Double-layer invisible cloak based on polarization conversion and phase modulation metasurface

Abstract

The present invention relates to a double-layer structure stealth cloak based on polarization conversion and phase modulation metasurface, comprising an outer transmission layer and an inner reflection layer, wherein the space enclosed by the reflection layer is a stealth area; the transmission layer is formed by splicing a plurality of plane transmission metasurfaces, and the reflection layer is formed by splicing a plurality of plane reflection metasurfaces; both the plane transmission metasurface and the plane reflection metasurface have polarization conversion and phase modulation functions, so that the incident wave is reflected multiple times in the area between the transmission layer and the reflection layer without entering the stealth area, and finally returns to the original propagation path for emission, thereby making the object in the stealth area invisible; the plane transmission metasurface and the plane reflection metasurface compensate for the path difference caused by multiple reflections through phase modulation. The stealth cloak has a simple structure, is easy to implement, has good stealth performance, and has a wide range of applications.

Description

Technical Field

[0001] The present invention belongs to the technical field of electromagnetic stealth, and in particular relates to a double-layer structure stealth cloak based on polarization conversion and phase modulation metasurface. Background Art

[0002] In recent years, with the progress in fields such as materials science, optics and electromagnetism, the research on stealth technology has made remarkable progress. There are three traditional methods for designing stealth cloaks, including transformation optics, conformal mapping and scattering cancellation.

[0003] Transformation optics theory provides the theoretical basis and conceptual design for the first reported invisibility cloak by deriving spatial coordinate changes and adjusting the wave propagation path around the object. This method relies on the precise control of light to bypass the target object, making the object appear to disappear. However, transformation optics requires materials to have complex anisotropic properties, which is difficult to manufacture and still has challenges in the visible light band. Conformal mapping cloaks are based on virtual-real space transformations, which relax the requirements for anisotropic materials, but still require the material to have a gradually changing refractive index over a large range. This makes the design and manufacturing process complex and difficult to implement in practice. Invisibility cloaks based on scattering elimination technology hide objects by reducing the scattering properties of objects. This design is relatively simple, but its application range is limited and it is only applicable to objects with sizes close to the working wavelength. In order to reduce the difficulty of realizing transmissive invisibility cloaks, carpet-type invisibility cloaks based on quasi-conformal transformations have been proposed. This invisibility cloak hides objects on the ground plane by covering them with a metamaterial carpet. Although the design is simplified to a certain extent, the metamaterial carpet is still limited by its large size.

[0004] Metasurface is a two-dimensional planar array of metamaterials, which is composed of periodic or non-periodic artificial units with sub-wavelength size. Its thickness is much smaller than the working wavelength. It is a new type of artificial electromagnetic medium. The units that make up the metasurface can be arranged arbitrarily, which gives the metasurface a high degree of freedom in controlling electromagnetic waves. Through structural design and parameter adjustment, the amplitude, phase and polarization of electromagnetic waves can be changed. It has the advantages of low loss, low cost, thin and compact structure, broadband operation and flexible design. The invention of metasurface has alleviated the limitations of metamaterial stealth to a certain extent. It is lighter and more sensitive, and has been widely used in carpet-type stealth cloaks. However, carpet-type stealth cloaks can only work in reflected waves and cannot be truly invisible in transmitted waves. There are few public reports on transmission-type stealth cloaks based on metasurfaces. Although some progress has been made, the design and implementation of invisibility cloaks still face many challenges. More practical and efficient transmission stealth cloak design methods need to be explored and implemented. Summary of the invention

[0005] The purpose of the present invention is to provide a double-layer structure stealth cloak based on polarization conversion and phase modulation metasurface, which has a simple structure, is easy to implement, has good stealth performance and a wide range of applications.

[0006] In order to achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a double-layer structure stealth cloak based on polarization conversion and phase modulation metasurface, characterized in that it includes an external transmission layer and an internal reflection layer, and the space enclosed by the reflection layer is a stealth area; the transmission layer is composed of several planar transmission metasurfaces, and the reflection layer is composed of several planar reflection metasurfaces; both the planar transmission metasurface and the planar reflection metasurface have polarization conversion and phase modulation functions, so that the incident wave is reflected multiple times in the area between the transmission layer and the reflection layer without entering the stealth area, and finally returns to the original propagation path to be emitted, thereby forming a transmission-type stealth cloak to make objects in the stealth area invisible; the planar transmission metasurface and the planar reflection metasurface compensate for the path difference caused by multiple reflections through phase modulation.

[0007] Furthermore, the inner wall and the outer wall of the transmission layer are covered with a metal polarization grid, which is a metal structure composed of a plurality of metal straight strips arranged in parallel, and has a polarization screening function, completely transmits electromagnetic waves whose polarization direction is perpendicular to the metal polarization grid, and completely reflects electromagnetic waves whose polarization direction is parallel to the metal polarization grid; the metal polarization grid on the outer wall of the transmission layer is perpendicular to the polarization direction of the incident electromagnetic wave, and the metal polarization grid on the inner wall of the transmission layer is parallel to the polarization direction of the incident electromagnetic wave.

[0008] Furthermore, from the perspective of the two-dimensional planar structure of the invisible cloak, both the transmission layer and the reflection layer are rhombus structures with an inner vertex angle of 2α; the side length of the transmission layer is l t , the side length of the reflective layer is l r ; The side lengths of the transmission layer and the reflection layer meet the conditions: .

[0009] Furthermore, after the plane wave is incident from one side of the invisibility cloak, it is reflected multiple times in the area between the transmission layer and the reflection layer, and finally converges and emits from the other side of the invisibility cloak; the invisibility cloak is divided into an incident side and an exit side with the line connecting the two opposite inner vertex angles 2α in the invisibility cloak as the symmetry axis. After the plane wave is incident on the invisibility cloak from the incident side, there are two wave paths, including:

[0010] Path r1 is the incident wave passing through the incident side transmission layer and entering the interior of the invisible cloak, without touching the reflection layer, and directly passing through the exit side transmission layer to exit;

[0011] Path r2 is the incident wave passing through the incident side transmission layer and entering the interior of the invisible cloak, and then being completely reflected by the incident side reflection layer, the inner wall of the incident side transmission layer, the inner wall of the exit side transmission layer, and the exit side reflection layer in sequence, and finally passing through the exit side transmission layer to exit; each time the plane wave passes through the reflection layer, an orthogonal polarization conversion occurs, so that the plane wave can be completely reflected by the inner wall of the transmission layer and finally pass through the transmission layer; the compensation relationship between the transmission phase, reflection phase and path calculated for the two paths is:

[0012] (1)

[0013] (2)

[0014] in, and are the transmission phases of the incident side transmission layer and the exit side transmission layer, and are the reflection phases of the incident side reflection layer and the exit side reflection layer, is the wave number, is the free space wavelength.

[0015] Furthermore, the planar transmission metasurface is composed of M×N transmission units arranged periodically at equal intervals; the transmission unit is composed of two laminated flexible dielectric plates and a metal layer printed thereon, the upper side of the upper flexible dielectric plate and the lower side of the lower flexible dielectric plate are printed with mutually orthogonal metal polarization gratings, and a metal cross-cross oscillator rotated 45° is printed between the upper and lower flexible dielectric plates; the transmission unit can efficiently convert a linearly polarized incident wave into its orthogonal polarization transmission; adjusting the structural parameters of the metal cross-cross oscillator can cover the transmission phase modulation requirements within a range of 360°.

[0016] Furthermore, the planar reflective metasurface is composed of L×N reflective units arranged periodically at equal intervals; the reflective unit includes a flexible dielectric plate, the upper side of which is printed with a double-notch metal elliptical ring rotated 45°, and the lower side is all metal; the reflective unit can efficiently convert a linearly polarized incident wave into its orthogonal polarization reflection; adjusting the structural parameters of the double-notch metal elliptical ring can cover the reflection phase modulation requirements within a range of 360°.

[0017] Furthermore, the invisibility cloak is a planar symmetrical structure, which is divided into a mutually symmetrical incident side and an exit side by a symmetry plane perpendicular to the incident direction of the electromagnetic wave. The transmission layer is a polygonal structure and has at least two planar transmission metasurfaces that form a certain angle with the incident wave. The reflection layer is a polygonal structure and has at least two planar reflection metasurfaces.

[0018] Furthermore, in the transmission layer, for the surface that does not participate in the transmission or reflection of electromagnetic waves, no planar transmission metasurface is set; in the reflection layer, for the surface that does not participate in the reflection of electromagnetic waves, no planar reflection metasurface is set.

[0019] Furthermore, for all configurations of transmission-type stealth cloaks, a symmetry plane perpendicular to the incident direction of the electromagnetic wave is used to divide them into a mutually symmetrical incident side and an exit side, only the transmission layer and the reflection layer on the incident side are retained, and a plane metal floor is set at the position of the symmetry plane. The transmission layer and the reflection layer on the incident side and the plane metal floor form a carpet-type stealth cloak corresponding to the transmission-type stealth cloak of the corresponding configuration; after the incident wave is transmitted from the outside of the transmission layer, it is reflected multiple times in the area between the transmission layer, the reflection layer and the plane metal floor without entering the stealth area, and finally exits along the reverse direction of the original propagation path.

[0020] Compared with the prior art, the present invention has the following beneficial effects: the present invention provides a double-layer structure stealth cloak based on polarization conversion and phase modulation metasurface, the stealth cloak utilizes the polarization control and phase modulation of the metasurface, through polarization conversion and polarization screening characteristics of the polarization grid, so that the electromagnetic wave is reflected multiple times in the stealth device, thereby bypassing the stealth area, realizing the wavefront recovery of the plane wave in the transmission direction, and almost no reflection, realizing stealth, so that the objects in the stealth area can be effectively hidden, and it is suitable for working under reflection geometry and transmission geometry at the same time, therefore, the stealth effect is good and the application restrictions are few. The present invention has a simple structure, is easy to design and manufacture, is applicable to all electromagnetic wave frequency bands, and has strong practicality and broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a schematic diagram of the implementation principle of the transmission type invisible cloak according to an embodiment of the present invention;

[0022] Figure 2 This is a schematic diagram of the implementation principle of the carpet-type invisible cloak according to an embodiment of the present invention;

[0023] Figure 3 is a schematic structural diagram of a transmission unit in an embodiment of the present invention;

[0024] Figure 4 is the electromagnetic response of the transmission unit in the embodiment of the present invention;

[0025] Figure 5 is a schematic structural diagram of a reflection unit in an embodiment of the present invention;

[0026] Figure 6 is the electromagnetic response of the reflection unit in the embodiment of the present invention;

[0027] Figure 7 Schematic diagram of the structure of the transmission type invisible cloak in the first embodiment of the present invention;

[0028] Figure 8 This is a simulated electric field distribution diagram of a plane wave incident on a free space, a transmission layer, a reflection layer and a transmission type invisible cloak at a frequency of 11.6 GHz in Example 1 of the present invention;

[0029] Fig. 9 This is a schematic diagram of the structure of a carpet-type invisible cloak in the second embodiment of the present invention;

[0030] Fig.10 This is a simulated electric field distribution diagram of a plane wave incident on a metal plane, a reflective layer, and a carpet-type invisible cloak at a frequency of 11.6 GHz in the second embodiment of the present invention;

[0031] Fig.11 Schematic diagram of the structure of a transmission-type invisible cloak with an inner vertex angle of 60° in Embodiment 3 of the present invention;

[0032] Fig.12 is the electromagnetic response of the polarization grid of the transmission layer in the third embodiment of the present invention;

[0033] Fig.13 is the electromagnetic response of the reflection unit in the third embodiment of the present invention;

[0034] Fig.14 This is a simulated electric field distribution diagram of a plane wave incident on the transmission layer, the reflection layer and the transmission type invisible cloak at a frequency of 11.6 GHz in the third embodiment of the present invention;

[0035] Fig.15 Schematic diagram of various implementation structures of the transmission-type invisible cloak and the corresponding carpet-type invisible cloak in the embodiments of the present invention.

[0036] Among them: 1-transmission layer; 2-reflection layer; 3-stealth area; 4-high-reflection flat metal floor; 5-flexible dielectric plate of the transmission unit; 6-upper metal polarization grid of the transmission unit; 7-metal cross-cross oscillator of the transmission unit; 8-lower metal polarization grid of the transmission unit; 9-flexible dielectric plate of the reflection unit; 10-double-notch metal elliptical ring of the reflection unit; 11-metal surface of the reflection unit; 12-dielectric substrate of the polarization grid; 13-metal grid lines of the polarization grid. DETAILED DESCRIPTION

[0037] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0038] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0039] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0040] like Figure 1 As shown, this embodiment provides a double-layer structure stealth cloak based on polarization conversion and phase modulation metasurface, characterized in that it includes an external transmission layer 1 and an internal reflection layer 2, the external transmission layer 1 and the internal reflection layer 2 are nested inside and outside at a certain spatial distance, and the space enclosed by the reflection layer 2 is the stealth area 3. The transmission layer 1 is formed by splicing a number of planar transmission metasurfaces, and the reflection layer 2 is formed by splicing a number of planar reflection metasurfaces. Both the planar transmission metasurface and the planar reflection metasurface have high-efficiency polarization conversion and phase modulation functions, so that the incident wave is reflected multiple times in the area between the transmission layer and the reflection layer without entering the stealth area, and finally returns to the original propagation path to be emitted, thereby forming a transmission-type stealth cloak to make objects in the stealth area invisible. The planar transmission metasurface and the planar reflection metasurface compensate for the path difference caused by multiple reflections through appropriate phase modulation.

[0041] The planar transmission metasurface is composed of M×N transmission units arranged periodically with equal spacing; the planar reflection metasurface is composed of L×N reflection units arranged periodically with equal spacing; the transmission units and reflection units are designed to achieve high-efficiency polarization conversion and phase modulation functions.

[0042] The inner wall and the outer wall of the transmission layer 1 are covered with a metal polarization grid. The metal polarization grid is a metal structure composed of a plurality of metal straight strips arranged in parallel, and has a polarization screening function. It completely transmits electromagnetic waves whose polarization direction is perpendicular to the metal polarization grid, and completely reflects electromagnetic waves whose polarization direction is parallel to the metal polarization grid; the metal polarization grid on the outer wall of the transmission layer 1 is perpendicular to the polarization direction of the incident electromagnetic wave, and the metal polarization grid on the inner wall of the transmission layer 1 is parallel to the polarization direction of the incident electromagnetic wave.

[0043] Figure 1 The two-dimensional implementation principle diagram of the transmission type invisible cloak provided in this embodiment. Figure 1 As shown, the transmission layer and the reflection layer are both rhombus structures with an inner vertex angle of 2α. The side length of the transmission layer is l t , the side length of the reflective layer is l r .

[0044] The design method and stealth mechanism of the transmission type stealth cloak are as follows:

[0045] For ease of understanding, Figure 1 The long arrows in the figure represent the propagation paths of the beams.

[0046] like Figure 1 As shown in the figure, after the plane wave is incident from one side of the invisibility cloak, it is reflected multiple times in the area between the transmission layer and the reflection layer, and finally converges and emits from the other side of the invisibility cloak; the line connecting the two opposite inner vertex angles 2α in the invisibility cloak is used as the symmetry axis to divide the invisibility cloak into the incident side and the output side. After the plane wave is incident on the invisibility cloak from the incident side, there are two wave paths, including:

[0047] Path r1 is the incident wave passing through the incident side transmission layer and entering the interior of the invisible cloak, without touching the reflection layer, and directly passing through the exit side transmission layer to exit;

[0048] Path r2 is the incident wave passing through the incident side transmission layer and entering the interior of the invisibility cloak, and then being completely reflected by the incident side reflection layer, the inner wall of the incident side transmission layer, the inner wall of the exit side transmission layer, and the exit side reflection layer in turn, and finally passing through the exit side transmission layer to exit. Each time the plane wave passes through the reflection layer, an orthogonal polarization conversion occurs, so that the plane wave can be completely reflected by the inner wall of the transmission layer and finally pass through the transmission layer. In order to ensure that the plane wave converges on the right side of the invisibility cloak and restores equal phase propagation, the compensation relationship between the transmission phase, reflection phase and path calculated by the two paths is:

[0049] (1)

[0050] (2)

[0051] in, and are the transmission phases of the incident side transmission layer and the exit side transmission layer, and are the reflection phases of the incident side reflection layer and the exit side reflection layer, is the wave number, is the free space wavelength. In order to ensure that the incident wave contacting the reflective layer propagates along the path r2, the side lengths of the transmission layer and the reflective layer meet the conditions: ; Wherein, α is half of the inner vertex angle 2α of the transmission layer and the reflection layer of the rhombus structure, l t is the side length of the transmission layer, l r is the side length of the reflection layer, and csc represents the cosecant function.

[0052] The designed transmission-type stealth cloak is a horizontally symmetrical structure. According to the mirror principle, the symmetry plane (the symmetry axis from the perspective of the two-dimensional plane structure) of the constructed transmission-type stealth cloak is replaced with a highly reflective flat metal floor 4, thereby realizing the construction of a carpet-type stealth cloak. Figure 2 A schematic diagram of the two-dimensional implementation principle of the carpet-type invisibility cloak provided in this embodiment.

[0053] The following three embodiments are used to describe in detail the specific implementation process of the double-layer structure stealth cloak based on polarization conversion and phase modulation metasurface provided by the present invention, including a transmission-type stealth cloak, a carpet-type stealth cloak, and a transmission-type stealth cloak with an inner vertex angle of 60°. The three examples are simulated by FDTD full-wave numerical simulation using simulation software. All three examples are periodic structures along the x-direction with a period of 6 mm. Therefore, in order to improve the calculation efficiency, a period is selected in the x-axis direction and periodic boundary conditions are set for simulation verification.

[0054] Embodiment 1

[0055] This embodiment is the design and verification of a transmission-type invisible cloak.

[0056] Figure 3 FIG. 1 is a schematic diagram of the structure of the transmission unit constituting the planar transmission metasurface in this embodiment. Figure 3 As shown, the transmission unit is composed of two laminated flexible dielectric plates 5 and a metal layer printed therein, an upper metal polarization grid 6 is printed on the front of the upper flexible dielectric plate, and a lower metal polarization grid 8 is printed on the back of the lower flexible dielectric plate. The upper metal polarization grid 6 and the lower metal polarization grid 8 are orthogonal to each other, and a metal cross-cross oscillator 7 rotated 45° is printed between the upper and lower flexible dielectric plates. The metal cross-cross oscillator 7 is composed of two metal straight strips of different lengths that cross vertically. The transmission unit can efficiently convert linearly polarized incident waves into their orthogonal polarization transmissions; by adjusting the structural parameters of the cross-cross oscillator, the transmission phase modulation requirements within a range of 360° can be covered.

[0057] Specifically, the central frequency of the transmission unit is designed to be 12 GHz. The flexible dielectric plate is a polytetrafluoroethylene glass cloth plate with a dielectric constant of ε r =4.4, the electrical tangent loss is tanδ=0.007, the thickness is h=1mm, and the unit period is p=6mm.

[0058] Preferably, the length of the long metal strip is t l =6mm, the length of the short metal strip is t s=4.6mm, the width of the metal strips is w=0.5mm, and the angle between the short metal strip and the Y axis is α=45°; the metal polarization grid is composed of four metal straight strips, the width of the first and fourth metal strips is s2=0.65mm, the width of the second and third metal strips is s1=1.4mm, the distance between the two middle metal strips is g1=0.7mm, and the distance between the metal strips on both sides is g2=0.6mm.

[0059] Figure 4 is the electromagnetic response of the transmission unit in this embodiment. Figure 4 Describes the amplitude and phase response of the transmission unit under plane wave vertical incidence (0°) and 45° oblique incidence. Cross-polarization transmission amplitude of the transmission unit under Y-polarized plane wave vertical incidence | T xy | Greater than 0.9 in the range of 10-12.2GHz; reflection amplitude of the transmission unit under X-polarized plane wave incidence |R xx | The simulation results show that the unit can convert the plane wave polarized along the Y direction (perpendicular to the upper polarization grid) into cross-polarization and transmit it, with a transmission efficiency greater than 90%; the plane wave polarized along the X direction (parallel to the upper polarization grid) is totally reflected. Under 45° oblique incidence, the cross-polarization transmission amplitude is greater than 0.9 in the range of 12-12.7 GHz, and the transmission phase is slightly reduced. At a frequency of 11.6 GHz, the transmission unit can provide nearly 100% high-efficiency polarization conversion for the incident wave and the transmission phase is close to 0, which meets the design requirements of formula (1).

[0060] The planar reflective metasurface is composed of L×N reflective units arranged periodically with equal spacing. Figure 5 FIG. 1 is a schematic diagram of the structure of the reflection unit constituting the planar reflection metasurface in this embodiment. Figure 5 As shown, the reflection unit includes a flexible dielectric plate 9, the upper side of which is printed with a double-notched metal elliptical ring 10 rotated 45°, and the lower side is full metal 11. The reflection unit can efficiently convert the linear polarization incident wave into its orthogonal polarization reflection; by adjusting the structural parameters of the double-notched metal elliptical ring, the reflection phase modulation requirements within a range of 360° can be covered.

[0061] Specifically, the center frequency of the reflector unit is designed to be 12 GHz. The flexible dielectric plate is a polytetrafluoroethylene glass cloth plate with a dielectric constant of ε r =4.4, the electrical tangent loss is tanδ=0.007, the thickness is h=2mm, and the unit period is p=6mm.

[0062] Preferably, the main body shape of the double-notched metal elliptical ring is an elliptical ring, and the semi-major axis of the outer diameter of the elliptical ring is Ra =2.3mm, semi-minor axis is R b =1.8mm; two notches are cut at both ends of the long axis, the width of the notch is w=0.15mm, and the end points of the two notches are connected inward with a metal straight bar. The length of each metal straight bar is d=1mm, and the width is consistent with the difference between the inner and outer diameters of the elliptical ring t=0.2mm; the angle between the long axis of the elliptical ring and the y-axis is θ=45°.

[0063] Figure 6 is the electromagnetic response of the reflection unit in this embodiment. Figure 6 The amplitude and phase response of the reflector unit under plane wave vertical incidence (0°) and 45° oblique incidence are described. Under vertical incidence, the cross-polarization reflection amplitude of the reflector unit |R yx |In the range of 10.6-14GHz, it is greater than 0.9; and under 45° oblique incidence, the cross-polarization reflection amplitude of the reflection unit is greater than 0.9 in the range of 11-14GHz, and the reflection phase decreases. At a frequency of 11.6GHz, the reflection phase is -120°. At this time, the reflection unit can provide nearly 100% high-efficiency polarization conversion for the incident wave, and the reflection phase meets the design requirements of formula (2).

[0064] The transmission unit and the reflection unit are used to respectively form a planar transmission metasurface and a planar reflection metasurface to form the transmission type invisible cloak of the first embodiment. Figure 7 FIG. 1 is a schematic diagram of the structure of the transmission type invisible cloak of Example 1. Figure 7 As shown in the figure, the transmission type invisible cloak is composed of a transmission layer and a reflection layer nested inside and outside, and the space wrapped by the reflection layer is the invisible area. The transmission layer is composed of four planar transmission metasurfaces, and the reflection layer is composed of four planar reflection metasurfaces. The transmission layer and the reflection layer are both diamond structures with an inner vertex angle of 90°. Each planar transmission metasurface is composed of 12×36 identical transmission units and has a size of 72×216mm. 2 Each planar reflective metasurface consists of 5×36 identical reflective units and measures 30×216 mm 2 The polarization grating on the outer wall of the transmission layer is parallel to the X-axis, and the polarization grating on the inner wall of the transmission layer is perpendicular to the X-axis.

[0065] Figure 8 1 is a simulated electric field distribution diagram of the Y-polarized plane wave incident on the free space, the transmission layer, the reflection layer and the transmission type invisible cloak at a frequency of 11.6 GHz in Example 1. Figure 8As shown in the figure, under the incidence of plane waves, the transmission layer is consistent with the wavefront of free space, indicating that the transmission layer is completely transparent to plane waves; while the reflection layer shows strong scattering and phase distortion under the incidence of plane waves; after the transmission layer is nested outside the reflection layer, the transmission type stealth cloak constructed alleviates the strong scattering caused by the reflection layer, restores the plane wavefront, which is consistent with the wavefront of free space, and the transmission type stealth cloak is perfectly hidden; among them, the stealth area wrapped by the reflection layer always maintains a state of no wave entering, therefore, any object in the stealth area can be perfectly hidden.

[0066] Embodiment 2:

[0067] According to the mirror principle, the horizontal symmetry axis of the constructed transmission-type invisible cloak is replaced with a flat metal floor, so that the carpet-type invisible cloak can be constructed. This embodiment is the design and verification of a carpet-type double-layer structure invisible cloak.

[0068] Fig. 9 FIG. 2 is a schematic diagram of the structure of the carpet-type invisible cloak of the second embodiment. Fig. 9 As shown in the figure, two planar transmission metasurfaces are placed on a planar metal floor to form a transmission layer; two planar reflection metasurfaces form a reflection layer and are placed inside the transmission layer. Both the transmission layer and the reflection layer are triangular structures with an inner vertex angle of 90°. Each planar transmission metasurface consists of 12×36 identical transmission units with a size of 72×216mm 2 Each planar reflective metasurface consists of 5×36 identical reflective units and measures 30×216 mm 2 .

[0069] Fig.10 In Example 2, at a frequency of 11.6 GHz, the reflected electric field distribution diagram of a Y-polarized plane wave incident on a flat metal floor, a reflective layer, and a carpet-type invisible cloak. Fig.10 As shown in the figure, a plane wave incident on a flat metal floor is mirror-reflected, and the reflected wave is a plane wavefront; while the reflective layer exhibits strong scattering under the incidence of a plane wave, and the plane wave is reflected in different directions. When covered with a carpet-like invisibility cloak, the invisibility cloak on the metal floor is almost transparent, and the wavefront of the reflection field is restored, close to the reflected wavefront of the metal floor. The results show that any object on a flat metal floor can be perfectly hidden after being wrapped in an invisibility cloak.

[0070] Embodiment three:

[0071] This embodiment is a design of a transmission type invisible cloak with an inner vertex angle of 60°, which verifies that the present invention can realize a double-layer structure invisible cloak with different inner vertex angles. In the third embodiment, the transmission layer is composed of a polarization grid with a transmission phase close to 0°, which is a simplified design of the present invention.

[0072] Fig.11FIG. 1 is a schematic diagram of the structure of a transmission type invisible cloak with an inner vertex angle of 60° in Example 3. Fig.11 As shown in the figure, the transmission type invisible cloak is composed of a transmission layer and a reflective layer nested inside and outside, and the space wrapped by the reflective layer is the invisible area. Among them, the reflective layer is composed of four planar reflective metasurfaces, and the reflective layer is a diamond structure with an inner vertex angle of 60°. Each planar reflective metasurface is composed of 4×36 identical reflective units, with a total size of 24×216mm 2 The transmission layer is made of four polyimide dielectric substrates printed with metal polarization grids, forming a diamond structure with an inner vertex angle of 60°. The dielectric constant of the dielectric substrate is ε r =4.2, the electrical tangent loss is tanδ=0.007, the thickness is 0.1mm, and the size of each dielectric substrate is 84×216mm 2 Each metal polarization grid is a structure composed of multiple metal strips arranged periodically, each metal strip is 0.2 mm wide, the spacing between the metal strips is 1 mm, and is etched on a copper foil with a thickness of 0.035 mm; the polarization grid is parallel to the X-axis.

[0073] Fig.12 This is the electromagnetic response of the metal polarization grid in Example 3. Here, a 6 mm×6 mm polarization grid unit is taken and a periodic boundary condition is applied, and simulation software is used for simulation. Fig.12 The amplitude and phase responses of the polarization grating under plane wave vertical incidence (0°) and 30° oblique incidence are described. Under vertical incidence, the transmission loss of the polarization grating for y-polarized waves is less than 0.01 dB, and the transmission phase is about 0°; the reflection loss for x-polarized waves is less than 0.02 dB, and the reflection phase is about 180°. The simulation results show that the polarization grating has broadband, high transmittance and low loss polarization selection characteristics, which meets the design requirements.

[0074] Fig.13 is the electromagnetic response of the reflection unit in Example 3. Preferably, the parameter of the reflection unit is R a =R b =2mm, w=0.15mm, d=1.3mm, t=0.2mm. Fig.13 The amplitude and phase response of the reflector unit in Example 3 under plane wave vertical incidence (0°) and 30° oblique incidence are described. Under vertical incidence, the cross-polarization reflection amplitude |R xy | is greater than 0.9 in the range of 11-14 GHz; and under 30° oblique incidence, the cross-polarization reflection amplitude of the reflection unit is greater than 0.9 in the range of 11.1-14 GHz, and the reflection phase decreases. At a frequency of 11.6 GHz, the reflection phase is -90°, at which time the reflection unit can provide nearly 100% high-efficiency polarization conversion for the incident wave, and the reflection phase meets the design requirements of formula (2).

[0075] Fig.14 : This is the simulated electric field distribution diagram of Example 3 at a frequency of 11.6 GHz when a Y-polarized plane wave is incident on the transmission layer, the reflection layer and the transmission type invisible cloak respectively. Fig.14 As shown, strong scattering and energy shielding are observed after the Y-polarized wave enters the reflective layer; while after the Y-polarized wave enters the transmission layer and the transmission-type stealth cloak, a uniform plane transmission wavefront is observed, which is consistent with the free space plane wave, indicating that the amplitude and phase of the transmission field are restored; the stealth area wrapped by the reflective layer remains in a state where no wave enters, and any object in the stealth area can be perfectly hidden. The results show that the transmission-type stealth cloak with an inner vertex angle of 60° in this embodiment 3 also has good stealth performance.

[0076] The present invention can be transformed into a variety of transmissive stealth cloaks with different configurations, the top angle of the structure is adjustable, and each transmissive stealth cloak can construct its corresponding carpet-type stealth cloak. Fig.15 The following are some possible schematic diagrams of the realization of transmission-type invisibility cloaks and carpet-type invisibility cloaks. Fig.15 As shown, the stealth cloak is a planar symmetrical structure, which is divided into a mutually symmetrical incident side and an exit side by a symmetry plane perpendicular to the incident direction of the electromagnetic wave. The transmission layer is a polygonal structure and has at least two planar transmission metasurfaces that form a certain angle with the incident wave. The reflection layer is a polygonal structure and has at least two planar reflection metasurfaces. In the transmission layer, for the surface that does not participate in the transmission or reflection of the electromagnetic wave, a planar transmission metasurface may not be set; in the reflection layer, for the surface that does not participate in the reflection of the electromagnetic wave, a planar reflection metasurface may not be set. For all configurations of transmission-type stealth cloaks, a symmetry plane perpendicular to the incident direction of the electromagnetic wave is used to divide them into a mutually symmetrical incident side and an exit side, and only the transmission layer and the reflection layer on the incident side are retained, and a planar metal floor is set at the position of the symmetry plane. Then, the transmission layer and the reflection layer on the incident side and the planar metal floor form a carpet-type stealth cloak corresponding to the transmission-type stealth cloak of the corresponding configuration. According to the design method and steps of the present invention, corresponding transmission-type invisible cloak and carpet-type invisible cloak examples can be obtained, but they are not intended to be limiting of the present invention.

[0077] In summary, the method proposed in the present invention solves the complex design and manufacturing problems of the transmissive invisible cloak, and it is applicable to both terahertz and optical frequencies, opening up an unprecedented approach for the design of transmissive cloaks and carpet cloaks.

[0078] The above is only a preferred embodiment of the present invention, and does not limit the present invention in other forms. Any technician familiar with the profession may use the above disclosed technical content to change or modify it into an equivalent embodiment with equivalent changes. However, any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention without departing from the technical solution of the present invention still belongs to the protection scope of the technical solution of the present invention.

Claims

1. A double-layer invisible cloak based on polarization conversion and phase modulation metasurface, characterized in that: It includes an external transmission layer and an internal reflection layer, and the space enclosed by the reflection layer is a stealth area; the transmission layer is formed by splicing a plurality of planar transmission metasurfaces, and the reflection layer is formed by splicing a plurality of planar reflection metasurfaces; both the planar transmission metasurface and the planar reflection metasurface have polarization conversion and phase modulation functions, so that the incident wave is reflected multiple times in the area between the transmission layer and the reflection layer without entering the stealth area, and finally returns to the original propagation path to be emitted, thereby forming a transmission-type stealth cloak to make the object in the stealth area invisible; the planar transmission metasurface and the planar reflection metasurface compensate for the path difference caused by multiple reflections through phase modulation; After the plane wave is incident from one side of the invisibility cloak, it is reflected multiple times in the area between the transmission layer and the reflection layer, and finally converges and emits from the other side of the invisibility cloak; the invisibility cloak is a plane symmetrical structure, and the invisibility cloak is divided into an incident side and an output side by the symmetry axis. After the plane wave is incident on the invisibility cloak from the incident side, there are two wave paths, including: Path r1 is the incident wave passing through the incident side transmission layer and entering the interior of the invisible cloak, without touching the reflection layer, and directly passing through the exit side transmission layer to exit; Path r2 is the incident wave passing through the incident side transmission layer and entering the interior of the invisible cloak, and then being completely reflected by the incident side reflection layer, the inner wall of the incident side transmission layer, the inner wall of the exit side transmission layer, and the exit side reflection layer in sequence, and finally passing through the exit side transmission layer to exit; each time the plane wave passes through the reflection layer, an orthogonal polarization conversion occurs, so that the plane wave can be completely reflected by the inner wall of the transmission layer and finally pass through the transmission layer; the compensation relationship between the transmission phase, reflection phase and path calculated for the two paths is: (1) (2) in, and are the transmission phases of the incident side transmission layer and the exit side transmission layer, and are the reflection phases of the incident side reflection layer and the exit side reflection layer, is the wave number, is the free space wavelength.

2. The double-layer structure invisible cloak based on polarization conversion and phase modulation metasurface according to claim 1 is characterized in that: The inner wall and the outer wall of the transmission layer are covered with a metal polarization grid. The metal polarization grid is a metal structure composed of a plurality of metal straight strips arranged in parallel, has a polarization screening function, completely transmits electromagnetic waves whose polarization direction is perpendicular to the metal polarization grid, and completely reflects electromagnetic waves whose polarization direction is parallel to the metal polarization grid; the metal polarization grid on the outer wall of the transmission layer is perpendicular to the polarization direction of the incident electromagnetic wave, and the metal polarization grid on the inner wall of the transmission layer is parallel to the polarization direction of the incident electromagnetic wave.

3. The double-layer structure invisible cloak based on polarization conversion and phase modulation metasurface according to claim 1 is characterized in that: From the perspective of the two-dimensional plane structure of the invisible cloak, the transmission layer and the reflection layer are both rhombus structures with an inner vertex angle of 2α; the side length of the transmission layer is l t , the side length of the reflective layer is l r ; The side lengths of the transmission layer and the reflection layer meet the conditions: .

4. The double-layer structure invisible cloak based on polarization conversion and phase modulation metasurface according to claim 1 is characterized in that: The planar transmission metasurface is composed of M×N transmission units arranged periodically with equal spacing; the transmission unit is composed of two laminated flexible dielectric plates and a metal layer printed thereon, the upper side of the upper flexible dielectric plate and the lower side of the lower flexible dielectric plate are printed with mutually orthogonal metal polarization gratings, and a metal cross-cross oscillator rotated 45° is printed between the upper and lower flexible dielectric plates; the transmission unit can efficiently convert a linearly polarized incident wave into its orthogonal polarization transmission; adjusting the structural parameters of the metal cross-cross oscillator can cover the transmission phase modulation requirements within a range of 360°.

5. The double-layer structure invisible cloak based on polarization conversion and phase modulation metasurface according to claim 1 is characterized in that: The planar reflective metasurface is composed of L×N reflective units arranged periodically with equal spacing; the reflective unit includes a flexible dielectric plate, on the upper side of which is printed a double-notched metal elliptical ring rotated 45°, and the lower side is all metal; the reflective unit can efficiently convert a linearly polarized incident wave into its orthogonal polarized reflection; adjusting the structural parameters of the double-notched metal elliptical ring can cover the reflection phase modulation requirements within a range of 360°.

6. The double-layer structure invisible cloak based on polarization conversion and phase modulation metasurface according to claim 1 is characterized in that: The invisibility cloak is a planar symmetrical structure, which is divided into a mutually symmetrical incident side and an exit side by a symmetry plane perpendicular to the incident direction of the electromagnetic wave. The transmission layer is a polygonal structure and has at least two planar transmission metasurfaces that form a certain angle with the incident wave. The reflection layer is a polygonal structure and has at least two planar reflection metasurfaces.

7. The double-layer structure invisible cloak based on polarization conversion and phase modulation metasurface according to claim 1 is characterized in that: In the transmission layer, for the surface that does not participate in the transmission or reflection of electromagnetic waves, no planar transmission metasurface is set; in the reflection layer, for the surface that does not participate in the reflection of electromagnetic waves, no planar reflection metasurface is set.

8. The double-layer structure invisible cloak based on polarization conversion and phase modulation metasurface according to claim 1 is characterized in that: For all configurations of transmission-type stealth cloaks, a symmetry plane perpendicular to the incident direction of the electromagnetic wave is used to divide them into a mutually symmetrical incident side and an exit side, and only the transmission layer and the reflection layer on the incident side are retained. A plane metal floor is set at the position of the symmetry plane. The transmission layer and the reflection layer on the incident side and the plane metal floor form a carpet-type stealth cloak corresponding to the transmission-type stealth cloak of the corresponding configuration; after the incident wave is transmitted from the outside of the transmission layer, it is reflected multiple times in the area between the transmission layer, the reflection layer and the plane metal floor without entering the stealth area, and finally exits along the reverse direction of the original propagation path.

Images