A tunable circular dichroism metasurface and a design method thereof

Abstract

The application relates to a tunable circular dichroism super surface and a design method thereof. The super surface is formed by periodically extending a square unit structure in two orthogonal directions on a horizontal plane of a silicon dioxide substrate to form a polysilicon grating structure layer. The square grating unit comprises a main beam and a pair of similar block-shaped epitaxial bodies which are rotationally symmetric about the center, are arranged on the two sides of the main beam, and have bottom edges attached to the main beam. The design parameters of the square grating unit include a grating period, a thickness, a width of the main beam, a shape of the block-shaped epitaxial body, a relative position of the block-shaped epitaxial body, and a similarity ratio adjustment factor. The grating structure is obtained by optimizing the design parameters by using a numerical simulation software with circular dichroism as the optimization target. The application utilizes the circular dichroism mode generated by the two-dimensional grating structure of the rotationally symmetric block-shaped body to realize nearly 100% circular dichroism. By introducing the similarity ratio adjustment factor, the dynamic modulation of the CD spectrum and the integration of the output polarization state of a single device are further realized.

Description

Technical Field

[0001] This invention relates to a tunable circular dichroic metasurface based on the principle of bound states (BICs) in a continuous domain, belonging to the field of polarization optical metamaterials technology. Background Technology

[0002] Chiral metasurfaces, including both three-dimensional and two-dimensional planar structures, possess strong intrinsic chirality, leading to structural distortions in three-dimensional / two-dimensional space, typically exhibiting strong circular dichroism and strong optical rotation. Their chiral characteristic states usually depend on circular dichroism resonance modes based on periodicity, height, or other structural parameters. The focus is often on broadband and high circular dichroism, rather than on controllability, because once fabricated, they become fixed, lacking the flexibility to adjust circular dichroism. Existing designs typically rely on tunable materials to achieve dynamic control of circular dichroism.For example, applying external voltages to graphene and other atomically sensitive materials (see: Electrical access to critical coupling of circularly polarized waves in graphene chiral metamaterials[J]. Science Advances, 2017, 3(9): e1701377.), liquid crystals (see: Liquid-crystal-loaded chiral metasurfaces for reconfigurable multiband spin-selective light absorption[J]. Optics Express, 2018, 26(19): 25305.); injecting additional photons based on the physical properties of the materials or through chemical reactions (see: Near‐Infrared Light‐Driven Three‐Dimensional Soft Photonic Crystals Loaded with Upconversion Nanoparticles[J]. Advanced Optical Materials, 2022, 10(9): 2102475.); and triggering phase transitions in phase change materials by adjusting the temperature (see: Wavelength-tunable infrared chiral metasurfaces with (Phase-change materials[J]. Optics Express, 2023, 31(13): 21118.) Switching between different chiral modes; and assembling or fabricating chiral structures on flexible material substrates (such as polydimethylsiloxane and hydrogels) and applying external forces to produce overall deformation (see: Mechano-tunable chiral metasurfaces via colloidal assembly[J]. Nature Materials, 2021, 20(7): 1024–1028.). However, these design methods suffer from the drawback of producing weak chirality or having poor tuning ability. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this invention provides a chiral metasurface and its design method that enables dynamic control of circular dichroism (CD) and the polarization state of emitted light.

[0004] The technical solution to achieve the objective of this invention is to provide a design method for a tunable circular dichroic metasurface, comprising the following steps: (1) Construction of grating unit structure and design parameters A square grating unit is constructed, comprising a main beam and a pair of similarly shaped block epitaxial bodies. The block epitaxial bodies are regular shapes that can be parameterized and are rotationally symmetric about the center. They are disposed on both sides of the main beam, with their bottom edges respectively attached to the main beam. The design parameters of the square grating unit structure include the grating period P, the grating thickness T, the width W of the main beam, and the shape, relative position, and similarity ratio adjustment factor η of the block epitaxial bodies. (2) Parameter optimization Using circular dichroism as the optimization objective, an optimization algorithm was employed to optimize the design parameters of the square grating unit structure through numerical simulation software, resulting in the square grating unit. (3) Constructing metasurfaces By periodically extending the square unit structure in two orthogonal directions on the horizontal plane of a silicon dioxide substrate, a polycrystalline silicon grating structure layer is formed, resulting in a circular dichroic metasurface.

[0005] The parameterizable regular-shaped block extensional bodies described in this invention include triangles, trapezoids, semicircles, and squares.

[0006] In a preferred embodiment of the present invention, the design parameters of the triangular block-shaped extension body are: base length, height on the base, distance of the base angle near the center from the center, and distance of the vertex from the center; the design parameters of the trapezoidal block-shaped extension body are: base length, top length, height of the trapezoid, distance of the base angle near the center from the center, and distance of the vertex near the center from the center; the center is the geometric center of the square unit of the grating.

[0007] The optimization algorithms described in this invention include genetic algorithms, particle swarm optimization, and Bayesian optimization algorithms.

[0008] The technical solution of the present invention also includes a tunable circular dichroic metasurface obtained according to the above design method.

[0009] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The chiral metasurface provided by the present invention consists of a two-dimensional grating structure with rotationally symmetric bulk extension, which achieves nearly 100% circular dichroism by utilizing the circular dichroism mode generated by the structure, and can be linearly tuned by BIC mode.

[0010] 2. The metasurface provided by this invention, through the introduction of an asymmetry factor between the bulk extensions in its grating structure, further integrates dynamic modulation of the CD spectrum and the output polarization state of a single device. The CD spectrum of the asymmetric grating structure retains the linear dispersion characteristics of the symmetric grating structure and maintains its splitting characteristics even under normal incidence. Within a small angular range around normal incidence, the output polarization state at the wavelength of the split CD peak undergoes a rapid transition from linear to circular. The metasurface provided by this invention provides topological protection for the linear dispersion of the CD spectrum caused by BICs, and the center wavelength and peak width of the CD resonance can be flexibly adjusted by changing the geometric parameters.

[0011] 3. The metasurface provided by the present invention expands the range of design possibilities because the peak shape (center wavelength, half width at half maximum) of the unsplit principal circular dichroism peak under normal incidence can be adjusted by the shape of the bulk epitaxial body. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the structure of a tunable circular dichroic metasurface provided in an embodiment of the present invention; Figure 2 , 3 These are, respectively, a three-dimensional and a top view schematic diagram of the square grating unit in the polycrystalline silicon grating structure layer provided in the embodiments of the present invention; Figure 4 These are the photonic band diagram and the corresponding quality factor diagram of the wavelength-tunable circular dichroism metasurface unit structure provided in Embodiment 1 of the present invention. Figure 5 This is a scan of the circular dichroism spectrum of the wavelength-tunable circular dichroism metasurface with respect to the incident angle provided in Embodiment 1 of the present invention; Figure 6 This is a scan of the circular dichroism spectrum (1.5-1.6 μm) of the wavelength-tunable circular dichroism metasurface (center wavelength 1550 nm) provided in Embodiment 2 of the present invention with respect to the incident angle (-5° to 5°). Figure 7 The circular dichroism spectrum (1.6-1.7 μm) of the mid-wavelength tunable circular dichroism metasurface (center wavelength 1650 nm) provided in Embodiment 3 of the present invention with respect to the incident angle (-5° to 5°); Figure 8 The circular dichroism spectrum (1.6-1.7 μm) of the mid-wavelength polarization state dual tunable circular dichroism metasurface with respect to a large incident angle (-8° to 8°) provided in Embodiment 4 of the present invention; Figure 9 This is a scan of the circular dichroism spectrum (1.6-1.7 μm) of the wavelength polarization-state tunable circular dichroism metasurface provided in Embodiment 4 of the present invention with respect to a small incident angle (-0.3° to 0.3°); Figure 10 This shows the variation trend of the transmission spectrum (wavelength range 1620-1660 nm) of each polarization component when incident at a small angle (-0.3° to 0.3°, every 0.05°). Figure 11 This is a top view schematic diagram of the structure of the square grating unit provided in Embodiment 5 of the present invention; Figure 12 This is a scan of the circular dichroism spectrum (1.6-1.7 μm) of the mid-wavelength tunable circular dichroism metasurface (center wavelength 1635 nm) provided in Embodiment 5 of the present invention with respect to the incident angle (-5° to 5°). In the figure: 1 is the polycrystalline silicon grating structure layer, 2 is the silicon dioxide substrate layer; 3 is the main beam; 4 is a pair of similarly shaped block epitaxial bodies. Detailed Implementation

[0013] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments. Example 1

[0014] This embodiment provides a design method for a tunable circular dichroic metasurface based on a grating structure.

[0015] See appendix Figure 1 This is a schematic diagram of the structure of a tunable circular dichroic metasurface provided in this embodiment; it is a planar two-dimensional subwavelength silicon grating structure, which includes a polycrystalline silicon grating structure layer 1 and a silicon dioxide substrate 2 in sequence along the incident direction of light; the polycrystalline silicon grating structure layer is composed of two orthogonal (X, Y directions) periodically extended square units in the horizontal plane.

[0016] The design method for tunable circular dichroic metasurfaces includes the following steps: 1. Constructing the grating unit structure and design parameters See appendix Figure 2 and 3These are, respectively, a three-dimensional and top-view schematic diagram of the square grating unit in the polycrystalline silicon grating structure layer provided in this embodiment; along the light incident direction (the z-direction perpendicular to the metasurface), it sequentially includes a polycrystalline silicon grating structure layer 2 and a silicon dioxide substrate layer 1. The square grating unit includes a main beam 3 and a pair of similarly shaped block epitaxial bodies 4. The direction along the main beam of the grating is the x-direction (normal), and the direction perpendicular to the main beam of the grating in the plane of the metasurface is the y-direction (tangential); in this embodiment, the block epitaxial bodies are triangular, and the triangular epitaxial bodies are similarly shaped and rotate symmetrically distributed on both sides of the main beam of the grating, with the base of the triangles respectively attached to both sides of the main beam; the grating has a period of p, a width of w, and a thickness of T. The shape and position of the triangular extension body are determined by the following four shape parameters: base length u, base height h, distance δ of the base angle near the center from the center, and eccentricity distance dx of the vertex. The similarity ratio of a pair of similar triangular extension bodies on both sides of the grating main beam is controlled by the asymmetry factor η. The wave vector k of the incident light is determined by the elevation angle θ and the azimuth angle φ. When controlling it, φ is required to be 0°, that is, θ changes along the grating normal direction (x direction).

[0017] 2. Parameter optimization Using circular dichroism as the optimization objective, an optimization algorithm is employed to optimize the design parameters of the square grating unit structure through numerical simulation software, thereby obtaining the square grating unit.

[0018] The center wavelength range of this invention is the near-infrared band of 1400–1700 nm. The structural parameters of the grating unit are optimized within a typical range using a combination of simulation models and optimization algorithms.

[0019] The parameter that has the greatest impact on the center wavelength is the width w of the grating main beam, which can be used to obtain preliminary design parameters using the following empirical formula: wl center = a1× w 2 +a2× w + a3 Among them, wl center With the center wavelength as the reference, the empirical fitting coefficients a1 = -3.4656, a2 = 2.8546, and a3 = 1.1668, allowing for circular dichroism modulation within a broadband range of ±100 nm from the center wavelength.

[0020] Similar bulk epitaxial bodies can be controlled using an asymmetry factor η (where η is the similarity ratio of the bulk epitaxial bodies extending on both sides of the grating main beam). For symmetric structures with an asymmetry factor η = 1, broadband circular dichroism modulation can be achieved by changing the incident angle. For asymmetric structures with an asymmetry factor η not equal to 1, broadband circular dichroism modulation can be achieved not only by changing the incident angle over a wide range (|θ|∈(1°, 10°)), but also by changing the incident angle over a small range (|θ| < 1°) to achieve polarization state modulation at a fixed wavelength, where the incident angle θ needs to be changed along the grating normal direction (x-direction).

[0021] This embodiment provides a wavelength-tunable circular dichroic metasurface with a center wavelength of 1600 nm. After optimization using FDTD (Ansys, America) and COMSOL (COMSOL Inc., Sweden) numerical simulation software, the specific parameters of the square grating unit are as follows: grating period p = 900 nm; grating thickness T = 330 nm; width of the grating main beam w = 200 nm; length of the triangle base u = 250 nm; height of the triangle base h = 200 nm; distance of the near-center base angle from the center δ = 0 nm; vertex eccentricity distance dx = 0 nm; asymmetry factor η = 1.

[0022] 3. Constructing Metasurfaces By periodically extending the square unit structure in two orthogonal directions on the horizontal plane of a silicon dioxide substrate, a polycrystalline silicon grating structure layer is formed, resulting in a circular dichroic metasurface.

[0023] See appendix Figure 4 Figures (a) and (b) show the photonic band structure and quality factor of the corresponding photonic bands of the mid-wavelength tunable circular dichroism metasurface unit structure provided in this embodiment. The metasurface with the structural parameters provided in this embodiment exhibits four different resonance modes in the wavelength range of 1400–1700 nm, primarily utilizing modes 1 and 2. The wavelength tuning principle of the tunable circular dichroism metasurface is derived from… Figure 4 It is evident that Mode 1 (BICs mode) and Mode 2 (circular dichroism mode based on guided mode resonance) form a Dirac cone band structure and are degenerate at the gamma point (Γ point, i.e., when the wave vector is incident perpendicularly). The BICs mode is protected by C2 rotational symmetry and is hidden during normal incidence; however, when the incident angle is changed, the symmetry of the structure is broken, and the BICs mode is excited. The single circular dichroism peak originally supported by the circular dichroism mode is thus split by the BICs mode. As the incident angle gradually increases, the quality factor of the BICs mode continuously decreases, and the radiation range continuously widens, resulting in linear dispersion of the split circular dichroism peak.

[0024] See appendix Figure 5 This is a scan of the circular dichroism spectrum (1.4–1.7 μm) of the wavelength-tunable circular dichroism metasurface (center wavelength 1600 nm) in this embodiment with respect to the incident angle (-20°–20°); [The image is derived from...] Figure 5 As can be seen, bright circular dichroism bands can be observed over a wide range (-20° to 20°, 1500 to 1700 nm).

[0025] Example 2 This embodiment provides a wavelength-tunable circular dichroic metasurface with a center wavelength of 1550 nm.

[0026] According to the metasurface structure and design method provided in Example 1, the specific parameters of its grating-shaped metasurface unit are as follows: grating period p = 900 nm; grating thickness T = 330 nm; w = 160 nm; triangle base length u = 245 nm; triangle base height h = 296 nm; reference point eccentricity δ = 0 nm; vertex eccentricity dx = 0 nm; asymmetry factor η = 1.

[0027] For its incident angle and circular dichroism spectrum, please refer to the appendix. Figure 6 As shown.

[0028] Example 3 This embodiment provides a wavelength-tunable circular dichroic metasurface with a center wavelength of 1650 nm.

[0029] According to the grating structure and design method provided in Example 1, the specific parameters of the grating-shaped metasurface unit are as follows: grating period p = 900 nm; grating thickness T = 330 nm; width of the grating main beam w = 234 nm; length of the triangle base u = 300 nm; height of the triangle base h = 116 nm; reference point eccentricity δ = -217 nm; vertex eccentricity dx = -144 nm; asymmetry factor η = 1.

[0030] For its incident angle and circular dichroism spectrum, please refer to the appendix. Figure 7 As shown.

[0031] Example 4 This embodiment provides a wavelength-transmitting polarization-state tunable circular dichroic metasurface (wavelength range 1620~1660 nm).

[0032] According to the grating structure and design method provided in Example 1, the specific parameters of the grating-shaped metasurface unit are as follows: grating period p = 900 nm; grating thickness T = 330 nm; width of the grating main beam w = 200 nm; length of the triangle base u = 250 nm; height of the triangle base h = 200 nm; eccentricity distance of the reference point δ = 0 nm; eccentricity distance of the vertex dx = 0 nm; asymmetry factor η = 1.2.

[0033] See appendix Figure 8 and 9 The figures show the circular dichroism spectra as the incident angle changes, for large and small incident angles, respectively. The figures demonstrate that even at large incident angles, the circular dichroism still exhibits good linear modulation capability with wavelength.

[0034] See appendix Figure 10 , is the cross-polarized transmittance (T LR and T RL ) and co-polarized transmittance (T LL and T RR As can be seen, when the incident angle changes within a small range, the polarization state of the outgoing light undergoes a dramatic change, that is, the proportions of cross-polarized transmittance and co-polarized transmittance gradually change. At the split circular dichroism peak, the changes from circular polarization to linear polarization and from linear polarization to circular polarization are realized, respectively.

[0035] Example 5 This embodiment provides a wavelength-tunable circular dichroic metasurface (center wavelength 1635 nm).

[0036] See appendix Figure 11 This is a top view schematic diagram of the square grating unit in the polycrystalline silicon grating structure layer provided in this embodiment. In this embodiment, the block epitaxial body 4 is trapezoidal, and the two epitaxial bodies are similar in shape and are distributed symmetrically on both sides of the grating main beam 3. The base of the trapezoid is attached to both sides of the main beam. The period of the grating is p, the width is w, and the thickness is T. The shape and position of the trapezoidal epitaxial body are determined by the following adjustable parameters: the length of the base of the trapezoid u, the length of the top side v, the height of the base of the trapezoid h, the distance δ of the base angle near the center from the center, and the distance dx of the vertex eccentricity. The similarity ratio of the epitaxial bodies is controlled by the asymmetry factor η.

[0037] Following the design method provided in Example 1, and after optimization using FDTD (Ansys, America) and COMSOL (COMSOL Inc., Sweden) numerical simulation software, the specific parameters of the grating-shaped metasurface unit are as follows: p = 900 nm, T = 330 nm, w = 200 nm; u = 250 nm, v = 100 nm, h = 200 nm; δ = 0 nm, dx = 30 nm; η = 1.

[0038] See appendix Figure 12 This is a scan of the circular dichroism spectrum (1.6-1.7 μm) of the wavelength-tunable circular dichroism metasurface (center wavelength 1635 nm) in this embodiment with respect to the incident angle (-5° to 5°); it can be seen that for grating metasurfaces with different bulk structures, the circular dichroism still has good linear modulation capability with wavelength.

[0039] In the technical solution of this invention, the blocky extension body can be a regular shape that can be parameterized and characterized, such as a triangle, trapezoid, semicircle, or square.

Claims

1. A design method for a tunable circular dichroic metasurface, characterized in that... Includes the following steps, (1) Construction of grating unit structure and design parameters A square grating unit is constructed, comprising a main beam and a pair of similarly shaped block epitaxial bodies. The block epitaxial bodies are regular shapes that can be parameterized and are rotationally symmetric about the center. They are disposed on both sides of the main beam, with their bottom edges respectively attached to the main beam. The design parameters of the square grating unit structure include the grating period P, the grating thickness T, the width W of the main beam, and the shape, relative position, and similarity ratio adjustment factor η of the block epitaxial bodies. (2) Parameter optimization Using circular dichroism as the optimization objective, an optimization algorithm was employed to optimize the design parameters of the square grating unit structure through numerical simulation software, resulting in the square grating unit. (3) Constructing metasurfaces By periodically extending the square unit structure in two orthogonal directions on the horizontal plane of a silicon dioxide substrate, a polycrystalline silicon grating structure layer is formed, resulting in a circular dichroic metasurface.

2. The design method for a tunable circular dichroic metasurface according to claim 1, characterized in that: The parameterizable regular-shaped blocky extensional body is a triangle, trapezoid, semicircle, or square.

3. The design method for a tunable circular dichroic metasurface according to claim 1, characterized in that: The optimization algorithm mentioned is a genetic algorithm, a particle swarm optimization algorithm, or a Bayesian optimization algorithm.

4. The design method of a tunable circular dichroic metasurface according to claim 2, characterized in that: The design parameters of the triangular block-shaped extension body are: base length, height on the base, distance of the base angle near the center from the center, and distance of the vertex from the center; the center is the geometric center of the grating square unit.

5. The design method for a tunable circular dichroic metasurface according to claim 2, characterized in that: The design parameters of the trapezoidal block extension are: base length, top length, height of the trapezoid, distance of the base angle near the center from the center, and distance of the vertex near the center from the center; the center is the geometric center of the grating square unit.

6. A tunable circular dichroic metasurface obtained by the design method according to claim 1.

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