A Reconfigurable Ultra-Wideband Metasurface with Dual Functions of Absorbing Waves and Polarization Conversion

Abstract

The present invention proposes a reconfigurable ultra-wideband metasurface with dual functions of wave absorption and polarization conversion, which solves the problem that it is difficult for existing dual-functional reconfigurable metasurfaces to achieve ultra-wideband wave absorption and ultra-wideband polarization conversion functions. It includes a first dielectric substrate with graphene patches printed on the upper surface and a first vanadium dioxide annular patch printed on the lower surface, second, third, and fourth dielectric substrates with second, third, and fourth vanadium dioxide annular patches printed on the lower surfaces respectively, and a fifth dielectric substrate with a metal bottom plate printed on the lower surface. When the Fermi level of graphene is 0 eV and the temperature of vanadium dioxide is 70 °C, the metasurface realizes broadband wave absorption. When the Fermi level of graphene is 1 eV and the temperature of vanadium dioxide is 50 °C, the metasurface realizes ultra-wideband linear-circular polarization conversion. The present invention realizes ultra-wideband wave absorption and ultra-wideband linear-circular polarization conversion by using a metasurface structure, and can be used for terahertz wave absorption and terahertz communication.

Description

Technical Field

[0001] The present invention belongs to the technical field of terahertz multifunctional devices, and relates to a reconfigurable ultra-wideband metasurface, specifically to a reconfigurable ultra-wideband metasurface with dual functions of wave absorption and polarization conversion, which can be used in the fields of terahertz wave absorption, terahertz communication, etc. Technical Background

[0002] The regulation of terahertz waves is an important research direction in the field of electromagnetic wave regulation. Compared with traditional terahertz devices, terahertz metasurfaces have the advantages of simple design and small volume. By designing terahertz metasurfaces with excellent structures and properties, some characteristics of electromagnetic waves can be regulated. For example, an absorptive metasurface is used to achieve the absorption of electromagnetic radiation, and a polarization converter is used to change the polarization state of electromagnetic waves. For a metasurface, integrating two functions onto one metasurface to form a dual-functional reconfigurable metasurface will improve its usability. At the same time, for a dual-functional reconfigurable metasurface, the wider the working bandwidth of each function, the more applicable scenarios there are, and ultra-wideband wave absorption and ultra-wideband polarization conversion are widely used, such as electromagnetic shielding and satellite communication.

[0003] A common method to realize a dual-functional reconfigurable metasurface is to introduce materials with adjustable characteristics during the design of the metasurface. Since the Fermi level of graphene changes with the bias voltage, resulting in a change in the conductivity of graphene, and the conductivity of vanadium dioxide undergoes a sudden change under the influence of temperature, graphene and vanadium dioxide are often used in the design of dual-functional reconfigurable metasurfaces. For example, in the patent application with the application publication number CN 114824819 A and the name "Dual-functional metasurface based on the absorption and polarization conversion characteristics of graphene and VO2", a square vanadium dioxide patch with cracks is printed on the upper surface of the first dielectric substrate, and four groups of a total of eight graphene resonant rings are printed on the lower surface. By regulating the Fermi level of graphene, wave absorption is achieved, and by regulating the temperature of vanadium dioxide, linear-linear polarization conversion is achieved. However, in this invention, due to the use of four groups of graphene patches of equal size and single-layer vanadium dioxide patches, the resonant frequency points of wave absorption and linear-linear polarization conversion generated are fewer, the bandwidth is narrower, and the application scenarios are limited, and broadband wave absorption and broadband communication cannot be achieved.

[0004] To broaden the working bandwidth of the reconfigurable dual-functional reconfigurable metasurface, the existing invention is to increase the resonant frequencies of wave absorption and polarization conversion. For example, in the patent application with the publication number CN 114759356 A and the title "Metamaterial Unit and Planar Metamaterial with Dual Functions of Wave Absorption and Linear Polarization Conversion", the wave absorption bandwidth is broadened by introducing resistance, and the polarization conversion bandwidth is broadened by using a multimode resonator. The relative bandwidth of wave absorption of this invention is 40%, and the relative bandwidth of polarization conversion is 50%. Both the wave absorption and polarization conversion bandwidths are broadened, but it is still impossible to achieve ultra-wideband wave absorption and ultra-wideband polarization conversion. Moreover, the method of broadening the bandwidth in this invention is not applicable to the terahertz band.

[0005] Since graphene and vanadium dioxide can realize the reconfigurable dual-functional metasurface, but many existing studies can only achieve narrowband wave absorption and narrowband polarization conversion. Summary of the Invention

[0006] The purpose of the present invention is to propose a reconfigurable ultra-wideband metasurface with dual functions of wave absorption and polarization conversion to solve the technical problem of narrow bandwidth existing in the prior art in view of the problems existing in the prior art.

[0007] To achieve the above purpose, the technical solution adopted by the present invention includes N×N periodically arranged metasurface units, N≥2; characterized in that: the metasurface unit includes a first dielectric substrate 1 with a graphene patch 6 printed on the upper surface and a first vanadium dioxide annular patch 7 printed on the lower surface, a second dielectric substrate 2 with a second vanadium dioxide annular patch 8 printed on the lower surface, a third dielectric substrate 3 with a third vanadium dioxide annular patch 9 printed on the lower surface, a fourth dielectric substrate 4 with a fourth vanadium dioxide annular patch 10 printed on the lower surface, and a fifth dielectric substrate 5 with a metal bottom plate 11 printed on the lower surface; the graphene patch 6 is composed of an I-shaped graphene patch 61 located on the two diagonals of the first dielectric substrate 1 and a strip-shaped graphene patch 62 not connected to it; the five dielectric substrates all adopt a square structure; the perimeters of the four vanadium dioxide annular patches are not equal; the I-shaped graphene patch 61 in the intersecting graphene patch 6 forms a high-frequency band polarization conversion resonance, and the strip-shaped graphene patch 62 forms a low-frequency band linear-circular polarization conversion resonance, realizing the ultra-wideband linear-circular polarization conversion characteristic. Through the four mutually stacked vanadium dioxide annular patches, the large-sized resonant ring regulates the low-frequency wave absorption resonance point, and the small-sized resonant ring regulates the high-frequency wave absorption resonance point, realizing the ultra-wideband wave absorption characteristic of the metasurface.

[0008] For the above reconfigurable ultra-wideband metasurface with dual functions of wave absorption and polarization conversion, the four vanadium dioxide annular patches all adopt an equi-width square ring structure, and the centers of the four vanadium dioxide annular patches are all located on the central normal line of the dielectric substrate where they are located.

[0009] The above-mentioned reconfigurable ultra-wideband metasurface with dual functions of wave absorption and polarization conversion, the first vanadium dioxide annular patch 7, the second vanadium dioxide annular patch 8, the third vanadium dioxide annular patch 9, and the fourth vanadium dioxide annular patch 10, their perimeter sizes increase or decrease in sequence.

[0010] The above-mentioned reconfigurable ultra-wideband metasurface with dual functions of wave absorption and polarization conversion, the graphene patch 6, the widths of the two transverse branches of the I-shaped graphene patch 61 it contains are equal to the width of the longitudinal branch; the width of the strip-shaped graphene patch 62 is greater than the widths of the three branches of the I-shaped graphene patch 61; the lengths of the I-shaped graphene patch 61 and the strip-shaped graphene patch 62 are not equal.

[0011] The above-mentioned reconfigurable ultra-wideband metasurface with dual functions of wave absorption and polarization conversion, the graphene patch 6, the midpoint of the cross-shaped structure formed by the I-shaped graphene patch 61 and the strip-shaped graphene patch 62 it contains is located on the central normal line of the first dielectric substrate 1.

[0012] The above-mentioned reconfigurable ultra-wideband metasurface with dual functions of wave absorption and polarization conversion, the strip-shaped graphene patch 62, a rectangular slit for the I-shaped graphene patch 61 to pass through is etched at the position of the midpoint connection line of its two long sides.

[0013] Compared with the prior art, the present invention has the following advantages:

[0014] In the present invention, the stacked structure of the vanadium dioxide annular patch greatly broadens the wave absorption bandwidth, and multiple graphene patches are combined and arranged to broaden the bandwidth of the line-circular polarization conversion by respectively regulating the resonance frequencies of the low-frequency and high-frequency line-line to circular polarization conversion. When the metasurface is used as a wave absorber, the wave absorption rate can be greater than 95% in the range of 1.43 - 8.96 THz, and when the metasurface is used as a polarization converter, the line-circular polarization conversion with an ellipticity greater than 0.95 can be achieved in the range of 1.24 - 3.57 THz. Description of the Drawings

[0015] Figure 1 It is a schematic diagram of the overall structure of the present invention.

[0016] Figure 2 It is a top view schematic diagram of the graphene patch of the present invention.

[0017] Figure 3 It is a top view of the vanadium dioxide annular patch of the present invention.

[0018] Figure 4 It is a wave absorption rate curve diagram when the present invention shows the wave absorption function.

[0019] Figure 5It is the ellipticity curve graph when the present invention exhibits the line - circular polarization conversion function.

[0020] Figure 6 They are the wave absorption rate graphs and ellipticity graphs of the present invention under different polarization modes and different incident angles. Specific Embodiments

[0021] The following further elaborates in detail in conjunction with the attached drawings and specific embodiments.

[0022] Refer to Figure 1 , the present invention includes a first dielectric substrate 1, a second dielectric substrate 2, a third dielectric substrate 3, a fourth dielectric substrate 4, and a fifth dielectric substrate 5 which are arranged in sequence from top to bottom and in contact with each other. The materials of the five - layer dielectric substrates are polymethacrylimide, and the dielectric constant is 1.05; the thickness of the first dielectric substrate is 1μm, and a graphene patch 6 is printed on the upper surface of the first dielectric substrate, and a first vanadium dioxide annular patch 7 is printed on the lower surface of the first dielectric substrate; the thickness of the second dielectric substrate is 7μm, and a second vanadium dioxide annular patch 8 is made on the lower surface of the second dielectric substrate 2, the thickness of the third dielectric substrate is 7μm, and a third vanadium dioxide annular patch 9 is made on the lower surface of the third dielectric substrate 3, the thickness of the fourth dielectric substrate is 7μm, and a fourth vanadium dioxide annular patch 10 is made on the lower surface of the fourth dielectric substrate 4; the thickness of the fifth dielectric substrate is 9μm, and a metal bottom plate 11 is made on the lower surface of the fifth dielectric substrate 5, the thickness of the metal bottom plate is 0.3μm, and the metal material is gold.

[0023] Refer to Figure 2 , in order to ensure the purity of the reflected circularly polarized wave, the I - shaped graphene patch 61 and the strip - shaped graphene patch 62 are cross - distributed on the diagonal of the first dielectric substrate 1. The lengths of the two ends of the I - shaped patch 61 are a = 12μm and the width d1 = 4μm, the length of the middle rectangular part of the I - shaped graphene patch 61 is l = 23.5μm, and the width is also d1. The length of the strip - shaped graphene patch 62 is b = 14μm, and the width d2 = 7μm. The period p of the dielectric substrate is 31μm. In order to broaden the line - circular polarization conversion bandwidth, there is a gap d3 = 2μm between the I - shaped graphene patch 61 and the strip - shaped graphene patch.

[0024] Refer to Figure 3 , in order to avoid resonance between non - adjacent vanadium dioxide annular patches as much as possible, the four vanadium dioxide annular patches are arranged from high to low according to the size from small to large. And to make the design method have reference value, the widths of the square vanadium dioxide patches are all w = 1.5μm, and the thicknesses are all 0.9μm. The side length s1 of the first vanadium dioxide annular patch 7 is 10μm, the side length s2 of the second vanadium dioxide annular patch 8 is 16μm, the side length s3 of the third vanadium dioxide annular patch 9 is 24μm, and the side length s4 of the fourth vanadium dioxide annular patch 10 is 29μm.

[0025] The working principle of the present invention is as follows: The combined regulation of wave absorption and linear-circular polarization conversion is achieved by using two tunable materials, graphene and vanadium dioxide. The change in the Fermi level of graphene mainly affects the linear-circular polarization conversion performance, while the metallic or insulating state of vanadium dioxide mainly affects the wave absorption performance. The wave absorption rate mainly depends on the reflectivity R(ω) and the transmittance T(ω). The calculation formula for the wave absorption rate is as follows:

[0026] A(ω) = 1 - R(ω) - T(ω)

[0027] Since this invention is a reflective metasurface, when the thickness of the metal bottom plate is greater than the skin depth of the electromagnetic wave, the transmittance T(ω) = 0. Therefore, the key point to achieve efficient wave absorption lies in reducing the reflectivity R(ω). Embedding four vanadium dioxide patches in the dielectric substrate can make the electromagnetic wave resonate in the dielectric substrate layer and be quickly dissipated. Measuring the performance of polarization conversion requires using the Stokes parameters, and the Stokes parameters are defined as S = [I Q U V] T , where I represents the total electromagnetic wave intensity, Q represents the polarization component in the x direction, U represents the polarization component in the direction of the 45° angle formed by the xoy plane and the x axis, and V represents the component of the right-handed circularly polarized wave; the specific calculation formulas for these four parameters are as follows:

[0028] I = |E x (ω)| 2 +|E y (ω)| 2

[0029] Q = |E x (ω)| 2 -|E y (ω)| 2

[0030]

[0031]

[0032] |E x (ω)| and |E y (ω)| in the above formulas respectively represent the amplitudes of the electric field components in the x and y directions of the electric field. respectively represent the phases corresponding to the x-polarized and y-polarized electromagnetic waves. The ellipticity is the result of further derivation based on the Stokes parameters, and its expression is

[0033]

[0034] The value range of χ is from -1 to 1. When χ is closer to 1, it indicates that the right-handed circularly polarized wave is more perfect. When χ is closer to -1, it indicates that the left-handed circularly polarized wave is more perfect. And the bandwidth of the line-circular polarization conversion is broadened by means of multiple graphene patches, and the bandwidth of wave absorption is broadened by stacking multiple vanadium dioxide patches.

[0035] 1. Simulation conditions and content

[0036] The technical effects of the present invention will be described below in combination with simulation experiments:

[0037] Simulation 1: For the present invention, the frequency range is set to 1.0 - 9.5 THz, the temperature of vanadium dioxide is set to 70 °C, the Fermi level of graphene is set to 0 eV, the dielectric constants of graphene, vanadium dioxide, and gold in the terahertz frequency band are all given by the Drude model, the incident direction of the electromagnetic wave is the vertical direction, the wave absorption performance of the present invention is simulated using the Floquet mode in CST, and the port is set at a certain distance from the unit surface to simulate the far-field condition. The simulation model is as Figure 4 (a) shown, and the simulation results are as Figure 4 (b) shown. [[ID=…]]

[0038] Simulation 2: For the present invention, the frequency range is set to 1.0 - 3.7 THz, the Fermi level of graphene is set to 0 eV, the temperature of the vanadium dioxide annular patch is set to 50 °C. At this time, the vanadium dioxide annular patch shows an insulating state. The dielectric constants of graphene, vanadium dioxide, and gold in the terahertz frequency band are all given by the Drude model, the incident direction of the electromagnetic wave is the vertical direction, and the line-circular polarization conversion performance of the present invention is simulated using the Floquet mode in CST. The simulation model is as Figure 5 (a) shown, and the simulation results are as Figure 5 (b) shown.

[0039] Simulation 3: Respectively simulate the wave absorption rate curves when the metasurface shows wave absorption function and the ellipticity curves when it shows line-circular polarization conversion under the TE polarization mode and the TM polarization mode, with the incident angles of 0°, 10°, 20°, 30°, 40°, 50°, and 60°. The frequency range for simulating the wave absorption rate curves is set to 1.0 - 9.5 THz, and the frequency range for simulating the ellipticity curves is 1.0 - 3.7 THz. The simulation results are as Figure 6 (a)-(d).

[0040] 2. Analysis of simulation results

[0041] Referring to Figure 4 , where Figure 4 After the linearly polarized wave in (a) is incident vertically on the metasurface structure along the -z direction, there is no reflected wave. Figure 4In (b), it shows that the metasurface has an absorption rate greater than 95% for incident waves in the frequency range of 1.43 - 8.96 THz, achieving ultra-wideband high-efficiency wave absorption, and the relative bandwidth reaches 145%. The electric field distributions at the resonant peak points of 1.61 THz, 3.82 THz, and 8.52 THz of the absorption rate curve are analyzed respectively. The resonance at the low frequency of 1.61 THz comes from the interaction between the third vanadium dioxide patch and the fourth vanadium dioxide patch, while the resonance generated at 3.82 THz comes from the interaction between the second vanadium dioxide patch and the third vanadium dioxide patch. The resonance at the high frequency of 8.52 THz comes from the internal resonance of the first vanadium dioxide patch. That is to say, the generation of ultra-wideband wave absorption in the present invention comes from the broadening of multiple resonant peaks with close frequencies.

[0042] Refer to Figure 5 , where Figure 5 After the linearly polarized wave in (a) is incident vertically on the metasurface structure along the -z direction, a circularly polarized reflected wave will be generated. Figure 5 In (b), it shows the ellipticity curve of the linear - circular polarization conversion of the present invention. The present invention can achieve efficient ultra-wideband linear - circular polarization conversion with an absolute value of ellipticity greater than 0.95 in the frequency range of 1.24 - 3.57 THz. In Figure 5 (b), the ellipticity curves of the first dielectric substrate with only I-shaped graphene patches and only strip-shaped graphene patches on the upper surface are also given. It can be found that the I-shaped graphene patches regulate the low-frequency part of the ultra-wideband linear - circular polarization conversion, and the strip-shaped graphene patches regulate the high-frequency part of the broadband linear - circular polarization conversion.

[0043] Refer to Figure 6 , as Figure 6 (a) shows that in the case of TE polarization mode incidence, as the incident angle gradually increases from 0°, when the incident angle is less than 40°, the metasurface can always maintain an absorption efficiency greater than 95% and a relative bandwidth above 140%, achieving good ultra-wideband wave absorption. However, as the incident angle is greater than 40°, the absorption rate decreases in both the high-frequency band and the low-frequency band, and the bandwidth with an absorption rate greater than 95% rapidly decreases, but the overall absorption rate is still greater than 80%; as Figure 6 (b) shows that when the electromagnetic wave in TE mode is incident for linear - circular polarization conversion, the absolute value of ellipticity can be maintained greater than 0.95 within the incident angle range of 0 - 20°. When the incident angle is greater than 20°, the relative bandwidth of the polarization conversion efficiency will decrease significantly; as Figure 6 (c) shows that for TM polarization wave incidence, efficient ultra-wideband wave absorption can also be achieved within the range of 0 - 40° in the wave absorption function; as Figure 6(d) As shown, in the case of TM polarized wave incidence, the ultra-wideband line-to-circular polarization conversion performance can be achieved when the incident angle is between 0° and 30°; whether it is the oblique incident electromagnetic wave in TE mode or TM mode, the metasurface can always maintain high wave absorption and polarization conversion performance, and the performance of TM wave in wave absorption and polarization conversion is slightly better than that of TE wave. This is because the resonance generated by the incident wave on the metasurface is mainly magnetic resonance, and magnetic resonance is more easily absorbed or used to achieve line-to-circular polarization conversion. For the wave absorption function, it can achieve efficient ultra-wideband wave absorption when the incident angle is between 0° and 40°. For polarization conversion, when the incident angle is between 0° and 20°, it can maintain broadband and efficient line-to-circular polarization conversion.

[0044] The above description is only the preferred embodiment of the present invention and does not limit the present invention. For those of ordinary skill in the art, several deformations and improvements can be made without departing from the innovative concept of the present invention, but these changes all fall within the protection scope of the present invention.

Claims

1. A reconfigurable ultra-wideband metasurface with dual functions of wave absorption and polarization conversion, comprising N×N periodically arranged metasurface units, where N≥2; characterized in that: The metasurface unit includes a first dielectric substrate (1) with a graphene patch (6) printed on its upper surface and a first vanadium dioxide annular patch (7) printed on its lower surface, a second dielectric substrate (2) with a second vanadium dioxide annular patch (8) printed on its lower surface, a third dielectric substrate (3) with a third vanadium dioxide annular patch (9) printed on its lower surface, a fourth dielectric substrate (4) with a fourth vanadium dioxide annular patch (10) printed on its lower surface, and a fifth dielectric substrate (5) with a metal bottom plate (11) printed on its lower surface; the graphene patch (6) is composed of an I-shaped graphene patch (61) located on two diagonals of the first dielectric substrate (1) and a strip-shaped graphene patch (62) not connected to it; the five dielectric substrates all adopt a square structure; the perimeters of the four vanadium dioxide annular patches are not equal; the I-shaped graphene patch (61) in the intersecting graphene patch (6) forms a high-frequency polarization conversion resonance, and the strip-shaped graphene patch (62) forms a low-frequency line-circular polarization conversion resonance to achieve the ultra-wideband line-circular polarization conversion characteristic. Through the four mutually stacked vanadium dioxide annular patches, the resonance ring with a larger size regulates the low-frequency absorption resonance point, and the resonance ring with a smaller size regulates the high-frequency absorption resonance point to achieve the ultra-wideband absorption characteristic of the metasurface.

2. The reconfigurable ultra-wideband metasurface with both microwave absorption and polarization conversion functions according to claim 1, wherein The four vanadium dioxide annular patches all adopt a square ring structure with equal width, and the centers of the four vanadium dioxide annular patches are all located on the central normal line of the dielectric substrate where they are located.

3. The reconfigurable ultra-wideband metasurface with dual functions of wave absorption and polarization conversion according to claim 2, characterized in that, The perimeters of the first vanadium dioxide annular patch (7), the second vanadium dioxide annular patch (8), the third vanadium dioxide annular patch (9), and the fourth vanadium dioxide annular patch (10) increase or decrease in sequence.

4. A reconfigurable ultra-wideband metasurface with dual functions of wave absorption and polarization conversion according to claim 1, characterized in that, For the graphene patch (6), the widths of the two transverse branches of the I-shaped graphene patch (61) it contains are equal to the width of the longitudinal branch; the width of the strip-shaped graphene patch (62) is greater than the widths of the three branches of the I-shaped graphene patch (61); the lengths of the I-shaped graphene patch (61) and the strip-shaped graphene patch (62) are not equal.

5. A reconfigurable ultra-wideband metasurface with both microwave absorption and polarization conversion functions, as claimed in claim 4, wherein For the graphene patch (6), the midpoint of the cross-shaped structure formed by the I-shaped graphene patch (61) and the strip-shaped graphene patch (62) it contains is located on the central normal line of the first dielectric substrate (1).

6. The reconfigurable ultra-wideband metasurface with dual functions of wave absorption and polarization conversion according to claim 5, characterized in that For the strip-shaped graphene patch (62), a rectangular slit for the passage of the I-shaped graphene patch (61) is etched at the position of the midpoint connection of its two long sides.

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