Bifunctional Metasurface with Tunable Linear Circular Dichroism Based on Graphene

Abstract

The present invention proposes a dual-functional metasurface with tunable linear circular dichroism based on graphene. Its characteristics are as follows: It is composed of a bottom metal plate (1), a polyimide layer (2), and a top patterned graphene layer (3), and the layers are closely attached to each other. The present invention takes into account the oblique incidence that inevitably exists in practical applications into the design of the device. As a new degree of freedom, it realizes 90% linear dichroism and 80% circular dichroism respectively, increases the functional integration degree, and reduces the requirements for electromagnetic wave incidence. This metasurface has the advantages of simple structure, wide application range, and easy integration. It belongs to the field of terahertz metamaterials and can be widely applied to various related directions such as signal processing, optical computing, and terahertz switches.

Description

(1) Technical Field

[0001] The present invention relates to a dual-functional metasurface with tunable linear circular dichroism based on graphene, which can simultaneously achieve linear dichroism and circular dichroism in different directions. Among them, the linear dichroism is tunable in the range of 0-90%, and the circular dichroism is tunable in the range of 0-80%. It has the characteristics of a wide tunable range and the simultaneous realization of dual functions, and can be widely used in the design of various optical computations, signal processing, terahertz switches, and other aspects. It belongs to the field of terahertz metamaterials. (2) Background Art

[0002] In recent years, due to its multifunctionality, ultrathinness, and easy integration, metasurfaces have been increasingly widely used in various fields. With the development of two-dimensional materials, such as graphene and black phosphorus, being extensively integrated onto metasurfaces to achieve the tunability of various functions, these tunable metasurfaces can achieve efficient electromagnetic wave manipulation in terms of amplitude, phase, and polarization. Tunable metasurfaces are very meaningful for the multifunctional applications of future engineering optics.

[0003] Dichroism refers to the polarization-dependent absorption characteristics of materials under the irradiation of orthogonally polarized electromagnetic waves. This phenomenon widely exists in nature, such as DNA, proteins, and even viruses, etc., all having this phenomenon. However, the dichroism in nature is weak and difficult to detect. Therefore, we need to design devices with strong dichroism to achieve functional applications such as substance detection.

[0004] In 2014, Li et al. reported an example in the article "An ultra-thin chiral metamaterial absorber with high selectivity for LCP and RCP waves" that provides absorption only for one circularly polarized wave and obtains a large circular dichroism through extrinsic chirality. In 2021, Zhang et al. proposed an actively tunable dual-functional metareflector in the article "Actively tunable bi-functional metamirror in a terahertz band", realizing a better active switching between CD and cross-polarization conversion.

[0005] The present invention discloses a dual-functional metasurface with tunable linear and circular dichroism based on graphene, which can be used in various fields such as imaging display, terahertz switches, optical computing, biological detection, and sensing. It adopts a simple three-layer structure, with a patterned graphene layer on the top, polyimide in the middle, and a reflective metal layer at the bottom, having the advantages of simple structure and easy integration. By designing the pattern of the top graphene layer to be anisotropic and taking the incident angle of electromagnetic waves as a new degree of freedom, linear dichroism and circular dichroism can be achieved simultaneously in two directions, and at the same time, an adjustable effect is obtained. Compared with the prior art, the present invention achieves the purpose of simultaneously tunable dual functions under a relatively simple structural design. In addition, in practical applications, the incident wave cannot always maintain normal incidence. Therefore, it is very meaningful for us to take this inevitable oblique incidence in practical applications as a new degree of freedom to increase the functions of the device. (III) Summary of the Invention

[0006] Aiming at the disadvantages of the current metasurface with single function and complex structure, the purpose of the present invention is to provide a dual-functional metasurface with tunable linear and circular dichroism based on graphene.

[0007] The purpose of the present invention is achieved as follows:

[0008] What is shown is a dual-functional metasurface with tunable linear and circular dichroism based on graphene, characterized in that: this is a metasurface that can simultaneously achieve tunable linear dichroism and circular dichroism. From bottom to top, there are a bottom metal plate (1), a polyimide dielectric layer (2), and a patterned graphene layer (3), and each layer is closely attached. The designed periodic lateral period and longitudinal period P of the dual-functional metasurface with tunable linear and circular dichroism based on graphene are both 178 μm. The bottom layer selects gold with a thickness of 0.2 μm as the reflective layer, and its function is to prevent the incident electromagnetic wave from transmitting out. The dielectric constant of polyimide is 3.5, and the thickness h 2 = 40 μm. The Fermi level E F of the top patterned graphene layer is set to 1 eV, the thickness of graphene is 0.34 nm, the relaxation time is 1 ps, the top graphene is formed by cutting off a semicircle from half of a regular hexagon, the side length of the regular hexagon is w = 70 μm, and the radius of the cut-off semicircle is r = 50 μm.

[0009] The technical solution adopted by the present invention is:

[0010] Step (1): First, study the characteristics of the dielectric constant, conductivity, etc. of single-layer graphene, select the target working frequency to be 0.95 - 1.15 THz, and within the selected working range, study different anisotropic structures to match the two functions of linear dichroism and circular dichroism, and record the data.

[0011] Step (2): Use software such as Matlab to perform numerical calculations on the properties of graphene, obtain the relationship between dichroism, graphene, and the corresponding operating frequency, and establish a graphene material model.

[0012] Step (3): Use the simulation software CST for modeling and simulation. Through numerical calculations, obtain the relationship between the incident electromagnetic wave frequency, incident angle, linear dichroism, and circular dichroism. Optimize the structural parameters by scanning and matching to obtain the optimal solution.

[0013] Step (4): Compare and record the data, and verify and summarize the final result scheme.

[0014] Beneficial effects of the present invention:

[0015] 1. The dual-functional metasurface can simultaneously achieve two functions of linear dichroism and circular dichroism without the need to switch between functions, avoiding crosstalk between functions.

[0016] 2. The linear dichroism of the dual-functional metasurface is adjustable in the range of 0-90%, and the circular dichroism is adjustable in the range of 0-80%.

[0017] 3. The dual-functional metasurface only adopts a three-layer structure, with the characteristic of simple structure. One layer of graphene can achieve two functions.

[0018] 4. The absorber adopts a two-dimensional periodic structure, which is convenient for large-scale integration. (IV) BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a schematic diagram of the unit structure of the present invention. 1: Metal layer; 2: Polyimide dielectric material; 3: Patterned graphene layer. The period of the structural unit is P = 178 μm, and the thickness h of the metal layer 1 = 0.2 μm, the thickness h of the polyimide dielectric 2 = 40 μm, the Fermi level E of the top graphene layer F is 1 eV, the side length of the top half regular hexagon is w = 70 μm, and the radius of the cut semi-circle is r = 50 μm.

[0020] Figure 2 is a top view of the unit of the structure of the present invention. The Fermi level E of the top graphene layer F is 1 eV, the side length of the top half regular hexagon is w = 70 μm, and the radius of the cut semi-circle is r = 50 μm.

[0021] Figure 3 is the absorption and CD spectrum diagram when the circularly polarized wave is incident on the metasurface at an incident angle of 20° along the x-z plane, and the Fermi level of the top graphene is 1 eV.

[0022] Figure 4Absorption and LD spectrum when a linearly polarized wave is incident on the metasurface at an incident angle of 45° along the y-z plane. The Fermi level of the top graphene is 1 eV.

[0023] Figure 5 It is the spectrum of the change of CD with the Fermi level of graphene when a circularly polarized wave is incident on the metasurface at an incident angle of 20° along the x-z plane.

[0024] Figure 6 It is the spectrum of the change of LD with the Fermi level of graphene when a linearly polarized wave is incident on the metasurface at an incident angle of 45° along the y-z plane. (V) Specific implementation manners

[0025] The following further clarifies the specific implementation manners and steps of the present invention with reference to the accompanying drawings.

[0026] The present invention designs a dual-functional metasurface with adjustable linear and circular dichroism based on graphene. The specific implementation steps include:

[0027] Figure 1 It is a schematic diagram of the unit structure of a dual-functional metasurface with adjustable linear and circular dichroism based on graphene. The unit structure period of the dual-functional metasurface is P, and the bottom gold layer (1) is used as the substrate with a thickness set to h 1 = 0.2 μm. The thickness of the metal plate should be greater than the skin depth of the incident wave to block the transmitted electromagnetic wave. The dielectric constant of the polyimide (2) is 3.5, and the thickness is set to h 2 = 40 μm. The top patterned graphene (3) is formed by cutting off a semi-circle from half of a regular hexagon. The side length of the hexagon is w = 70 μm, the radius of the semi-circle is r = 50 μm, the Fermi level of graphene is 1 eV, and the relaxation time is 1 ps.

[0028] The dual-functional metasurface with adjustable linear and circular dichroism based on graphene proposed by the present invention operates in the 0.95 - 1.15 THz band, and the selected period P = 175 μm.

[0029] Figure 2 It is a top view of the unit structure of the designed metasurface. The patterned graphene is formed by cutting off a semi-circle from half of a regular hexagon. The side length of the hexagon is w = 70 μm, the radius of the semi-circle is r = 50 μm, the Fermi level of graphene is 1 eV, the relaxation time is 1 ps, and the thickness of the single-layer graphene is 0.34 nm.

[0030] The working principle of the present invention is as follows:

[0031] The bottom layer uses gold with a certain thickness as the substrate. The thickness of the gold is greater than the skin depth of the incident electromagnetic wave. Therefore, the electromagnetic wave cannot pass through the designed dual-functional metasurface, that is, the transmittance of the electromagnetic wave is zero. Therefore, the circular dichroism calculation formula under the incidence of a circularly polarized wave can be expressed as:

[0032] A RCP = 1 - r LR -r RR , (1)

[0033] A LCP = 1 - r LL -r RL , (2)

[0034] CD = A RCP -A LCP , (3)

[0035] Similarly, the linear dichroism under the incidence of linearly polarized waves can be expressed as

[0036] A y = 1 - r xy -r yy , (4)

[0037] A x = 1 - r xx -r yx , (5)

[0038] LD = A y -A x , (6)

[0039] In Figure 3 , for the circular dichroism of the dual-functional metasurface, it is mainly caused by the interaction between the graphene structures on both sides and the incident circularly polarized waves, generating electrical resonance. The graphene on both sides has a nearly 90% absorption effect on the right-handed circularly polarized wave at 1.05 THz, while a large amount of reflection occurs for the left-handed circularly polarized wave, with only about 10% absorption rate. Therefore, a circular dichroism of up to about 80% is finally achieved.

[0040] In Figure 4 , for the linear dichroism of the dual-functional metasurface, it is mainly caused by the interaction between the horizontally placed graphene structure in the middle and the incident linearly polarized waves, generating electrical resonance. The horizontally placed graphene structure in the middle has a nearly 100% absorption of the y-polarized wave at 1.047 THz, while a large amount of reflection occurs for the incident x-polarized wave, with only about 10% absorption. Therefore, the final linear dichroism reaches about 90%.

[0041] In Figure 5 , when the circularly polarized wave is incident on the metasurface at an incident angle of 20° along the x-z plane, we scanned the circular dichroism at different Fermi levels and found that the circular dichroism of the designed dual-functional metasurface can be tuned in the range of 0 - 80%. When the Fermi level reaches 1 eV, the circular dichroism reaches a maximum of 0.8, achieving the best circular dichroism function.

[0042] Such asFigure 6 As shown, when a linearly polarized wave is incident on the metasurface at an incident angle of 45° along the y-z plane, we scanned the linear dichroism at different Fermi levels. The linear dichroism of the designed dual-functional metasurface can be adjusted in the range of 0 - 90%. When the Fermi level is 1 eV, the linear dichroism reaches a maximum of 0.9, achieving the best linear dichroism function.

Claims

1. A dual-functional metasurface based on graphene with tunable linear circular dichroism, characterized in that: The metasurface sequentially includes a bottom reflective metal layer (1), a middle polyimide dielectric layer (2), and a top patterned graphene layer (3) from bottom to top, and each layer is closely attached; the lateral period and longitudinal period of the metasurface are both P, the bottom reflective metal layer is a gold substrate with a thickness of h1; the dielectric constant of the middle polyimide dielectric layer is 3.5 and the thickness is h2; the top patterned graphene layer is formed by cutting off half a circle from half a hexagon, the side length of the hexagon is w, the radius of the semi-circle is r, the Fermi level of graphene is 1 eV, the thickness is 0.34 nm, and the relaxation time is 1 ps.

2. A dual-functional metasurface based on graphene with tunable linear circular dichroism according to claim 1, characterized in that The lateral period and longitudinal period of its single structure are both P = 178 μm.

3. A dual-functional metasurface based on graphene with tunable linear circular dichroism according to claim 1, characterized in that The thickness h1 of the bottom reflective metal layer is 0.2 μm, which is greater than the skin depth of the incident wave, and the thickness h2 of the middle polyimide dielectric layer is 40 μm.

4. A dual-functional metasurface based on graphene with tunable linear circular dichroism according to claim 1, characterized in that The side length w of the hexagon is 70 μm, and the radius r of the cut-off semi-circle is 50 μm.

5. A dual-functional metasurface based on graphene with tunable linear circular dichroism according to claim 1, characterized in that The dual-functional metasurface operates in the frequency band of 0.95 - 1.15 THz.

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