as shown in Figure 4 Fig. (a) shows the normalized acoustic intensity distribution of the acoustic vortex in the xz plane for l=1, -1, 2, -2, and the acoustic vortex is focused at the preset 15λ0, Figure 4 Fig. (b) shows the normalized acoustic 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 vortices. The coordinates of each point on a planar regular polygon can be written as:

[0081]

[0082] where q represents the number of sides of the polygon, and p represents the bending degree of the side. θ n represents the polar angle of the nth focal point, as shown in Figure 5 Fig. (a) shows that when q=3, p=10, the planar regular triangle acoustic vortex for l=1, -1, 2, -2 is successfully constructed by the present application, as shown inFigure 5 In the middle (b), when q=4, p=15, l=1, -1, 2, -2, the planar square acoustic vortex is successfully constructed by the present application.

[0083] Similarly, three-dimensional acoustic vortices can also be constructed by the same method. Non-circular symmetric perfect acoustic vortices can provide more degrees of freedom, and at the same time, multiple acoustic vortices can be generated and superimposed simultaneously, which has important application value in particle manipulation and acoustic communication.

[0084] The superlens is prepared by using a photosensitive resin material. According to the design method of the present application, a Helmholtz resonant cavity or a spatially curled structure can be used as a superlens unit to manufacture the superlens in air. In addition, an underwater superlens can also be prepared. The superlens is constructed by using photosensitive resins with different heights. The blocks of photosensitive resins with different heights are used as phase control superlens units to meet the requirements of needle focusing phase shift. In addition to PR resin, other conventional 3D printing materials can also be used for photosensitive resin.

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

[0086]

[0087] wherein n represents the refractive index of the longitudinal wave of the photosensitive resin material used for preparing the superlens, is the phase shift generated when a vertical incident plane wave passes through a PR resin with a height of h, and λ0 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 the PR resin are 1130 kg / m 3 , 2.3 GPa and 0.45, respectively. According to the formula in the third edition of "Fundamentals of Acoustics" P337-340.

[0089]

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

[0091] The application also provides a multi-focus acoustic metasurface based on complex amplitude coding designed by the design method.

[0092] In conclusion, the application provides an acoustic metasurface capable of realizing spatial multi-point focusing, including design principles, numerical calculation and production. The acoustic metasurface is made of photosensitive resin material, and the number, position, relative intensity and phase of the focal points are adjustable, realizing multi-focus synthetic acoustic needle or acoustic vortex, which is expected to obtain important applications in the fields of acoustic detection, imaging, medical treatment and the like.

[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 actual needs. However, it is obvious to those skilled in the art that the specific details do not have to be used to implement the application. In other examples, in order to avoid confusion of the application, well-known components, structures or parts are not specifically described, and are within the technical solution limited by the claims of the application.

[0094] Changes and variations made by those skilled in the art without departing from the spirit and scope of the application shall be within the scope of protection of the appended claims of the application. In the above description, a large number of specific details are set forth in order to provide a thorough understanding of the application. However, it is obvious to those skilled in the art that the specific details do not have to be used to implement the application. In other examples, in order to avoid confusion of the application, well-known technologies are not specifically described, such as specific construction details, operating conditions and other technical conditions.

[0095] The principles and implementation modes of the application are described by applying specific examples, and the above description of the examples is only used to help understand the method of the application and its core idea; at the same time, for those skilled in the art, according to the idea of the application, there will be changes in specific implementation modes and application ranges. In conclusion, the content of the specification should not be understood as a limitation of the application.

Claims

1. A design method for a multi-focal acoustic metalens based on complex amplitude coding, characterized in that: A multi-focal acoustic meta-lens is prepared using a photosensitive resin material. The number, position, relative intensity, and phase of the focal points of the multi-focal acoustic meta-lens are adjustable to achieve multi-focal synthetic acoustic needles or acoustic vortices. The method specifically includes the following steps: A modulation phase formula is constructed based on the focal coordinates, focal length, and wave number in the background medium. 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 the 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 achieved. 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 whose specific value does not change the focal coordinates and focal length. (x, y) represents the 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 encoding 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, specifically including: The initial transmission coefficient matrix t(x,y) is constructed as a superposition of two pure phase distributions to obtain two pure phase distributions; Use superpixels 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 sound fields 4. The design method of a multi-focal acoustic metalens 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 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), Respectively represent the amplitude and phase of the incident sound field focused at any multiple points; The transmission coefficient matrix t(x,y) is constructed as the superposition of two pure phase distributions: Where, Indicates 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 method of constructing binary complementary matrices M1 and M2 using superpixels specifically includes: Four consecutive basic pixel elements are used as a 2×2 superpixel unit, and the numbers 1, 2, 3, and 4 are used to distinguish these four basic pixel elements. 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 positions corresponding to odd numbers 1 and 3 are filled with "0", and the positions corresponding to even numbers 2 and 4 are filled with "1", thereby obtaining the first binary matrix M1; conversely, the positions corresponding to odd numbers 1 and 3 are filled with "1", and the positions corresponding to even numbers 2 and 4 are filled with "0", thereby obtaining 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 metalens 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 specifically includes: After selecting the needle-shaped focus center, half 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 focal intervals 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% within the focusing range is obtained.

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 by adjusting the position of the focus on any closed trajectory specifically includes: 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, an acoustic vortex can be synthesized.

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 preparing a multi-focal acoustic meta-lens using a photosensitive resin material further comprises: 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 metalens based on complex amplitude coding designed by the design method according to any one of claims 1 to 9.

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