Terahertz broadband polarization converter based on dual polarization and application thereof in space imaging

Abstract

The invention discloses a dual polarization-based terahertz broadband polarization converter metasurface, which is an efficient terahertz wave linear polarization converter based on a square bracket-shaped metal patch unit structure array, and is formed by adding a pair of orthogonal metal gratings on the upper side and the lower side of the square bracket-shaped metal patch unit structure array. According to the invention, the orthogonal metal grating and the square bracket-shaped metal patch form a Fabry-Perot cavity, so that cross polarization reflection and co-polarization transmission can be effectively prevented, and the transmission of cross polarization waves is remarkably increased. The amplitude coding metasurface is constructed by changing the amplitude and utilizing the transmission characteristics of incidence of different linear polarization waves. Under the incidence of linear polarization terahertz waves, a dual-polarization space imaging function is realized. The designed metasurface shows the characteristics of ultra wide band and high transmission efficiency, and has potential application value in the field of wireless communication.

Description

Technical Field

[0001] The present invention belongs to the technical field of electromagnetic metamaterials, and particularly relates to a terahertz broadband polarization converter metasurface based on dual polarization and its application in spatial imaging. Background Art

[0002] Terahertz is a region located between microwaves and infrared in the electromagnetic wave spectrum, and is an electromagnetic wave with a wavelength range of 0.03 - 3.00 mm and a frequency range of 0.1 - 10.0 THz. Due to the advantages of low photon energy, high penetration, water absorption, and high resolution, terahertz waves have been widely used in fields such as non-destructive testing, imaging, wireless communication, and security detection. A metasurface is a two-dimensional artificial planar structure composed of sub-wavelength units, which has characteristics such as thin thickness, light weight, and compact structure, and can simultaneously control the amplitude, phase, and polarization state of electromagnetic waves. In the applications of many terahertz functional devices, it is necessary to arbitrarily control and manipulate the polarization state of terahertz waves. Resonant metasurfaces can be used to design polarization converters, but their conversion efficiency and bandwidth functions are low. In most studies, multi-layer structures are used to improve efficiency and expand the bandwidth.

[0003] Generally, there are single-layer structures and reflective polarization converters. However, for single-layer structures, the transmission efficiency and reflection efficiency are similar, and it is difficult to obtain a high transmission efficiency. Reflective polarization converters are prone to showing a high working efficiency, but due to the problem of incident wave interference, they are not applicable in many situations. Although the current research on terahertz polarization converters and their applications has greatly improved the working efficiency of transmissive phase gradient metasurfaces, it is impossible to simultaneously achieve characteristics such as ultra-wideband, high transmission, and dual-polarization spatial imaging, which greatly limits the practical applications. Therefore, the present invention designs a terahertz metasurface with high efficiency, wideband, and dual polarization. By using the amplitude difference between different units, metasurface units with different amplitudes can be constructed, realizing the function of metasurface spatial imaging. Further, by using the amplitude difference between different units and the transmission characteristics of different linearly polarized waves incident, a coded metasurface is constructed, thereby realizing the dual-polarization spatial imaging function. Therefore, the designed metasurface has great application potential in the field of terahertz communication. Summary of the Invention

[0004] The purpose of the present invention is to provide a terahertz broadband polarization converter metasurface based on dual polarization, and the second purpose of the present invention is to use it for spatial imaging.

[0005] Based on the above purposes, the present invention adopts the following technical solutions:

[0006] The terahertz broadband polarization converter metasurface based on dual polarization includes a plurality of cell structure plasmas distributed in an array, and the cell structure is a square structure; each cell structure includes a first metal grating, a first dielectric plate, a bracket-shaped metal patch, a second dielectric plate and a second metal grating connected in sequence, and the first metal grating and the second metal grating are orthogonally arranged; the bracket-shaped metal patch is composed of two bracket-shaped corner bracket structures, and these two bracket-shaped corner bracket structures are centrosymmetric.

[0007] Preferably, the bracket-shaped metal patch is arranged along the diagonal of the dielectric plate; both the first dielectric plate and the second dielectric plate are square structures, and the two arm lengths of the two bracket-shaped corner bracket structures are equal.

[0008] Preferably, both the first dielectric plate and the second dielectric plate are polyimide dielectric plates, the dielectric constant of the dielectric plate is 3.5, and the loss tangent is 0.0027.

[0009] Further preferably, the thickness H of the first dielectric plate and the second dielectric plate is 30 μm.

[0010] Preferably, the first metal grating, the second metal grating and the bracket-shaped metal patch are all copper with a conductivity of 5.8×10 7 S / m and a thickness of t = 0.3 μm.

[0011] Preferably, the unit size P of the cell structure is 100 μm, the widths of the first metal grating and the second metal grating are W = 12 μm, and the interval d between adjacent grating bars is 20 μm.

[0012] Preferably, in the bracket-shaped metal patch, the length and width of the bracket-shaped corner bracket structure are L = 60 - 66 μm, W = 5 - 12 μm, and the angle α between the two arms of the bracket-shaped metal patch is 90° - 102°.

[0013] The present invention also discloses the application of the terahertz broadband polarization converter metasurface based on dual polarization in space imaging. By using the amplitude difference between different units to construct a coded supercell, the space imaging function can be realized.

[0014] By using the amplitude difference between different units and the transmission characteristics of different linearly polarized waves incident, a coded metasurface is constructed to realize the dual-polarization space imaging function.

[0015] Compared with the prior art, the present invention has the following technical effects:

[0016] 1) In the present invention, a dielectric plate is used to separate a metal grating and a bracket-shaped metal patch. A plasma antenna with two bracket-shaped corner bracket structures forms multiple ion resonance structures, and finally a high-efficiency broadband polarization converter with a three-layer structure is designed, which can convert x-polarized incident electromagnetic waves into pure y-polarized waves.

[0017] 2) The orthogonal metal grating and the bracket-shaped metal patch in the present invention form a Fabry - Perot cavity, which can effectively block cross-polarization reflection and co-polarization transmission, and significantly increase the transmission of cross-polarized waves.

[0018] 3) The present invention uses the finite element numerical simulation software CST to study its polarization conversion characteristics. The designed metasurface exhibits the characteristics of ultra-wideband and high transmission efficiency. The results show that it can enable the polarization converter to operate in the range of 0.36 - 1.24 THz, with a relative bandwidth reaching 110%, a transmission coefficient greater than 0.8, and a polarization conversion rate close to 100%.

[0019] 4) By changing the structural parameters of the polarization converter, the present invention constructs a "1" "0" amplitude-encoded metasurface and constructs an amplitude-encoded metasurface using the transmission characteristics of different linearly polarized wave incidences. This encoded metasurface realizes the function of dual-polarization spatial imaging under the incidence of linearly polarized terahertz waves.

[0020] Therefore, the metasurface proposed by the present invention has great application potential in the field of terahertz communication. Brief Description of the Drawings

[0021] To more clearly illustrate the technical solutions of the present invention, the following will briefly introduce the drawings required in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those skilled in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0022] Figure 1 a is a schematic structural diagram of the metasurface unit cell of the present invention, and 1b is a schematic structural diagram of the intermediate sub-surface structure layer. Figure 1 c is an overall array diagram of the metasurface of the present invention;

[0023] Figure 2 are the transmission coefficient diagram, polarization conversion rate, polarization angle ellipticity angle, and reflectivity of the metasurface provided by the present invention;

[0024] Figure 3 is the physical mechanism of the interference model of the three-layer structure provided by the present invention;

[0025] Figure 4 is the analysis result of the response of the single-layer bracket-shaped corner bracket structure to electromagnetic waves;

[0026] Figure 5 The analysis result of the response of the grating structure to electromagnetic waves;

[0027] Figure 6 The analysis result of the response of the three-layer structure to electromagnetic waves;

[0028] Figure 7 a shows the influence of the arm length L between the angled brackets-shaped corner brackets provided by the present invention on the unit transmission efficiency, Figure 7 b shows the influence of the width W between the angled brackets-shaped corner brackets provided by the present invention on the unit transmission efficiency, Figure 7 c shows the influence of the angle α between the angled brackets-shaped corner brackets provided by the present invention on the unit transmission efficiency;

[0029] Figure 8 Schematic diagram of the intermediate sub-surface structure layer between two coding "1" and "0" units with different amplitudes provided by the present invention;

[0030] Figure 9 a shows the transmission amplitude difference between two different units formed by changing the parameters L and W in the same frequency band provided by the present invention, and 9b shows the transmission phase difference between two different units formed by changing the parameters L and W in the same frequency band;

[0031] Figure 10 Intermediate structure array diagram of 30×25 units of the "HLTU" pattern composed of coding "1" and "0" units with different amplitudes provided by the present invention;

[0032] Figure 11 Simulated near-field imaging effect of 30×25 units of the "HLTU" pattern composed of coding "1" and "0" units with different amplitudes provided by the present invention;

[0033] Figure 12 Intermediate structure array diagram of 32×30 units composed of the transmission characteristics of coding "1" and "0" units with different amplitudes and different linearly polarized waves incident provided by the present invention;

[0034] Figure 13 a shows the simulated near-field imaging effect of 32×30 units composed of coding "1" and "0" units with different amplitudes and the transmission characteristics when the x linearly polarized wave is incident, and 13b shows the simulated near-field imaging effect of 32×30 units composed of coding "1" and "0" units with different amplitudes and the transmission characteristics when the y linearly polarized wave is incident. Specific implementation manners

[0035] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments. The technical solutions of the present invention are described in detail. The embodiments described with reference to the accompanying drawings are exemplary and should not be construed as limiting the present invention. All other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0036] Embodiment 1

[0037] The terahertz broadband polarization converter metasurface based on dual polarization provided in this embodiment includes a plurality of unit cell structures arranged in an array. See Figure 1 , Figure 1 Figure a shows the overall structural schematic diagram of the metasurface unit cell structure. The unit cell is composed of two metal gratings, a bracket-shaped metal patch, and two dielectric plates. The unit cell structure includes a first metal grating, a first dielectric plate, a bracket-shaped metal patch, a second dielectric plate, and a second metal grating connected in sequence from top to bottom. The first metal grating and the second metal grating are orthogonally arranged; the bracket-shaped metal patch is composed of two plasma antennas with bracket-shaped corner bracket structures, and these two bracket-shaped corner bracket structures are centrosymmetric.

[0038] In this embodiment, polyimide with a thickness of H = 30 μm is used as the dielectric plate. Among them, the dielectric constant of the dielectric plate is ε = 3.5, and the loss tangent is δ = 0.0027. Due to the low dielectric loss of polyimide, this dielectric plate film is used to separate the metal grating and the bracket-shaped metal surface. The two identical gratings on the front and back are orthogonally placed. The broadband of the grating is W = 12 μm, the adjacent gap of the grating is d = 20 μm, the materials of the grating and the bracket-shaped metal patch are both copper, and the conductivity of copper is σ = 5.8×10 7 S / m, and the metal thickness is t = 0.3 μm.

[0039] In the bracket-shaped metal patch, the two bracket-shaped corner bracket structures are centrosymmetric. See Figure 1 Figure b, the sub-surface diagram of the middle structure of the metasurface. See Figure 1 Figure c, the metasurface array diagram. In this embodiment, the specific dimension parameters of the unit cell structure are: P = 100 μm, L = 66 μm, W = 12 μm, α = 102°.

[0040] Performance test results

[0041] See Figure 2 Figure a, which shows the cross-polarization and co-polarization transmission coefficients of the present invention. From Figure 2 Figure a, it can be seen that in the range of 0.36 THz to 1.23 THz, the transmission coefficient of cross-polarization is above 0.87, while the co-polarization is below 0.2.

[0042] See Figure 2b, The polarization conversion rate diagram of the present invention is shown. From Figure 5 b, it can be seen that in the range of 0.2 THz to 1.4 THz, the polarization conversion ratio (PCR) of the model is close to 1, indicating that the designed metasurface can effectively work as a broadband linear polarization converter in the terahertz band.

[0043] See Figure 2 c, The polarization angle and ellipticity angle diagram provided by the present invention is shown. It can be seen that in the range of 0.2 THz to 1.4 THz, the polarization angle fluctuates around ±90°, while the ellipticity angle is approximately equal to 0 within the two polarization conversion frequency bands, proving that the transmitted wave is a linearly polarized wave. This means that the metasurface of the present invention can realize the conversion of x-polarized incident waves into almost pure y-polarized waves within a broadband range.

[0044] See Figure 2 d, The reflectivity of the present invention is shown, with four low peaks of the reflection coefficient at 0.387 THz, 0.605 THz, 0.92 THz, and 1.154 THz.

[0045] See Figure 3 , The interference diagram of the physical model of the three-layer structure for clarifying the physical process of polarization conversion of the present invention is shown. It shows the interference process of electromagnetic waves in the three-layer metasurface structure of the present invention. The first layer of the metasurface structure is composed of the first metal grating and the first dielectric plate, the bracket-shaped metal patch is the second layer structure, and the third layer of the metasurface structure is composed of the second metal grating and the second dielectric plate. Figure 3 In, the top and bottom metal gratings can be regarded as x and y selectors. The intermediate layer unit structure can be regarded as a polarization converter, which can convert x(y) waves into y(x) waves. When the x wave (Ei,x) is incident vertically along the -z direction, a part is converted into the Y wave (Et,y1) and passes through the intermediate layer, and the other part is reflected by the intermediate layer (Er,x1) and returns to interact with the top layer. The bottom grating ensures the transmission of pure y waves. The incident x wave is almost completely converted into the y wave and passes through the bottom grating. The x-wave terahertz incident wave can be converted into cross-polarized y waves within an ultra-wide frequency range, and the PCR can be as high as 1. Obviously, the multi-layer structure is an excellent choice for broadband and efficient linear polarization conversion devices operating in the terahertz band. By repeating the above process for multiple reflections and transmissions, the incident x-polarized wave is finally almost completely converted into the y-polarized wave, effectively improving the transmission efficiency over a wide operating bandwidth.

[0046] See Figure 4 a, The analysis result of the response of the bracket-shaped corner bracket structure of the single-layer metasurface structure of the present invention to electromagnetic waves is shown. When the electromagnetic wave is incident along the -z direction, within the frequency range of (0.2 - 1.6) THz, the cross-polarization transmission coefficient is less than 0.5, which is due to the inherent limitations of the single-layer structure.

[0047] See Figure 4 b, the analysis result of the transmission coefficient when the electromagnetic wave is incident along the +z direction is exactly the same as the result shown in Figure 4 (a), indicating that this single-layer structure has the property of being insensitive to the incident direction.

[0048] See Figure 5 a, The figure shows the analysis result of the response of the angled bracket structure of the double-layer metasurface structure of the present invention and the bottom grating being a horizontal grating structure to electromagnetic waves. At 0.65 THz and 1.26 THz, the transmittance reaches 0.78 and 0.83 respectively, and the co-polarized transmittance is almost zero. The x-polarized terahertz wave will be reflected back to the L-shaped metasurface by the rear grating. This part of the reflected wave will undergo polarization conversion again when it reaches the L-shaped metasurface before it can be transmitted through the rear grating. Therefore, this cross-polarization transmission enhancement effect benefits from the resonant cavity formed inside the double-layer structure.

[0049] See Figure 5 b, The figure shows the analysis result of the response of the angled bracket structure of the double-layer metasurface structure of the present invention and the bottom grating being a vertical grating structure to electromagnetic waves. When the x-polarized wave is incident, the cross-polarization transmittance is almost zero. The appearance of this phenomenon is attributed to the polarization selection characteristic of the rear grating, and the grating only allows terahertz waves with a specific polarization direction to pass through.

[0050] See Figure 6 a, The figure shows the transmission diagram of the incident light of the metasurface of the present invention propagating along the -z direction. In the range of 0.36 - 1.23 THz, the cross-polarization transmission coefficient t yx is greater than 0.87, that is, the incident x-polarized electromagnetic wave is basically converted into a y-polarized transmitted wave. In the working frequency band, the cross-polarization transmission coefficient t xy and the co-polarization transmission coefficients t xx 、t yy are basically limited to below 0.2, which is beneficial to improving the working efficiency of the metasurface.

[0051] See Figure 6 b, The figure shows the transmission diagram of the incident light of the metasurface of the present invention propagating along the +z direction. It can be seen that the cross-polarization transmission coefficient is completely opposite to that in Figure 6 (a), that is, the incident x-polarized wave is completely reflected, and the incident y-polarized wave is converted into an x-polarized transmitted wave. Similarly, the co-polarization transmission coefficient still remains below 0.2.

[0052] See Figure 7 a, The figure shows the influence of the arm length L of different angled bracket-shaped corner brackets provided by the present invention on the unit. It can be seen that the unit has better effects when the arm length L of the angled bracket-shaped corner bracket is between 60 - 66 μm.

[0053] SeeFigure 7 b. The figure shows the influence of different widths of W provided by the present invention on the unit. It can be seen that the effect is very good when W is in the range of 5 - 12 um.

[0054] See Figure 7 c. The figure shows the influence of different angles of α provided by the present invention on the unit. It can be seen that the effect is very good when α is in the range of 90° - 102°.

[0055] See Figure 8 . The figure shows two different transmission amplitude encoding units provided by the present invention. Among them, the encoding with high amplitude is "1", and the encoding with low amplitude is "0".

[0056] See Figure 9 . The figure shows the transmission amplitude curves and phase curves of these two units in the frequency range of 0.5 - 1.2 THz when two different transmission amplitude encoding units provided by the present invention are irradiated by linearly polarized terahertz waves.

[0057] See Figure 10 . The figure shows the array diagram of the "HLTU" pattern structure provided by the present invention. The "HLTU" pattern is composed of encoded metasurfaces with two different amplitudes. The amplitude difference between the observation unit "1" and the unit "0" is relatively large in the range of 0.4 - 1.2 THz. Therefore, when arbitrarily selecting a frequency f = 1 THz, using the amplitude difference between the unit "1" and the unit "0", an encoded metasurface is constructed. The letter part is arranged with units encoded with amplitude "1", and the rest outside the letter is arranged with units encoded with amplitude "0". The encoded metasurface is composed of 30 × 25 units. The parameters of the unit "1" and the unit "0" are respectively: L = 66 um, W = 12 um; L = 25 um, W = 4.5 um. The angle α between the two arms of the bracket-shaped metal patch of the two units is 102°.

[0058] See Figure 11 . The figure shows the near-field imaging diagram obtained by simulating the "HLTU" of the present invention. The red part corresponds to the encoded unit with high amplitude, and the black part corresponds to the encoded unit with low amplitude.

[0059] See Figure 12, The figure shows the array diagram of the "HU" pattern structure provided by the present invention. The "HU" pattern is composed of coded metasurfaces with 3 different amplitudes. Because the grating has polarization selection characteristics, the grating only allows terahertz waves with a specific polarization direction to pass through. Therefore, by cleverly mirroring the metasurface units representing "1", the response characteristics of the incident electromagnetic waves can be significantly changed. The combination of this operation, the introduction of unit "0", and the careful design of the unit arrangement and coding method enables the units at different positions of the metasurface to have a differential effect on the incident wave, thus precisely shaping a specific image in space. Among them, for the letter part, the unit arrangement of the metasurface units with amplitude coding of "1" and the mirror operation of the unit with "1" are selected, and for the rest outside the letter, the unit arrangement with amplitude coding of "0" is used. The coded metasurface is composed of 32×30 units.

[0060] See Figure 13 , The figure shows the near-field imaging diagram obtained by simulating the "HU" provided by the present invention. It can be observed that 13a is the simulation near-field imaging effect of obtaining the "H" structure pattern when the x-polarized wave is incident, and 13b is the simulation near-field imaging effect of obtaining the "U" structure pattern when the y-polarized wave is incident. The red part corresponds to the coded units with high amplitude, and the black part corresponds to the coded units with low amplitude.

Claims

1. Terahertz broadband polarization converter metasurface based on dual polarization, characterized in that It includes a plurality of cell structures distributed in an array, and the cell structures are in a square structure; each cell structure includes a first metal grating, a first dielectric plate, a bracket-shaped metal patch, a second dielectric plate and a second metal grating connected in sequence, and the first metal grating and the second metal grating are orthogonally arranged; the bracket-shaped metal patch is composed of two bracket-shaped corner bracket structures, and these two bracket-shaped corner bracket structures are centrosymmetric.

2. The terahertz broadband polarization converter metasurface based on dual polarization according to claim 1, wherein The bracket-shaped metal patch is distributed along the diagonal of the dielectric plate; both the first dielectric plate and the second dielectric plate are in a square structure, and the arm lengths of the two arms of the two bracket-shaped corner bracket structures are equal.

3. The terahertz broadband polarization converter metasurface based on dual polarization according to claim 1, wherein Both the first dielectric plate and the second dielectric plate are polyimide dielectric plates.

4. The terahertz broadband polarization converter metasurface based on dual polarization according to claim 1, wherein The first metal grating, the second metal grating, and the bracket-shaped metal patch are all made of copper with a conductivity of 5.8×10 7 S / m, and the thickness is t = 0.3 um; the thickness H of the first dielectric plate and the second dielectric plate is 30 um.

5. The terahertz broadband polarization converter metasurface based on dual polarization according to claim 1, wherein The unit size P of the cell structure is 100 um; the widths of the first metal grating and the second metal grating are W = 12 um, and the gap d between adjacent grating bars is 20 um.

6. The terahertz broadband polarization converter metasurface based on dual polarization according to claim 1, characterized in that In the angled bracket-shaped metal patch, the length and width of the angled bracket-shaped corner bracket structure are L = 60 - 66 um and W = 5 - 12 um, and the angle between the two arms of the angled bracket-shaped metal patch = 90 0 - 102 0 .

7. Application of the dual-polarization-based terahertz broadband polarization converter metasurface according to any one of claims 1-6 in spatial imaging, characterized in that An encoded metasurface is constructed by using the amplitude difference between different units, so as to realize the spatial imaging function.

8. The application according to claim 7, wherein An encoded metasurface is constructed by using the amplitude difference between different units and the transmission characteristics of different linearly polarized waves incident, so as to realize the dual-polarization spatial imaging function.

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