Total-space multifunctional terahertz metasurface

Abstract

The invention relates to the technical field of artificial electromagnetic metasurfaces, in particular to a full-space multifunctional terahertz metasurface. The metasurface is composed of a plurality of metasurface units arranged periodically, and each metasurface unit sequentially comprises a first metal structure layer, a first middle dielectric layer, a grating layer, a second middle dielectric layer and a second metal structure layer from top to bottom. The first metal structure layer and the second metal structure layer are the same in structure and each comprise two C-shaped metal rings, and the two C-shaped metal rings are connected in a staggered mode to form a disconnected and staggered S shape. The first intermediate dielectric layer and the second intermediate dielectric layer are consistent in thickness; the grating layer comprises vanadium dioxide and copper which are alternately arranged, and multifunctional dynamic regulation and control of transmission, reflection and the whole space are achieved by controlling phase change of the vanadium dioxide. The metasurface provided by the invention can respectively realize the functions of polarization conversion, linear-circular polarization wave conversion, circular dichroism, asymmetric transmission and the like in different states.

Description

Technical Field

[0001] The present invention relates to the technical field of artificial electromagnetic supersurfaces, and in particular to a full-space multifunctional terahertz supersurface, specifically a full-space multifunctional terahertz supersurface based on a cascaded Fabry-Perot cavity structure. Background Art

[0002] As an ultra-thin two-dimensional array planar structure, metasurface has attracted extensive attention in academia and industry due to its unique ability to control the phase, polarization state and propagation mode of electromagnetic waves, as well as its easy processing and integration. It has shown great application potential in the fields of holographic imaging, polarization transformation, asymmetric transmission, perfect absorption and vortex beam generation. However, existing research mainly focuses on the application of transmission and reflection metasurfaces, and this single space utilization limits its application scope. In recent years, with the improvement of the integration and performance requirements of terahertz metasurfaces, the concept of full-space metasurfaces has gradually emerged.

[0003] Traditional full-space metasurface designs rely on tunable materials such as vanadium dioxide (VO2), graphene, photosensitive semiconductors, and liquid crystals to achieve switching between transmission and reflection spaces, but these methods often cannot independently control both at the same time, and have a narrow working frequency band and complex structure. At present, there are still technical gaps in full-space metasurfaces in terms of bandwidth, functional diversity, and mechanism clarity. Summary of the invention

[0004] The present invention aims to improve at least one of the technical problems existing in the prior art. To this end, the present invention proposes a full-space multifunctional terahertz metasurface. Specifically, it is a full-space multifunctional terahertz metasurface based on a cascaded Fabry-Perot cavity structure, which realizes the switching of transmission, reflection and full-space regulation by controlling the phase change of vanadium dioxide. When vanadium dioxide is in a metallic state, the present invention can convert linear or circular polarization waves in the range of 0.9THz to 1.9THz into their orthogonal polarization waves in the reflection space, with a polarization conversion rate greater than 90% and a relative bandwidth of 71.43%; when vanadium dioxide is in an insulating state, the metasurface can convert linear polarization waves into circular polarization waves in the transmission space, and has asymmetric transmission characteristics when circular polarization waves are incident. It is particularly worth mentioning that the metasurface can not only work alone in the transmission or reflection space, but also regulate the two spaces at the same time. When vanadium dioxide is in a metallic state, in order to demonstrate its full-space regulation capability in a wide frequency band, the generation of focused vortex beams is realized in the upper surface space, and high-quality imaging is achieved in the lower surface space. At the same time, by adjusting the relative rotation angle of the upper and lower metal structure layers, efficient polarization conversion is achieved in the reflection space, and circular dichroism is exhibited in the transmission space.

[0005] The technical solution is as follows: According to a first aspect of the present invention, a full-space multifunctional terahertz metasurface is provided, wherein the metasurface is composed of a plurality of periodically arranged metasurface units, wherein the metasurface unit is composed of two Fabry-Perot resonant cavities cascaded, and the metasurface units are sequentially composed of a first metal structure layer, a first intermediate dielectric layer, a grating layer, a second intermediate dielectric layer, and a second metal structure layer from top to bottom; wherein, The first metal structure layer and the second metal structure layer have the same structure, both comprising two C-shaped metal rings, the two C-shaped metal rings are staggered and connected to form a disconnected and dislocated "S" shape; The first intermediate dielectric layer and the second intermediate dielectric layer have the same thickness; The grating layer includes vanadium dioxide and copper that are alternately arranged. When the vanadium dioxide is in a metallic state, the metasurface realizes a polarization conversion function of 0.9THz-1.9THz; when the vanadium dioxide is in an insulating state, in the transmission space, the metasurface can convert linearly polarized waves into circularly polarized waves, and has asymmetric transmission characteristics when circularly polarized waves are incident.

[0006] In a possible implementation manner of the first aspect, The arrangement period of the metasurface units is p=40 μm to 60 μm, preferably, the arrangement period p=50 μm; wherein, The thickness of the first metal structure layer, the second metal structure layer and the grating layer are not less than 0.2 μm.

[0007] Preferably, the thickness of the first intermediate dielectric layer and the second intermediate dielectric layer ranges from 24 μm to 26 μm.

[0008] The thickness of the first intermediate dielectric layer and the second intermediate dielectric layer is in the range of 25 μm.

[0009] In a possible implementation manner of the first aspect, in each metasurface unit, geometric centers of the first metal structure layer, the second metal structure layer, and the grating layer are all located on the same vertical line.

[0010] In a possible implementation manner of the first aspect, two C-shaped metal rings located on the surfaces of the first intermediate dielectric layer and the second intermediate dielectric layer are identical in shape, size, and position.

[0011] In a possible implementation of the first aspect, the opening angle range of the C-shaped metal ring is 100° to 140°, the outer diameter of the C-shaped metal ring is 12μm to 15μm, the inner diameter of the metal ring is 5μm to 8μm, and the distance between the centers of the two metal rings is 12μm to 18μm.

[0012] Preferably, the outer diameter of the C-shaped metal ring is 13.8 μm, the inner diameter is 7 μm, the center distance between the two metal rings is 16 μm, and the opening angle of the C-shaped metal ring is 120°.

[0013] In a possible implementation manner of the first aspect, any side edge of the first intermediate dielectric layer or the second intermediate dielectric layer in the horizontal direction is used as a reference line, and the two C-shaped metal rings in an overall S-shape are arranged at an angle of 45° to the reference line to ensure that the polarization conversion efficiency of linear polarization reaches 90%, and at the same time, the polarization conversion efficiency of linear polarization waves and circular polarization waves is achieved to be above 90%.

[0014] In a possible implementation of the first aspect, in the grating layer, the grating is in the shape of strips, each vanadium dioxide strip and each copper strip constitute a grating period, and when the grating period is 5, the width of the copper strip is 5 μm, and the width of the vanadium dioxide strip is 5 μm, which is used to achieve switching among transmission, reflection and full space.

[0015] In a possible implementation manner of the first aspect, the materials of the first intermediate dielectric layer and the second intermediate dielectric layer are both polyimide, with a relative dielectric constant of 3.5 and a tangent loss in the range of 0.001 to 0.005.

[0016] In a possible implementation manner of the first aspect, the material of the C-shaped metal ring is any one of gold, silver, copper, and aluminum.

[0017] Preferably, the material of the C-shaped metal ring is copper.

[0018] The full-space multifunctional terahertz metasurface of the present invention, when the vanadium dioxide in the grating layer is in a metallic state, the grating composed of alternating VO2-copper can be equivalent to a metal reflective surface, reflecting and converting the incident forward and reverse electromagnetic waves into their cross-polarized waves, thereby realizing the polarization conversion function. In particular, it has the same effect on circularly polarised waves and linearly polarised waves.

[0019] When the vanadium dioxide in the grating layer is in an insulating state, the VO2-copper alternating grating layer can be regarded as a grating composed only of metal. In the transmission space, when the incident wave is a circularly polarized wave, it has asymmetric transmission properties when propagating in the forward (+z) and reverse (-z) directions. When the incident wave is a linearly polarized wave, due to the effect of the cascaded Fabry-Perot cavity structure, the linearly polarized wave is converted into the corresponding circularly polarized wave, and a linear-circular conversion occurs.

[0020] The present invention provides a full-space multifunctional terahertz metasurface, which is formed by periodically arranging metasurface units in the same plane. At the same time, a cascaded Fabry-Perot cavity structure of two metal structure layers, two dielectric layers and one grating is adopted in the metasurface unit. The metal layer and the dielectric layer are reasonably designed, and the multifunctional dynamic regulation in reflection, transmission and full space is realized by controlling the VO2 phase change in the middle grating layer.

[0021] Compared with the prior art, the full-space multifunctional terahertz metasurface provided by the present invention has the following advantages: (1) Multifunctionality. By changing the phase change state of vanadium dioxide, it can realize multifunctional dynamic control of reflection, transmission and full space. When vanadium dioxide is in a metallic state, it has polarization conversion function for both circularly polarized waves and linearly polarized waves in the reflection space. When vanadium dioxide is in an insulating state, the metasurface has asymmetric transmission characteristics for circularly polarized waves and linear-circular conversion characteristics for linearly polarized waves in the transmission space.

[0022] (2) Full-space characteristics. The designed metasurface can work not only in the reflection space or the transmission space alone, but also in the transmission space and the reflection space at the same time. When vanadium dioxide is in a metallic state, the upper and lower surface spaces are independent of each other. Vortex beams can be generated in the upper surface space, while imaging functions can be realized in the lower surface space. When vanadium dioxide is in an insulating state, by breaking the spatial symmetry, the upper and lower metal structure layers produce a rotation angle difference, realizing efficient polarization conversion in the reflection space and high transmittance circular dichroism in the transmission space.

[0023] (3) The metasurface device of the present invention has good oblique incidence characteristics and can be used in complex working environments. It has a small structure and is easy to integrate. It has a symmetrical structure and a periodic arrangement design, and also has the advantage of being easy to process.

[0024] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0026] Figure 1 This is a schematic diagram of the overall structure of the full-space multifunctional terahertz metasurface of the present invention; Figure 2 for Figure 1An exploded diagram of a metasurface periodic unit structure in the metasurface structure shown; Figure 3 for Figure 2 A schematic diagram of the structural design of the first metal structure layer of the metasurface unit shown; Figure 4 for Figure 2 Schematic diagram of the design of the metasurface unit grating layer structure; Figure 5 The linear polarization reflection coefficient and polarization conversion rate when vanadium dioxide is in a metallic state in the simulation experiment of the present invention; Figure 6 are the circular polarization reflection coefficient and polarization conversion rate when vanadium dioxide is in a metallic state in the simulation experiment of the present invention; Figure 7 This is an asymmetric transmission curve when the vanadium dioxide is in an insulating state and the incident wave is a circularly polarized wave in the simulation experiment of the present invention; Figure 8 This is a line-circle conversion result diagram when vanadium dioxide is in an insulating state and the incident wave is a linearized wave in the simulation experiment of the present invention; Fig. 9 This is the effect of the number of gratings on the asymmetric transmission (AT) parameter when vanadium dioxide is in the insulating state; Fig.10 This is a diagram showing the influence of the rotation angle of the first metal structure layer on the circular dichroism (CD) parameters when vanadium dioxide is in an insulating state in the simulation experiment of the present invention; Fig.11 The reflection parameters and the reflection polarization conversion rate when the rotation angle of the first metal structure layer is 80° in the vanadium dioxide insulating state in the simulation experiment of the present invention; Fig.12 This is a diagram showing the relationship between the rotation angle of the "S"-shaped metal structure layer and the reflection phase of the circularly polarized wave when the vanadium dioxide is in a metallic state in the simulation experiment of the present invention; Fig.13 The amplitude and phase difference of the unit "1" and "0" after the "S"-shaped metal structure layer is rotated when the vanadium dioxide is in a metallic state in the simulation experiment of the present invention; Fig.14 When vanadium dioxide is in a metallic state in the simulation experiment of the present invention, a focused vortex beam is formed in the upper surface space, and a "West" pattern is formed in the lower surface space; Fig.15 This is a diagram showing the influence of changing the parameters of the corresponding structure in the metal structure layer on the polarization conversion rate when the vanadium dioxide is in a metallic state in the simulation experiment of the present invention.

[0027] Reference numerals: The first metal structure layer 100 , the first intermediate dielectric layer 200 , the grating layer 300 , the vanadium dioxide 310 , the copper 320 , the second intermediate dielectric layer 400 and the second metal structure layer 500 . DETAILED DESCRIPTION

[0028] The embodiments of the present invention are described in detail below. The embodiments described with reference to the accompanying drawings are exemplary. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0029] It should be noted that when an element is referred to as being "fixed to" another element, it may be directly on the other element or there may be an intermediate element. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be an intermediate element at the same time.

[0030] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which the present invention belongs. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0031] The terms "first", "second", "third", etc. in the specification and claims of the present application and the drawings are used to distinguish different objects rather than to describe a specific order. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a series of steps or units are included, or optionally, steps or units not listed are included, or optionally, other steps or units inherent to these processes, methods, products or devices are included.

[0032] Only the part relevant to the present application is shown in the accompanying drawings, but not all of the content. Before discussing the exemplary embodiments in more detail, it should be mentioned that some exemplary embodiments are described as processing or methods depicted as flow charts. Although the flow chart describes each operation (or step) as a sequential process, many of the operations therein can be implemented in parallel, concurrently or simultaneously. In addition, the order of each operation can be rearranged. When its operation is completed, the process can be terminated, but it can also have additional steps not included in the accompanying drawings. The process can correspond to a method, function, procedure, subroutine, subprogram, etc.

[0033] Example 1 See also Figure 1 to Figure 2 The present invention proposes a full-space multifunctional terahertz metasurface based on a cascaded Fabry-Perot cavity structure, comprising a plurality of metasurface units, wherein the plurality of metasurface units are periodically arranged in the same plane, and the number of the transverse and longitudinal arrangements is equal. The overall structure of the metasurface is as follows: Figure 1 As shown, Figure 2The exploded diagram of the metasurface periodic unit structure. The metasurface unit is composed of two cascaded Fabry-Perot resonant cavities, and the metasurface unit is composed of the first metal structure layer 100, the first intermediate dielectric layer 200, the grating layer 300, the second intermediate dielectric layer 400 and the second metal structure layer 500 from top to bottom. The first metal structure layer 100 and the first intermediate dielectric layer 200 on the grating layer 300 are completely the same as the second intermediate dielectric layer 400 and the second metal structure layer 500 under the grating layer 300. There is no gap between adjacent structural layers, wherein the first metal structure layer 100 and the second metal structure layer 500 each include two C-shaped metal rings, and the two C-shaped metal rings are staggered and connected to form a disconnected and dislocated "S" shape; The first intermediate dielectric layer 200 and the second intermediate dielectric layer 400 have the same thickness; The grating layer 300 includes vanadium dioxide 310 and copper 320 that are alternately arranged. When the vanadium dioxide is in a metallic state, the metasurface realizes a polarization conversion function of 0.9THz-1.9THz; when the vanadium dioxide is in an insulating state, in the transmission space, the metasurface can convert linearly polarized waves into circularly polarized waves, and has asymmetric transmission characteristics when circularly polarized waves are incident.

[0034] The arrangement period of the metasurface unit is p=50 μm, the thickness of the first metal structure layer and the second metal structure layer are both 0.2 μm, the thickness t1 of the first intermediate dielectric layer and the second intermediate dielectric layer are both 25 μm, and the thickness t2 of the intermediate grating layer is 0.2 μm.

[0035] The first metal structure layer and the second metal structure layer (hereinafter referred to as metal structure layer) can be made of gold, silver, copper, aluminum and other materials. In this embodiment, copper is selected with a thickness of 0.2 μm and a conductivity of 5.8×10 7 Different material selections may have corresponding effects on device performance, which is also within the scope of protection of the present invention.

[0036] Reference Figure 3 As shown, the metal structure layer is composed of two C-shaped metal rings with an angle of 240°, wherein the outer diameter of the C-shaped metal ring is r1=13.8μm, the inner diameter is r2=7μm, and the center distance between the two C-shaped metal rings is m=16μm.

[0037] Any side edge of the first intermediate dielectric layer or the second intermediate dielectric layer in the horizontal direction is used as a reference line, and the two C-shaped metal rings in an overall S-shape are arranged at an angle of 45° to the reference line to ensure that the polarization conversion efficiency of linear polarization reaches 90%, and at the same time, the polarization conversion efficiency of linear polarization waves and circular polarization waves is achieved at more than 90%.

[0038] The material of the first intermediate dielectric layer and the second intermediate dielectric layer is polyimide, and the relative dielectric constant thereof is 3.5.

[0039] Reference Figure 4 As shown, the grating layer is composed of vanadium dioxide and copper alternately. In this embodiment, the gratings (copper and vanadium dioxide) of the grating layer are all rectangular strips, wherein one vanadium dioxide strip and one copper strip form a grating period, and the width d of the copper strip and the vanadium dioxide strip is 5 μm. It should be noted that in this embodiment, the grating period is 5, that is, there are 5 groups of vanadium dioxide strips-copper strips. Changing the grating period (the number of grating groups) will affect the device performance, which is also within the scope of protection required by the present invention.

[0040] The following is a description of the technical effects of the present invention in combination with simulation experiments: 1. Simulation conditions and contents: The simulation software used is CST, and the Floquet mode is used to simulate the infinite array and the various functions of the metasurface. When the temperature of vanadium dioxide is higher than 360K, vanadium dioxide is in a metallic state with an electrical conductivity of σ=2×10 5 S / m, the corresponding dielectric constant can be described by the Drude model, that is: , where = 12 is the relative dielectric constant of VO2 at infinite frequency, is the angular frequency of the incident wave, is the plasma frequency of VO2, is the collision frequency, which is 5.75×10 13 rad / s.

[0041] When the temperature of vanadium dioxide is lower than 360K, vanadium dioxide is in an insulating state with an electrical conductivity of σ=200 S / m, and electromagnetic waves are incident vertically on the metasurface unit along the -z direction.

[0042] Simulation 1: When vanadium dioxide is in a metallic state, the metasurface exhibits a polarization conversion function, and the co-polarized wave is converted into a cross-polarized wave under vertical incidence, such as Figures 5 and 6 As shown in the simulation, the present invention has a polarization conversion function in the reflection space, wherein Figure 5 is the co-polarization reflection coefficient of the linearly polarized wave , cross polarization reflection coefficient and the polarization conversion rate ( ), Figure 6 is the co-polarization reflection coefficient of circularly polarized wave , cross polarization reflection coefficient and the polarization conversion rate ( ).

[0043] Simulation 2: When the vanadium dioxide in the grating layer is in an insulating state, the metasurface has an asymmetric transmission effect on the circularly polarized wave in the transmission space. Its asymmetric transmission curve is shown in Figure 7 When the incident wave is a linearly polarized wave, due to the effect of the Fabry-Perot cavity structure, the corresponding linearly polarized wave can be converted into a corresponding circularly polarized wave. In order to characterize the degree of linear-circular polarization wave conversion, the normalized ellipticity is introduced To measure, When , they represent the complete left-hand circularly polarized wave and right-hand circularly polarized wave respectively, and the axial ratio AR is defined to estimate the circular polarization performance. ,in When AR is less than 3dB, the polarized wave is circularly polarized. The corresponding ellipticity and axis are as follows: Figure 8 shown.

[0044] Simulation 3, changing the number of gratings in the grating layer, analyzing its impact on the AT parameters, the results are as follows Fig. 9 shown.

[0045] Simulation 4: When vanadium dioxide is in an insulating state, the rotation angles of the upper and lower metal structure layers are misaligned, breaking the spatial symmetry and realizing full-space applications. The metasurface has selective polarization conversion characteristics in the reflection space, and dual-band circular dichroism in the transmission space, such as Fig.10 and Fig.11 shown.

[0046] Simulation 4: When the vanadium dioxide in the grating layer is in a metallic state, the rotation angle of the "S"-shaped metal structure layer is changed, and its cross-polarization reflection coefficient and phase are analyzed. The results are as follows: Fig.12 and Fig.13 By regularly arranging the metal structure layers on the upper and lower surfaces, a focused vortex beam is formed in the upper surface space, and a "West" pattern imaging is formed in the lower surface space, as shown in FIG. Fig.14 shown.

[0047] Simulation 6, changing the corresponding structural parameters in the metal structure layer, analyzing its impact on device performance, the results are as follows Fig.15 shown.

[0048] 2. Analysis of simulation results: refer to Figures 5 and 6 When vanadium dioxide is in a metallic state, the middle grating layer is equivalent to a metal backplane, which prevents the incident wave from transmitting. Only the first three layers of the metasurface structure work, becoming a reflective metasurface. At this time, the device exhibits dual-polarization conversion characteristics, and exhibits strong polarization conversion performance for both linearly polarized waves and circularly polarized waves. When the incident wave is a linearly polarized wave, its co-polarization reflection coefficient is , cross-polarization reflection coefficient Polarization conversion rate PCR Figure 5 As shown, the polarization conversion rate PCR is greater than 92% in the bandwidth of 0.93THz to 1.92THz, the bandwidth is 0.99THz, and the relative bandwidth is 69.47%. The PCRs at the three resonance peaks of 1.79THz, 1.36THz and 1.03THz are 0.961, 0.945 and 0.955 respectively.

[0049] To further explore the performance of the device, we analyzed the reflection coefficient of circularly polarized waves in reflection mode, e.g. Figure 6 As shown, the polarization conversion rate PCR is greater than 97% within the bandwidth of 0.98THz to 1.86THz, the bandwidth is 0.88THz, and the relative bandwidth is 64.8%, among which the PCR at the three resonance peaks of 1.03THz, 1.36THz and 1.79THz is close to 1.

[0050] refer to Figures 7 and 8 When vanadium dioxide is in an insulating state, the conductivity of VO2 is 200 S / m, and when the incident wave is a circularly polarized wave, the present invention exhibits asymmetric transmission characteristics in the transmission space, and the corresponding transmission parameters are as follows: Figure 7 As shown in (a), the AT parameters of the CP wave at 0.95 THz and 1.77 THz are 0.8 and -0.81 respectively, while the AT parameter are - 0.83 and 0.87 respectively. The AT parameter of the linearly polarized wave is almost zero in the entire frequency range. Figure 7 Therefore, the designed structure has significant dual-band asymmetric transmission performance only for CP waves.

[0051] When the incident wave is a linearly polarized wave, the linearly polarized wave incident through the metasurface will be transmitted and converted into a circularly polarized wave, which has a linear-circular conversion function. Due to the influence of the grating layer, different effects will be produced on the incident y-polarized wave (TE wave) and x-polarized wave (TM wave). When the incident wave is a y-polarized wave, the linearly polarized wave is converted into a right-hand circularly polarized wave in the range of 1.66THz to 1.83THz; when the incident wave is an x-polarized wave, the linearly polarized wave is converted into a left-hand circularly polarized wave in the range of 1.7THz to 1.87THz, and is converted into a right-hand circularly polarized wave in the range of 0.93THz to 0.99THz. The axial ratio and ellipticity are as follows: Figure 8 As shown. TE wave (transverse electric wave) has AR < 3dB in the range of 1.66 to 1.83THz, and the ellipticity is approximately -1. TM wave (transverse magnetic wave) has AR < 3dB in the range of 1.7TH to 1.87THz, and the ellipticity is approximately 1. In the range of 0.93THz to 0.99THz, AR < 3dB, and the ellipticity is approximately -1, realizing the line-circle conversion function.

[0052] Reference Fig. 9 In order to study the cause of the AT transmission effect, we studied the effect of the number of gratings in the unit on its performance. When there is no grating in the middle of the metasurface, the AT parameter is 0. When the number of unit gratings is 1, an obvious AT phenomenon appears at 0.95THz, and the amplitude reaches 0.8, indicating that the key to the asymmetric transmission of the present invention is the grating layer. With the increase of the number of gratings, obvious AT phenomenon also appears at 0.95THz and 1.77THz. With the increase of the grating period, the trend of the asymmetric transmission parameter stabilizes at around 0.8, indicating that the increase in the number of gratings makes the AT phenomenon more obvious.

[0053] refer to Figure 10 to Figure 11 The designed metasurface can be used in both reflection space and transmission space. When vanadium dioxide is in an insulating state, circular dichroism appears in the transmission space by misaligning the rotation angles of the upper and lower metal structure layers, and polarization conversion characteristics appear in the reflection space. Fig.10 When the circularly polarized wave is incident along the -z direction, the first metal structure layer rotates counterclockwise, and the second metal structure layer remains unchanged, the circular dichroism change curve shows that as the rotation angle increases, at 0.95THz, the transmission of RCP continues to increase, and the transmission of LCP decreases, that is, the circular dichroism continues to increase. At 1.78THz, the situation is the opposite, the transmission of LCP continues to increase, and the transmission of RCP decreases. When the first metal structure layer rotates 80°, the CD value reaches a maximum of 0.82. At the same time, the selective polarization conversion characteristics are exhibited in the reflection space of the two frequency points. When the LCP wave is incident, the polarization conversion rate of the LCP wave at 0.95THz is nearly 1, such as Fig.11 As shown in (b), the polarization conversion rate at 1.78 THz is also close to 1, but Fig.11 As shown in (a), r -+ The value is lower but relative to r ++ The value of is an order of magnitude different, which makes the PCR higher, but in practical applications, the polarization conversion at this frequency is generally not considered; for RCP wave incidence, the situation is similar to the above, the polarization conversion rate at 1.78THz is close to 1, and a high PCR value appears at 0.95THz.

[0054] refer to Figure 12 to Figure 14 When vanadium dioxide is in a metallic state, the upper and lower surface spaces of the grating layer are independent of each other, and more diverse designs and applications can be made for the upper and lower surface spaces. A focused vortex beam is generated in the upper surface space, and imaging design is performed on the lower surface space. According to the PB phase principle, for the incident circularly polarized wave, when the metal structure layer rotates at an angle of When , the unit structure will obtain a phase increment twice the rotation angle, that is, By adjusting the rotation angle of the metal structure layer, the full phase change from 0° to 360° can be achieved on the metasurface. The "S"-shaped structure is rotated 7 times in a counterclockwise direction with a step length of 22.5° to obtain 8 basic units. The phase difference between adjacent units is about 45°, and the cross-polarization amplitude is maintained at 0.95, as shown in Figure 2. Fig.12 As shown. With 8 basic unit structures for phase arrangement, the focal length f is selected as 1500 microns, the frequency point is 1.4THz, and a 32*32 array structure is constructed. The simulation results are shown as follows Fig.14 As shown in (a) in the figure. In particular, when the incident wave is a linearly polarized wave in the lower surface space, the "S"-shaped structure is rotated 135° counterclockwise along the Z axis, and the polarization conversion amplitude of the structure is almost zero, and the phase is the same as the original structure, as shown in Fig.13 As shown, the unit is encoded as "0" and the original unit structure is encoded as "1". The metasurface is encoded by arranging structural units with amplitude differences, and then specific information is encoded to achieve terahertz imaging. The "West" pattern is constructed on the lower surface space. The inner area of ​​the word is constructed by the unit "1" with high amplitude coding, while "0" is used as a unit with low amplitude coding in the outer area. The simulation results are shown in Fig.14 (b) shown.

[0055] Reference Fig.15 When vanadium dioxide is in a metallic state, the parameters of the corresponding structure in the metal structure layer are changed to analyze its effect on the device performance. Fig.15 As shown in (a), the outer diameter r1 of the "C"-shaped ring in the metal structure layer is changed. As r1 continues to increase, the polarization conversion bandwidth increases, the linear polarization conversion rate continues to decrease, and the bandwidth tends to move toward low frequency. When r1 increases to 13.8μm, the polarization conversion bandwidth and PCR are optimal. Fig.15 (b) in the figure shows the influence of changing the inner diameter r2 of the metal ring in the metal structure layer on the circular polarization conversion rate. It can be seen from the figure that as the inner diameter of the metal ring increases, the bandwidth of the low frequency band increases, and the PCR increases. When m increases to 9μm, the two metal rings separate, the circular polarization conversion rate drops rapidly, and only one resonance peak appears at the 2.5THz frequency point.

[0056] It should be noted that the full-space multifunctional terahertz metasurface proposed in the present invention can expand the working frequency of the metasurface to the microwave or optical band by proportionally enlarging or reducing it, which also falls within the protection scope of this patent.

[0057] In the description of the present invention, it is to be understood that the terms “center”, “longitudinal”, “lateral”, “length”, “width”, “thickness”, “up”, “down”, “front”, “back”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”, “inside”, “outside”, “clockwise”, “counterclockwise”, “axial”, “radial”, “circumferential”, etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation to the invention.

[0058] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "illustrative embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example.

[0059] Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Mentioning "embodiment" in this article means that the specific features, structures or characteristics described in conjunction with the embodiment may be included in at least one embodiment of the present embodiment application. The appearance of this phrase in various positions in the specification is not necessarily the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It can be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0060] Although the embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the claims and their equivalents.

Claims

1. A full-space multifunctional terahertz metasurface, the metasurface consisting of a number of periodically arranged metasurface units, characterized in that: The metasurface unit is composed of two Fabry-Perot resonant cavities cascaded, and the metasurface unit is composed of a first metal structure layer, a first intermediate dielectric layer, a grating layer, a second intermediate dielectric layer, and a second metal structure layer from top to bottom; wherein, The first metal structure layer and the second metal structure layer have the same structure, both comprising two C-shaped metal rings, the two C-shaped metal rings are staggered and connected to form a disconnected and dislocated "S" shape; The first intermediate dielectric layer and the second intermediate dielectric layer have the same thickness; The grating layer includes alternately arranged vanadium dioxide and copper. When the vanadium dioxide is in a metallic state, the metasurface realizes a polarization conversion function of 0.9THz to 1.9THz; when the vanadium dioxide is in an insulating state, in the transmission space, the metasurface can convert linearly polarized waves into circularly polarized waves, and has asymmetric transmission characteristics when circularly polarized waves are incident.

2. The full-space multifunctional terahertz metasurface according to claim 1, characterized in that: The arrangement period of the metasurface units is p=40 μm to 60 μm, wherein: The thickness of the first metal structure layer, the second metal structure layer and the grating layer are not less than 0.2 μm; The thickness of the first intermediate dielectric layer and the second intermediate dielectric layer ranges from 24 μm to 26 μm.

3. The full-space multifunctional terahertz metasurface according to claim 1, characterized in that: In each metasurface unit, the geometric centers of the first metal structure layer, the second metal structure layer and the grating layer are all located on the same vertical line.

4. The full-space multifunctional terahertz metasurface according to claim 2, characterized in that: The two C-shaped metal rings located on the surfaces of the first intermediate dielectric layer and the second intermediate dielectric layer are identical in shape, size and position.

5. The full-space multifunctional terahertz metasurface according to claim 1, characterized in that: The opening angle range of the C-shaped metal ring is 100° to 140°, the outer diameter of the C-shaped metal ring is 12 μm to 15 μm, and the inner diameter is 5 μm to 8 μm.

6. The full-space multifunctional terahertz metasurface according to claim 1, characterized in that: Any side of the first intermediate dielectric layer or the second intermediate dielectric layer in the horizontal direction is used as a reference line, and the two C-shaped metal rings in an overall S-shape are arranged at an angle of 45 degrees to the reference line.

7. The full-space multifunctional terahertz metasurface according to claim 1, characterized in that: In the grating layer, the grating is in strip shape, each vanadium dioxide strip and each copper strip form a grating period, and when the grating period is 5, the width of the copper strip is 5 μm, and the width of the vanadium dioxide strip is 5 μm.

8. The full-space multifunctional terahertz metasurface according to claim 1, characterized in that: The first intermediate dielectric layer and the second intermediate dielectric layer are both made of polyimide, with a relative dielectric constant of 3.5 and a tangent loss ranging from 0.001 to 0.

005.

9. The full-space multifunctional terahertz metasurface according to claim 1, characterized in that: The material of the C-shaped metal ring is any one of gold, silver, copper and aluminum.

10. The full-space multifunctional terahertz metasurface according to claim 9, characterized in that: The material of the C-shaped metal ring is copper.

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