A dynamically tunable broadband strong circular dichroism chiral metasurface

Abstract

The application provides a dynamically tunable broadband strong circular dichroism chiral metasurface. A single period structure of the metasurface is sequentially composed of a gold substrate 1, a TOPAS dielectric layer 2, a single-layer graphene 3, a PMI dielectric layer 4, a TOPAS dielectric layer 5, an open circular arc gold split ring 6, an open U-shaped vanadium dioxide ring 7 and a single-layer graphene disc 8 from bottom to top. The open circular arc gold split ring and the open U-shaped vanadium dioxide ring in the structure strongly react to incident circularly polarized light, form a broadband and high numerical value circular dichroism, and the circular dichroism can be changed by changing the period of the unit structure, the Fermi energy level of the single-layer graphene and the conductivity of the open U-shaped vanadium dioxide ring, so that the height of the circular dichroism wave peak is tunable. The structure has potential application prospects in realizing tunable dynamic imaging, constructing a chiral optical detector and other communication fields.

Description

Technical Field

[0001] This invention relates to a dynamically tunable broadband strong circular dichroic chiral metasurface. With its precise control over circularly polarized light, the chiral metasurface is widely used in fields such as chiral molecule detection and recognition, circular polarization imaging, spin photonic devices, optical encryption and anti-counterfeiting, terahertz communication, and chiral catalysis. It also shows important application value in biosensing, quantum optics, and information security. Background Technology

[0002] Chirality describes the geometric characteristic that an object and its mirror image cannot be perfectly superimposed through rotation, translation, or other means. In the electromagnetic field, circular dichroism (CD) is generally used to measure the strength of chirality, which is the difference in absorption characteristics of a material for left-handed circularly polarized light (LCP) and right-handed circularly polarized light (RCP). Compared to naturally occurring chiral molecules such as proteins, amino acids, and viruses, artificially constructed chiral metamaterials can elicit stronger circular dichroism signals, showing significant application potential in areas such as biomolecular detection, polarization imaging, and super-resolution spectroscopy. Metasurfaces are two-dimensional artificial electromagnetic structures that, through the precise design of subwavelength structural units, can flexibly control the amplitude, phase, and polarization state of electromagnetic waves. Compared to traditional three-dimensional metamaterials, metasurfaces have advantages such as low loss, simple fabrication processes, and significant electromagnetic field localization effects, with particularly outstanding advantages in achieving chiral responses in the 0.1-10 THz terahertz band. Terahertz photons, with their low energy and significant non-ionizing properties, hold great promise for applications in biomedical detection. However, their interaction with natural chiral molecules is weak, necessitating the use of artificial metasurfaces to effectively enhance chiral signals. Early chiral metasurfaces often employed three-dimensional helical structures or multilayer metal resonant units to achieve significant circular dichroism, but these structures have significant limitations: firstly, three-dimensional nanofabrication processes are complex and costly, making large-scale fabrication difficult; secondly, most devices have fixed functions, and the chiral response cannot be dynamically controlled after fabrication; and thirdly, some designs, while pursuing high CD values, often suffer from narrowing operating bandwidth or decreased energy efficiency. Therefore, the design of novel chiral metasurface devices is developing in a diversified direction, exhibiting a trend from complex three-dimensional structures to simple two-dimensional structures, from fixed functions to dynamically tunable ones, and from single metal structures to composites of dielectric and functional materials. This aims to improve device performance while overcoming key challenges such as high fabrication difficulty, limited functionality, and insufficient control flexibility.

[0003] This invention designs a broadband, strong circular dichroism chiral metasurface. In its structure, the open arc-shaped gold split ring and the open U-shaped vanadium dioxide ring strongly respond to incident circularly polarized light, forming broadband, high-value circular dichroism. Furthermore, the circular dichroism of the unit structure can be adjusted by tuning the Fermi level of the monolayer graphene and the conductivity of vanadium dioxide. At the same time, the circular dichroism can also be changed by altering the period of the unit structure, the thickness of the dielectric layer, and the size of the open arc-shaped gold split ring, thus achieving adjustable circular dichroism peak height. Summary of the Invention

[0004] This invention designs a broadband strong circular dichroism chiral metasurface and provides a broadband, strong circular dichroism tunable structure. The circular dichroism of this structure can achieve a CD greater than 0.8 in the range of 3.98 THz to 9.14 THz, and a CD greater than 0.9 in the range of 4.54 THz to 8.12 THz. The maximum CD value is achieved at 7.24 THz, which is 0.924.

[0005] To solve the above problems, the present invention is achieved through the following technical solution:

[0006] This broadband, strongly circularly dichroic chiral metasurface is composed of a multilayer structure. Its single periodic structure, from bottom to top, consists of a gold substrate 1, a TOPAS dielectric layer 2, a monolayer graphene 3, a PMI dielectric layer 4, a TOPAS dielectric layer 5, an open arc-shaped gold cleaved ring 6, an open U-shaped vanadium dioxide ring 7, and a monolayer graphene disk 8. The open arc-shaped gold cleaved ring and the open U-shaped vanadium dioxide ring in the structure exhibit a strong response to incident circularly polarized light, resulting in broadband, high-value circular dichroism.

[0007] As a preferred option, the broadband strongly circular dichroic chiral metasurface has a period P = 23 μm.

[0008] Preferably, the thickness of the unit structure gold substrate 1 is h1=0.2μm, the thickness of the TOPAS dielectric layer 2 is h2=0.5μm, the thickness of the monolayer graphene 3 and the monolayer graphene disk 8 is h3=0.34nm, the thickness of the PMI dielectric layer 4 is h4=8μm, the thickness of the TOPAS dielectric layer 5 is h5=0.2μm, the thickness of the open arc-shaped gold split ring 6 is h6=1μm, and the thickness of the open U-shaped vanadium dioxide ring 7 is h7=0.1μm.

[0009] Preferably, the length and width of the monolayer graphene layer 3 are both 23 μm, the radius R3 of the monolayer graphene disk 8 is 4 μm, and the Fermi level E of the monolayer graphene layer 3 and the monolayer graphene disk 8 is... F =0.1ev, carrier relaxation time τ=0.1ps.

[0010] Preferably, a coordinate axis is established with the exact center of the unit structure as the origin. The opening angle of the open arc-shaped gold split ring 6 is θ1=212°, the deflection angle relative to the positive z-axis is θ2=-40°, the outer radius is R1=9μm, the inner radius is R2=6μm, and the width is W=3μm; the conductivity of the open U-shaped vanadium dioxide ring 7 is σ=2×10 5 S / m, side length L1=7μm, L2=1.4μm, opening width W1=0.8μm, W2=0.4μm, W3=1μm.

[0011] Preferably, the material of the gold substrate 1 is gold, with an electrical conductivity of 4.561 × 10⁻⁶. 7 S / m; The dielectric constants of TOPAS dielectric layer 2 and TOPAS dielectric layer 5 are both ε = 2.35. The material of the dielectric layer is polyethylene cyclic olefin copolymer (TOPAS). TOPAS is a transparent, rigid amorphous thermoplastic copolymer with excellent optical properties and is suitable for advanced terahertz applications. The dielectric constant of PMI dielectric layer 4 is ε = 1.1. The material of the dielectric layer is polymethacrylamide (PMI). PMI is a lightweight, closed-cell rigid foam plastic with excellent mechanical properties, heat distortion temperature and chemical stability. It is widely used as an ideal core material for high-performance sandwich structure composite materials. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the three-dimensional structure of a dynamically tunable broadband strongly circular dichroic chiral metasurface.

[0013] Figure 2 It is a top view of a dynamically tunable broadband strongly circular dichromatic chiral metasurface.

[0014] Figure 3 The open arc-shaped gold split ring 6 has an opening angle θ1 = 212°, a deflection angle θ2 = -40° relative to the positive z-axis, an outer radius R1 = 9 μm, an inner radius R2 = 6 μm, and a width W = 3 μm; the open U-shaped vanadium dioxide ring 7 has a conductivity σ = 2 × 10⁻⁶. 5 S / m, side length L1=7μm, L2=1.4μm, opening width W1=0.8μm, W2=0.4μm, W3=1μm; Fermi level E of monolayer graphene F =0.1ev, carrier relaxation time τ=0.1ps, left-hand and right-hand circularly polarized light absorptivity of the device and circular dichroism diagram of the device.

[0015] Figure 4 The graph shows the change in the peak height of circular dichroism when the dielectric layer thickness, the size of the open arc-shaped gold split ring, and the size of the open U-shaped vanadium dioxide ring remain constant, and the period P of the device is changed.

[0016] Figure 5 The graph shows the change in the peak height of circular dichroism when the thickness of the dielectric layer, the size of the open arc-shaped gold split ring, and the size of the open U-shaped vanadium dioxide ring remain constant, and the Fermi level of the tuned monolayer graphene is changed.

[0017] Figure 6 The graph shows the change in the peak height of circular dichroism when the dielectric layer thickness, the size of the open arc-shaped gold split ring, and the size of the open U-shaped vanadium dioxide ring remain constant, and the conductivity of the tuned vanadium dioxide is changed. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific examples and the accompanying drawings.

[0019] This invention provides a dynamically tunable broadband strong circular dichroic chiral metasurface. Its single periodic structure consists of, from bottom to top, a gold substrate 1, a TOPAS dielectric layer 2, a monolayer graphene 3, a PMI dielectric layer 4, a TOPAS dielectric layer 5, an open arc-shaped gold split ring 6, an open U-shaped vanadium dioxide ring 7, and a monolayer graphene disk 8.

[0020] As an example, the dimensions of each unit structure are as follows: lattice period P = 23 μm, thickness h1 = 0.2 μm of gold substrate 1, thickness h2 = 0.5 μm of TOPAS dielectric layer 2, thickness h3 = 0.34 nm of monolayer graphene 3 and monolayer graphene disk 8, thickness h4 = 8 μm of PMI dielectric layer 4, thickness h5 = 0.2 μm of TOPAS dielectric layer 5, and thickness h6 = 1 μm of open arc-shaped gold split ring 6. The thickness of the open U-shaped vanadium dioxide ring 7 is h7=0.1μm. The opening angle of the open arc-shaped gold split ring 6 is θ1=212°, the deflection angle relative to the positive z-axis is θ2=-40°, the outer radius is R1=9μm, the inner radius is R2=6μm, the width is W=3μm, the side length of the open U-shaped vanadium dioxide ring 7 is L1=7μm, L2=1.4μm, the opening width is W1=0.8μm, W2=0.4μm, and W3=1μm.

[0021] As an example, the gold substrate 1 is made of gold with an electrical conductivity of 4.561 × 10⁻⁶. 7 S / m; The dielectric constants of TOPAS dielectric layers 2 and 5 are both ε = 2.35, and the material of both dielectric layers is polyethylene cyclic olefin copolymer (TOPAS); The dielectric constant of PMI dielectric layer 4 is ε = 1.1, and the material of the dielectric layer is polymethacrylamide (PMI); The Fermi level E of monolayer graphene layer 3 and monolayer graphene disk 8 is... F =0.1ev, carrier relaxation time τ=0.1ps; conductivity σ=2×10⁻⁶ of open U-shaped vanadium dioxide ring 75 S / m.

[0022] The calculation of circular dichroism can be obtained from the formula CD = Alcp - Arcp, where Alcp and Arcp represent the absorptivity of the chiral device for left-handed and right-handed circularly polarized light, respectively. Simultaneously, there is the formula A(ω) = 1 - R(ω) - T(ω), where A(ω), R(ω), and T(ω) represent the absorptivity, reflectivity, and transmittance, respectively. Since the primary function of the gold substrate is to prevent electromagnetic waves entering the structure from being transmitted out, T(ω) = 0. Therefore, the absorptivity formula can be simplified to A(ω) = 1 - R(ω).

[0023] In this embodiment, the circular dichroism under circularly polarized light incidence is as follows: Figure 3 As shown, the simulation results were calculated using CST StudioSuite 2020 software. By tuning the Fermi level of monolayer graphene and the conductivity of vanadium dioxide, and simultaneously changing the period of the unit structure, the thickness of the dielectric layer, and the dimensions of the open arc-shaped gold split ring and the open U-shaped vanadium dioxide ring, the unit structure can achieve a CD greater than 0.8 in the range of 3.98 THz to 9.14 THz, and a CD greater than 0.9 in the range of 4.54 THz to 8.12 THz. The maximum CD value is achieved at 7.24 THz, which is 0.924.

Claims

1. A dynamically tunable broadband strongly circular dichroic chiral metasurface. Its characteristics are: It consists of a multi-layered structure, from bottom to top: a gold substrate 1, a TOPAS dielectric layer 2, a monolayer graphene 3, a PMI dielectric layer 4, a TOPAS dielectric layer 5, an open arc-shaped gold cleaved ring 6, an open U-shaped vanadium dioxide ring 7, and a monolayer graphene disk 8. A coordinate axis is established with the center of the unit structure as the origin, constituting this terahertz actively tuned multifunctional chiral metasurface device. Incident light is incident perpendicularly. This part is crucial for forming strong circular dichroism.

2. The broadband strongly circular dichroic chiral metasurface according to claim 1, characterized in that: The overall unit structure has a period P = 23 μm, the thickness of the gold substrate 1 is h1 = 0.2 μm, the thickness of the TOPAS dielectric layer 2 is h2 = 0.5 μm, the thickness of the monolayer graphene 3 and the monolayer graphene disk 8 is h3 = 0.34 nm, the thickness of the PMI dielectric layer 4 is h4 = 8 μm, the thickness of the TOPAS dielectric layer 5 is h5 = 0.2 μm, the thickness of the open arc-shaped gold split ring 6 is h6 = 1 μm, and the thickness of the open U-shaped vanadium dioxide ring 7 is h7 = 0.1 μm.

3. The broadband strongly circular dichroic chiral metasurface according to claim 1, characterized in that: The gold substrate 1 is made of gold with an electrical conductivity of 4.561 × 10⁻⁶. 7 S / m is the reflective layer, which makes the transmittance of the entire unit structure 0, ensuring that the incident electromagnetic waves are reflected.

4. The broadband strongly circular dichroic chiral metasurface according to claim 1, characterized in that: The dielectric constants of both TOPAS dielectric layer 2 and TOPAS dielectric layer 5 are ε = 2.

35. The material of both dielectric layers is polyethylene cyclic olefin copolymer (TOPAS). TOPAS is a transparent, rigid amorphous thermoplastic copolymer with superior optical properties, suitable for advanced terahertz applications.

5. The broadband strongly circular dichroic chiral metasurface according to claim 1, characterized in that: The monolayer graphene layer 3 has a length and width of 23 μm, the monolayer graphene disk 8 has a radius R3 = 4 μm, and the Fermi level E of the monolayer graphene layer 3 and the monolayer graphene disk 8 is... F =0.1ev, carrier relaxation time τ=0.1ps.

6. The broadband strongly circular dichroic chiral metasurface according to claim 1, characterized in that: The dielectric constant of the PMI dielectric layer 4 is ε = 1.

1. The material of the dielectric layer is polymethacrylamide (PMI). PMI is a lightweight, closed-cell rigid foam plastic with excellent mechanical properties, heat distortion temperature and chemical stability. It is widely used as an ideal core material for high-performance sandwich structure composite materials.

7. The broadband strongly circular dichroic chiral metasurface according to claim 1, characterized in that: The open arc-shaped gold split ring 6 has an opening angle θ1 = 212°, a deflection angle θ2 = -40° relative to the positive z-axis, an outer radius R1 = 9 μm, an inner radius R2 = 6 μm, and a width W = 3 μm; the open U-shaped vanadium dioxide ring 7 has a conductivity σ = 2 × 10⁻⁶. 5 S / m, side length L1=7μm, L2=1.4μm, opening width W1=0.8μm, W2=0.4μm, W3=1μm.

8. The broadband strongly circular dichroic chiral metasurface according to claim 1, characterized in that: Based on the electrically tunable properties of graphene (Fermi level modulation) and the phase transition characteristics of vanadium dioxide (temperature modulation), a response with a CD greater than 0.8 can be achieved in the range of 3.98 THz to 9.14 THz, and a response with a CD greater than 0.9 in the range of 4.54 THz to 8.12 THz. The maximum CD value is achieved at 7.24 THz, with a value of 0.

924. Furthermore, the broadband strong circular dichroism can be tuned by changing the period of the unit cell structure, the thickness of the dielectric layer, and the thickness, radius, and opening angle of the open arc-shaped gold split ring.

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