Reflection type polarization independent controllable holographic metasurface based on electric tuning

Abstract

The invention discloses an X and Y polarization independent control programmable metasurface unit, and signal output is expanded through an IIC bus mode, so that each metasurface unit in a metasurface array can independently input an electric control signal. Two diodes are integrated in the X direction and the Y direction in the units respectively, 180-degree phase mutation can be achieved by changing voltages at the two ends of the diodes, and coding in the X direction and coding in the Y direction are independent and do not interfere with each other. The coding metasurface is constructed by utilizing the polarization independent control and programmable characteristics of the unit. The metasurface can achieve the holographic imaging effect through a specific coding array, and the holographic imaging real-time switching effect can be achieved by changing the output signal array of the FPGA. The metasurface can achieve dynamic holography under X polarization incidence and can also achieve dynamic holography under Y polarization incidence, polarization is independently controlled, the transmission amount of information can be increased through the characteristic, and the metasurface can also be used for a polarization encryption method in microwave imaging. The invention provides a new scheme for expanding a dynamic holographic channel.

Description

Technical Field

[0001] The present invention relates to the field of optical holographic imaging, and in particular to an electrically tunable reflective polarization-independent controllable holographic metasurface. Technical Background

[0002] As an emerging platform, metasurfaces have a strong ability to control electromagnetic waves. Researchers have done a lot of research on metasurfaces and used them to make optical devices to achieve the desired functions, such as flat lenses, beam controllers, absorbers and cloaks, holograms and imagers. Among them, dynamic metasurfaces can dynamically generate different functions under external stimuli, so dynamic metasurfaces have also become a research direction for many researchers. So far, there have been many attempts to use dynamic metasurfaces to achieve multiple functions, using tunable materials to make tunable metasurface absorbers, dynamic holographic imaging, and communication transmission, etc.

[0003] Digital programmable metasurface is one of the dynamic metasurfaces. This concept was first proposed in 2014. By integrating diodes in the metasurface unit structure and adding bias voltages to both ends of the diodes to change the conduction state of the diodes, the electric field characteristics can be changed. Through the external input of electrical signals from FPGA, the characteristics of electromagnetic waves can be dynamically regulated by real-time coding. Due to the programmable characteristics and real-time operability of the metasurface, digital coding and programmable metasurfaces have been widely studied in the past few years, and even extended to the field of acoustics, from functional verification to equipment design and system applications.

[0004] However, in current programmable metasurface designs, the phase distributions of orthogonally polarized electromagnetic waves are coupled to each other, or programmability is only possible under specific polarization conditions. Therefore, a single-polarization programmable metasurface can only provide an effective information transmission channel to process multiple tasks in serial. In addition, due to the limited number of control interfaces that an FPGA can provide, most programmable metasurfaces to date have only a few independent control channels, so they are usually used to implement some easy-to-implement functions. These two problems greatly limit the programmability of programmable metasurfaces, making it difficult to implement complex EM functions, and also severely restrict their ability to perform multiple tasks in parallel.

[0005] We designed a programmable metasurface unit with independent control of X and Y polarization, and extended the signal output through the IIC bus mode, so that each metasurface unit in the metasurface array can independently input an electrical control signal. We integrated two diodes in the X and Y directions respectively in these units. By changing the voltage across the diodes, a 180° phase mutation can be achieved, and the coding in the X direction and the Y direction is independent and does not interfere with each other. Using the polarization-independent control and programmable characteristics of this unit, a coded metasurface is constructed. This metasurface can achieve dynamic holography under X-polarized incidence and can also achieve dynamic holography under Y-polarized incidence, and the polarization is independently controlled. This characteristic can increase the amount of information transmission and can also be used in the polarization encryption method in microwave imaging. Summary of the Invention

[0006] The object of the present invention is to provide an electrically tunable reflective polarization-independent controllable holographic metasurface, which can solve one or more of the above technical problems.

[0007] In order to achieve the above object, the technical solution proposed by the present invention is as follows:

[0008] An electrically tunable reflective polarization-independent controllable holographic metasurface, characterized in that the structure of the electrically tunable reflective polarization-independent controllable holographic metasurface unit is composed of four metal layers, three dielectric layers and four PIN diodes connected to the top layer;

[0009] The first layer of metal of the structure of the electrically tunable reflective polarization-independent controllable holographic metasurface unit is composed of a rectangular metal sheet with a groove and four metal strips;

[0010] In the first layer of metal structure, the four metal strips are connected to the rectangular metal sheet in the groove through four PIN diodes;

[0011] The first layer of metal structure is respectively connected to the second layer, the third layer and the fourth layer through vias;

[0012] The second layer of metal of the structure of the electrically tunable reflective polarization-independent controllable holographic metasurface unit is a metal sheet with vias and serves as the ground layer of the metasurface unit structure;

[0013] The third layer of metal copper plate of the structure of the electrically tunable reflective polarization-independent controllable holographic metasurface unit serves as the wiring layer for the X-direction diodes;

[0014] The fourth layer of metal copper plate of the structure of the electrically tunable reflective polarization-independent controllable holographic metasurface unit serves as the wiring layer for the Y-direction diodes;

[0015] The three dielectric layers are all dielectric substrates with a cubic structure;

[0016] Define the center of the top-layer trenched metal as the origin, and the four metal bar structures are symmetric about the center of the trenched metal;

[0017] The electrically tunable reflective polarization-independent controllable holographic metasurface can use an FPGA (Field Programmable Gate Array) to control the voltage across the PIN diodes of each unit of the electrically tunable reflective polarization-independent controllable holographic metasurface, realizing the polarization-independent controllable dynamic holographic imaging function;

[0018] The metasurface unit structure is composed of metal-dielectric-metal-dielectric-metal-dielectric-metal. The metal is pure copper, and the dielectric is FR4 dielectric material.

[0019] When the metasurface is irradiated by a plane electromagnetic wave at 4.75 GHz, the phase delays all differ by 180°;

[0020] When the metasurface unit is irradiated by a single-polarized electromagnetic wave at 4.75 GHz, its reflection amplitude exceeds 0.8;

[0021] The period of the metasurface unit is 15 mm;

[0022] The thickness of the four-layer copper metal of the unit is 0.035 mm;

[0023] The thickness of the first-layer dielectric of the unit is 2.66 mm;

[0024] The thickness of the second-layer dielectric of the unit is 0.1 mm;

[0025] The thickness of the third-layer dielectric of the unit is 0.1 mm;

[0026] The side length of the square metal sheet with a trench on the top layer of the unit is 12 mm;

[0027] The groove length of the square metal sheet with a trench on the top layer of the unit is 3 mm;

[0028] The groove width of the square metal sheet with a trench on the top layer of the unit is 1.2 mm;

[0029] The length of the metal bars around the square metal sheet with a trench on the top layer of the unit is 3.45 mm;

[0030] The width of the metal bars around the square metal sheet with a trench on the top layer of the unit is 0.5 mm;

[0031] The distance between the top-layer metal bar and the trench of the unit, which is the width for connecting the PIN diode, is 0.8 mm;

[0032] The model of the PIN diode is Skyworks “SMP1320-079LF”;

[0033] The model of the FPGA is Xilinx ZYNQ 7020;

[0034] The polarization-independent controllable dynamic holographic imaging is characterized in that under the action of the metasurface, both X-polarized or Y-polarized incident waves can achieve the effect of holographic imaging, and the dynamic holographic imaging effect can be achieved by changing the coding array of the metasurface according to the FPGA;

[0035] The electrically tunable reflective polarization-independent controllable holographic metasurface, and its polarization-independent controllable dynamic holographic imaging includes the following steps: Step 1: Select a target image, and obtain the coded phase distribution of the target image through the GS algorithm; Step 2: Change the state of the diode of the metasurface unit through the FPGA, and load the coded phase information onto the metasurface; Step 3: Irradiate the metasurface with a specific X-polarized or Y-polarized plane wave, and obtain the target image electric field distribution on the pre-set imaging plane; Step 4: Corresponding to the coded phase distributions generated by different images, change the coded phase array of the metasurface; Step 5: On the pre-set imaging plane, the target image corresponding to the switched coded phase distribution can be obtained, and the effect of dynamic holographic imaging can be achieved;

[0036] The technical effects of the present invention are:

[0037] (1) The present invention uses PCB processing technology, which is easy to fabricate and process.

[0038] (2) In the present invention, the FPGA is used to perform real-time phase encoding control on the metasurface, which is programmable and can realize dynamic regulation of the metasurface phase distribution.

[0039] (3) In the present invention, by using the different phase responses of the electrically tunable metasurface unit in X polarization and Y polarization, polarization-independent controllable dynamic holographic imaging is realized.

[0040] The polarization-independent controllable dynamic holographic imaging proposed by the present invention can realize X and Y polarization multiplexing to increase the information transmission volume, providing an idea for realizing multi-channel holographic imaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The specification drawings forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention.

[0042] In the drawings:

[0043] Figure 1 It is a schematic diagram of polarization-independent controllable dynamic holographic imaging.

[0044] Among them, in Figure 1 when X-polarized light is incident, the electric field distribution of the target image can be obtained on the imaging plane. According to the dynamic regulation of the coding phase distribution of the metasurface by the FPGA, dynamic holographic imaging switching is realized. When Y-polarized electromagnetic waves are incident, corresponding imaging effects can also be obtained, and the X and Y polarizations are independently controllable.

[0045] Figure 2 It is a flowchart of the GS algorithm based on the Green's function.

[0046] Figure 3 It is the unit structure diagram of an electrically tunable reflective polarization-independent controllable dynamic holographic metasurface.

[0047] Figure 4 It is the electric field distribution of the metasurface unit under the incidence of Y-polarized electromagnetic waves.

[0048] Figure 5 It is the reflection characteristic curve of the electrically tunable reflective polarization-independent controllable dynamic holographic metasurface unit.

[0049] Figure 6 They are the four target letter images of "C", "J", "L", and "U".

[0050] Figure 7 They are the coding phase distributions of the four target letter images of "C", "J", "L", and "U" obtained by iterating the GS algorithm based on the Green's function operator.

[0051] Figure 8 They are the electric field distribution diagrams of the four target letter images of "C", "J", "L", and "U" calculated by the forward propagation theory according to the above coding phase distribution.

[0052] Figure 9 They are the electric field distribution diagrams obtained by simulation.

[0053] Figure 10 They are the metasurface samples for experimental testing, the microwave anechoic chamber environment, and the experimental testing device.

[0054] Figure 11 They are the scanned electric field distribution diagrams obtained by experimental testing. Specific implementation manner

[0057] In order to make the purpose, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the embodiments and the accompanying drawings. The illustrative embodiments of the present invention and their descriptions are only used to explain the present invention and are not intended to limit the present invention.

[0058] Figure 1Schematic diagram of polarization-independent controllable dynamic holographic imaging. According to the encoded phase distribution obtained by iterative calculation using the GS algorithm, the voltage across the two ends of the corresponding metasurface unit diode is controlled by an FPGA, so that the phase distribution of the encoded metasurface array conforms to the calculation result. When an X-polarized electromagnetic wave is incident, the corresponding image electric field distribution can be obtained on the imaging plane. By changing the input encoded array through the FPGA, the image electric field distribution after switching can be obtained on the imaging plane. When a Y-polarized electromagnetic wave is incident, the result is similar to the above, and the holographic imaging of the X-polarized electromagnetic wave incident and the holographic imaging of the Y-polarized electromagnetic wave incident are independently controllable.

[0059] Figure 2 Flowchart of the GS iterative algorithm based on the Green's function operator. Through this algorithm process, the encoded phase distribution of the four-letter images shown in Figure 7 the figure can be iteratively calculated, and the phase distribution can be encoded into the metasurface by controlling the voltage across the diode by an FPGA.

[0060] Figure 3 The structure and structure parameters of the metasurface unit are as follows. After optimization, the parameters are: p = 15 mm, L1 = 12 mm, L2 = 3 mm, L4 = 0.5 mm, D = 1.2 mm, g = 0.8 mm, r = 0.25 mm. The thickness of the copper metal layer is 0.035 mm, the thickness of the first dielectric layer is 2.66 mm, and the thicknesses of the second and third dielectric layers are both 0.1 mm.

[0061] Figure 4 The electric field distribution of the metasurface unit under the incidence of a Y-polarized electromagnetic wave. Due to the central symmetry of the unit structure, the electric field distribution of the metasurface unit under the incidence of an X-polarized electromagnetic wave is centrosymmetric with it. From left to right and top to bottom are the electric field distributions when the diodes in the X and Y directions are both turned on, the electric field distributions when the diodes in the X and Y directions are both turned off, the electric field distribution when the diode in the X direction is turned on and the diode in the Y direction is turned off, and the electric field distribution when the diode in the X direction is turned off and the diode in the Y direction is turned on.

[0062] Figure 5 Reflectance curves of the electrically tunable reflective polarization-independent controllable dynamic holographic metasurface unit. From left to right and top to bottom are the reflection phase characteristics when the state of the diode in the X direction changes, the reflection amplitude characteristics when the state of the diode in the X direction changes, the reflection phase characteristics when the state of the diode in the Y direction changes, and the reflection amplitude characteristics when the state of the diode in the Y direction changes. It can be seen from the figure that at 4.75 GHz for this unit, by changing the voltage across the diode, a 180° phase flip can be obtained, and the reflection amplitude is above 0.8, meeting the basic requirements of the above holographic imaging GS algorithm.

[0063] Figure 6The four target letter images of "C", "J", "L", and "U" serve as the input electric fields for the above GS algorithm process.

[0064] Figure 7 The four target letter images of "C", "J", "L", and "U" are used to control the diodes through the FPGA according to the encoded phase distribution obtained by iterating the GS algorithm based on the Green's function operator, so that the phase distribution of the metasurface is the same as it.

[0065] Figure 8 It is the electric field distribution diagram calculated by the forward propagation theory of the four target letter images of "C", "J", "L", and "U" according to the above encoded phase distribution.

[0066] Figure 9 The corresponding electric field distribution diagrams obtained by simulation in CST Studio are the simulated holographic electric field distribution diagrams of the letter patterns of "C", "J", "L", and "U" under the incidence of X - polarized light with X - direction encoding and the simulated holographic electric field distribution diagrams of the letter patterns of "C", "J", "L", and "U" under the incidence of Y - polarized light with Y - direction encoding.

[0067] Figure 10 They are the metasurface sample for experimental testing, the microwave anechoic chamber environment, and the experimental testing device. The horn emits electromagnetic waves, the probe receives the electric field intensity at that position, and the controller moves the rotating lead screw to scan the planar electric field.

[0068] Figure 11 They are the scanned electric field distribution diagrams obtained by experimental testing, which are the scanned electric fields corresponding to the incidence of Y - polarized electromagnetic waves when encoding diodes in the Y - direction and the scanned electric fields corresponding to the incidence of X - polarized electromagnetic waves when encoding diodes in the X - direction respectively.

[0069] The present invention proposes and designs a reflective polarization - independent controllable programmable metasurface, which can independently control the electromagnetic wave characteristics of X - polarization or Y - polarization by regulating the conduction states of the diodes in the X or Y direction. Relying on the programmability of this metasurface, we have realized dynamic holography independently controlled in the X and Y polarization directions. First, an improved GS holographic algorithm is proposed using the Green's function as the light propagation operator. This algorithm can be applied not only in the near - field but also in the far - field, and has a wider application range compared with the traditional algorithms using Fourier transform and Fresnel diffraction integral as the propagation operators. The theoretical verification of the algorithm is carried out through Matlab, and the rationality of the theory is verified by full - wave simulation in CST Studio. Under the incidence of X - polarized and Y - polarized lights respectively, the corresponding holographic images of "C", "J", "L", and "U" can be independently reconstructed. The experimental test results are in line with the theoretical calculation and full - wave simulation results. It is worth mentioning that the polarization - independent regulation characteristic of this metasurface can also be used in the field of polarization encryption for microwave imaging, and the programmable characteristic can also enable real - time changes of information, which is beneficial to the development of real - time communication technology.

[0070] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A reflective polarization-independent controllable holographic metasurface based on electrical tunability, characterized in that: The electrically tunable reflective polarization-independent controllable holographic metasurface unit structure consists of four metal layers, three dielectric layers and four PIN diodes connected to the top layer; The first metal layer of the electrically tunable reflective polarization-independent controllable holographic metasurface unit structure is composed of a grooved rectangular metal sheet and four metal strips; The four metal strips in the first metal structure are connected to the rectangular metal sheet with grooves through four PIN diodes; The first layer of metal structure is connected to the second layer, the third layer and the fourth layer respectively through through holes; The second metal layer of the electrically tunable reflective polarization-independent controllable holographic metasurface unit structure is a metal sheet with through holes, which serves as the ground layer of the metasurface unit structure; The third metal copper plate of the electrically tunable reflective polarization-independent controllable holographic metasurface unit structure is used as the wiring layer of the X-direction diode; The fourth metal copper plate of the electrically tunable reflective polarization-independent controllable holographic metasurface unit structure is used as the wiring layer of the Y-direction diode; The three dielectric layers are all dielectric substrates with a cubic structure; The center of the top-layer grooved metal is defined as the origin, and the four metal strip structures are symmetrical about the center of the grooved metal; The electrically tunable reflective polarization-independent controllable holographic metasurface can utilize FPGA (field programmable gate array) to control the voltage across the PIN diode of each unit of the electrically tunable reflective polarization-independent controllable holographic metasurface, thereby realizing a polarization-independent controllable dynamic holographic imaging function.

2. The supersurface according to claim 1, characterized in that The metasurface unit structure is composed of metal-medium-metal-medium-metal-medium-metal-medium-metal, the metal is pure copper, and the medium is FR4 medium material.

3. The super surface unit according to claim 2, characterized in that: When the metasurface is irradiated by a 4.75 GHz plane electromagnetic wave, the phase delays differ by 180°.

4. The super surface unit according to claim 3, characterized in that: When the metasurface unit is irradiated with a 4.75 GHz single-polarized electromagnetic wave, its reflection amplitude exceeds 0.

8.

5. The super surface unit according to claim 3, characterized in that: The metasurface unit period is 15 mm; The thickness of the four-layer copper metal of the unit is 0.035 mm; The thickness of the first dielectric layer of the unit is 2.66 mm; The thickness of the second dielectric layer of the unit is 0.1 mm; The thickness of the third dielectric layer of the unit is 0.1 mm; The square metal sheet with grooves on the top layer of the unit has a side length of 12 mm; The groove length of the square metal sheet with grooves on the top layer of the unit is 3 mm; The width of the groove of the square metal sheet with grooves on the top layer of the unit is 1.2 mm; The length of the metal strips around the square metal sheet with grooves on the top layer of the unit is 3.45 mm; The width of the metal strips around the square metal sheet with grooves on the top layer of the unit is 0.5 mm; The distance between the top metal strip of the unit and the trench, i.e. the width of the connected PIN diode, is 0.8 mm; The model of the PIN diode is Skyworks "SMP1320-079LF".

6. The model of the FPGA according to claim 1 is Xilinx ZYNQ 7020.

7. The polarization-independent controllable dynamic holographic imaging according to claim 1, characterized in that: Under the action of the metasurface, X-polarized or Y-polarized incident waves can achieve the effect of holographic imaging, and changing the coding array of the metasurface according to the FPGA can achieve the effect of dynamic holographic imaging.

8. According to claim 5, the electrically tunable reflective polarization-independently controllable holographic metasurface, wherein the polarization-independently controllable dynamic holographic imaging comprises the following steps: Step 1: Select the target image and obtain the coding phase distribution of the target image through the GS algorithm; Step 2: Change the state of the metasurface unit diode through FPGA to load the encoded phase information onto the metasurface; Step 3: Irradiate the metasurface with a specific X-polarized or Y-polarized plane wave to obtain the target image electric field distribution on a pre-set imaging plane; Step 4: Generate coded phase distributions through different images and change the coded phase array of the metasurface accordingly; Step 5: On a preset imaging plane, a target image corresponding to the switched coded phase distribution can be obtained, thereby achieving a dynamic holographic imaging effect.

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