Coding Moire metasurface capable of dynamically controlling ultrasonic beam and preparation method thereof

Abstract

The invention discloses a coding Moire metasurface capable of dynamically controlling an ultrasonic beam and a preparation method of the coding Moire metasurface. The coding Moire metasurface comprises two same coding metasurfaces, the coding metasurface is composed of '0' and '1' with periodic coding sequences; each coding unit can be composed of a water column and a photosensitive resin column which are the same in height, and the stainless steel frame serves as a hard boundary condition between the coding units. By changing the coding sequences and relative angles of the two coding metasurfaces, the period of the moire pattern of the coding moire metasurface is changed, so that the dynamic control of different transmission ultrasonic beams is realized, for example, two or four beams of transmission waves are generated in water, and the elevation angle and azimuth angle of the transmission waves are continuously controlled. The method is simple in processing process and can be widely applied to the fields of multi-channel acoustic communication, remote dynamic detection and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of acoustic metasurfaces, and in particular, relates to a coded moiré metasurface capable of dynamically manipulating ultrasonic beams. Background Art

[0002] Acoustic wavefront manipulation has attracted widespread attention from researchers and engineers due to its applications in acoustic communications, acoustic holography, biomedical imaging, and other fields. Once it appeared, acoustic metasurfaces quickly became a "ubiquitous tool" in the field of wavefront processing. These artificially designed, subwavelength-scale acoustic metasurfaces with periodic or non-periodic structure arrays have shown powerful capabilities for amplitude and phase manipulation of acoustic waves. However, their acoustic response is often unmodifiable, and once manufactured, the geometry of their unit cells is fixed, which limits their applicability in dynamic acoustic wave manipulation scenarios.

[0003] For dynamic and multifunctional application scenarios, tunable acoustic metasurfaces show great application potential and value. Researchers have invested a lot of effort in designing tunable or reconfigurable acoustic metasurfaces. For example, by adjusting the size of the cavity in the Helmholtz resonator or the effective acoustic path length of the curled space structure metasurface, the phase distribution of the metasurface is controlled; by adjusting the distance between the double-layer metasurfaces, the wavefront of the sound wave when it reaches the second metasurface is controlled, thereby changing the manipulation effect of the double-layer metasurface. However, the structural units of these tunable or reconfigurable acoustic metasurfaces are usually complex and are mainly applicable to airborne acoustic scenarios. At ultrasonic operating frequencies, it is basically not feasible to manufacture such complex subwavelength units, which makes dynamic ultrasonic operation a daunting challenge.

[0004] In recent years, an acoustic metasurface based on the moiré effect has emerged, which has brought great convenience to the field of dynamic acoustic wave manipulation. The acoustic moiré metasurface consists of two phase-modulated metasurfaces in series, in which the phase-modulated metasurface acts as a moiré diffraction element. Its uniqueness lies in that it can achieve dynamic wavefront manipulation effects simply by rotating the metasurface without changing its own parameters, which greatly reduces the requirements for the processing accuracy of the structural unit. Compared with traditional acoustic metasurfaces, acoustic moiré metasurfaces are more flexible and tunable in dynamic beam manipulation, providing new applications and opportunities for customized tunable devices.

[0005] Although acoustic moiré metasurfaces have great flexibility and tunability, existing technologies still face certain limitations. Specifically, once the diffractive element is prepared, its function is fixed. Therefore, there is a need for an acoustic moiré metasurface with tunable functions, dynamic wavefront control, and a simple and compact structure. Summary of the invention

[0006] In order to solve the above problems, the present invention provides a coded moiré metasurface that can dynamically manipulate ultrasonic beams and a preparation method thereof, which realizes dynamic manipulation of the incident ultrasonic wavefront by changing the rotation angle between the two metasurfaces that constitute it.

[0007] In order to achieve the above object, the present invention provides the following technical solutions:

[0008] A coded moiré metasurface capable of dynamically manipulating an ultrasonic beam, the coded moiré metasurface comprising two identical coded metasurfaces, each having a periodic coding sequence; by changing the coding sequences and relative angles of the two coded metasurfaces, the period of the moiré pattern of the coded moiré metasurface changes, thereby achieving dynamic manipulation of different transmitted ultrasonic beams.

[0009] In one embodiment of the present invention, the coded moiré metasurface is composed of two coded metasurfaces having periodic coding sequences.

[0010] Specifically, the coding metasurface contains two coding units “0” and “1”, corresponding to phase shifts of 0 and π, respectively. The two coding metasurfaces have the same phase gradient ξ=[ξ x ,ξ y ] T On the xy plane. According to the order in which the ultrasonic beam passes through, two coded metasurfaces are defined as metasurface 1 and metasurface 2, and metasurface 1 is set in the global coordinate system r = [x, y, z] T When the wave vector is k 0 When a plane ultrasonic beam is incident on the metasurface 1, the tangential wave vector components of the incident beam and the transmitted beam are and They satisfy the following equations:

[0011]

[0012] in, is the diffraction term in the reciprocal lattice vector of the metasurface 1. x ,p y and m,n (m,n∈Z) are the period and diffraction order in the x and y directions respectively. When metasurface 2 rotates counterclockwise by an angle Φ relative to metasurface 1, it is equivalent to the local coordinate system r′=[x′,y′,z′] T =[xcosΦ-ysinΦ,xsinΦ+ycosΦ,z] T , rotates clockwise relative to metasurface 1 Angle Φ. The tangential wave vector component of the transmitted beam is and The following equations are satisfied:

[0013]

[0014] in is the rotation matrix. is the diffraction term in the metasurface 2 reciprocal lattice vector. i,j (i,j∈Z) are the period and diffraction order in the x and y directions respectively. By coordinate transformation, the transmission tangential wave vector component in the global coordinate r is obtained

[0015] In global coordinates, the incident and transmitted tangential wave vector components passing through the coded moiré metasurface satisfy the following formulas:

[0016]

[0017] The wave vector of corresponds to the radiating propagation beam, The wave vector corresponds to the evanescent surface wave. Assume the wavelength is λ 0 The plane wave is incident on the coded moiré metasurface, and the elevation angle θ and azimuth angle of the transmitted beam are for:

[0018]

[0019] Therefore, when θ = 90°, the critical angle Φ c for:

[0020]

[0021] It can be seen that by continuously changing the rotation angle between the two coded metasurfaces, the coded moiré metasurface can achieve dynamic manipulation of the transmitted beam. The initial angle of the encoded moiré metasurface can be changed by rotating the entire metasurface, so the transmitted beam can be dynamically scanned in the entire transmission space.

[0022] The present invention also provides a method for preparing a coded moiré metasurface that can dynamically manipulate ultrasonic beams, using cylindrical photosensitive resin (PR resin for short) of the same height as the coding unit of the coded metasurface, and using a stainless steel frame with a periodic perforated structure as the acoustic hard boundary between the coded metasurface units. The stainless steel frame stuffed with the periodically arranged PR resin is placed in water to meet the phase gradient requirements of the coded metasurface.

[0023] When a vertically incident ultrasonic plane wave passes through a PR resin column and a water column with the same height h, a certain phase difference will be generated between them, which can be expressed as:

[0024]

[0025] Where n represents the refractive index of sound waves in PR resin relative to the longitudinal wave of water;

[0026] 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; the mass density ρ, Young's modulus E and Poisson's ratio σ of stainless steel are 7800kg / m 3 , 200GPa and 0.25. According to the formula in the third edition of Fundamentals of Acoustics, P337-340:

[0027]

[0028] Where λ and μ are Lamm constants, the longitudinal propagation velocity c of the sound wave in PR resin and stainless steel can be obtained. L are approximately 2785m / s and 5547m / s respectively; the mass density of water is ρ w =998kg / m 3 , the speed of sound is c w =1500m / s. Furthermore, according to It can be found that the refractive index of the longitudinal wave in the PR resin is about 0.54. Therefore, the stainless steel can be approximately regarded as the acoustic hard boundary between the PR resin and the water. According to equation (7), the height when the phase difference between the PR resin column and the water column is π can be obtained as h = 1.07λ 0 =8.2mm. In order to obtain the same height of water column, we set the thickness of stainless steel frame to be the same as that of PR resin.

[0029] Specifically, by designing coding sequences with different periods and embedding PR resin columns into the predetermined punching positions of the stainless steel frame, we were able to construct two types of coded moiré metasurfaces: one is a stripe-type coded moiré metasurface, and the other is a chessboard-type coded moiré metasurface. These two types of coded moiré metasurfaces can split an incident ultrasonic beam into two and four beams, respectively, and can achieve dynamic manipulation of the transmitted beam.

[0030] In one embodiment of the present invention, the PR resin column is prepared by 3D printing technology; and the stainless steel frame is prepared by punching periodic circular holes on a stainless steel plate.

[0031] Beneficial effects of the present invention

[0032] 1) Based on the moiré principle, the present invention designs a coded moiré metasurface, which is composed of two identical coded metasurfaces with periodic coding sequences. The coded metasurface is composed of a water column, a PR resin column and a stainless steel frame, wherein the water column and the PR resin column act as coding units "0" and "1", and the stainless steel frame acts as an acoustic hard boundary between the units. When the incident plane ultrasonic wave passes through the coded moiré metasurface, the period of the moiré pattern of the coded moiré metasurface changes by changing the coding sequence and relative angle of the two coded metasurfaces, thereby realizing dynamic manipulation of different transmitted ultrasonic beams. For example, when a beam of ultrasonic plane wave is incident on the stripe-type and chessboard-type coded moiré metasurfaces, it is split into two and four beams of transmitted ultrasonic waves, respectively, and the transmitted ultrasonic waves are dynamically manipulated by changing the relative rotation angle.

[0033] 2) When the incident ultrasonic plane wave passes through the stripe-type coded moiré metasurface, one incident ultrasonic wave is split into two beams, and the two ultrasonic waves have C about the z-axis. 2 Rotational symmetry. By changing the rotation angle between the two coded metasurfaces, the directions of the two transmitted ultrasonic beams can be dynamically adjusted, and the elevation angle of the transmitted beam can vary between 0° and 90°. When the incident ultrasonic plane wave passes through the chessboard-type coded moiré metasurface, one incident ultrasonic beam is divided into four beams, and the four ultrasonic beams have C about the z-axis. 4 Rotational symmetry. By changing the rotation angle between the two coding metasurfaces, the directions of the four transmitted ultrasonic beams can be dynamically adjusted, and the elevation angle of the transmitted beam can also be varied between 0° and 90°.

[0034] 3) The present invention provides an underwater coded moiré metasurface that can be continuously controlled and has a simple and compact structure. The processing process is simple and can be widely used in multi-channel acoustic communication, remote dynamic detection and other fields. It can promote the development of dynamic ultrasonic manipulation technology and provide more accurate and adjustable solutions for acoustic communication, detection and other related fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 (a1) is a single-layer coded metasurface with a stripe pattern; (a2) a stripe-type coded moiré metasurface with a rotation angle of Φ; (a3) ​​is a schematic diagram of the stripe-type coded moiré metasurface dynamically manipulating two transmitted beams; (b1) is a single-layer coded metasurface with a checkerboard pattern; (b2) a checkerboard-type coded moiré metasurface with a rotation angle of Φ; (b3) is a schematic diagram of the checkerboard-type coded moiré metasurface dynamically manipulating four transmitted beams.

[0036] Figure 2Figure 2 shows the transmission tangential wave vector distribution through the stripe-type coded moiré metasurface in reciprocal space for rotation angles (a1) 10°, (a2) 20° and (a3) ​​30°, and the transmission tangential wave vector distribution through the chessboard-type coded moiré metasurface at rotation angles (b1) 10°, (b2) 15° and (b3) 20°. The inset shows the corresponding far-field radiation pattern.

[0037] Figure 3 (a) is the transmission phase difference between PR resin and water column and the simulated value of the transmittance of plane ultrasound through PR resin as a function of height h. The inset is a schematic diagram of the water column and PR resin column.

[0038] (b) is a schematic diagram of the experimental device, and the inset is a sample photo of the coded moiré metasurface;

[0039] (c) is a photograph of a stainless steel frame with periodic cylindrical holes;

[0040] (d) is a photograph of a coded metasurface with striped and checkerboard patterns.

[0041] (e) Photos of the stripe-type and checkerboard-type coded moiré metasurfaces for rotation angles of 30° and 20°, respectively.

[0042] Figure 4 Normalized simulated sound intensity distribution of the transmitted acoustic field through the fringe-type encoded moiré metasurface on the maximum radiation plane for rotation angles (a1) 10°, (b1) 20° and (c1) 30°. Normalized simulated sound intensity and phase distribution of the transmitted acoustic field on the xy plane at z = 16λ for rotation angles (a2) 10°, (b2) 20° and (c2) 30°. Normalized measured sound intensity and phase distribution for rotation angles (a3) ​​10°, (b3) 20° and (c3) 30°.

[0043] Figure 5 The normalized simulated sound intensity distribution of the transmitted sound field through the chessboard-coded moiré metasurface on the maximum radiation plane is for rotation angles of (a1) 10°, (b1) 15° and (c1) 20°; the normalized simulated sound intensity and phase distribution of the transmitted sound field on the xy plane of z=16λ are for rotation angles of (a2) 10°, (b2) 15° and (c2) 20°; the normalized measured sound intensity and phase distribution are for rotation angles of (a3) ​​10°, (b3) 15° and (c3) 20°. DETAILED DESCRIPTION

[0044] The structure of the present invention will be further described in detail below in conjunction with specific embodiments. It should be understood that the following embodiments are only exemplary descriptions and explanations of the present invention, and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are included in the scope that the present invention is intended to protect.

[0045] Example 1

[0046] like Figure 1 As shown in the figure, the coded metasurfaces with stripe patterns and checkerboard patterns are composed of "0" and "1" coding units. When two identical coded metasurfaces with stripe patterns and checkerboard patterns are stacked together at a certain rotation angle, their corresponding coded moiré metasurfaces produce moiré patterns with quasi-periodic properties in one dimension and two dimensions, respectively. When a plane ultrasonic wave is incident on the stripe-type and checkerboard-type coded moiré metasurfaces, the incident ultrasonic wave will be split into two and four beams, respectively, and as the rotation angle changes, the elevation angle and azimuth angle of the two and four transmitted ultrasonic waves will change accordingly.

[0047] The phase gradient of the periodic coding metasurface can be expressed as ξ = [ξ x ,ξ y ] T According to the order in which the ultrasonic beam passes through, we define two coded metasurfaces, metasurface 1 and metasurface 2, and set the metasurface 1 in the global coordinate system r = [x, y, z] T When the wave vector is k 0 When a plane ultrasonic beam is incident on the metasurface 1, the tangential wave vector components of the incident beam and the transmitted beam are and The following equations are satisfied:

[0048]

[0049] in, is the diffraction term in the reciprocal lattice vector of the metasurface 1. x ,p y and m,n (m,n∈Z) are the period and diffraction order in the x and y directions respectively. When metasurface 2 rotates counterclockwise by an angle Φ relative to metasurface 1, it is equivalent to the local coordinate system r′=[x′,y′,z′] T =[xcosΦ-ysinΦ,xsinΦ+ycosΦ,z] T , rotates clockwise relative to metasurface 1 Angle Φ. The tangential wave vector component of the transmitted beam is and The following equations are satisfied:

[0050]

[0051] in is the rotation matrix. is the diffraction term in the metasurface 2 reciprocal lattice vector. i,j (i,j∈Z) are the period and diffraction order in the x and y directions respectively. By coordinate transformation, the transmission tangential wave vector component in the global coordinate r is obtained Therefore, in global coordinates, the incident and transmitted tangential wave vector components through the coded moiré hypermeter satisfy the following formulas:

[0052]

[0053] The wave vector of corresponds to the radiating propagation beam, The wave vector corresponds to the evanescent surface wave. Assume the wavelength is λ 0 The plane wave is incident on the coded moiré metasurface, and the elevation angle θ and azimuth angle of the transmitted beam are for:

[0054]

[0055] Therefore, when θ = 90°, the critical angle Φ c for:

[0056]

[0057] For simplicity, we assume that the incident wave is normal incidence (k i =[0,0] T ), and λ 0 =7.5mm. The periods of the stripe-type and chessboard-type coded moiré metasurfaces are set to p x =6mm and p x =p y =6mm. The phase gradients of the stripe-type and chessboard-type coded moiré metasurfaces can be calculated as [±1.25k 0 ,0] T and [±1.25k 0 ,±1.25k 0 ] T For the phase gradient, m=n=0 is the main diffraction order.

[0058] Figure 2 The blue dotted circle in the middle represents the isofrequency curve of free space, and the transmitted wave vector in the blue dotted circle corresponds to the radiated propagation beam. Figure 2 (a) shows the distribution of the transmitted tangential wave vector in the reciprocal space according to Equations (1), (2) and (3) when an ultrasonic beam is incident on the stripe-type coded moiré metasurface at rotation angles (a1) 10°, (a2) 20° and (a3) ​​30°. Figure 2(b) in the figure shows the distribution of the transmitted tangential wave vector in the reciprocal space according to equations (1), (2) and (3) when an ultrasonic wave beam is incident on the chessboard-coded moiré metasurface at rotation angles (b1) 10°, (b2) 15° and (b3) 20°. The inset shows their corresponding far-field radiation patterns. Obviously, it can be seen that the two radiated propagation wave vectors are relative to k z With C 2 Rotational symmetry.

[0059] For the stripe-type coded moiré metasurface, when the rotation angle is 10°, 20° and 30°, according to equations (4) and (5), we can obtain 2 The elevation angle and azimuth of the radiation propagation wave vector after coupling with metasurface 2 are (12.6°, 95°), (25.7°, 100°) and (40.3°, 105°) respectively. Similarly, for the chessboard-type coded moiré metasurface, when the rotation angles are 10°, 15° and 20°, we can obtain the value of k 2 The elevation angle and azimuth angle corresponding to the radiation propagation wave vector after coupling with metasurface 2 are (12.6°, 95°), (25.7°, 100°) and (40.3°, 105°), respectively. As the rotation angle increases, when the radiation propagation wave vector just moves to the blue virtual circle, the radiation propagation wave becomes an evanescent surface wave mode. According to equation (6), the critical angle of the stripe-type coded moiré metasurface can be obtained to be 47.2°. Therefore, for rotation angles of 0° to 47.2°, the stripe-type coded moiré metasurface can achieve dynamic control of the elevation angle of the transmitted acoustic wave from 0° to 90°. Similarly, the critical angle of the chessboard-type coded moiré metasurface is 32.9°. The chessboard-type coded moiré metasurface can achieve dynamic control of the elevation angle of the transmitted acoustic wave from 0° to 90° within the range of rotation angles of 0° to 32.9°.

[0060] Example 2

[0061] The present invention verifies the feasibility of the designed stripe-type and chessboard-type coded moiré metasurfaces through simulation and experiments.

[0062] like Figure 3 As shown in the illustration of (a), we introduced coding units "0" and "1" to construct a coded moiré metasurface, where the coding "0" and "1" are water columns and PR resin columns, respectively. When a vertically incident plane wave passes through the PR resin column and the water column at the same height, a certain phase difference is generated between them. For the structure of the present invention, the acoustic-solid coupling module of the COMSOL multi-physics simulation software was used for simulation verification, in which the pressure acoustics and solid mechanics modules were applied to water and PR resin, respectively. The structural simulation and experimental verification were both carried out in water, and the operating frequency was set to 200kHz (corresponding to the wavelength λ 0=7.5mm). The mass density ρ, Young's modulus E and Poisson's ratio σ of PR resin are 1130kg / m 3 , 2.3GPa and 0.45; the mass density ρ, Young's modulus E and Poisson's ratio σ of stainless steel are 7800kg / m 3 , 200 GPa and 0.25; the mass density of water is ρ w =998kg / m 3 , the speed of sound is c w =1500m / s. The radius of the water column and PR resin column is λ 0 / 6.

[0063] Figure 3 (a) shows the simulation results of the transmission phase difference between the PR resin column and the water column and the functional relationship between the transmittance of the plane ultrasonic wave through the PR resin and the height h. The solid line represents the phase difference after the incident plane wave is incident on the water column and the PR resin column, corresponding to the coordinate axis on the left. The dotted line represents the transmittance of the incident plane wave when it is incident on the PR resin column, corresponding to the coordinate axis on the right. As h increases, the transmission phase difference becomes larger, while the transmittance remains at a relatively high level. When h = 1.07λ 0 =8.2mm, it is marked as a five-pointed star on the way, the phase difference between the water column and the PR resin column is π, and the water column and PR resin column at this height are used as coding units 0 and 1. Referring to equation (7), the corresponding curve of the phase difference theoretical result is drawn, corresponding to Figure 3 (a) The dotted line in the middle shows that the theoretical results are basically consistent with the simulation results.

[0064] Figure 3 (b) shows a schematic diagram of the experimental setup, with the inset being a sample photo of the coded moiré metasurface. The acoustic field measurement process of the experiment was carried out in a transparent water tank with dimensions of 0.6 m × 0.6 m × 1 m. Sound-absorbing materials were placed around the water tank to reduce reflections from the tank walls. The excitation signal was generated by a data acquisition card (NIPXIe-5423) controlled by a computer, which was then amplified by a power amplifier and sent to the ultrasonic transducer. The ultrasonic transducer had a diameter of 10 cm and an operating frequency of 200 kHz. The coded moiré metasurface was attached to the surface of the transducer plate and immersed in water. A hydrophone controlled by a three-dimensional stepper motor was used to measure the acoustic pressure field with a step size of 1 mm. The computer recorded the output signal of the hydrophone through a data acquisition card (NIPXIe-5105).

[0065] Figure 3 (c) shows two stainless steel frames with periodic cylindrical holes. The stainless steel frames act as acoustic hard boundaries to prevent coupling between the encoding units. The thickness of the stainless steel frames is set to 1.07λ 0 , the radius of the hole is set to λ 0 / 6, which matches the thickness and radius of the coding unit. The distance between two adjacent holes is 1 / 15λ 0 , the effective area of ​​the coded moiré metasurface sample is 3λ 0 The circular holes around the effective circular area are used to fix the two frames.

[0066] like Figure 3 As shown in (d), by inserting PR resins with different coding sequences, samples of coded metasurfaces with striped and checkerboard patterns can be obtained. Figure 3 (e) Shows photos of the stripe-type and checkerboard-type coded moiré metasurfaces at rotation angles of 30° and 20°, respectively.

[0067] Figure 4 (a1) shows the normalized simulated sound intensity distribution of the transmitted sound field on the maximum radiation plane when the plane ultrasound is incident on the stripe-coded moiré metasurface when the rotation angle is 10°. It can be seen that a plane ultrasound beam is split into two beams after being incident on the stripe-coded moiré metasurface. These two sound waves are about the z-axis C. 2 Rotational symmetry.

[0068] Figure 4 (a2) shows that at z = 16λ 0 On the xy plane, corresponding to Figure 4 The normalized simulated intensity and phase distribution of the transmitted sound field in the dashed area of ​​(a1). Two bright spots can be observed, and the directions of their phase gradients are consistent with their respective azimuths. The elevation and azimuth angles of the propagation beam in the second quadrant are (13.1°, 94.3°), which are in good agreement with the theoretical values ​​(12.6°, 95°).

[0069] Figure 4 (a3) shows the normalized measured sound intensity and phase distribution in the scan area, corresponding to Figure 4 The solid square area in (a2). The experimental measurement results are basically consistent with the simulation results. In the experimental results, the intensity difference between the two bright spots may be caused by the tilt between the transducer and the sample surface, and the speckle may be caused by the processing error of the PR resin column and the sound wave reflection from the water cylinder wall, hydrophone and stainless steel frame.

[0070] Figure 4 (b1) and (c1) show the normalized simulated sound intensity distribution of the transmitted acoustic field on the maximum radiation plane when the plane ultrasound is incident on the stripe-type coded moiré metasurface when the rotation angle is 20° and 30°.

[0071] Figure 4 (b2, c2) and (b3, c3) show the normalized simulation of the transmitted sound field and the measured sound intensity and phase distribution, respectively. Figure 4(a1) is normalized. For rotation angles of 20° and 30°, the elevation and azimuth angles of the propagation beam in the second quadrant are (25.7°, 99.7°) and (39.5°, 104.2°), respectively, which are in good agreement with the theoretical values ​​(25.7°, 100°) and (40.3°, 105°). The measurement results are also basically consistent with the simulation results. With the increase of the rotation angle, the elevation and azimuth angles of the transmitted beam increase, while the intensity of the beam decreases. Therefore, the stripe-type coded moiré metasurface with the ability to dynamically manipulate two beams of transmitted acoustic waves by adjusting the rotation angle is verified.

[0072] Figure 5 Figures (a1-c1) show the normalized simulated sound intensity distribution of the transmitted sound field on the maximum radiation plane when a plane ultrasonic wave is incident on the chessboard-coded moiré metasurface and splits into four beams when the rotation angle is 10°, 15°, and 20°, respectively. These four sound waves have C 4 Rotational symmetry.

[0073] Figure 5 (a2-c2) show the normalized simulated sound intensity and phase distribution of the transmitted sound field when the rotation angle is 10°, 15° and 20° respectively. Their elevation and azimuth angles They are (18.6°, 50.8°), (26.3°, 51.3°) and (36°, 55.3°), which are basically consistent with the theoretical values ​​(17.9°, 140°), (27.5°, 142.5°) and (37.9°, 145°). As the rotation angle increases, the elevation and azimuth angles of the transmitted sound wave increase, while the intensity decreases.

[0074] Figure 5 (a3-c3) show the normalized measured sound intensity and phase distribution of the transmitted sound field when the rotation angle is 10°, 15° and 20°, respectively, corresponding to Figure 5 (a2-c2) Solid square area. The measured results are in good agreement with the simulation results, which shows that the chessboard-type coded moiré metasurface can dynamically manipulate the four transmitted acoustic waves by changing the rotation angle.

[0075] Therefore, the stripe-type and chessboard-type coded moiré metasurfaces designed in the present invention can achieve dynamic control effects of two and four transmitted beams, respectively.

[0076] The above is an explanation of the embodiments of the present invention. However, the present invention is not limited to the above embodiments. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A coded moiré metasurface that can dynamically manipulate ultrasonic beams, the coded moiré metasurface comprising two identical coded metasurfaces, the coded metasurface having a periodic coding sequence, and the period of the moiré pattern of the coded moiré metasurface changes by changing the coding sequence and relative angle of the two coded metasurfaces, thereby achieving dynamic manipulation of different transmitted ultrasonic beams.

2. The coded moiré metasurface according to claim 1, characterized in that: The coding metasurface comprises two coding units "0" and "1", corresponding to phase shifts of 0 and π respectively; The two coding metasurfaces have the same phase gradient ξ=[ξ x ,ξ y ] T In the xy plane; According to the order in which the ultrasonic beam passes through, two coded metasurfaces are defined as metasurface 1 and metasurface 2, and metasurface 1 is set in the global coordinate system r = [x, y, z] T middle; When a planar ultrasonic beam with wave vector k0 is incident on metasurface 1, the tangential wave vector components of the incident beam and the transmitted beam are and The following equations are satisfied: in, is the diffraction term in the reciprocal lattice vector of the metasurface 1; p x ,p y and m,n (m,n∈Z) are the period and diffraction order in the x and y directions respectively; When the hypersurface 2 rotates counterclockwise by an angle Φ relative to the hypersurface 1, it is equivalent to the local coordinate system r′=[x′,y′,z′] T =[xcosΦ-ysinΦ,xsinΦ+ycosΦ,z] T , rotates clockwise relative to metasurface 1 Angle Φ; The tangential wave vector component of the transmitted beam is and The following equations are satisfied: in is the rotation matrix; is the diffraction term in the 2-reciprocal lattice vector of the metasurface; i,j (i,j∈Z) are the period and diffraction order in the x and y directions respectively; Through coordinate transformation, we get the transmission tangential wave vector component in the global coordinate r: In global coordinates, the incident and transmitted tangential wave vector components passing through the coded moiré metasurface satisfy the following formulas: The wave vector of corresponds to the radiating propagation beam, The wave vector of corresponds to the evanescent surface wave; A plane wave with a wavelength of λ0 is incident on the coded moiré metasurface. The elevation angle θ and azimuth angle of the transmitted beam are for: When θ=90°, the critical angle Φ c for: By continuously changing the rotation angle between the two coded metasurfaces, the coded moiré metasurface achieves dynamic manipulation of the transmitted beam; azimuth The initial angle of the metasurface can be changed by rotating the entire encoded moiré metasurface, and the transmitted beam can be dynamically scanned in the entire transmission space.

3. The coded moiré metasurface according to claim 1 or 2, characterized in that: The coded moiré metasurface is of stripe type, and the incident ultrasonic wave is divided into two beams, and the two ultrasonic wave beams have C2 rotational symmetry about the z-axis; by changing the rotation angle between the two coded metasurfaces, the directions of the two transmitted ultrasonic beams are dynamically adjusted, and the elevation angle of the transmitted beam can be changed between.

4. The coded moiré metasurface according to claim 1 or 2, characterized in that: The coded moiré metasurface is of a chessboard type, and the incident ultrasonic wave is divided into four beams, and the four ultrasonic wave beams have C4 rotational symmetry about the z-axis; by changing the rotation angle between the two coded metasurfaces, the directions of the four transmitted ultrasonic wave beams are dynamically adjusted, and the elevation angle of the transmitted wave beam can be varied between 0° and 90°.

5. A method for preparing a coded moiré metasurface capable of dynamically manipulating an ultrasonic beam as claimed in any one of claims 1 to 4, characterized in that: include: 1) Using cylindrical photosensitive resins of the same height as coding units of the coding metasurface; 2) A stainless steel frame with a periodic perforated structure is used as the acoustic hard boundary between the coding metasurface units.

6. The preparation method according to claim 5, characterized in that: It also includes placing a stainless steel frame plugged with periodically arranged PR resin into water to meet the phase gradient requirements of the coding metasurface; When a vertically incident ultrasonic plane wave passes through a PR resin column and a water column with the same height h, a certain phase difference will be generated, which can be expressed as: Where n represents the refractive index of sound waves in PR resin relative to the longitudinal wave of water.

7. The preparation method according to claim 6, characterized in that: The thickness of the stainless steel frame is consistent with that of the PR resin to obtain the same height of the water column.

8. The preparation method according to claim 6, characterized in that: The mass density ρ, Young's modulus E and Poisson's ratio σ of the PR resin are 1130 kg / m 3 , 2.3GPa and 0.45; The mass density ρ, Young's modulus E and Poisson's ratio σ of the stainless steel frame are 7800 kg / m 3 , 200GPa and 0.25; According to the formula: Among them, λ and μ are Lammi constants; The longitudinal propagation velocity c of the acoustic wave in PR resin and stainless steel L are 2785m / s and 5547m / s respectively; the mass density of water is ρ w =998kg / m 3 , the speed of sound is c w =1500m / s; according to The refractive index of longitudinal waves in PR resin is found to be 0.54; stainless steel is considered as the acoustic hard boundary between PR resin and water; According to equation (7), when the phase difference between the PR resin column and the water column is π, the height is h = 1.07λ0 = 8.2 mm.

9. The preparation method according to claim 8, characterized in that: Coding sequences with different periods are designed, and PR resin columns are embedded into the predetermined punching positions of the stainless steel frame to construct two types of coded moiré metasurfaces: a stripe-type coded moiré metasurface and a chessboard-type coded moiré metasurface. The coded moiré metasurfaces can split an incident ultrasonic beam into two and four beams, respectively, and can realize dynamic manipulation of the transmitted beam.

10. The preparation method according to claim 6, characterized in that: The PR resin column is prepared by 3D printing technology; and the stainless steel frame is prepared by punching periodic circular holes on a stainless steel plate.

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