Terahertz wave band regulation metasurface and design method thereof

Abstract

The invention discloses an artificial metasurface for terahertz light wave regulation and control and a design method of the artificial metasurface. The metasurface is an array formed by a plurality of monomer structures, and each monomer structure comprises a reflection substrate layer; a polyimide dielectric layer; and patterning the top layer. The device is characterized in that the patterned top layer is composed of an outer-layer circular ring composed of metal and a conductivity-adjustable active material and an inner-layer circular ring composed of a conductivity-adjustable active material; and the reflection regulation and control of the terahertz wave can be completed by controlling the stepping rotation angle of the two circular rings and the conductivity change of the active material. According to the invention, the design is simple, and the modulation capability of terahertz wave reflection vortex beams is greatly improved.

Description

Technical Field

[0001] The present invention relates to an artificial metasurface for regulating electromagnetic waves in the terahertz band and a design method thereof, and in particular to a metasurface for performing vortex beam regulation on electromagnetic waves in the terahertz band and a design method thereof. Background Art

[0002] Currently, the use of artificial metasurfaces to manipulate terahertz waves has become a common technique in the terahertz field. Among these, the research on using artificial metasurfaces to modulate incident terahertz waves into vortex beams has great potential for application. Conventional artificial metasurfaces are static in their modulation of vortex beams; once the metasurface is fabricated, the topological charge of the vortex beam cannot be altered.

[0003] To overcome this problem, the industry generally adopts the method of introducing active materials into the metasurface structure, and taking advantage of the different properties of the active materials under different external conditions to achieve the modulation of the topological charge number of the vortex beam. However, according to current reports, the incident terahertz electromagnetic wave needs to be converted from a circularly polarized wave, that is, a CP wave, to a transverse electric wave or a transverse magnetic wave, that is, a TE wave or a TM wave, in order to complete the adjustment of the topological charge number. At the same time, the adjustment range of the topological charge number is very small. For example, it can only complete the switching of the topological charge number from 1 to 2; finally, it can only complete unidirectional switching. For example, when the active material changes from a high conductivity to a low conductivity state, the topological charge number changes from 1 to 2, and the reverse switching cannot be achieved. Summary of the Invention

[0004] To address these issues, the present invention provides an artificial metasurface and its design method. This design eliminates the need to convert incident terahertz waves from CP waves to TM or TE waves. Instead, the reflected vortex beam can be switched between different topological charge numbers simply by changing the temperature. Furthermore, bidirectional switching of the topological charge number can be achieved by presetting two angles within the basic structure without changing the basic structure. This significantly enhances the flexibility of terahertz wave control.

[0005] The present invention can be achieved by providing a solution for an artificial metasurface:

[0006] An artificial metasurface for controlling reflected vortex beams in the terahertz band, which is composed of n monomer structures according to the instructions attached Figure 3The structure is arranged counterclockwise in a matrix, where n is equal to or greater than 8. Each monomer structure comprises three layers, from bottom to top: a reflective substrate layer, a polyimide layer, and a patterned top layer. The patterned top layer is characterized by an inner and outer ring structure: the inner ring is made of an electrically conductive active material with two symmetrical notches about its center, each corresponding to a central angle α; the outer ring is made of metal and also has two symmetrical notches about its center, filled with the same active material as the inner ring. The central angle β is defined by the values of α and β to ensure that the structure's reflection coefficient for terahertz light waves within the 0.85±0.03THz frequency band is greater than 60%, thereby ensuring reflection intensity. There is a central rotation angle difference Δa between the inner rings of adjacent monomer structures in the metasurface, and a central rotation angle difference Δb between the outer rings, and Δa = 180l1 / n, Δb = 180l2 / n; the active material must have two states of high conductivity and low conductivity. In the high conductivity state, the conductivity must be at least of the same order of magnitude as the conductivity of the outer ring metal; in the low conductivity state, the conductivity must be low enough to ensure that the geometric rotation of the active material has a negligible ability to control the phase of the terahertz wave compared with the outer ring metal material; the l1 and l2 are the topological charge numbers of the reflected light beam that are expected to be obtained when the active material is in the high and low conductivity states, respectively.

[0007] In the above structure: the thickness of the reflective substrate layer is not less than 0.2 μm.

[0008] In the above structure: the reflective substrate layer is aluminum.

[0009] In the above structure, the metal of the outer ring of the patterned top layer is aluminum.

[0010] In the above structure, the material of the inner ring of the patterned top layer is a superconducting material.

[0011] In the above structure: the superconducting material is niobium nitride.

[0012] In the above structure, the central angle α is 1.5° and β is 44-46°.

[0013] In order to obtain the above-mentioned artificial metasurface, the present invention further provides a design method for the above-mentioned metasurface, which is characterized by specifically comprising the following steps in sequence:

[0014] n monomer structures according to claim 1 are provided, n≥2 l ; where l is the maximum value among l1, l2, and 3, and the n monomer structures differ only in the inner and outer ring rotation angles a and b.

[0015] Among the n monomer structures, the inner and outer ring rotation angles of the m-th monomer structure are set to: am = (m-1) Δa, bm = b0 + (m-1) Δb, wherein Δa = 180l1 / n, Δb = 180l2 / n; m ≤ n and is a natural number, i.e., Figure 3 In the metasurface shown, there is a center rotation angle difference Δa between the inner rings of adjacent monomer structures, and a center rotation angle difference Δb between the outer rings.

[0016] The value of b0 can be set to any value in the range of (-ππ], that is, among the n monomer structures, the inner and outer ring rotation angles a1 and b1 of the first monomer structure do not need to be equal.

[0017] The n monomer structures are as shown in the attached instructions. Figure 3 The shown method is arranged counterclockwise to form a matrix.

[0018] The present invention provides an artificial metasurface structure and a corresponding design method thereof. By calculating the sizes of the inner and outer ring step rotation angles Δa and Δb of each monomer structure in advance, the artificial metasurface can conveniently and bidirectionally adjust the topological charge number of the reflected vortex light beam by simply adjusting the conductivity of the active material without changing the incident circularly polarized terahertz wave. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1-2 A three-dimensional view and a top view of a monomer structure in the artificial metasurface provided by the present invention;

[0020] Figure 3 A top view of the monomer structures in the artificial metasurface provided by the present invention arranged in counterclockwise order;

[0021] Figure 4 Schematic diagram of 8 monomer structures with different inner and outer ring rotation angles

[0022] Figure 5 Because the instruction manual is attached Figure 4 Schematic diagram of the metasurface formed by the arrangement of 8 different monomer structures shown;

[0023] Figure 6-7 The instructions are attached Figure 4 The reflectivity simulation results of the eight different monomer structures at room temperature and low temperature;

[0024] Figure 8-9 The instructions are attached Figure 4 The simulation results of the phase difference of the eight different monomer structures at room temperature and low temperature;

[0025] Figure 10 and Figure 12 Attached to the instruction manual Figure 5Schematic diagram of the phase difference of different monomer structures of the metasurface at room temperature and superconducting low temperature;

[0026] Figure 11 and Figure 13 Attached to the instruction manual Figure 5 Reflection vortex phase distribution diagram of the metasurface at room temperature and superconducting low temperature.

[0027] Figure 14-15 This is the reflection vortex phase distribution diagram of the metasurface described in the specification

[0037] at room temperature and superconducting low temperature.

[0028] Figure 16-17 These are the reflection vortex phase distribution diagrams of the corresponding metasurface at room temperature and superconducting low temperature when the step angles Δa = 67.5° and Δb = 45° as described in the specification

[0039] .

[0029] Figure 18-19 This is the reflection vortex phase distribution diagram of the supersurface composed of 16 monomer structures described in the specification

[0041] at room temperature and superconducting low temperature. DETAILED DESCRIPTION

[0030] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in combination with specific embodiments and with reference to the accompanying drawings. The description omits the introduction of the well-known technologies for those skilled in the art.

[0031] The artificial super surface of the present invention is composed of n kinds of monomer structures. Figure 1-2 As shown, the monomer structure includes 3 layers, from bottom to top: aluminum reflective substrate layer, polyimide dielectric layer, dielectric constant ε = 2.93 + 0.13i, and patterned top layer. Among them, the top view of the aluminum reflective substrate layer and the polyimide dielectric layer is a square with a side length p = 164μm; the outer ring of the patterned top layer is aluminum, and the gap of the outer ring is filled with superconducting material niobium nitride, and the inner ring is superconducting material niobium nitride. The specific geometric parameters are shown in the figure; the thickness of the reflective substrate layer, polyimide dielectric layer, and patterned top layer are 0.2μm, 70μm, and 0.2μm, respectively. The n monomer structures are different only in the values of the rotation angles a and b of the inner and outer rings in the patterned top layer. The above n monomer structures are rotated counterclockwise in sequence according to the instructions attached. Figure 3 The hypersurface is obtained by arranging them into a matrix. Where n can be any natural number greater than 7. Figure 3 The 8 monomer structures shown in the figure are only examples, and the attached specification Figure 3 Only the 1st, 2nd, 3rd and n-2, n-1, nth monomer structures are shown, and not all monomer structures are fully labeled, which does not affect the understanding of the arrangement.

[0032] Example 1: We take the design of an artificial metasurface whose topological charge number for reflecting a vortex beam at room temperature is 1 and whose topological charge number becomes 2 at low temperatures close to superconducting as an example to introduce the artificial metasurface and its design method of the present invention.

[0033] First design as shown in the instruction manual Figure 4 The eight monomer structures shown are represented by 1-8. Monomer structures with different numbers differ only in the rotation angle of the patterned top layer. The rotation angles (a, b) of the inner and outer rings are (0°11.25°)(45°33.75°)(90°56.25°)(135°78.75°)(0°101.25°)(45°123.75°)(90°146.25°)(135°168.75°), that is, the step rotation angles Δa and Δb of the inner and outer rings between the two adjacent structures are 45° and 22.5° respectively. The eight structures are arranged according to the instructions attached. Figure 5 The 24×24 matrix structure is arranged in the form of a terahertz beam to reflect the incident terahertz beam into a vortex beam. The simulation calculation results of the reflectivity and phase difference of the above 8 monomer structures at room temperature and superconducting low temperature are shown in the attached manual. Figure 6-9 shown.

[0034] The theoretical principle is: when the temperature is room temperature, the conductivity of niobium nitride is very small compared to that of metal aluminum. At this time, the metal outer ring in each monomer structure has an obvious phase modulation effect on the reflection of the terahertz wave, and the phase modulation effect of the inner ring can be ignored. Therefore, based on the Pancharatnam-Berry phase theory: For reflective metasurfaces: when the incident wave is a circularly polarized wave, that is, a CP wave, the polarization mode of the reflected wave is the same as the polarization mode of the incident wave, and the patterned rotation angle of the metasurface unit is Δa, and the corresponding phase shift is ±Δ2a, where + is an LCP wave and – is an RCP wave. At this time, combined with the attached instructions Figure 8 As you can see, the instruction manual is attached. Figure 5 The phase difference of each monomer structure is as shown in the appendix of the manual. Figure 10 As shown in the figure, one rotation of the space plane corresponds to 360°, thus forming a reflected vortex beam with a topological charge number of 1. The corresponding simulation results are shown in the attached manual. Figure 11 shown.

[0035] When the temperature drops to the superconducting critical point, niobium nitride approaches the superconducting state, and the conductivity increases rapidly. It begins to have a phase modulation effect on the reflection of terahertz waves like a metal material. Therefore, the outer ring can be regarded as a whole metal ring. At this time, the 22.5° step rotation angle of the outer ring is meaningless, and the inner ring plays the main role in the phase modulation of the terahertz beam. Therefore, combined with the attached instructions Figure 9 It is known that the instructions are attached at this time Figure 5 The phase difference of each monomer structure is as shown in the appendix of the manual. Figure 12As shown in the figure, one rotation of the space plane corresponds to 720°, thus forming a reflected vortex beam with a topological charge number of 2. The simulation results are shown in the attached manual. Figure 13 shown.

[0036] Example 2: Design a metasurface whose topological charge number for reflecting a vortex beam is 2 at room temperature and becomes 1 at low temperatures close to superconductivity.

[0037] The same scheme as described in Example 1: First, 8 monomer structures are designed, represented by 1-8 respectively, and the monomer structures with different labels only have different rotation angles of the patterned top layer. However, in this example, the step rotation angles of the inner and outer rings between each two adjacent monomer structures are 22.5° and 45°, respectively, that is, the values of the step angles Δa and Δb in Example 1 are interchanged. Therefore, the rotation angles (a, b) of the inner and outer rings of the 8 monomer structures 1-8 are respectively (0°11.25°)(22.5°56.25°)(45°101.25°)(67.5°146.25°)(90°11.25°)(112.5°56.25°)(135°101.25°)(157.5°145.25°). The 8 monomer structures are arranged according to the instructions attached. Figure 5 The metasurface can be obtained by arranging them into a 24×24 matrix structure.

[0038] When the temperature is room temperature, the outer metal ring in each monomer structure has a significant phase modulation effect on the reflection of the terahertz wave, and the phase modulation effect of the inner ring is negligible. Compared with Example 1, Δb becomes 45° Based on the Pancharatnam-Berry phase theory, at this time the specification appendix Figure 5 The phase difference of adjacent monomer structures is 90°, as shown in the attached manual. Figure 12 As shown, one rotation of the space plane corresponds to 720°, thus forming a reflected vortex beam with a topological charge number of 2. When the temperature drops to the superconducting critical point, Δa becomes 22.5° compared with Example 1. Figure 5 The phase difference of adjacent monomer structures is 45°, as shown in the attached manual. Figure 10 As shown in the figure, one rotation of the space plane corresponds to 360°, thus forming a reflected vortex beam with a topological charge number of 1. The simulation results at room temperature and superconducting low temperature are shown in the appendix of the manual. Figure 14-15 shown.

[0039] As shown in Examples 1 and 2, 8 monomer structures are prepared according to the instructions. Figure 5 The topological charge numbers l1 and l2 of the vortex beam at low and room temperature depend on the values of Δa and Δb. The calculation formula is Δa = 180l1 / 8; Δb = 180l2 / 8; that is, the rotation angle (a) between the inner and outer rings of the eight monomer structures is m ,bm ) are respectively a m =(m-1)Δa, bm=11.25+(m-1)Δb, m≤8 and is a natural number. Furthermore, if the topological charge number of the reflected vortex beam is switched from l=2 at room temperature to l=3 near the superconducting low temperature, then it is only necessary to rotate the inner and outer rings by step angles Δa=67.5° and Δb=45° to set the corresponding 8 monomer structures; the corresponding simulation results at room temperature and superconducting low temperature are respectively attached to the specification. Figure 16-17 Conversely, to switch the topological charge from l = 3 at room temperature to l = 2 near the superconducting low temperature, one only needs to swap Δa and Δb, i.e., Δa = 45° and Δb = 67.5°. Therefore, unlike traditional single-way adjustment, the present invention allows for convenient bidirectional adjustment of the topological charge.

[0040] Example 3: Design a metasurface whose topological charge number for reflecting a vortex beam is 1 at room temperature and becomes 4 at low temperatures close to superconductivity.

[0041] Similar to the scheme described in Example 1, the monomer structure is designed first. If 8 monomer structures are still designed, when the topological charge number is 4 at low temperature, then Δa=90° so that the phase difference between two adjacent monomer structures is 180°. However, at this time, the phase difference between adjacent monomer structures is too large, resulting in the inability to form vortex light between the reflected beams. Instead, 8 discrete light beams are formed, and the phase difference between each two adjacent vertical beams is 180°. Therefore, in order to reduce the phase difference between adjacent monomer structures and satisfy the topological charge number of 4 at the same time, it is only necessary to increase the number of monomer structures. For example, 16 monomer structures are designed, of which the first monomer structure (ab) is (0°5.25°), and the remaining 15 monomers are rotated in sequence according to the step angles Δa=45° and Δb=11.25°. The 16 monomer structures are arranged according to the instructions attached. Figure 3 In this case, the instructions are attached Figure 3 The value of n is 16. Based on the Pancharatnam-Berry phase theory, one rotation of the space plane at room temperature and low temperature corresponds to 360° and 1440° respectively. Therefore, the topological charge number of the reflected vortex beam at room temperature is 1, and the topological charge number becomes 4 at superconducting low temperature. At the same time, due to the 25° phase difference between adjacent monomer structures at room temperature, the phase continuity of the vortex beam is better at this time. The simulation results at room temperature and superconducting low temperature are shown in the attached manual. Figure 18-19 shown.

[0042] As shown in Example 3, when a larger topological charge number is required, this can be achieved by increasing the number of monomer structures. If the number of monomer structures is n, then the inner and outer ring rotation angles of the n monomer structures are am = (m-1)Δa and bm = b0 + (m-1)Δb, respectively, where Δa = 180l1 / n and Δb = 180l2 / n; m ≤ n and is a natural number.

[0043] In the embodiments of the present invention, the incident waves are all vertically incident, so the values of a and b of the patterned top layer are the rotation angles of the incident wave in the plane perpendicular to the propagation direction. Because the incident wave used in the present invention is a circularly polarized wave, that is, a CP wave, rather than a TE wave or a TM wave, which has no directionality in the plane perpendicular to the propagation direction, the values of a and b have no effect on the reflectivity of the incident wave. At the same time, since the reflection phase difference between adjacent monomer structures depends only on the value of Δa or Δb, and Examples 1-3 all show the phase distribution of the final vortex beam, it is consistent with the theoretical design. Therefore, this application only shows the reflectivity and phase diagrams of the 8 monomer structures in Example 1. At the same time, in the above embodiments, the incident wave is an LCP wave. When it is RCP, the instructions are attached. Figure 3 The counterclockwise arrangement order of the monomer structures shown in the figure can be changed to a clockwise arrangement order.

[0044] The parameters described in the above embodiments, such as the side length of the monomer structure, the inner and outer radii of the outer ring, and the inner and outer radii of the inner ring, are merely numerical values used in the embodiments. Modifications to these geometric parameters using the principles of this document, while ensuring effective results, do not limit the present invention. Furthermore, the above design principles, by modifying geometric parameters to ensure reflectivity, can also be applied to other electromagnetic bands, such as radar bands. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this invention are intended to be included within the scope of protection of this invention.

Claims

1. An artificial metasurface for regulating reflective vortex beams in the terahertz band, comprising n monomer structures arranged counterclockwise in a matrix as shown in FIG3 of the specification, where n ≥ 8, and each monomer structure comprises three layers, from bottom to top: a reflective substrate layer, a polyimide layer, and a patterned top layer, characterized in that: The patterned top layer consists of two circular rings: the inner ring, made of a tunable conductivity active material, has two symmetrical notches about its center, each corresponding to an angle α. The outer ring, made of metal, also has two symmetrical notches about its center, filled with the same active material as the inner ring. Each notch has an angle β. The values of α and β are limited to ensure that the structure's reflection coefficient for terahertz light waves in the 0.85±0.03THz frequency band is greater than 60%, thereby ensuring reflection intensity. The inner rings of adjacent monomer structures in the metasurface have a central rotation angle difference ∆a, and the outer rings have a central rotation angle difference ∆b, and ∆a = 180l1 / n, ∆b = 180l2 / n; the active material must have two states of high conductivity and low conductivity. In the high conductivity state, the conductivity must be at least of the same order of magnitude as the conductivity of the outer ring metal; in the low conductivity state, the conductivity must be low enough to ensure that the geometric rotation of the active material has negligible phase control ability on the terahertz wave compared with the outer ring metal material; the l1 and l2 are the topological charge numbers of the reflected light beam that are expected to be obtained when the active material is in the high and low conductivity states, respectively.

2. The artificial supersurface structure according to claim 1, wherein: The thickness of the reflective substrate layer is not less than 0.2 μm.

3. The artificial supersurface structure according to claim 1, wherein: The reflective substrate layer is made of aluminum.

4. The artificial supersurface structure according to claim 1, wherein: The metal of the outer ring of the patterned top layer is aluminum.

5. The artificial metasurface structure according to claim 1, wherein: The material of the inner ring of the patterned top layer is a superconducting material.

6. The artificial metasurface structure according to claim 5, wherein: The superconducting material is niobium nitride.

7. The artificial metasurface structure according to claim 1, wherein: The central angle α is 1.5° and β is 44-46°.

8. A method for designing an artificial metasurface structure according to claim 1, characterized in that: The specific steps include: First, set n types of monomer structures described in claim 1, n≥2 l ; where l is the maximum value among l1, l2, and 3, and the n monomer structures differ only in the rotation angles of the inner and outer rings; Second, among the n monomer structures, the inner and outer ring rotation angles of the m-th monomer structure are set to: am = (m-1)∆a, bm = b0 + (m-1)∆b, where ∆a = 180l1 / n and ∆b = 180l2 / n; m ≤ n and is a natural number, that is, in the metasurface shown in Figure 3 of the specification, there is a central rotation angle difference ∆a between the inner rings of adjacent monomer structures and a central rotation angle difference ∆b between the outer rings; Third, the value of b0 can be set to any value in the range of (-π π], that is, among n monomer structures, the inner and outer ring rotation angles a1 and b1 of the first monomer structure do not need to be equal; Fourth, n monomer structures are arranged counterclockwise to form a matrix as shown in FIG3 of the specification.

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