An acoustic focusing coding metasurface and its reconfiguration method

Abstract

The present invention relates to an acoustically focused coding metasurface and its reconfiguration method. The metasurface comprises several regions linearly arranged along a first direction, each region containing one or more coding units. The coding units are divided into first and second coding units, with a reflection phase difference of π. The regions containing different types of coding units are arranged alternately. The coding units comprise slotted circular tubes arranged along the first direction, with their lengths extending perpendicular to the plane formed by the first and second directions. Compared with existing technologies, this invention offers advantages such as adjustable focus position and reduced wear.

Description

Technical Field

[0001] The present invention relates to the field of acoustics, and in particular to an acoustic focusing coding metasurface and a reconfigurable method thereof. Background Art

[0002] In recent years, acoustic metasurfaces with unique sound field control capabilities have attracted widespread attention and have become a research hotspot at home and abroad. Many unique acoustic functions can be achieved based on acoustic metasurfaces, such as acoustic stealth, sound isolation, perfect sound absorption, one-way sound propagation, and sound focusing. Among them, the acoustic focusing effect can concentrate sound energy in a certain local space and form a high-energy area. As an important acoustic technology, it has been widely used in medical ultrasound diagnosis and treatment, industrial ultrasound non-destructive testing, and detection. In order to achieve sound focusing at a specific location, the acoustic focusing metasurface must have a transmission (or reflection) phase and amplitude distributed according to a specific law. Therefore, each subunit used to construct the acoustic focusing metasurface needs to be designed with a specific geometric configuration, which also makes the structure of the acoustic focusing metasurface more complex. When the sound wave propagates in a narrow sound channel, the higher viscous sound loss will lead to lower acoustic focusing efficiency. In recent years, the emergence of digitally coded metasurfaces has, to a certain extent, solved the structural complexity problems of traditional acoustic metasurfaces. By performing specific coding arrangements on the two coding units (coding unit 0 and coding unit 1), sound field control functions such as sound beam splitting, unidirectional sound transmission, and sound focusing can be achieved.

[0003] Furthermore, once the structure of a conventional acoustically focused coding metasurface is fabricated, its focal position cannot be adjusted. Changing the focal position requires removing and re-arranging the previously arranged coding units before installing them, which inevitably causes unnecessary wear and tear on the coding units and is time-consuming and labor-intensive. Summary of the Invention

[0004] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and to provide an acoustic focusing coding metasurface with adjustable focal position and low wear and tear and a reconfigurable method thereof.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] An acoustic focusing coding metasurface includes a plurality of regions linearly arranged along a first direction, each region includes one or more coding units, and the coding units are divided into a first coding unit and a second coding unit. The reflection phase difference between the two coding units is π.

[0007] The coding unit in the same region is either the first coding unit 0 or the second coding unit 1, and regions containing different types of coding units are arranged alternately;

[0008] The encoding unit includes a slotted circular tube, which is arranged along a first direction. The length extension direction of the slotted circular tube is perpendicular to the plane formed by the first direction and the second direction. The rotation angles of the slotted circular tubes of the two encoding units are different. The absolute value of the difference between the reflection phase of the slotted circular tube of the first encoding unit and the reflection phase of the slotted circular tube of the second encoding unit is the reflection phase difference of the two encoding units, and the reflection phase is determined based on the rotation angle.

[0009] For two adjacent regions, the centers of the regions meet the following conditions:

[0010]

[0011] Among them, x ±i Indicates area A ±i The distance between the center of the metasurface and the center of the metasurface, x ±(i+1) Indicates area A ±(i+1) The distance between the center of the region and the center of the metasurface, i represents the sequence number of the region, λ is the wavelength of the sound wave when it propagates in the air, F x and F y are the horizontal and vertical coordinates of the focus.

[0012] Furthermore, the acoustic path difference between two adjacent regions of the metasurface and the focus satisfies:

[0013] l ±(i+1) -l ±i =0.5λ(i=0, 1, 2...)

[0014] Among them, l ±i Indicates area A ±i is the distance between the center and the focus, i represents the sequence number of the region, and λ is the wavelength of the sound wave when it propagates in the air.

[0015] Furthermore, area A ±i The distance between the center and the focus is expressed as:

[0016]

[0017] Among them, x ±i Indicates area A ±i The distance between the center of and the center of the metasurface.

[0018] Furthermore, the encoding unit further includes a hard acoustic boundary layer, and a first distance is separated between the axis of the slotted circular tube and the hard acoustic boundary layer.

[0019] Furthermore, the hard acoustic boundary layer is provided on the lower side of the encoding unit.

[0020] Furthermore, the first distance is 11 mm.

[0021] Furthermore, the first direction and the second direction are perpendicular to each other, the first direction is the x-axis direction, and the second direction is the y-axis direction.

[0022] Furthermore, the coding unit at the center of the metasurface is numbered 1, and the center line of the coding unit coincides with the y-axis direction.

[0023] Furthermore, the slit circular tube is an axisymmetric structure.

[0024] Another aspect of the present invention provides a reconfigurable method based on the above-mentioned acoustic focusing coding metasurface, which realizes metasurface reconstruction under axial focusing and off-axis focusing conditions. The method comprises the following steps:

[0025] The focal length is adjusted to the required set value. When the focal length changes, the center coordinates of each area in the metasurface are calculated in sequence based on the conditions satisfied by the center of the area. The arrangement sequence of the coding units is determined based on the center coordinates of each area. The number and type of coding units in each area are determined according to the arrangement sequence. The corresponding slotted circular tube is rotated, and the rotation angle of the slotted circular tube is changed to change the reflection phase of the slotted circular tube, thereby realizing the change of the type of coding unit and completing the reconstruction of the metasurface.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] (1) The present invention proposes a reconfigurable acoustic focusing coding metasurface based on a slotted circular tube and a metasurface reconfiguration method. The arrangement sequence of the coding units can be changed by rotating the slotted circular tube, which can not only move the focal position on the central axis of the metasurface, but also move the focal position in the off-axis direction, thereby realizing effective adjustment of the focusing focal position.

[0028] (2) The metasurface and metasurface reconfiguration method of the present invention change the arrangement sequence of the coding units by rotating the slit circular tube. Compared with the existing focus adjustment method of removing and rearranging the combination and then installing it, the wear on the coding units is reduced and the service life of the coding units is extended.

[0029] (3) The metasurface and metasurface reconfiguration method proposed in the present invention realize the effective adjustment of the focus position with low wear and high efficiency, and have broad application prospects in the fields of acoustic coherence tomography, acoustic holographic microscopy, ultrasonic cranial imaging, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a structural schematic diagram of the present invention;

[0031] Figure 2 Schematic diagram of the coding unit structure of the reconfigurable acoustic focusing coding metasurface;

[0032] Figure 3 This is the phase relationship diagram of the slotted circular tube of the present invention, where: Figure 3 (a) shows the relationship between the reflection phase and reflectivity when a plane sound wave with a frequency of 4000 Hz is incident on the slotted circular tube and the rotation angle α (0° to 180°) of the slotted circular tube. Figure 3 (b) is Figure 3 The reflected sound pressure field distribution diagram of point A and point B in (a);

[0033] Figure 4 is the scattered sound intensity distribution diagram of the present invention, wherein, Figure 4 (a), Figure 4 (b), Figure 4 (c) are the focal length F y The scattered sound intensity distribution diagram at 35cm, 40cm, and 45cm; Figure 4 (d), Figure 4 (e), Figure 4 (f) are the focal length F y Normalized sound intensity distribution curve of the x-direction section through the focal point when the distance is 35cm, 40cm, and 45cm;

[0034] Figure 5 The half-maximum width (FWHM) varies with the focal length F y Change relationship diagram;

[0035] Figure 6 is the scattered sound intensity distribution diagram when Gaussian sound beams of different frequencies are incident on the reconfigurable acoustic focusing coding metasurface, where: Figure 6 The frequency of (a) is 3400 Hz, Figure 6 The frequency of (b) is 3600Hz, Figure 6 The frequency of (c) is 3800Hz, Figure 6 The frequency of (d) is 4000Hz, Figure 6 The frequency of (e) is 4300Hz, Figure 6 The frequency of (f) is 4600Hz, Figure 6 The frequency of (g) is 4900Hz, Figure 6 The frequency of (h) is 5200Hz;

[0036] Figure 7 is the focal length F y A graph showing the relationship between the sound wave frequency and the sound wave frequency;

[0037] Figure 8 A Gaussian sound beam with a frequency of 4000 Hz is incident on the focal length F y =40cm reconfigurable acoustic focusing coding metasurface, the scattered sound intensity distribution diagram and the normalized sound intensity distribution curve of the x-direction section through the focus, where Figure 8 (a), Figure 8 (b), Figure 8 (c) are F x =2cm,F x =4cm,F x =Distribution of scattered sound intensity when 6cm; Figure 8 (d) Figure 8 (e), Figure 8 The (f) are F x =2cm,F x =4cm,F x =6cm, the normalized sound intensity distribution curve of the x-direction section through the focus. DETAILED DESCRIPTION

[0038] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0039] To achieve efficient, tunable acoustic focusing, it's essential to design a reconfigurable acoustic focusing metasurface with a simple structure. A reconfigurable coded metasurface containing only two coding units, 0 and 1, offers a novel approach for achieving this. Based on this approach, the present invention proposes an acoustic focusing coded metasurface and its reconfiguration method to address existing problems with acoustic focusing metasurfaces, such as unadjustable focal position, complex structure, and high acoustic loss.

[0040] The metasurface of the present invention can completely reflect vertically incident sound waves and focus the sound energy at the focal point. By rotating the slotted circular tube to change the arrangement sequence of the coding units, the focal position can be effectively adjusted, ultimately achieving an adjustable sound focusing effect. The structural diagram of the metasurface used in the present invention is shown in FIG. Figure 1 shown.

[0041] The acoustic focusing coding metasurface (hereinafter referred to as metasurface) comprises a plurality of regions A arranged linearly along a first direction (i.e., the x-axis direction). i , which only contains two coding units composed of slotted circular tubes: coding unit 0 and coding unit 1. The rotation angles of the slotted circular tubes in the two coding units are different, and their reflection phase difference is π and both have high reflectivity. i The coding units have the same type, and coding units 0 and 1 are arranged alternately in adjacent areas. The coding units include a plurality of slotted circular tubes linearly and periodically arranged along a first direction (i.e., the x-axis direction), and the length extension direction of the slotted circular tubes is perpendicular to the plane formed by the first direction (i.e., the x-axis direction) and the second direction (i.e., the y-axis direction) (i.e., the xoy plane).

[0042] The focal length adjustment range of the metasurface of the present invention is primarily affected by the number of coding units. As the number of coding units increases, the types of arrangement sequences of the metasurface coding units increase accordingly, and thus the adjustment range of the focal position also increases accordingly. Conversely, as the number of coding units decreases, the adjustment range of the focal position gradually decreases, and eventually, the acoustic focusing effect may no longer be achieved.

[0043] According to Huygens Fresnel principle, when sound waves are incident on the coding unit, a sound scattering effect will occur. Each coding unit can be regarded as a secondary sound source, and the total scattered sound field is the superposition of the sound fields generated by these secondary sound sources. In order to achieve the sound focusing effect, the scattered sound waves need to be focused at the set focus (F x ,F y ), so the reflection phase difference between two adjacent areas in the reconfigurable acoustic focusing coding metasurface is π, that is, the acoustic path difference between the two adjacent areas and the focus must satisfy:

[0044] l ±(i+1) -l ±i =0.5λ(i=0,1,2...) (1)

[0045] Among them, l ±i Indicates area A ±i is the distance between the center and the focus, i represents the sequence number of the region, and λ is the wavelength of the sound wave when it propagates in the air.

[0046] Area A ±i The distance between the center and the focus is expressed as:

[0047]

[0048] Among them, x ±i Indicates area A ±i The distance between the center of and the center of the metasurface.

[0049] Therefore, from the above analysis, it can be concluded that for two adjacent regions, the centers of the regions meet the following conditions:

[0050]

[0051] Among them, x ±i Indicates area A ±i The distance between the center of the metasurface and the center of the metasurface, x ±(i+1) Indicates area A ±(i+1) The distance between the center of the region and the center of the metasurface, i represents the sequence number of the region, λ is the wavelength of the sound wave when it propagates in the air, F x and F y are the horizontal and vertical coordinates of the focus.

[0052] The encoding unit of the metasurface consists of a rotatable slotted circular tube and a hard acoustic boundary layer, which together form a reflective metasurface encoding unit. The inner diameter of the slotted circular tube is R i , outer diameter is R o The slot width is w, the distance between the slotted circular tube axis and the hard acoustic boundary layer is h, and the counterclockwise angle of the slot around the tube axis is α. Different angles of rotation of the slotted circular tube around the tube axis produce different phase modulation effects on the scattered sound field. Unit structures with a reflection phase difference of π and high reflectivity are selected as the first encoding unit 0 and the second encoding unit 1, respectively.

[0053] The present invention also proposes a metasurface reconfiguration method based on the above metasurface, which realizes metasurface reconstruction under axial focusing and off-axis focusing conditions. The method comprises the following steps:

[0054] The focal length is adjusted to the required set value. When the focal length changes, the center coordinates of each area in the metasurface are calculated in sequence based on the conditions satisfied by the center of the area. The arrangement sequence of the coding units is determined based on the center coordinates of each area. The number and type of coding units in each area are determined according to the arrangement sequence. The corresponding slotted circular tube is rotated, and the rotation angle of the slotted circular tube is changed to change the reflection phase of the slotted circular tube, thereby realizing the change of the type of coding unit and completing the reconstruction of the metasurface.

[0055] For axial focusing, when the focus is located at the central axis of the metasurface x = 0, that is, F x =0, then the focal coordinates are (0, F y ), then area A -i With area A i Symmetric about the y-axis, so only the area A to the right of the y-axis needs to be i Conduct research and analysis. y When changes, according to Determine the area A of the reconfigurable acoustic focusing coding metasurface i The center coordinate x i , further according to the coordinates x of the center of the region i Determine each area A i The coding unit number contained in .

[0056] For off-axis focusing, when the focus deviates from the central axis of the metasurface x = 0, that is, F x ≠0, then the focal coordinates are (F x , F y ), then area A -i With area A i It is no longer symmetric about the y-axis, so it is necessary to ±i Conduct research and analysis. xWhen the coordinates of the centers of two adjacent regions change, the A of each region in the reconfigurable acoustic focusing coding metasurface is determined according to the formula between the coordinates of the centers of two adjacent regions. ±i The center coordinate x ±i , further according to the coordinates x of the center of the region ±i Determine each area A ±i The coding unit number contained in .

[0057] In adjacent areas of the metasurface, coding units 0 and 1 are alternately arranged, so that the reflection phase difference between two adjacent areas is π and the acoustic path difference is 0.5λ, finally completing the construction of the reconfigurable acoustic focusing coding metasurface.

[0058] like Figure 1 As shown, the metasurface of the present invention comprises a plurality of Figure 1 Area A (linear arrangement in the x-axis direction) i , that is, A -3 ,A -2 ,A -1 , A0, A1, A2, A3…. The metasurface of the present invention only contains two coding units composed of slotted circular tubes: coding unit 0 and coding unit 1. The rotation angles of the slotted circular tubes in the two coding units are different, and the reflection phase difference is π and both have high reflectivity. i The coding units have the same type, and the coding units 0 and 1 are arranged alternately in adjacent areas. In some embodiments, a reconfigurable acoustic focusing coding metasurface consisting of 49 coding units (slit circular tubes) is constructed. The coding unit at the center of the metasurface is numbered 1, and the center line of the coding unit coincides with the y-axis. Then, 24 coding units numbered 2 to 25 are placed in sequence along the positive direction of the x-axis. Similarly, 24 coding units numbered -2 ​​to -25 are placed in sequence along the negative direction of the x-axis, and finally a reconfigurable acoustic focusing coding metasurface is constructed. The coding unit includes a number of coding units along the first direction (i.e. Figure 1 The slit circular tubes are arranged linearly and periodically in the x-axis direction, and the length extension direction of the slit circular tubes is perpendicular to the first direction (i.e. Figure 1 x-axis direction) and the second direction (i.e. Figure 1 The plane formed by the y-axis direction) (i.e. Figure 1 mid-xoy plane).

[0059] The schematic diagram of the unit structure used to construct the reconfigurable acoustic focusing coding metasurface in the present invention is as follows: Figure 2 The coding unit includes a slotted circular tube and a hard acoustic boundary layer. The inner diameter of the slotted circular tube is R i =9mm, outer diameter is R o=10mm, the gap width is w=10mm, the distance between the axis of the slotted circular tube and the hard acoustic boundary layer is h=11mm, and the angle of the slot rotating counterclockwise around the axis of the circular tube is α. The finite element numerical simulation method is used to study the sound scattering characteristics of the unit structure. Figure 2 The left and right sides of the unit structure shown are set to periodic boundary conditions; the hard acoustic boundary layer on the lower side is set to a hard acoustic boundary condition; due to the large impedance difference between solid materials and air, the boundaries of the slit circular tube are all set to hard acoustic boundary conditions.

[0060] Taking the operating frequency of 4000Hz as an example, when the plane sound wave is incident vertically from the upper side to the unit structure, Figure 3 (a) shows the relationship between the reflection phase (dotted line) and reflectivity (dashed line) of the slotted circular tube and the rotation angle α of the slotted circular tube. Since the slotted circular tube is an axisymmetric structure, it is only necessary to analyze the situation when it rotates half a circle, that is, the rotation angle α increases from 0° to 180°. As the rotation angle of the slotted circular tube increases, the reflectivity always maintains a high value close to 1, while the minimum value of the reflection phase is -0.21π and the maximum value is π. According to the phase requirements of the coding metasurface for the coding unit, a 32° ( Figure 3 At point A in (a), the reflection phase is -0.2π) and 100° ( Figure 3 At point B in (a), the reflection phase is 0.8π. The slotted circular tubes with a rotation angle of 0.8π serve as the coding unit 0 and coding unit 1 of the metasurface in the present invention, and the reflection phase difference between the two coding units is π. Figure 3 (b) is Figure 3 The reflected sound pressure field distribution diagram at points A and B in (a) shows that after the plane sound wave is incident on the two coding units, there is a phase difference of π in the reflected sound wave, thus satisfying the selection conditions of the coding unit in the coding metasurface.

[0061] The following is an experimental analysis of focus adjustment under axial focusing and off-axis focusing conditions:

[0062] 1. Adjustable axial focus

[0063] First, we analyze the situation when the focus is located at the central axis of the metasurface x = 0, that is, F x =0, then the focal coordinates are (0, F y ), then area A -i With area A i Symmetric about the y-axis, so only the area A to the right of the y-axis needs to be i Conduct research and analysis. Formula (2) can be written as:

[0064]

[0065] When the focal length F yWhen the change occurs, the area A of each region in the reconfigurable acoustic focusing coding metasurface is determined according to formula (3): i The center coordinate x i , further according to the coordinates x of the center of the region i Determine each area A i The coding unit number contained in . The coding unit 0 and coding unit 1 are alternately arranged in adjacent areas of the reconfigurable acoustic focusing coding metasurface, so that the reflection phase difference between the two adjacent areas is π and the sound path difference is 0.5λ. When the operating frequency is 4000Hz and the focal length F y When the range is 30cm to 50cm and the step length increases by 1cm, based on the above principle, Table 1 lists all the coding unit arrangement sequences of the reconfigurable acoustic focusing coding metasurface in the above case, and the width of each coding unit is 2cm. y =40cm as an example, the arrangement of the coding units of the reconfigurable acoustic focusing coding metasurface is described in detail. The coding unit 0 is placed in area A0 at the center of the metasurface, and the right half of area A0 contains 8 coding units 0 with serial numbers 1 to 8. Similarly, area A1 contains 4 coding units 1 with serial numbers 9 to 12, area A2 contains 5 coding units 0 with serial numbers 13 to 17, area A3 contains 2 coding units 1 with serial numbers 18 to 19, area A4 contains 4 coding units 0 with serial numbers 20 to 23, and area A5 contains 2 coding units 1 with serial numbers 24 to 25. Since the metasurface of the present invention is an axisymmetric structure, the area A arranged along the negative direction of the x-axis is -1 ,A -2 ,A -3 ,A -4 ,A -5 It is completely symmetrical with the areas A1, A2, A3, A4, and A5 arranged along the positive direction of the x-axis. According to the structural symmetry, the focal length F can be obtained. y Similarly, when the focal length is other values, under axial focusing, the arrangement sequence of the encoding units of the metasurface of the present invention is shown in Table 1.

[0066] Table 1 Arrangement sequence of adjustable axial focus encoding units

[0067]

[0068]

[0069] The acoustic focusing coding metasurface is reconstructed based on the coding unit arrangement sequence listed in Table 1, and the acoustic focusing effect of the metasurface in the present invention is analyzed using the finite element numerical simulation method. When a Gaussian acoustic beam with a frequency of 4000 Hz is incident on the reconfigurable acoustic focusing coding metasurface, Figure 4 (a), Figure 4(b), Figure 4 (c) are the focal length F y The scattered sound intensity distribution diagram at 35cm, 40cm, and 45cm. It can be seen from the figure that the metasurface can focus the sound energy on the central axis, achieving a good sound focusing effect. The focus coordinates in the simulation are (0, 35cm), (0, 39.5cm), and (0, 44.2cm), respectively, indicating that the focus position of the metasurface (i.e., the focal length value) is basically consistent with the design expected value. In order to better evaluate the sound focusing effect, at the focus y = F y Make a section parallel to the x-axis at the intersection and calculate the normalized sound intensity on the section. Figure 4 (d), Figure 4 (e), Figure 4 (f) are the focal length F y When the focal length is 35cm, 40cm, and 45cm, the normalized sound intensity distribution curve of the x-direction section through the focal point shows that the sound intensity value is the largest at x = 0, and the sound intensity is symmetrically distributed about the axis x = 0. The corresponding half-maximum width (FWHM) is calculated to be 39.65mm, 43.74mm, and 47.04mm, respectively, indicating that the half-maximum width gradually increases with the increase of the focal length. The focal length F of the reconfigurable acoustic focusing coding metasurface in the present invention is y When the distance increases from 30cm to 50cm, all acoustic focusing coding metasurfaces are constructed based on the coding unit arrangement sequence in Table 1 and finite element numerical simulation is performed. Figure 5 is the half-maximum width (FWHM) which changes with the focal length F y It can be seen from the graph that the half-peak width increases gradually with the increase of focal length when the focal length is less than 48cm, but after the focal length is greater than 48cm, the half-peak width gradually decreases. This is mainly because the number of encoding units will have a certain impact on the sound focusing effect and the focus adjustment range.

[0070] The focal length F in the present invention is analyzed below y =40cm metasurface focusing performance at different frequencies to analyze its bandwidth characteristics, Figure 6 This is the scattered sound intensity distribution diagram when Gaussian sound beams with frequencies of 3400Hz, 3600Hz, 3800Hz, 4000Hz, 4300Hz, 4600Hz, 4900Hz, and 5200Hz are incident on the reconfigurable acoustic focusing coding metasurface. It can be seen that the metasurface can achieve acoustic focusing on the central axis at all frequencies. Figure 7 is the focal length F y The relationship between the acoustic frequency and the focus position is shown to gradually shift upward as the operating frequency increases, that is, the focal length gradually increases. The above analysis shows that the metasurface of the present invention can achieve axial acoustic focusing over a wide frequency range, overcoming the narrow operating frequency band problem commonly encountered in traditional focusing metasurfaces.

[0071] 2. Adjustable off-axis focus

[0072] Secondly, the off-axis acoustic focusing situation when the focal point deviates from the central axis of the metasurface x = 0 is analyzed, that is, F x ≠0, then the focal coordinates are (F x , F y ), then area A -i With area A i It is no longer symmetric about the y-axis, so it is necessary to ±i Conduct research and analysis. Still with focal length F y = 40cm metasurface as an example, the focal coordinates are (F x ,40cm). When F x When the coordinates of the centers of two adjacent regions change, the A of each region in the reconfigurable acoustic focusing coding metasurface is determined according to the formula between the coordinates of the centers of two adjacent regions. ±i The center coordinate x ±i , further according to the coordinates x of the center of the region ±i Determine each area A ±i Table 2 lists the horizontal coordinates of the focus F x 2cm, 4cm, 6cm and F y When =40cm, the coding unit arrangement sequence of the acoustic focusing coding metasurface can be reconstructed. Figure 8 When a Gaussian beam with a frequency of 4000 Hz is incident on the reconfigurable acoustic focusing coding metasurface, the scattered sound intensity distribution diagram and the normalized sound intensity distribution curve of the x-direction section through the focus are shown. x The scattered sound intensity distribution diagrams when the distance is 2cm, 4cm, and 6cm are as follows: Figure 8 (a), Figure 8 (b) Figure 8 As shown in (c), it can be seen that the focus gradually deviates from the central axis in the horizontal direction. The focus coordinates in the simulation results are (2.3cm, 40cm), (4.6cm, 40cm), and (6.5cm, 40cm), respectively, indicating that the simulation results are in good agreement with the theoretical predictions. The corresponding normalized sound intensity distribution curves of the x-direction section through the focus are as follows: Figure 8 (d), Figure 8 (e), Figure 8 As shown in (f), we can see that at the horizontal coordinate F x The sound intensity is maximum at .

[0073] Table 2 Arrangement sequence of adjustable off-axis focusing coding units

[0074]

[0075]

[0076] By analyzing the two examples of "adjustable axial focusing" and "adjustable off-axis focusing" above, it is shown that simply by rotating the slotted circular tube in each individual encoding unit by a certain angle, it is possible to switch between encoding unit 0 and encoding unit 1, thereby forming different encoding unit arrangement sequences. This allows the focal position to be moved not only on the central axis of the metasurface, but also in the off-axis direction, achieving effective adjustment of the focal position. This solves the problems of existing acoustic focusing metasurfaces, such as unadjustable focal position, complex structure, and high acoustic loss, and can meet the significant high-performance requirements of the new generation of acoustic focusing devices. It has broad application prospects in fields such as acoustic coherence tomography, acoustic holographic microscopy, and ultrasonic brain imaging.

[0077] The above describes in detail the preferred embodiments of the present invention. It should be understood that those skilled in the art can make numerous modifications and variations based on the concepts of the present invention without inventive effort. Therefore, any technical solutions that can be derived by those skilled in the art through logical analysis, reasoning, or limited experimentation based on the concepts of the present invention and the prior art should be within the scope of protection defined by the claims.

Claims

1. An acoustic focusing coding metasurface, characterized in that: The metasurface includes a plurality of regions linearly arranged along a first direction, each region includes one or more coding units, and the coding units are divided into a first coding unit and a second coding unit. The reflection phase difference between the two coding units is π. The coding unit in the same region is either the first coding unit 0 or the second coding unit 1, and regions containing different types of coding units are arranged alternately; The encoding unit includes a slotted circular tube, which is arranged along a first direction. The length extension direction of the slotted circular tube is perpendicular to the plane formed by the first direction and the second direction. The rotation angles of the slotted circular tubes of the two encoding units are different. The absolute value of the difference between the reflection phase of the slotted circular tube of the first encoding unit and the reflection phase of the slotted circular tube of the second encoding unit is the reflection phase difference of the two encoding units, and the reflection phase is determined based on the rotation angle. For two adjacent regions, the centers of the regions meet the following conditions: Among them, x ±i Indicates area A ±i The distance between the center of the metasurface and the center of the metasurface, x ±(i+1) Indicates area A ±(i+1) The distance between the center of the region and the center of the metasurface, i represents the sequence number of the region, λ is the wavelength of the sound wave when it propagates in the air, F x and F y are the horizontal and vertical coordinates of the focus.

2. The acoustic focusing coding metasurface according to claim 1, wherein: The acoustic path difference between two adjacent areas of the metasurface and the focus satisfies: l ±(i+1) -l ±i =0.5λ,i=0,1,2… Among them, l ±i Indicates area A ±i The distance between the center and the focal point.

3. The acoustic focusing coding metasurface according to claim 2, wherein: Area A ±i The distance between the center and the focus is expressed as:

4. The acoustic focusing coding metasurface according to claim 1, wherein: The encoding unit further includes a hard acoustic boundary layer, and a first distance is separated between the axis of the slotted circular tube and the hard acoustic boundary layer.

5. The acoustic focusing coding metasurface according to claim 4, wherein: The hard acoustic boundary layer is provided on the lower side of the encoding unit.

6. The acoustic focusing coding metasurface according to claim 4, wherein: The first distance is 11 mm.

7. The acoustic focusing coding metasurface according to claim 1, wherein: The first direction and the second direction are perpendicular to each other, the first direction is the x-axis direction, and the second direction is the y-axis direction.

8. The acoustic focusing coding metasurface according to claim 7, characterized in that: The coding unit at the center of the metasurface is numbered 1, and the center line of the coding unit coincides with the y-axis direction.

9. The acoustic focusing coding metasurface according to claim 1, wherein: The slit circular tube is an axisymmetric structure.

10. A reconfigurable method based on the acoustic focusing coding metasurface according to any one of claims 1 to 9, characterized in that: The method realizes metasurface reconstruction under axial focusing conditions and off-axis focusing conditions, and the method comprises the following steps: The focal length is adjusted to the required set value. When the focal length changes, the center coordinates of each area in the metasurface are calculated in sequence based on the conditions satisfied by the center of the area. The arrangement sequence of the coding units is determined based on the center coordinates of each area. The number and type of coding units in each area are determined according to the arrangement sequence. The corresponding slotted circular tube is rotated, and the rotation angle of the slotted circular tube is changed to change the reflection phase of the slotted circular tube, thereby realizing the change of the type of coding unit and completing the reconstruction of the metasurface.

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