Ultrabroadband polarization converter metasurfaces for focusing lenses and spatial imaging

Abstract

The application discloses an ultra-wideband polarization converter metasurface for converging lenses and spatial imaging. The cell of the metasurface is composed of two metal gratings, two pairs of L-shaped metal patches and two dielectric plates. The metasurface of the application works in the range of 0.5 to 1.8 THz, the transmission coefficient is greater than 0.87, the relative bandwidth reaches 113%, the polarization conversion rate remains close to 100%, and the phase covers 360°. The metasurface has the advantages of simple structure, small size, high bandwidth, good polarization characteristics, easy integration, low loss and simple preparation process. The metasurface combines the changes of geometric phase and amplitude, which can not only realize two-dimensional focusing lenses by using phase changes, but also realize spatial imaging functions by using amplitude changes. The research results show that the metasurface provides a new way for the flexibility of terahertz wave polarization and phase manipulation, and opens up a new way for the dynamic operation of terahertz wave fronts.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electromagnetic metamaterials, and particularly relates to a super-wideband polarization converter metasurface for converging lenses and spatial imaging. BACKGROUND

[0002] The proposal of metamaterials provides an effective way for the manipulation of THz beams, and has far-reaching significance in the fields of material science, electromagnetics and optics. In recent years, in-depth research has been conducted on metasurface-assisted polarization modulators / conversioners. However, the current terahertz metasurface polarization converters have low transmission efficiency, narrow working bandwidth, single polarization mode and high sensitivity to the direction of incident waves, which greatly limits their practical application. Therefore, under the premise of maintaining high efficiency, how to expand the bandwidth and improve the flexibility of terahertz wave manipulation is still one of the hotspots in the research of metasurfaces.

[0003] Generally, reflection-type phase gradient metasurfaces (PGMS) are easy to exhibit high working efficiency, but due to the problem of interference of incident waves, they are not suitable for many occasions. Therefore, it is of great significance to research high-efficiency wideband transmission-type PGMS. In the present application, we propose a PGMS with multiple plasmonic resonances to realize a super-wideband high-efficiency polarization converter for linear polarization. The transmission efficiency and bandwidth of the PGMS are simultaneously improved by multiple plasmonic resonances, so as to realize a super-wideband high-efficiency linear polarization converter of metasurface. SUMMARY

[0004] The purpose of the present application is to provide a super-wideband polarization converter metasurface for converging lenses and spatial imaging.

[0005] In order to achieve the above purpose, the present application adopts the following technical scheme:

[0006] The super-wideband polarization converter metasurface for converging lenses and spatial imaging comprises a plurality of cell structures of plasmon, each cell structure being a square; the cell structure comprises, from top to bottom, a first metal grating, a first dielectric plate, an L-shaped metal patch, a second dielectric plate and a second metal grating, which are sequentially connected; the first metal grating and the second metal grating are arranged orthogonally; the L-shaped metal patch is composed of two large Ls and two small Ls, wherein the two large Ls are centrally symmetric, the two small Ls are located between the two large Ls, and the two small Ls are oriented in the same direction and the two sides of the small L are perpendicular to the two sides of one of the large Ls.

[0007] Preferably, the L-shaped metal patch is arranged along the diagonal of the dielectric plate; the dielectric plate is a square structure, and the two arms of the two large L-shaped patches and the two small L-shaped patches are equal in length.

[0008] Preferably, the medium plate is a F4B medium plate, the dielectric constant of the medium plate is 3, and the loss tangent is 0.002.

[0009] Further preferably, the thickness h of the medium plate is 30um.

[0010] Preferably, the first metal grating, the second metal grating and the L-shaped metal patch are all made of metal copper with an electrical conductivity of 5.8*10 7 S / m, and the thickness is t=0.3um.

[0011] Preferably, the unit size p of the cell structure is 100um.

[0012] Preferably, in the L-shaped metal patch, the length and width of the large L-shaped patch are L1=68um and W1=9um respectively, and the length and width of the small L-shaped patch are L2=28um and W2=6um respectively.

[0013] Preferably, the width of the first metal grating and the second metal grating is W3=6um, and the gap d between two adjacent grating strips is 20um.

[0014] The coding metasurface is constructed by using the amplitude difference between different units, so as to realize the spatial imaging function.

[0015] By scaling up or down the geometric parameters of the entire intermediate layer structure, a plurality of different unit structures are designed to construct a two-dimensional focusing superlens, so as to realize the function of a converging lens.

[0016] Compared with the prior art, the present application has the following technical effects:

[0017] The present application separates the metal grating and the L-shaped metal patch by the medium plate, and the multiple plasmonic resonance structures are composed of two large L-shaped and two small L-shaped plasmonic antennas, and finally a three-layer structure of high-efficiency broadband PGMS is designed, which can convert x-polarized incident electromagnetic waves into pure y-polarized waves; and the physical mechanism of the efficient operation of the three-layer structure of the present application is verified by the established Fabry-Perot interference model, which can work in an ultra-wide frequency band of 0.5-1.8THz, the size of the cross-polarization transmission coefficient is greater than 0.87, the relative bandwidth reaches 113%, the polarization conversion rate remains close to 100%, and the polarization conversion ratio (PCR) is greater than 99.2%. In addition, the polarization converter metasurface combines the geometric phase and amplitude parameter change, not only realizes a two-dimensional converging lens at 1.6THZ, but also realizes a spatial imaging function at 0.59073THz by using the amplitude change.

[0018] Secondly, the application has the advantages of simple structure, small size, high bandwidth, good polarization characteristics, easy integration, low loss and simple preparation process, and provides a new idea for flexibility of terahertz wave polarization and phase manipulation, and can be applied to super-wideband anomalous refraction, converging lens, orbital angular momentum, imaging and communication. BRIEF DESCRIPTION OF DRAWINGS

[0019] In order to more clearly illustrate the technical solutions of the present application, the drawings needed in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor on the basis of these drawings.

[0020] Figure 1 a is a structural schematic diagram of the metasurface cell of the present application, and 1b is a structural schematic diagram of the intermediate sub-surface structure layer;

[0021] Figure 2 In order to change the intermediate sub-surface Figure 2 a is a pair of L-shaped structures, and 2b is two pairs of mutually symmetrical L-shaped structures;

[0022] Figure 3 Analysis result of response of two pairs of double L-shaped structures of single-layer structure to electromagnetic waves;

[0023] Figure 4 The transmission coefficient of the three structures is compared; Figure 1 and Figure 2 The transmission coefficient of the three structures is compared;

[0024] Figure 5 The transmission coefficient of the three structures is compared;

[0025] Figure 6 The interference model of the surface simulated by MATLAB is provided;

[0026] Figure 7 The interference model of the surface simulated by MATLAB is provided;

[0027] Figure 8 a is the influence of the arm length and W1 between different L1 provided by the present application on the unit transmission efficiency, and 8b is the influence of the distance between different small L on the unit transmission efficiency, Figure 8 c is the influence of the width between different w1 provided by the present application on the unit transmission efficiency;

[0028] Figure 9a is the transmission amplitude difference of two different units provided by the application by changing parameters L1, W1, L2, W2, 9b is the transmission phase difference of two different units provided by the application by changing parameters L1, W1, L2, W2;

[0029] Figure 10 The 33x25 unit middle structure array pattern of "1" and "0" units encoded by different amplitudes provided by the application to form the "IUTL" pattern;

[0030] Figure 11 The simulation near-field imaging effect of the 33x25 unit middle structure array pattern of "1" and "0" units encoded by different amplitudes provided by the application to form the "IUTL" pattern;

[0031] Figure 12 Ten unit structures are provided by the application for constructing a focusing lens design by uniformly reducing or enlarging the overall unit structure. The transmission amplitude of the 10 unit structures is constant at about 0.75, and the phase covers 2π;

[0032] Figure 13 a is the 19x19 two-dimensional focusing superlens middle layer structure diagram constructed by 10 units provided by the application, which is symmetric about the x and y axes. 13b is the cross-polarized electric field density distribution in the xoz plane, 13c is the measured focal length of the cross-polarized electric field density distribution in the xoz plane, which is 810.8μm, and 13d is the cross-polarized electric field density distribution in the xoy plane. DETAILED DESCRIPTION

[0033] The application will be further described below in conjunction with the drawings and specific embodiments. The technical solutions of the application are described in detail with reference to the embodiments described in the drawings, which are exemplary and cannot be understood as limiting the application. All other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of the application.

[0034] Example 1

[0035] The super wideband polarization converter metasurface provided by the embodiment for converging lens and spatial imaging in the terahertz range includes a plurality of cell structures arranged in an array, as shown in Figure 1 , Figure 1a shows the overall structure of the metasurface cell structure, the cell is composed of two metal gratings, two pairs of L-shaped metal patches and two dielectric plates. The cell structure includes a first metal grating, a first dielectric plate, an L-shaped metal patch, a second dielectric plate and a second metal grating connected in turn from top to bottom, and the first metal grating and the second metal grating are arranged orthogonally; the L-shaped metal patch is composed of two large L-shaped plasmonic antennas and two small L-shaped plasmonic antennas, wherein the two large L-shaped plasmonic antennas are centrally symmetric, the two small L-shaped plasmonic antennas are arranged between the two large L-shaped plasmonic antennas, the two small L-shaped plasmonic antennas are oriented in the same direction and the two sides of the small L-shaped plasmonic antennas are perpendicular to the two sides of one of the large L-shaped plasmonic antennas respectively, and the two small L-shaped plasmonic antennas and the two large L-shaped plasmonic antennas are symmetrically distributed on the diagonal of the cell.

[0036] In this embodiment, F4B with a thickness of h = 30 um is used as the dielectric plate, wherein the dielectric constant of the dielectric plate is ε = 3.0 and the loss tangent is δ = 0.002. Due to the low dielectric loss of F4B, the dielectric plate film is used to separate the metal gratings and the two pairs of double L-shaped metal surfaces. The two same gratings on the front and back surfaces are arranged orthogonally, the width of the grating is W3 = 6 um, the adjacent gap of the grating is d = 20 um, and the material of the grating and the L-shaped metal patch is copper, and the conductivity of copper is σ = 5.8 × 10 7 S / m, and the metal thickness is t = 0.3 um.

[0037] In the L-shaped metal patch, the two large L-shaped plasmonic antennas are centrally symmetric, and the two small L-shaped plasmonic antennas are asymmetric structures, as shown in Figure 1 b, the intermediate structure sub-surface diagram Mode2 of the metasurface. In this embodiment, the specific size parameters of the cell structure are: P x = P y = 100 um, L1 = 68 um, L2 = 28 um, W1 = 9 um, and W2 = 6 um.

[0038] Comparative Example 1

[0039] The metasurface structure described in this comparative example is the same as that in Example 1, except that it is composed of two large L-shaped plasmonic antennas, wherein the two large L-shaped plasmonic antennas are symmetric about the origin, as shown in Figure 2 a, which shows the intermediate structure sub-surface diagram Mode1 of the metasurface. In this comparative example, the specific size parameters of the cell structure are: P x = P y = 100 um, L1 = 68 um, and W1 = 9 um.

[0040] Comparative Example 3

[0041] The metasurface structure described in this comparative example is the same as that in Example 1, except that it is composed of two large L-shaped plasmonic antennas and two small L-shaped plasmonic antennas, wherein the two large L-shaped plasmonic antennas are symmetric about the origin, and the two small L-shaped plasmonic antennas are also symmetric about the origin, and the structure is a symmetric structure, as shown in Figure 2b, the sub-surface diagram of the intermediate structure of the metasurface Mode3 is shown. In the present comparative example, the specific size parameters of the cell structure are: P x = P y = 100 um, L1= 68 um, L2= 28 um, W1= 9 um, W2= 6 um.

[0042] Performance test results

[0043] Referring to Figure 3 a, the analysis results of the response of the two pairs of double-L-shaped structure of the single-layer structure of the metasurface of the present application to electromagnetic waves are shown. When the electromagnetic waves are incident along the +z direction, the cross-polarization transmission coefficient is less than 0.5 in the frequency range of (0.2-1.8) THz, which is due to the inherent limitation of the single-layer structure.

[0044] Referring to Figure 3 b, the analysis results of the transmission coefficient when the electromagnetic waves are incident along the -z direction are exactly the same as those shown in Figure 3 (a), which shows that the single-layer structure has the property of being insensitive to the incident direction.

[0045] The transmission coefficients of Mode1, Mode2 and Mode3 prepared in Example 1 and Comparative Example 1 and Comparative Example 2 are tested, respectively. The results are shown in Figure 4 .

[0046] Referring to Figure 4 , the comparative curve diagram of the transmission coefficients of the metasurfaces Mode1, Mode2 and Mode3 of the present application is shown. According to the relative bandwidth formula It can be seen that the two pairs of symmetric structures Mode3 and the pair of symmetric and pair of asymmetric structures Mode2 have expanded the bandwidth to a certain extent relative to the pair of symmetric structures Mode1, wherein the relative bandwidths of Mode1, Mode2 and Mode3 are 35%, 111% and 113%, respectively, and the transmission efficiencies of Mode2 and Mode3 are both above 80%, both having good effects.

[0047] Referring to Figure 5 a, the cross-polarization and co-polarization transmission coefficients of the present application are shown. From Figure 5 a, it can be seen that in the range of 0.5 THz to 1.8 THz, the cross-polarization transmission coefficient is above 0.87, while the co-polarization is below 0.2.

[0048] Referring to Figure 5 b, the polarization conversion rate diagram of the present application is shown. From Figure 5 b, it can be seen that in the range of 0.2 THz to 1.8 THz, the polarization conversion ratio (PCR) of Mode3 is close to 1, indicating that the designed metasurface can work effectively as a broadband linear polarization converter in the terahertz wave band.

[0049] Referring to Figure 5 c, the polar angle-elliptic angle diagram provided by the present application is shown. It can be seen that the polar angle fluctuates around ±90° in the range of 0.4-1.8 THz, while the elliptic angle is about equal to 0 in the two polarization conversion frequency bands, which proves that the transmitted wave is linearly polarized wave. This means that the metasurface of the present application can convert the x-polarized incident wave into almost pure y-polarized wave in a wide frequency range.

[0050] Referring to Figure 5 d, the multi-ion resonance diagram provided by the present application is shown. It can be seen that there are four resonances at 0.565 THz, 1.023 THz, 1.430 THz and 1.695 THz.

[0051] Referring to Figure 5 e, the polarization conversion rate diagram of Mode 2 is shown. From Figure 5 e, it can be seen that the polarization conversion ratio (PCR) of Mode 2 is close to 1 in the range of 0.36 THz to 1.8 THz.

[0052] Referring to Figure 5 f, the polarization conversion rate diagram of Mode 1 is shown. From Figure 5 f, it can be seen that the polarization conversion ratio (PCR) of Mode 1 is also close to 1 in the range of 0.15 THz to 0.45 THz.

[0053] Referring to Figure 6 , the Fabry-Perot interference model diagram of the present application in illustrating the physical process simulation of polarization conversion is shown. The interference process of electromagnetic wave in the three-layer structure metasurface of the present application is shown. The first layer metasurface structure is composed of the first metal grating and the first dielectric plate, the L-shaped metal patch is the second layer structure, the second metal grating and the second dielectric plate constitute the third layer metasurface structure, Figure 6 , the black bidirectional arrow represents the y-polarized electromagnetic wave, and the red dot represents the x-polarized component. After the electromagnetic wave is incident on the first layer metasurface, it is partially transmitted and partially reflected; the transmitted electromagnetic wave reaches the second layer structure and is again transmitted and reflected; the electromagnetic wave reflected by the second layer is reflected again after reaching the first layer; the electromagnetic wave reaching the third layer is also transmitted and reflected. The electromagnetic wave is transmitted and reflected multiple times in the three-layer structure, and the final transmission amount is the superposition of multiple transmission components, which can effectively improve the transmission efficiency at the resonance frequency.

[0054] Referring to Figure 7a, the transmission diagram of the incident light propagating along the +z direction of the super surface structure provided by the application is shown. In the range of 0.5-1.78 THz, the cross-polarization transmission coefficient tyx is greater than 0.87, that is, the incident x-polarized electromagnetic wave is basically converted into y-polarized transmission wave. In the working frequency band, the cross-polarization transmission coefficient txy and the co-polarization transmission coefficients txx and tyy are basically limited to below 0.2, which is conducive to improving the working efficiency of the super surface.

[0055] Referring to Figure 7 b, the transmission diagram of the incident light propagating along the -z direction of the super surface provided by the application is shown. It can be seen that the cross-polarization transmission coefficient and Figure 7 (a) is opposite, that is, the incident x-polarized wave is completely reflected, and the incident y-polarized wave is converted into x-polarized transmission wave. Similarly, the co-polarization transmission coefficients are still maintained below 0.2.

[0056] Referring to Figure 7 c, the theoretical transmission coefficient diagram simulated by establishing a Fabry-Perot interference model by MATLAB for the incident light propagating along the +z direction of the super wideband high-efficiency linear polarization converter based on the super surface provided by the application is shown. It can be seen that the theoretically calculated transmission coefficient is consistent with the simulation result in CST Microwave Studio 2021, and the difference may be due to the difference in the number of iterations of the two software CST and MATLAB.

[0057] Referring to Figure 7 d, the theoretical transmission coefficient diagram simulated by establishing a Fabry-Perot interference model by MATLAB for the incident light propagating along the -z direction of the super surface provided by the application is shown. It can be seen that the theoretically calculated transmission coefficient is consistent with the simulation result in CST Microwave Studio 2021, and the difference may be due to the difference in the number of iterations of the two software CST and MATLAB.

[0058] Referring to Figure 8 a, the influence of different L1 arm lengths on the unit provided by the application is shown. It can be seen that the unit has good effect when the L1 arm length is between 66 and 70 um.

[0059] Referring to Figure 8 b, the influence of different small L spacings on the unit provided by the application is shown. The distance z of the small L moving up and down is set. It can be seen that the effect is good when z is in the range of -3 to 3 um.

[0060] Referring to Figure 8 C, the influence of different W1 widths on the unit provided by the application is shown. It can be seen that the effect is good when w1 is in the range of 5 to 10 um.

[0061] Referring to Figure 9 a, two different transmissive amplitude coding units provided by the present application are shown. The coding of high amplitude is "1", and the coding of low amplitude is "0".

[0062] Referring to Figure 9 b, two different transmissive phase coding units provided by the present application are shown at 1.04 THz phase. The coding of red is "1", the coding of black is "0", and the phase difference between the two is close to 10°.

[0063] Referring to Figure 10 , an "IUTL" pattern structure array provided by the present application is shown. The "IUTL" pattern is composed of two different amplitude coding metasurfaces, which are constructed by using the amplitude difference between unit "1" and unit "0" at 0.59073 THz. In the coding metasurface, the letter part is arranged by using the unit with amplitude coding "1", and the remaining part except the letter is arranged by using the unit with amplitude coding "0". The coding metasurface is composed of 33x25 units. The parameters of unit "1" and unit "0" are respectively: L1=68um, W1=9um, L2=28um, W2=6um; L1=30um, W1=3.97um, L2=11.91um, W2=2.65um;

[0064] Referring to Figure 11 , the near-field imaging diagram obtained by simulation of the "IUTL" provided by the present application is shown. The red part corresponds to the coding unit with high amplitude, and the blue part corresponds to the coding unit with low amplitude.

[0065] Referring to Figure 12 , ten different units that can realize amplitude above 0.74 and phase covering 360° at 1.6 THz are shown by scaling up or down the geometric parameters of the whole structure provided by the present application.

[0066] Table 1 shows the specific parameters of ten units for constructing a focusing lens design by scaling up or down the whole unit structure provided by the present application.

[0067] Table 1 Unit structure size

[0068]

[0069] Referring to Table 1, the table shows the specific parameters of ten different units that can realize amplitude above 0.74 and phase covering 360° at 1.6 THz provided by the present application. A 19x19 two-dimensional focusing superlens is constructed by using the ten units, which is symmetrical about the x and y axes. The focal length and the initial phase of the focusing lens are preset to 800um and-180°, respectively.

[0070] Referring toFigure 13 a, the figure illustrates the intermediate layer structure of the designed two-dimensional focusing superlens array provided by the present application at 1.6 THz.

[0071] Referring to Figure 13 b, the figure illustrates the cross-polarization electric field distribution of the two-dimensional focusing superlens provided by the present application in the xoz plane, and it can be seen that the incident plane wave is converged to the focal point.

[0072] Referring to Figure 13 c, the figure illustrates the cross-polarization RMS electric field distribution of the two-dimensional focusing superlens provided by the present application in the xoz plane, and the actual focal length of the cross-polarization electric field distribution in the xoz plane is measured to be 810.6 μm, which is close to the theoretical value of 800 μm, showing good subwavelength focusing characteristics.

[0073] Referring to Figure 13 d, the figure illustrates the cross-polarization electric field distribution of the two-dimensional focusing superlens provided by the present application in the xoy plane, and it can be seen that the incident plane wave is converged to the focal point.

Claims

1. A metasurface for an ultra-wideband polarization converter used in converging lenses and spatial imaging, characterized in that, It includes multiple cellular structures, each of which is square; the cellular structure includes a first metal grating, a first dielectric substrate, an L-shaped metal patch, a second dielectric substrate, and a second metal grating connected sequentially from top to bottom, the first metal grating and the second metal grating being orthogonally arranged; the L-shaped metal patch is composed of two large Ls and two small Ls, wherein the two large Ls are centrally symmetrical, and the two small Ls are spaced apart between the two large Ls, the two small Ls are oriented in the same direction and the two sides of the small Ls are perpendicular to the two sides of one of the large Ls respectively.

2. The ultra-wideband polarization converter metasurface for converging lenses and spatial imaging according to claim 1, characterized in that, The L-shaped metal patches are distributed along the diagonal of the dielectric substrate; the dielectric substrate has a square structure, and the two arms of the two large L-shaped patches and the two small L-shaped patches are of equal length.

3. The ultra-wideband polarization converter metasurface for converging lenses and spatial imaging according to claim 1, characterized in that, The dielectric substrate is an F4B dielectric substrate.

4. The ultra-wideband polarization converter metasurface for converging lenses and spatial imaging according to claim 1, characterized in that, The first metal grating, the second metal grating, and the L-shaped metal patch all have an electrical conductivity of 5.8 × 10⁻⁶. 7 The metal copper has a thickness of S / m and a thickness of t = 0.3 μm; the dielectric substrate has a thickness of h = 30 μm.

5. The ultra-wideband polarization converter metasurface for converging lenses and spatial imaging according to claim 1, characterized in that, The unit size of the cell structure is p = 100 μm; the width of the first metal grating and the second metal grating is W3 = 6 μm, and the gap between two adjacent grating strips is d = 20 μm.

6. The ultra-wideband polarization converter metasurface for converging lenses and spatial imaging according to any one of claims 1-5, characterized in that, By utilizing the amplitude differences between different units, a coded metasurface is constructed to achieve spatial imaging capabilities.

7. The ultra-wideband polarization converter metasurface for converging lenses and spatial imaging according to any one of claims 1-5, characterized in that, By proportionally enlarging or reducing the geometric parameters of the entire intermediate layer structure, multiple different unit structures are designed to construct a two-dimensional focusing superlens, thereby realizing the function of a converging lens.

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