A Multilayer Broadband Tunable Anti-Reflection Impedance Matching Method and Metasurface Structure

Abstract

The present invention relates to a multi-layer broadband tunable anti-reflection impedance matching method and a metasurface structure. The method includes designing a broadband multi-layer anti-reflection thin film using the Chebyshev matching layer theory, and engraving a metasurface composed of periodically arranged metal resonant rings on the front surface of each dielectric plate. By optimizing the structural parameters of the resonant rings of the metasurface in each layer, the thickness of the anti-reflection matching layer is significantly reduced without affecting the anti-reflection characteristics. Compared with the prior art, the present invention has the advantages of ultra-wideband transmission enhancement, ultra-thin low loss, tunability, and applicability to multi-angle incidence.

Description

Technical Field

[0001] The present invention relates to the technical field of ground penetrating radar signal enhancement in non-destructive detection systems, and more particularly to a multi-layer broadband tunable anti-reflection impedance matching method and a metasurface structure. Background Art

[0002] Electromagnetic metamaterials have customized electromagnetic properties and can effectively manipulate electromagnetic waves, with anomalous refraction and reflection, coupling of propagating waves to surface waves, planar holograms, focusing lenses, the photonic spin Hall effect, and other properties. Metasurface functional electromagnetic devices have powerful wave manipulation capabilities, exhibit diverse functions, and are usually thin and flat, showing broad prospects in modern integrated optical applications.

[0003] In non-destructive detection fields such as through-wall radars and ground penetrating radars, and in some other impedance mismatch systems, eliminating unnecessary reflections at the interface between two media has great application value.

[0004] For example, the ground penetrating radar GPR with ultra-wideband radiation is an electromagnetic technology currently used for detecting and imaging buried objects, and its mainstream operating frequency is usually in the range of 10 - 5000 MHz. The impedance mismatch at the interface depends on the refractive index difference between these two media. To match the impedance of the two, traditional methods such as Chebyshev transmission lines are usually used to design single-layer or multi-layer dielectric anti-reflection films to match the load. The single-layer anti-reflection film is the simplest method, which requires the refractive index to meet specific conditions and the thickness of the dielectric layer to be one-quarter of the operating wavelength, but its operating bandwidth is usually very narrow. On the other hand, although the multi-layer anti-reflection film brings a wider operating bandwidth, its thickness also increases significantly, reducing its applicability in the gigahertz frequency band and lower frequency bands. Therefore, traditional methods often cannot have both bandwidth and device scale.

[0005] In addition, in existing non-destructive detection systems, such as in ground penetrating radar signal enhancement technology, a series of enhancements are mainly performed on the original signal through image processing, such as removing the direct wave and surface reflection wave based on obvious image features, background denoising, image filtering, and gain. These methods essentially only perform display enhancement on the original data and do not add more useful information. At the same time, some useful signals will inevitably be filtered out during noise removal, resulting in mistakes and omissions in subsequent information interpretation. Antennas with gain and frequency selectivity functions can also be manufactured using electromagnetic metamaterials, but when the gain signal propagates to the surface of the object to be detected, a large amount of the signal will still be reflected and cannot enter the interior of the object to be measured.

[0006] Therefore, impedance mismatch has become an urgent problem to be solved in the non-destructive detection field, and there is an increasing demand for electrically thin scale and broadband anti-reflection design. Using metasurface functional electromagnetic devices, a thin, light, transmission-enhanced, and ultra-wideband impedance matching layer can be manufactured. Summary of the Invention

[0007] The object of the present invention is to provide a multi-layer broadband tunable anti-reflection impedance matching method and a metasurface structure with enhanced ultra-wideband transmission, ultra-thin low loss, tunability, and applicability to multi-angle incidence, so as to overcome the defects existing in the above-mentioned prior art.

[0008] The object of the present invention can be achieved by the following technical solutions:

[0009] According to a first aspect of the present invention, there is provided a multi-layer broadband tunable anti-reflection impedance matching method, the method comprising the following steps:

[0010] Step S1, based on the small reflection theory, design a Chebyshev transmitter to determine the dielectric constant of each layer of the medium. Specifically, based on the correspondence between the total reflectivity and the Chebyshev polynomial, solve the reflection coefficient at each layer interface, and gradually calculate the impedance of each layer of the medium in the matching layer, and then calculate the dielectric constant of the matching layer;

[0011] Step S2, based on the multi-layer interference theory, after loading the metal resonant ring, determine the change in the transmission and reflection coefficients at the interface where it is located, and calculate the total reflection coefficient of the anti-reflection matching layer when the electromagnetic wave propagates from the detection scenario medium through the anti-reflection matching layer to the medium to be measured;

[0012] Step S3, based on the zero reflection theory obtained by the interference subtraction of electromagnetic waves, adjust the structural parameters of the metal resonant ring to ensure that when the size of the dielectric layer decreases, the change in the impedance matching frequency band is within a preset range;

[0013] Step S4, according to the zero reflection theory and the frequency adjustment requirement, adjust the structural parameters of the metal resonant ring without changing the thickness of the dielectric layer to shift the working frequency band of the anti-reflection matching layer.

[0014] Preferably, the step S1 is specifically:

[0015] Given the air impedance Z0 and the impedance Z of the medium to be measured L , the number of layers N of the Chebyshev transmitter, and the maximum allowable reflection coefficient A, by establishing a one-to-one correspondence between the expression of the total reflectivity Γ(θ) and the Chebyshev polynomial T N (secθ m cosθ), we get:

[0016] Γ(θ) = 2e -jNθ [Γ0cosNθ + Γ1cos(N - 2)θ +... + Γ n cos(N - 2n)θ +...]

[0017] = Ae -jNθ T N (secθm cosθ)

[0018] where θ is the phase delay when the electromagnetic wave propagates in each matching layer, and θ m is the phase corresponding to the minimum frequency in the passband response frequency band, defined as:

[0019] secθ m =cosh[1 / N*cosh -1 (|(lnZ L / Z0) / (2|A|)|)]

[0020] The reflection coefficient at each layer interface is calculated by the above formula, and then the formula Γ N =1 / 2*ln(Z n+1 / Z n ) is used to calculate the impedance of each layer of the matching layer step by step. Finally, the dielectric constant of the matching layer is calculated, and the thickness of each layer of the medium is one-quarter of the wavelength.

[0021] Preferably, the anti-reflection matching layer in step S2 is the total reflection coefficient of a single-layer anti-reflection broadband matching layer or a double-layer anti-reflection broadband matching layer, and the corresponding total reflection coefficients are respectively:

[0022] The total reflection coefficient of the single-layer anti-reflection broadband matching layer is:

[0023]

[0024] In the formula, the subscripts 0, 1, and 2 respectively represent the detection scenario medium, the anti-reflection matching layer, and the medium to be detected, r represents the reflection coefficient of the electromagnetic wave at the interface of different media, φ represents the phase corresponding to the reflection coefficient, and θ is the phase delay when the electromagnetic wave propagates in the anti-reflection matching layer medium;

[0025] The total reflection coefficient of the double-layer anti-reflection broadband matching layer is:

[0026]

[0027] In the formula, the subscripts 0, 1, and 2 respectively represent the detection scenario medium, the first-layer anti-reflection broadband matching layer, and the second-layer anti-reflection broadband matching layer, and the subscript es represents the whole formed by the second-layer anti-reflection broadband matching layer and the medium to be detected; r represents the reflection coefficient of the electromagnetic wave at the interface of different media, φ represents the phase corresponding to the reflection coefficient, and θ is the phase delay when the electromagnetic wave propagates in the first-layer anti-reflection broadband matching layer medium.

[0028] According to a second aspect of the present invention, there is provided a multi-layer broadband tunable anti-reflection metasurface structure, which is prepared by using the multi-layer broadband tunable anti-reflection impedance matching method. The metasurface structure includes a dielectric plate, and metal resonant rings arranged periodically are engraved on the front surface of the dielectric plate, forming an anti-reflection matching layer together with the dielectric plate; the metal resonant rings are metal resonant rings with adjustable structural parameters to achieve all-transmission of electromagnetic waves within an ultra-wide tunable frequency band.

[0029] The anti-reflection matching layer is a single-layer anti-reflection broadband matching layer or a double-layer anti-reflection broadband matching layer; the single-layer anti-reflection broadband matching layer includes a layer of dielectric plate, the first layer of metal resonant rings arranged periodically are engraved on the front surface of the dielectric plate, and the second layer of metal resonant rings arranged periodically are engraved on the back surface; the double-layer anti-reflection broadband matching layer includes a first-layer dielectric plate and a second-layer dielectric plate that are attached to each other. The first layer of metal resonant rings are engraved on the front surface of the first-layer dielectric plate, and the second layer of metal resonant rings are engraved on the front surface of the second-layer dielectric plate.

[0030] Both the first layer of metal resonant rings and the second layer of metal resonant rings are metal split-ring resonators, and their sizes are slightly smaller than the period length and their thicknesses are extremely thin.

[0031] Preferably, the thickness of the single-layer anti-reflection broadband matching layer under a 100% working bandwidth is less than 0.2 times the electromagnetic wave wavelength, and the thickness of the double-layer anti-reflection broadband matching layer is less than 0.35 times the electromagnetic wave wavelength.

[0032] Preferably, the anti-reflection matching layer is a single-layer anti-reflection broadband matching layer, and the second layer of metal resonant rings is a metal split-ring resonator with a straight-strip resonator.

[0033] Preferably, the metal split-ring resonator is a circular double-split-ring resonator with inner and outer nesting or a square double-split-ring resonator with inner and outer nesting.

[0034] Preferably, the arrangement periods of the first layer of metal resonant rings and the second layer of metal resonant rings are the same, and the metal resonant rings in the same unit in the two layers pass through the same central axis.

[0035] Preferably, the dielectric plate is a low-loss dielectric plate with a relative dielectric constant between 1 and 8.

[0036] Preferably, the thickness, dielectric constant of the dielectric plate, and the structural parameters of the metal resonant rings are adaptively adjusted according to the dielectric constant of the medium to be matched to achieve broadband anti-reflection.

[0037] Compared with the prior art, the present invention has the following advantages:

[0038] 1) Ultra-wideband transmission enhancement: The present invention is based on the combination of multi-layer Chebyshev transmitters and metamaterials. By using split-ring resonators to adjust the anti-transmission phase and amplitude at different dielectric interfaces, a thinner and lighter metasurface electromagnetic functional device with the same bandwidth is designed. Compared with the anti-reflection film in the prior art, the bandwidth has been significantly improved. Among them, the single-layer anti-reflection broadband matching layer has a relative bandwidth of 65%, and the double-layer anti-reflection broadband matching layer has a relative bandwidth of 100%. The working bandwidth has been greatly improved compared with the prior art, and the latter can match the 100% radiation bandwidth of the ground-penetrating radar antenna.

[0039] 2) Thin, light, and low-loss: The multi-layer broadband tunable anti-reflection metasurface structure designed in the present invention effectively replaces a part of the thickness of the dielectric layer. Since the thickness of the metasurface structure is less than 1 mm and can be ignored, a thinner and lighter matching layer is obtained, and the smaller thickness also greatly reduces the loss of the matching layer itself.

[0040] 3) Frequency tunability: The metal resonator ring metasurface structure used can effectively adjust the working frequency band range of the matching layer. Introducing the metal resonator ring surface causes the center working frequency of the Chebyshev transmitter with an original working frequency of 1 GHz - 2.5 GHz to shift by 13% - 27%, and the relative bandwidth is not affected and remains at about 100%.

[0041] 4) Strong robustness: The present invention introduces a metal resonator ring metasurface to adjust the amplitude and phase of the partial reflection coefficient and optimize the thickness of the dielectric plate. Therefore, there is a strong correlation between the structural parameters of the metal resonator ring and the thickness of the dielectric plate, and the thickness of the dielectric plate can be flexibly adjusted as needed. The structure of the metal resonator ring is not limited to circular split rings. For example, the second resonator rings of the single-layer and double-layer matching layers are different, and metal structures with similar electromagnetic resonance characteristics can be used. When the relative permittivity of the dielectric plate varies within the range of plus or minus 25%, the anti-reflection performance and working bandwidth are still good, indicating that the transmission enhancement performance of the matching layer is not strongly dependent on the relative permittivity of the dielectric plate, but the principle of gradual matching cannot be violated. The diversity of this metal resonator ring structure, the flexibility of the dielectric plate thickness selection, the insensitivity to the relative permittivity, and the wide range of material selection have great advantages in manufacturing and application.

[0042] 5) Multi-angle incidence: Due to the periodic arrangement of centrosymmetric metamaterial resonant units in the present invention, the broadband matching layer is not sensitive to the polarization mode of the incident wave under normal incidence. When the incident angle of the electromagnetic wave changes from 0° to 60°, the electromagnetic signal can achieve good broadband enhancement performance, and the working bandwidth can reach 160% in the TM polarization mode.

[0043] 6) Strong universality: The proposed metasurface structure can be used for matching between a variety of different media. By adjusting the thickness and dielectric constant of the dielectric layer and the structural parameters of the resonant ring, the coverage range of the dielectric constant to be matched can be from air to water. Description of the Drawings

[0044] Figure 1 It is a schematic diagram of the metal resonant ring structure; among them, Figure 1 a is a schematic diagram of the circular double-split ring resonant ring structure with inner and outer nesting, Figure 1 b is a schematic diagram of the square double-split ring resonant ring structure with inner and outer nesting, Figure 1 c is a circular metal resonant split ring with a straight resonator;

[0045] Figure 2 It is a schematic diagram of the Chebyshev transmitter; among them, Figure 2 a is a schematic diagram of the multi-layer Chebyshev transmitter, and the length l of each transmission line is a quarter of the dielectric wavelength λ; Figure 2 b is a schematic diagram of the single-layer impedance matching system with a metasurface introduced, and the thickness of the matching layer is 0.16λ; Figure 2 c is a schematic diagram of the double-layer impedance matching system with a metasurface introduced, and the thickness of the matching layer is 0.33λ;

[0046] Figure 3 It is the principle diagram of the reflection and transmission of electromagnetic waves at the interface of "air - matching layer - medium to be measured", among which the dotted line represents the embedded metasurface;

[0047] Figure 4 It is the structure and simulation result diagram of the single-layer antireflection impedance matching layer; among them, Figure 4 a is a schematic diagram of the structure of the single-layer antireflection impedance matching layer, and the middle layer is the matching layer medium with a thickness of l1 = 12 mm; Figure 4 b is the reflection coefficient characteristic of the interface where the two resonant rings are located, Figure 4 c is the phase corresponding to the reflection coefficient and the zero reflection phase condition characteristic, Figure 4 d is the reflectivity and transmittance of electromagnetic waves from air through the matching layer to the medium to be measured;

[0048] Figure 5 It is the double-layer matching layer designed by the traditional Chebyshev transmitter method and the reflection / transmittance result diagram; among them, Figure 5 a is a schematic diagram of the model, and the middle two layers are the matching layer media with thicknesses of l1 = 25.22 mm and l2 = 13.94 mm respectively, and the back end is the medium to be measured; Figure 5 b is the result diagram of the reflectivity and transmittance from air to the medium to be measured after using this matching layer, and the working bandwidth is 0.97 GHz - 3.03 GHz;

[0049] Figure 6is the reflectance / transmittance of the double-layer matching layer with metal resonant rings etched in front of the first-layer dielectric plate; where, Figure 6 a is a schematic diagram of the model. The middle two layers are the matching layer dielectrics with thicknesses of l1 = 15 mm and l2 = 13.94 mm respectively, and the back end is the medium to be measured; Figure 6 b is the reflectance and transmittance from air to the medium to be measured after using this matching layer, and the working bandwidth is 0.97 GHz - 3.06 GHz;

[0050] Figure 7 is the reflectance / transmittance of the double-layer matching layer with metal resonant rings etched in front of the second-layer dielectric plate; where, Figure 7 a is a schematic diagram of the model. The middle two layers are the matching layer dielectrics with thicknesses of l1 = 25.22 mm and l2 = 10 mm respectively, and the back end is the medium to be measured; Figure 7 b is the reflectance and transmittance from air to the medium to be measured after using this matching layer, and the working bandwidth is 1.06 GHz - 3.01 GHz;

[0051] Figure 8 is the reflectance / transmittance of the double-layer matching layer with metal resonant rings etched in front of both layers of dielectric plates; where, Figure 8 a is a schematic diagram of the model. The middle two layers are the matching layer dielectrics with thicknesses of l1 = 15 mm and l2 = 10 mm respectively, and the back end is the medium to be measured; Figure 8 b is the reflectance and transmittance from air to the medium to be measured after using this matching layer, and the working bandwidth is 1.07 GHz - 3.22 GHz;

[0052] Figure 9 is the reflectance / transmittance of the tunable double-layer matching layer with resonant rings etched in front of both layers of dielectric plates of the Chebyshev transmitter with a set working frequency of 2 GHz; where, Figure 9 a is a schematic diagram of the model. The middle two layers are the matching layer dielectrics with thicknesses of l1 = 25.22 mm and l2 = 13.94 mm respectively, and the back end is the medium to be measured; Figure 9 b is the reflectance and transmittance from air to the medium to be measured after using this matching layer, and the working bandwidth is 0.78 GHz - 2.32 GHz;

[0053] Figure 10 is the frequency shift of the reflectance of the Chebyshev transmitter loaded with a metasurface at set working frequencies of 1 GHz, 1.5 GHz, 2 GHz, and 2.5 GHz; where, Figure 10 a corresponds to the case of a working frequency of 1 GHz, and the thicknesses of the two layers of dielectric plates are 50.45 mm and 27.88 mm respectively; Figure 10 b corresponds to the case of a working frequency of 1.5 GHz, and the thicknesses of the two layers of dielectric plates are 33.63 mm and 18.58 mm respectively; Figure 10For the case where c corresponds to a working frequency of 2 GHz, the thicknesses of the two dielectric plates are 25.22 mm and 13.94 mm respectively; Figure 10 For the case where d corresponds to a working frequency of 2.5 GHz, the thicknesses of the two dielectric plates are 20.18 mm and 11.15 mm respectively;

[0054] Figure 11 In order to Figure 8 a Contour maps of reflectance when electromagnetic waves are incident at multiple angles from 0° to 60° in both transverse electric polarization and transverse magnetic polarization modes for a multi-layer broadband tunable antireflection metasurface structure under the a model structure; among them, Figure 11 a shows the characteristics of reflectance varying with angle in different frequency ranges when incident at multiple angles in the transverse electric polarization mode (simulation results), Figure 11 b shows the characteristics of reflectance varying with angle in different frequency ranges when incident at multiple angles in the transverse magnetic polarization mode (simulation results);

[0055] Figure 12 Schematic diagram and result display of the measurement experimental device for the transmission enhancement characteristics of a double-layer broadband antireflection metasurface sample; among them, Figure 12 a is the experimental measurement platform, Figure 12 b are the measured transmittance, absorbance, and reflectance curves;

[0056] Figure 13 For Figure 8 Experimental measurement results of contour maps of reflectance when electromagnetic waves are incident at multiple angles from 0° - 60° in both transverse electric polarization and transverse magnetic polarization modes for a double-layer broadband antireflection metasurface structure under the a model structure; among them, Figure 13 a shows the characteristics of reflectance varying with angle in different frequency ranges when incident at multiple angles in the transverse electric polarization mode (experimental results), Figure 13 b shows the characteristics of reflectance varying with angle in different frequency ranges when incident at multiple angles in the transverse magnetic polarization mode (experimental results). Detailed implementation manners

[0057] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0058] Embodiment 1

[0059] This embodiment provides a multi-layer broadband tunable antireflection impedance matching method, and the method includes the following steps:

[0060] Step S1: Based on the small reflection theory, design a Chebyshev transducer to determine the dielectric constant of each layer of the medium. Specifically: based on the correspondence between the total reflectivity and the Chebyshev polynomial, solve the reflection coefficient at each interface, and gradually calculate the impedance of each layer of the medium in the matching layer, and then calculate the dielectric constant of the matching layer;

[0061] Step S2: Based on the multi-layer interference theory, after loading the metal resonant ring, determine the change in the transmission and reflection coefficient at the interface where it is located, and calculate the total reflection coefficient of the anti-reflection matching layer when the electromagnetic wave propagates from the detection scene medium through the anti-reflection matching layer to the medium to be measured;

[0062] Step S3: Based on the zero-reflection theory obtained by subtracting the electromagnetic wave interference, adjust the structural parameters of the metal resonant ring to ensure that when the size of the dielectric layer decreases, the impedance matching frequency band basically does not change;

[0063] Step S4: According to the zero-reflection theory and the frequency adjustment requirements, adjust the structural parameters of the metal resonant ring without changing the thickness of the dielectric layer to shift the working frequency band of the anti-reflection matching layer.

[0064] In addition, this embodiment also provides a multi-layer broadband tunable anti-reflection metasurface structure, which is prepared by using the multi-layer broadband tunable anti-reflection impedance matching method described above. The metasurface structure includes a dielectric plate, and the front surface of the dielectric plate is engraved with periodically arranged metal resonant rings, which together with the dielectric plate form an anti-reflection matching layer; the metal resonant rings are metal resonant rings with adjustable structural parameters to achieve all-transmission of electromagnetic waves within an ultra-wide tunable frequency band;

[0065] The anti-reflection matching layer is a single-layer anti-reflection broadband matching layer or a double-layer anti-reflection broadband matching layer; the single-layer anti-reflection broadband matching layer includes a layer of dielectric plate, the front surface of the dielectric plate is engraved with the first layer of periodically arranged metal resonant rings, and the back surface is engraved with the second layer of periodically arranged metal resonant rings; the double-layer anti-reflection broadband matching layer includes two first-layer dielectric plates and second-layer dielectric plates that are bonded together. The front surface of the first-layer dielectric plate is engraved with the first layer of metal resonant rings, and the front surface of the second-layer dielectric plate is engraved with the second layer of metal resonant rings;

[0066] Both the first layer of metal resonant rings and the second layer of metal resonant rings are metal split resonant rings, and their sizes are slightly smaller than the period length and their thicknesses are extremely thin.

[0067] Next, the present invention will be introduced in detail.

[0068] The present invention introduces periodically arranged metal resonant rings (artificial electromagnetic material resonant units) engraved on the surface of a dielectric plate into a traditional Chebyshev transmitter, designs a thinner, broadband, and tunable impedance matching layer, and introduces corresponding electromagnetic resonances at the dielectric interface, thereby enabling the regulation of the amplitude and phase of incident electromagnetic waves, achieving the purpose of multi-beam interference cancellation on the incident surface, weakening the reflected wave, and enhancing the transmitted wave. The designed multi-layer broadband tunable anti-reflection metasurface structure can achieve excellent impedance matching effects within a working bandwidth of up to 100%. In impedance mismatch application scenarios such as ground penetrating radar, a new method is provided to cover the metasurface sample for the medium to be measured, thereby eliminating or weakening the strong reflection generated by the antenna signal on the ground surface and increasing the detection signal strength.

[0069] In a traditional Chebyshev transmitter, the thickness of each layer of dielectric is one-quarter wavelength. Taking a single-layer matching layer as an example, the thickness is strictly limited to one-quarter wavelength, and the relative dielectric constant is the square root of the product of the relative dielectric constants of air and the medium to be measured, which is very difficult to achieve in application scenarios.

[0070] After introducing the metal resonant ring into the Chebyshev transmitter in the present invention, the thickness of the dielectric layer can be flexibly adjusted by adjusting the presence or absence of the resonant ring and the structural parameters of the resonant ring. The resonant ring is engraved on a low-loss plate, such as a polytetrafluoroethylene fiberglass plate, a polytetrafluoroethylene ceramic plate, an acrylic plate, a polyimide plate, and other materials with low loss and a relative dielectric constant between 1 and 8. The dielectric constant and thickness of the plate can both be related to the structural parameters of the resonant ring, so there are no very strict requirements like traditional matching layers. These characteristics make the present invention very useful in actual detection signal enhancement applications.

[0071] The principle of the present invention is to introduce periodically arranged metal resonant units into a traditional Chebyshev transmitter, introduce corresponding electromagnetic resonances at the dielectric interface, thereby enabling the regulation of the amplitude and phase of incident electromagnetic waves, achieving the purpose of multi-beam interference cancellation on the incident surface, weakening the reflected wave, and enhancing the transmitted wave.

[0072] Applying the multi-beam interference theory, a Chebyshev transmitter can be designed to match air (with an impedance Z0 of 1 and a relative dielectric constant of 1) with the medium to be measured (e.g., an impedance Z L of 0.25 and a relative dielectric constant of 16). When the number of layers N of the transmitter, the maximum allowable reflection coefficient A, and Z0, Z L are set, by establishing a one-to-one correspondence between the reflection coefficient Γ n of each layer and the Chebyshev polynomial T N (secθ m cosθ), the total reflectance Γ(θ) is obtained as:

[0073] Γ(θ) = 2e-jNθ [Γ0cosNθ + Γ1cos(N - 2)θ +... + Γ n cos(N - 2n)θ +...]

[0074] = Ae -jNθ T N (secθ m cosθ)

[0075] where θ is the phase delay when the electromagnetic wave propagates in each matching layer, and θ m is the phase corresponding to the minimum frequency in the passband response frequency band, defined as:

[0076] secθ m = cosh[1 / N * cosh -1 (|(lnZ L / Z0) / (2|A|)|)]

[0077] The reflection coefficient at each interface can be calculated from the above formula, and then the impedance of each layer of the matching layer can be gradually calculated by the formula Γ N = 1 / 2 * ln(Z n+1 / Z n ). Finally, the dielectric constant of the last matching layer can also be calculated, and the thickness of each layer of the medium is one - quarter of the wavelength, as shown in Figure (2a).

[0078] For a single - layer traditional antireflection matching layer, the method is to design a quarter - wavelength matching layer, and its dielectric constant is the square root of the product of the dielectric constants of air and the medium to be matched.

[0079] Specifically for a two - layer Chebyshev transducer, the above general formula can be specialized. Set the maximum allowable reflection coefficient A to 0.1, and the total reflectance Γ(θ) is:

[0080]

[0081] By making it correspond one - by - one with the second - order Chebyshev polynomial T2(secθ m cosθ) = sec 2 θ m (1 + cos2θ)-1 and applying the definition formula of θ m and the impedance calculation formulas Γ0 = 1 / 2 * ln(Z1 / Z0) and Γ1 = 1 / 2 * ln(Z2 / Z1), we can get Z1 = 0.6726 and Z2 = 0.3717, and the corresponding dielectric constants of the two - layer dielectric plates are 2.21 and 7.24 respectively.

[0082] After the Chebyshev transmitter is designed, the present invention uses the principle of interference cancellation to guide the optimization principle of the metal resonant ring. The zero reflection condition derived from the multi-beam interference theory is the basis for the metasurface design.

[0083] When the electromagnetic wave propagates from the air through the matching layer to the medium to be measured, the total reflection coefficient calculation formula of the single-layer anti-reflection broadband matching layer is:

[0084]

[0085] Among them, the subscripts 0, 1, and 2 represent air, the matching layer, and the medium to be detected; r 01 , r 12 represents the reflection coefficient of the electromagnetic wave at the interface of different media, and t 01 represents the transmission coefficient of the electromagnetic wave at the interface of different media; φ 01 , φ 10 , φ 12 represent the phase corresponding to the reflection coefficient and the phase corresponding to the transmission coefficient, and θ is the phase delay when the electromagnetic wave propagates in the matching layer medium. The reflection coefficient, transmission coefficient, and their corresponding phases at each medium interface are as Figure 3 shown.

[0086] The total reflection coefficient calculation formula of the double-layer anti-reflection broadband matching layer is:

[0087]

[0088] In the formula, the subscripts 0, 1, and 2 represent air, the first-layer matching layer, and the second-layer matching layer respectively, the subscript es represents the whole formed by the second-layer matching layer and the medium to be measured, and θ is the phase delay when the electromagnetic wave propagates in the first-layer matching layer medium. It can be seen from the formula that the conditions for the electromagnetic wave interference to subtract and the reflection to be zero are expressed as r 01 = r 10 = r 12 and φ 10 + φ 12 - 2θ = 0. From Figure 4 the simulation results, it can be seen that when the amplitude and phase at the medium interface approximately satisfy the above two zero reflection conditions within the working frequency band, the reflectivity at the incident interface and the transmittance entering the medium to be measured are as Figure 4 d shown, and the results show that the structure of this embodiment has a good matching effect within the frequency band that satisfies the approximate zero reflection condition.

[0089] Embodiment 2

[0090] As a non - restrictive example, this embodiment presents a multi - layer broadband tunable antireflection metasurface structure. The metasurface structure includes a dielectric plate, and the front surface of the dielectric plate is engraved with periodically arranged metal resonant rings, which together with the dielectric plate form an antireflection matching layer; the metal resonant rings are metal resonant rings with adjustable structural parameters to achieve full transmission of electromagnetic waves within an ultra - wide tunable frequency band.

[0091] As Figure 4 shown, the antireflection matching layer in this embodiment is a single - layer antireflection broadband matching layer, which includes a dielectric plate. The front surface of the dielectric plate is engraved with the first - layer metal resonant rings arranged periodically, and the back surface is engraved with the second - layer metal resonant rings arranged periodically. The arrangement periods of the first - layer metal resonant rings and the second - layer metal resonant rings are the same, and the ones in the same unit of the two layers pass through the same central axis.

[0092] Among them, the first - layer metal resonant rings adopt Figure 1 the circular metal split - ring resonator in a a, the width of the resonant ring w1 = 3mm, the outer - ring radius r a = 14mm, the central angle Ф of the outer arc b = 85°, the central angle Ф of the inner arc Figure 1 = 60°, and the interval g between the inner and outer arcs = 3.5mm; the second - layer metal resonant rings adopt c the metal resonant split - ring with a straight - bar resonator in

[0093] c, the width of the resonant ring w2 = 2mm, the outer - ring radius r Figure 1 = 9mm, and the notch width s = 3mm. The period of the unit structure is p = 30mm, the thickness of the dielectric plate is t = 12mm, and the relative dielectric constant of the dielectric plate is 4.

[0094] From Figure 4 b and Figure 4 c, it can be seen that the zero - reflection condition of the single - layer antireflection broadband matching layer is satisfied best at 2GHz, where the phase condition is fully satisfied. Figure 4 The reflectivity curve in

[0095] d also shows that the reflectivity is almost zero at 2GHz and less than 10% within a relative bandwidth of 65%. Broadband transmission can be achieved through simple adjustment of the sizes of the two layers of resonant rings and the thickness of the dielectric plate. Figure 2 a and Figure 2As shown in Fig. 2b, the thickness of the metasurface device is reduced to 0.16λ, which is much thinner than the 0.25λ of the conventional Chebyshev transmitter, and the bandwidth is very impressive in the prior art involving a single-layer anti-reflection broadband matching layer. As a non-limiting example, Figure 4-8 The relative dielectric constants of the measured media shown are all 16, and the original reflectivity without the matching layer is all 36%.

[0096] The other settings in this embodiment are the same as those in Embodiment 1.

[0097] Example 3

[0098] like Figure 5 The figure shows a double-layer Chebyshev transmission line model, in which the thickness of the first dielectric plate is t=25.22mm, the relative dielectric constant of the dielectric plate is 2.21, and the thickness of the second dielectric plate is t=13.94mm, and the relative dielectric constant of the dielectric plate is 7.24. This model has a wider matching band in the existing traditional matching method, with a working bandwidth of 0.97-3.03GHz with a reflectivity lower than 0.1, a center frequency of 2GHz, and a relative bandwidth of 103%.

[0099] like Figure 6 As shown in FIG. 1 , the first layer of the double-layer anti-reflection broadband matching layer with reduced thickness is engraved with a periodic array of metal resonant rings on the front surface of the first dielectric plate. Figure 1 In the structure of a, the width of the resonant ring w1 = 4 mm, the radius of the outer ring r a =14mm, the center angle of the outer arc Ф a =80°, the center angle of the inner arc Ф b =80°, the interval between the inner and outer arcs is g=3.5mm. The period of the unit structure is p=30mm, the thickness of the first dielectric plate is t=15mm, the relative dielectric constant of the dielectric plate is 2.21, the thickness of the second dielectric plate is t=13.94mm, and the relative dielectric constant of the dielectric plate is 7.24. Figure 6 As shown in b, the transmittance is greater than 90% within a relative bandwidth of 104%, and the operating frequency band is 0.97-3.06 GHz.

[0100] The other settings in this embodiment are the same as those in Embodiment 1.

[0101] Example 4

[0102] like Figure 7 As shown in the figure, the second layer of the double-layer anti-reflection broadband matching layer with reduced thickness, the front surface of the second dielectric plate is engraved with a metal resonant ring, and the two middle layers are matching layer dielectrics with thicknesses of l1 = 25.22 mm and l2 = 10 mm respectively. Among them, the second layer of resonant ring adopts Figure 1 The circular metal split ring resonator in a has a width of w1 = 3 mm and an outer ring radius of ra = 10.5 mm, the central angle Ф of the outer arc a = 80°, the central angle Ф of the inner arc b = 60°, the interval g between the inner and outer arcs = 2 mm. The period of the unit structure is p = 30 mm, the thickness of the first dielectric plate is t = 25.22 mm, the relative dielectric constant of the dielectric plate is 2.21, the thickness of the first dielectric plate is t = 10 mm, and the relative dielectric constant of the dielectric plate is 7.24. As Figure 7 shown in b, the transmittance is greater than 90% within the relative bandwidth of 96%, and the operating frequency band is 1.06 - 3.01 GHz.

[0103] Other settings in this embodiment are the same as those in Embodiment 1.

[0104] Embodiment 5

[0105] As Figure 8 shown, it is a double-layer antireflection broadband matching layer, and metal resonant rings are engraved on both sides of the dielectric plate. The middle two layers are the matching layer dielectrics, with thicknesses of l1 = 15 mm and l2 = 10 mm respectively, and the backend is the dielectric to be measured. Among them, the first-layer resonant ring adopts Figure 1 the structure in a, the width of the resonant ring w1 = 4 mm, the radius r of the outer ring a = 14 mm, the central angle Ф of the outer arc a = 80°, the central angle Ф of the inner arc b = 80°, the interval g between the inner and outer arcs = 3.5 mm. The second-layer resonant ring adopts Figure 1 the structure in a, the width of the resonant ring w1 = 3 mm, the radius r of the outer ring a = 10.5 mm, the central angle Ф of the outer arc a = 80°, the central angle Ф of the inner arc b = 60°, the interval g between the inner and outer arcs = 2 mm. The period of the unit structure is p = 30 mm, the thickness of the first dielectric plate is t = 15 mm, the relative dielectric constant of the dielectric plate is 2.21, the thickness of the first dielectric plate is t = 10 mm, and the relative dielectric constant of the dielectric plate is 7.24. As shown in Figure (8b), the transmittance is greater than 90% within the relative bandwidth of 100%, and the operating frequency band is 1.07 - 3.22 GHz.

[0106] Other settings in this embodiment are the same as those in Embodiment 1.

[0107] Embodiment 6

[0108] As Figure 9 shown, it is a frequency-tunable double-layer antireflection broadband matching layer. The two dielectric plates are exactly the same as the Chebyshev transmitter (designed operating frequency is 2 GHz), and only metal resonant rings are engraved on both sides of the dielectric plate to achieve the function of frequency tuning. Among them, the first-layer resonant ring adopts Figure 1The structure in a, the width of the resonant ring w1 = 4 mm, the radius of the outer ring r a = 14 mm, the central angle Ф of the outer arc a = 80°, the central angle Ф of the inner arc b = 80°, the interval g between the inner and outer arcs = 3.5 mm. The second-layer resonant ring adopts Figure 1 the structure in a, the width of the resonant ring w1 = 3 mm, the radius of the outer ring r a = 10.5 mm, the central angle Ф of the outer arc a = 80°, the central angle Ф of the inner arc b = 60°, the interval g between the inner and outer arcs = 2 mm. The period of the unit structure is p = 30 mm, the thickness of the first-layer dielectric plate is t = 25.22 mm, the relative dielectric constant of the dielectric plate is 2.21, the thickness of the first-layer dielectric plate is t = 13.94 mm, and the relative dielectric constant of the dielectric plate is 7.24.

[0109] For a Chebyshev transmitter designed to operate at 1 GHz, after loading the metasurface, the operating frequency shifts to 0.87 GHz, with an offset of 13%; for a Chebyshev transmitter designed to operate at 1.5 GHz, after loading the metasurface, the operating frequency shifts to 1.23 GHz, with an offset of 18%; for a Chebyshev transmitter designed to operate at 2 GHz, after loading the metasurface, the operating frequency shifts to 1.55 GHz, with an offset of 22.5%; for a Chebyshev transmitter designed to operate at 2.5 GHz, after loading the metasurface, the operating frequency shifts to 1.84 GHz, with an offset of 26.4%. While tuning the frequency, the relative bandwidth of the above devices remains at about 100%, demonstrating the good tunable broadband performance of the metasurface.

[0110] Figure 11 Describes Figure 8The reflection spectrum images of electromagnetic waves incident at multiple angles in both TE and TM polarization modes for the structure shown in a. In the TE (transverse electric polarization) mode, as the incident angle increases to 45°, the relative bandwidth of the antireflection frequency band with a reflectivity less than 0.1 can still reach 84.76%. In the TM (transverse magnetic polarization) mode, when the incident angle is 20°, the relative working bandwidth is 101.37%. When the incident angle is 40°, the relative working bandwidth is 112.02%. When the incident angle is 60°, the relative working bandwidth broadens to 162.89%. In the TE polarization, due to the gradual change of the amplitude and phase of the reflection coefficient and transmission coefficient at the two interfaces where the metal rings are located with the increase of the incident angle, the reflectivity also gradually increases, so the working bandwidth decreases to a certain extent. This is mainly probably because the magnetic field component parallel to the metasurface decreases with the increase of the incident angle. For the TM polarization case, since the magnetic field vector remains parallel to the metasurface, the minimum value of the reflectivity only changes slightly and does not change with the increase of the incident angle. Therefore, the working bandwidth is not damaged and increases with the increase of the incident angle. Figure 13 The experimental measurement results shown are in good agreement with the simulation. It can be seen that this type of matching layer has a good transmission enhancement effect on electromagnetic waves with different frequencies, different polarization modes, and different incident angles, which will play an important advantage in practical non-destructive detection applications.

[0111] Figure 12 The measurement experimental device and experimental results for the matching layer. First, use Figure 8 the structure shown in a to fabricate a periodically arranged matching layer, and the resonant rings engraved in front of each dielectric plate are arranged in a 20*20 periodic pattern. The overall length and width of the matching layer sample are both 600 mm, and the total thickness is 25 mm. Considering the application scenarios in the actual non-destructive detection field, a foam brick with a certain humidity is used as the object to be measured, and the changes in reflectivity and transmittance before and after the placement of the matching layer are measured in the laboratory, and each measured value is normalized with an aluminum plate to obtain the reflectivity in different situations. The specific measurement device is as shown in Figure 12 a.

[0112] Figure 12b represents the measured transmittance, absorptance, and reflectance curves. Among them, the horizontal solid line is the original reflectance of the wet brick without the matching layer. The dotted line is the comparison of the corresponding simulation results. When there is no matching layer, the reflectance of the wet brick is about 40%, which indicates that the impedance mismatch on the ground surface causes unnecessary strong reflection, resulting in a large amount of energy being unable to enter the internal medium to be measured and being wasted, limiting the detection depth and sensitivity of the detection signal, and increasing the difficulty of detecting deeper and smaller objects by the detection system. By loading the double-layer ultra-wideband anti-reflection matching layer involved in the present invention, the reflectance on the surface of the incident interface reaches below 10% within the range of 1.21 GHz - 3.26 GHz, and the transmittance is basically greater than 90% within this frequency band. This is because the dielectric plates for engraving the metasurface are made of low-loss materials and have a relatively thin thickness, so the absorptance is very small. The matching layer shows excellent anti-reflection and transmission enhancement effects, and the reflectance is lower than the original reflectance of the wet brick when the matching layer is placed within the frequency band of 0.5 - 3.5 GHz, which indicates that this type of matching layer has great application value in the field of impedance mismatch. For example, it can enable the detection signal of the radar to detect deeper buried objects.

[0113] Other settings in this embodiment are the same as those in Embodiment 1.

[0114] In summary, the multi-layer broadband tunable anti-reflection metasurface structure designed by the present invention can effectively solve the impedance mismatch problem, significantly weaken the strong reflection at the interface of different media, and has the characteristics of being thin, light, transmission enhancement, and ultra-wideband. It is of great significance for improving the detection depth and detection sensitivity of non-destructive testing technologies such as ground penetrating radar. When the polarization mode of the electromagnetic wave changes and the incident angle increases, the present invention still maintains strong ultra-wideband transmission performance and has an obvious enhancement effect on the incident signal. Therefore, the present invention has many advantages that can be effectively applied to the field of non-destructive detection. It can be used for the detection of the main application objects of non-destructive detection such as wall, road, railway, and tunnel detection, and the detection of mines, pipelines, underground cavities, water accumulation, and underground buried objects.

[0115] The above is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention can easily think of various equivalent modifications or substitutions, and these modifications or substitutions should all be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A multi-layer broadband tunable anti-reflection impedance matching method, characterized in that, The method includes the following steps: Step S1: Based on the small reflection theory, design a Chebyshev transducer to determine the dielectric constant of each layer of the medium. Specifically, based on the correspondence between the total reflectivity and the Chebyshev polynomial, solve the reflection coefficient at each interface, and gradually calculate the impedance of each layer of the matching layer, and then calculate the dielectric constant of the matching layer; Step S2: Based on the multi-layer interference theory, after loading the metal resonant ring, determine the change in the transmission and reflection coefficient at its interface, and calculate the total reflection coefficient of the anti-reflection matching layer when the electromagnetic wave propagates from the detection scene medium through the anti-reflection matching layer to the medium to be measured; Step S3: Based on the zero reflection theory obtained by subtracting electromagnetic wave interference, adjust the structural parameters of the metal resonant ring to ensure that when the size of the dielectric layer decreases, the change in the impedance matching frequency band is within the preset range; Step S4: According to the zero reflection theory and the frequency adjustment requirements, adjust the structural parameters of the metal resonant ring without changing the thickness of the dielectric layer to shift the working frequency band of the anti-reflection matching layer; The specific content of step S1 is as follows: Given the air impedance Z0 and the impedance Z of the medium to be measured L , the number of layers N of the Chebyshev transducer, the maximum allowable reflection coefficient A, by establishing a one-to-one correspondence between the expression of the total reflectance Γ(θ) and the Chebyshev polynomial T N (secθ m cosθ), we obtain: Γ(θ) = 2e -jNθ [Γ0cosNθ + Γ1cos(N - 2)θ + … + Γ n cos(N - 2n)θ + …] = Ae -jNθ T N (secθ m cosθ) where θ is the phase delay when the electromagnetic wave propagates in each matching layer, and θ m is the phase corresponding to the minimum frequency in the passband response frequency band, and is defined as: secθ m = cosh[1 / N * cosh -1 (|(lnZ L / Z0) / (2|A|)|)] The reflection coefficient at each interface layer is calculated by the above formula, and then the formula Γ N = 1 / 2 * ln(Z n+1 / Z n ) is used to calculate step by step the impedance of each layer of the matching layer, and finally the relative permittivity of the matching layer is calculated. The thickness of each layer of the medium is one quarter of the wavelength.

2. The multi-layer broadband tunable anti-reflection impedance matching method according to claim 1, wherein The anti-reflection matching layer in step S2 is the total reflection coefficient of a single-layer anti-reflection broadband matching layer or a double-layer anti-reflection broadband matching layer, and the corresponding total reflection coefficients are respectively: The total reflection coefficient of the single-layer anti-reflection broadband matching layer is: In the formula, the subscripts 0, 1, and 2 respectively represent the detection scene medium, the anti-reflection matching layer, and the medium to be detected, r represents the reflection coefficient of the electromagnetic wave at the interface of different media, φ represents the phase corresponding to the reflection coefficient, and θ is the phase delay when the electromagnetic wave propagates in the anti-reflection matching layer medium; The total reflection coefficient of the double-layer anti-reflection broadband matching layer is: In the formula, the subscripts 0, 1, and 2 respectively represent the detection scene medium, the first-layer anti-reflection broadband matching layer, and the second-layer anti-reflection broadband matching layer, and the subscript es represents the whole formed by the second-layer anti-reflection broadband matching layer and the medium to be detected; r represents the reflection coefficient of the electromagnetic wave at the interface of different media, φ represents the phase corresponding to the reflection coefficient, and θ is the phase delay when the electromagnetic wave propagates in the first-layer anti-reflection broadband matching layer medium.

3. A metasurface structure based on the multi-layer broadband tunable anti-reflection impedance matching method according to claim 1, characterized in that, The metasurface structure includes a dielectric plate, and the front surface of the dielectric plate is engraved with periodically arranged metal resonant rings, which together with the dielectric plate form an anti-reflection matching layer; the metal resonant rings are metal resonant rings with adjustable structural parameters to achieve all transmission of electromagnetic waves in an ultra-wide adjustable frequency band.

4. The metasurface structure according to claim 3, characterized in that, The anti-reflection matching layer is a single-layer anti-reflection broadband matching layer or a double-layer anti-reflection broadband matching layer; The single-layer anti-reflection broadband matching layer includes a layer of dielectric plate, the front surface of the dielectric plate is engraved with the first-layer metal resonant rings arranged periodically, and the back surface is engraved with the second-layer metal resonant rings arranged periodically; The double-layer anti-reflection broadband matching layer includes two first-layer dielectric plates and second-layer dielectric plates that are fitted together. The front surface of the first-layer dielectric plate is engraved with the first-layer metal resonant rings, and the front surface of the second-layer dielectric plate is engraved with the second-layer metal resonant rings; Both the first-layer metal resonant rings and the second-layer metal resonant rings are metal split resonant rings, and their sizes are slightly smaller than the period length and the thickness is extremely thin.

5. The metasurface structure according to claim 3, wherein The thickness of the single-layer antireflection broadband matching layer under 100% of the working bandwidth is less than 0.2 times the electromagnetic wave wavelength, and the thickness of the double-layer antireflection broadband matching layer is less than 0.35 times the electromagnetic wave wavelength.

6. The metasurface structure according to claim 4, wherein The antireflection matching layer is a single-layer antireflection broadband matching layer, and the second-layer metal resonant ring is a metal resonant split ring with a straight-bar resonator.

7. The metasurface structure according to claim 4, wherein The metal resonant split ring is a circular double-split-ring resonator with inner and outer nesting or a square double-split-ring resonator with inner and outer nesting.

8. The metasurface structure according to claim 4, wherein The arrangement periods of the first-layer metal resonant ring and the second-layer metal resonant ring are the same, and the metal resonant rings in the same unit in the two layers pass through the same central axis.

9. The metasurface structure according to claim 3, characterized in that, The dielectric plate is a low-loss dielectric plate with a relative permittivity between 1 and 8.

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