A terahertz wave band regulated super surface and a design method thereof

Abstract

The application discloses an artificial super surface for terahertz light wave regulation and a design method thereof. The super surface is an array of a plurality of monomer structures, and the monomer structure comprises a reflective substrate layer, a polyimide medium layer and a patterned top layer. The patterned top layer is characterized by comprising an outer circular ring composed of metal and an electrically conductive adjustable active material and an inner circular ring composed of the electrically conductive adjustable active material. The reflection regulation of the terahertz wave can be completed by controlling the step rotation angle of the two circular rings and the electric conductance change of the active material. The application has simple design and greatly increases the modulation capacity of the terahertz wave reflection vortex light beam.

Description

Technical Field

[0001] This invention relates to an artificial metasurface for terahertz electromagnetic wave manipulation and its design method, and more particularly to a metasurface for vortex beam manipulation of terahertz electromagnetic waves and its design method. Background Technology

[0002] Currently, the manipulation of terahertz waves using artificial metasurfaces has become a common technique in the terahertz wave field. Among these techniques, the research on modulating incident terahertz waves into vortex beams using artificial metasurfaces has high potential application value. Traditional artificial metasurfaces provide static modulation of vortex beams; once the metasurface is fabricated, the topological charge number of the vortex beam cannot be changed.

[0003] To overcome this problem, the industry generally adopts the method of introducing active materials into metasurface structures, utilizing the different properties of active materials under different external conditions to modulate the topological charge number of vortex beams. However, current reports require the incident terahertz electromagnetic wave to be converted from a circularly polarized wave (CP wave) to a transverse electric wave or a transverse magnetic wave (TE wave or TM wave) to adjust the topological charge number. Furthermore, the range of adjustment for the topological charge number is very small; for example, it can only switch the topological charge number from 1 to 2. Finally, it can only switch in one direction; for example, when the active material changes from a high conductivity state to a low conductivity state, the topological charge number changes from 1 to 2, and it cannot switch in the opposite direction. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides an artificial metasurface and its design method. This metasurface eliminates the need to convert the incident terahertz wave from a CP wave to a TM or TE wave; it allows the reflected vortex beam to switch between different topological charge numbers simply by varying the temperature. Furthermore, it eliminates the need to modify the basic structure; bidirectional switching of the topological charge number can be achieved simply by pre-setting two angle values ​​within the basic structure. This significantly improves the flexibility of terahertz wave manipulation.

[0005] This invention can be achieved by providing an artificial metasurface:

[0006] An artificial metasurface for manipulating reflected vortex beams in the terahertz band, comprising n monomeric structures as per the instruction manual. Figure 3The structures shown are arranged in a counter-clockwise matrix, where n≥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 consists of inner and outer rings: the inner ring is made of an electrically tunable active material with two symmetrical notches about its center, each notch corresponding to a central angle α; the outer ring is made of metal, also with two symmetrical notches about its center, filled with the same active material as the inner ring, each notch corresponding to a central angle β. The values ​​of α and β are limited to ensure that the structure has a reflection coefficient greater than 60% for terahertz light waves in the 0.85±0.03THz frequency band, thus guaranteeing reflection intensity. In the metasurface, the inner ring of adjacent monomer structures has a center rotation angle difference Δa, and the outer ring has a center rotation angle difference Δb, where Δa = 180l1 / n and Δb = 180l2 / n. The active material must have two states: high conductivity and low conductivity. In the high conductivity state, the conductivity must be at least on the same order of magnitude as the conductivity of the outer ring metal. In the low conductivity state, the conductivity must be so low that the phase modulation capability of the terahertz wave by the geometric rotation of the active material is negligible compared with that of the outer ring metal material. l1 and l2 are the topological charge numbers of the reflected beams that are desired 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 inner ring of the patterned top layer is made of 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] To obtain the aforementioned artificial metasurface, the present invention also provides a design method for the aforementioned metasurface, characterized by comprising the following steps in sequence:

[0014] Provide n monomer structures as described in claim 1, where n ≥ 2 l ; where l is the maximum value among l1, l2, and l3, and the n single-unit structures differ only in the rotation angles a and b of the inner and outer rings.

[0015] In these n single-unit structures, the inner and outer ring rotation angles of the m-th single-unit structure are set as: am = (m-1)Δa, bm = b0 + (m-1)Δb, where Δa = 180l1 / n and Δb = 180l2 / n; m ≤ n and is a natural number, as specified in the instruction manual. Figure 3 In the metasurface shown, the inner rings of adjacent monolithic structures have a center rotation angle difference Δa, and the outer rings have a center rotation angle difference Δb.

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

[0017] The n monomer structures are as shown in the appendix of the instruction manual. Figure 3 The items are arranged in a counter-clockwise matrix as shown.

[0018] This invention provides an artificial metasurface structure and its corresponding design method. By pre-calculating the magnitudes of the inner and outer ring stepping rotation angles Δa and Δb of each individual structure, the artificial metasurface can conveniently and bidirectionally adjust the topological charge number of the reflected vortex beam by simply adjusting the conductivity of the active material without changing the incident circularly polarized terahertz wave. Attached Figure Description

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

[0020] Figure 3 A top view showing the individual structures in the artificial metasurface provided by this invention arranged in a counterclockwise order;

[0021] Figure 4 Schematic diagrams of eight different monolithic structures with varying inner and outer ring rotation angles.

[0022] Figure 5 As per the instruction manual Figure 4 A schematic diagram of a metasurface composed of eight different monomer structures;

[0023] Figure 6-7 Each is attached to the instruction manual Figure 4 Simulation results of reflectivity of the eight different monomer structures at room temperature and low temperature;

[0024] Figure 8-9 Each is attached to the instruction manual Figure 4 Simulation results of phase differences of the eight different monomer structures at room temperature and low temperature;

[0025] Figure 10 and Figure 12 Each comes with an instruction manual Figure 5A schematic diagram showing the phase difference of different monomer structures of the metasurface at room temperature and under superconducting low temperature.

[0026] Figure 11 and Figure 13 Included with instruction manual Figure 5 The reflection vortex phase distribution diagram of the metasurface at room temperature and superconducting low temperature.

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

[0037] at room temperature and superconducting low temperature.

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

[0039] .

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

[0041] at room temperature and superconducting low temperature. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings. The description of techniques known to those skilled in the art is omitted in the description.

[0031] The artificial metasurface described in this invention is composed of n monomeric structures. (See attached specification.) Figure 1-2 As shown, the monolithic structure comprises three layers, from bottom to top: an aluminum reflective substrate layer, a polyimide dielectric layer with a dielectric constant ε = 2.93 + 0.13i, and a patterned top layer. The aluminum reflective substrate layer and the polyimide dielectric layer are square in top view, with a side length p = 164 μm. The outer ring of the patterned top layer is made of aluminum, with a gap filled with niobium nitride superconducting material. The inner ring is also made of niobium nitride superconducting material. Specific geometric parameters are shown in the figure. The thicknesses of the reflective substrate layer, the polyimide dielectric layer, and the patterned top layer are 0.2 μm, 70 μm, and 0.2 μm, respectively. The n monolithic structures differ only in the rotation angles a and b of the inner and outer rings in the patterned top layer. The above n monolithic structures are arranged in a counter-clockwise order according to the instructions attached. Figure 3 Arranging the elements in the form of a matrix yields the hypersurface. Here, n can be any natural number greater than 7. (See the attached instruction manual.) Figure 3 The eight monomer structures shown are merely one example, and the instruction manual includes... Figure 3 Only the first, second, third, and n-2, n-1, and n-th monomer structures are shown, and not all monomer structures are fully labeled. However, this does not affect the understanding of the arrangement.

[0032] Example 1: We will take the design of an artificial metasurface with a topological charge number of 1 at room temperature and a topological charge number of 2 at near-superconducting low temperature as an example to introduce the artificial metasurface and its design method of the present invention.

[0033] First, design as per the instruction manual. Figure 4 The eight individual structures shown are denoted by 1-8. The only difference between the different numbered individual structures is the rotation angle of the patterned top layer. The rotation angles (a, b) of their 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°), and (135° 168.75°), respectively. That is, the step rotation angles Δa and Δb between adjacent structures are 45° and 22.5°, respectively. Arrange the eight structures according to the instructions attached... Figure 5 The beams are arranged in a 24×24 matrix structure to reflect the incident terahertz beam into a vortex beam. The simulation results of the reflectivity and phase difference of the above eight single-unit structures at room temperature and superconducting cryogenic temperatures are shown in the appendix to the instruction manual. Figure 6-9 As shown.

[0034] The theoretical principle is as follows: At room temperature, the conductivity of niobium nitride is very small compared to that of aluminum. At this temperature, the outer metal rings in each individual unit structure exhibit significant phase modulation of the terahertz wave reflection, while the phase modulation of the inner rings is negligible. Therefore, based on the Pancharatnam-Berry phase theory: for reflective metasurfaces: when the incident wave is a circularly polarized wave, i.e., a CP wave, the polarization of the reflected wave is the same as that of the incident wave, and the metasurface unit patterning rotation angle is Δa, with a corresponding phase shift of ±Δ2a, where + represents the LCP wave and – represents the RCP wave. At this point, referring to the appendix of the instruction manual... Figure 8 It can be seen that the instruction manual includes... Figure 5 The phase differences of each individual structure are shown in the attached instruction manual. Figure 10 As shown, a full rotation of the spatial plane corresponds to 360°, thus forming a reflected vortex beam with a topological charge number of 1. The corresponding simulation results are attached to the instruction manual. Figure 11 As shown.

[0035] As the temperature drops to the superconducting critical point, niobium nitride approaches the superconducting state, and its conductivity increases rapidly. It begins to exhibit phase modulation of terahertz wave reflection, similar to that of metallic materials. Therefore, the outer ring can be considered a single metallic ring, and at this point, the 22.5° step rotation angle of the outer ring is meaningless; the inner ring plays the primary role in phase modulation of the terahertz beam. Therefore, referring to the appendix to the instruction manual... Figure 9 It can be seen that the instruction manual is attached at this time. Figure 5 The phase differences of each individual structure are shown in the attached instruction manual. Figure 12As shown, a full rotation of the spatial plane corresponds to 720°, thus forming a reflected vortex beam with a topological charge number of 2. Simulation results are shown in the attached manual. Figure 13 As shown.

[0036] Example 2: Design a metasurface that reflects vortex beams at room temperature with a topological charge number of 2, and whose topological charge number becomes 1 at near-superconducting low temperatures.

[0037] Similar to the scheme described in Example 1: First, eight single-unit structures are designed, denoted by 1-8. The only difference between the different numbered single-unit structures is the rotation angle of the patterned top layer. However, in this example, the step rotation angles of the inner and outer rings between each pair of adjacent single-unit structures are 22.5° and 45°, respectively, which is equivalent to swapping the values ​​of the step angles Δa and Δb in Example 1. Therefore, the rotation angles (a, b) of the inner and outer rings of the eight single-unit structures 1-8 are (0° 11.25°), (22.5° 56.25°), (45° 101.25°), (67.5° 146.25°), (90° 11.25°), (112.5° 56.25°), (135° 101.25°), and (157.5° 145.25°). The eight single-unit structures are then arranged according to the instructions attached... Figure 5 The metasurface can be obtained by arranging the elements in a 24×24 matrix structure.

[0038] At room temperature, the outer metal rings in each individual unit structure exhibit significant phase modulation of terahertz wave reflection, while the phase modulation of the inner rings is negligible. Compared to Example 1, Δb becomes 45° based on the Pancharatnam-Berry phase theory, as indicated in the accompanying manual. Figure 5 The phase difference between adjacent monomer structures is 90°, as shown in the instruction manual. Figure 12 As shown, a full rotation of the spatial plane corresponds to 720°, thus forming a reflecting vortex beam with a topological charge number of 2. When the temperature drops to the superconducting critical point, Δa becomes 22.5° compared to Example 1. At this point, the instruction manual (attached) Figure 5 The phase difference between adjacent monomer structures is 45°, as shown in the instruction manual. Figure 10 As shown, a full rotation of the spatial plane corresponds to 360°, thus forming a reflected vortex beam with a topological charge number of 1. Simulation results at room temperature and superconducting cryogenic temperatures are shown in the appendix to the instruction manual. Figure 14-15 As shown.

[0039] As shown in Examples 1 and 2, eight monomer structures were constructed according to the instructions. Figure 5 For metasurfaces composed of various forms, the topological charge numbers l1 and l2 of vortex beams at low and room temperatures depend on the values ​​of Δa and Δb. The calculation formulas are Δa = 180l1 / 8; Δb = 180l2 / 8; that is, the rotation angle (a) between the inner and outer rings of the eight monomer structures. 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 superconducting cryogenic temperatures, then it is only necessary to rotate the inner and outer rings by steps Δa=67.5° and Δb=45° to set the corresponding 8 single-unit structures; the corresponding simulation results at room temperature and superconducting cryogenic temperatures are provided in the accompanying manual. Figure 16-17 Conversely, to switch the topological charge number from l=3 at room temperature to l=2 near the superconducting low temperature, it is only necessary to interchange Δa and Δb, i.e., Δa=45° and Δb=67.5°. Therefore, unlike traditional single-direction adjustment, this invention can conveniently adjust the topological charge number bidirectionally.

[0040] Example 3: Design a metasurface that reflects vortex beams at room temperature with a topological charge number of 1, and whose topological charge number becomes 4 at near-superconducting low temperatures.

[0041] Similar to the scheme described in Example 1, the individual unit structures are designed first. If 8 individual unit structures are still designed, and the topological charge number is 4 at low temperature, then Δa = 90° to make the phase difference between two adjacent individual unit structures 180°. However, at this time, the phase difference between adjacent individual unit structures is too large, causing the reflected beams to fail to form vortex light, instead forming 8 discrete beams, with a phase difference of 180° between each pair of adjacent vertical beams. Therefore, to reduce the phase difference between adjacent individual unit structures while satisfying the topological charge number of 4, it is only necessary to increase the number of individual unit structures. For example, 16 individual unit structures are designed, where the first individual unit structure (ab) is (0° 5.25°), and the remaining 15 individual units are rotated sequentially according to step angles Δa = 45° and Δb = 11.25°. The 16 individual unit structures are then arranged according to the instructions attached. Figure 3 Arranged in the following order, at this time, the instruction manual is attached. Figure 3 The value of n is 16. Based on the Pancharatnam-Berry phase theory, a complete rotation of the space plane at room temperature and at superconducting cryogenic temperatures corresponds to 360° and 1440°, respectively. Therefore, the topological charge number of the reflected vortex beam is 1 at room temperature and becomes 4 at superconducting cryogenic temperatures. Meanwhile, at room temperature, due to the 25° phase difference between adjacent monolithic structures, the phase continuity of the vortex beam is better. The simulation results at room temperature and superconducting cryogenic temperatures are shown in the appendix to the instruction manual. Figure 18-19 As shown.

[0042] As shown in Example 3, when a larger number of topological charges is required, it 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 these 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 are natural numbers.

[0043] In this embodiment of the invention, the incident waves are all perpendicularly incident. Therefore, the values ​​of a and b in the patterned top layer are the rotation angles of the incident waves in a plane perpendicular to the propagation direction. Since the incident waves used in this invention are circularly polarized waves, i.e., CP waves, rather than TE or TM waves, they have no directionality in a plane perpendicular to the propagation direction. Therefore, the values ​​of a and b have no effect on the reflectivity of the incident waves. Furthermore, since the reflection phase difference between adjacent single-unit structures depends only on the value of Δa or Δb, and embodiments 1-3 all show the phase distribution of the final vortex beam, which conforms to the theoretical design, this application only shows the reflectivity and phase diagrams of the eight single-unit structures in embodiment 1. Also, in the above embodiments, the incident waves are LCP waves; when RCP is used, the attached specification will be used. Figure 3 The counter-clockwise arrangement of the individual structures shown can be changed to a clockwise arrangement.

[0044] The side length of the single-unit structure, the inner and outer radii of the outer ring, and the inner and outer radii of the inner ring in the above embodiments are only the values ​​used in the embodiments. Modifications to the above geometric parameters based on the principles of this document, while ensuring effectiveness, are not intended to limit the invention. Furthermore, the above design principles, by modifying geometric parameters to ensure reflectivity, can also be applied to other electromagnetic bands, such as the radar band. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. An artificial metasurface for manipulating reflected vortex beams in the terahertz band, comprising n monomer structures arranged in a counterclockwise matrix, wherein n≥8, each monomer structure comprising 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 inner and outer rings: the inner ring is made of an electrically tunable active material with two symmetrical notches about the center, each notch corresponding to a central angle α; the outer ring is made of metal and also has two symmetrical notches about the center, filled with the same active material as the inner ring, each notch corresponding to a central angle β. The values ​​of α and β are limited to ensure that the reflection coefficient of the structure for terahertz light waves in the 0.85±0.03THz frequency band is greater than 60%, thus ensuring the reflection intensity. In the metasurface, the inner ring of adjacent monomer structures has a center rotation angle difference Δa, and the outer ring has a center rotation angle difference Δb, where Δa = 180l1 / n and Δb = 180l2 / n, where l1 and l2 are the desired topological charge numbers of the reflected beam when the active material is in a high-conductivity and low-conductivity state, respectively. The active material must have both high-conductivity and low-conductivity states, and in the high-conductivity state, the conductivity must be at least on the same order of magnitude as the conductivity of the outer ring metal.

2. The artificial metasurface according to claim 1, characterized in that: The thickness of the reflective substrate layer is not less than 0.2µm.

3. The artificial metasurface according to claim 1, characterized in that: The reflective substrate is made of aluminum.

4. The artificial metasurface according to claim 1, characterized in that: The metal of the outer ring of the patterned top layer is aluminum.

5. The artificial metasurface according to claim 1, characterized in that: The inner ring of the patterned top layer is made of a superconducting material.

6. The artificial metasurface according to claim 5, characterized in that: The superconducting material is niobium nitride.

7. The artificial metasurface according to claim 1, characterized in that: The central angle α is 1.5° and β is 44–46°.

8. A method for designing an artificial metasurface as described in claim 1, characterized in that, Specifically, the following steps are included in sequence: First, n kinds of monomer structures as described in claim 1 are provided, where n ≥ 2. l Where l is the maximum value among l1, l2, and l3, and the n monomer structures differ only in the rotation angles of the inner and outer rings; Second, among the n types of single-unit structures, the inner and outer ring rotation angles of the m-th single-unit structure are set as follows: am = (m-1) Δa, bm = b0 + (m-1)Δb, where Δa = 180l1 / n and Δb = 180l2 / n; m ≤ n and is a natural number; Third, the value of b0 can be set to any value within the range of (-π π], that is, in the n single-unit structures, the inner and outer ring rotation angles a1 and b1 of the first single-unit structure do not need to be equal; Fourth, arrange the n single-unit structures in a counterclockwise direction in the order from the first to the nth to form a matrix.

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