Broadband infrared polarization control metasurface based on chiral structure design
By designing a broadband infrared polarization-tunable metasurface with a chiral structure, the limitations on incident polarization angle and narrow bandwidth in existing technologies have been solved, enabling broadband control that is insensitive to incident polarization angle, expanding application scenarios and simplifying device structure.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- UNIV OF ELECTRONICS SCI & TECH OF CHINA
- Filing Date
- 2026-03-11
- Publication Date
- 2026-06-05
Smart Images
Figure CN122151271A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of artificial metamaterials and relates to polarization-controlled metasurfaces, specifically a broadband infrared polarization-controlled metasurface based on a chiral structure design. Background Technology
[0002] Infrared polarization modulation technology is a key optical technology that uses artificial means to precisely control the polarization state of electromagnetic waves (such as linear polarization, circular polarization, etc.). It utilizes the sensitivity of polarization state to the interaction between electromagnetic waves and matter (such as surface roughness, material, scattering characteristics, etc.) and has shown unique advantages in fields such as imaging, communication, remote sensing, biomedicine, industrial inspection, and military, thus attracting widespread attention and research.
[0003] Early polarization manipulation primarily relied on naturally occurring anisotropic materials to control the polarization of electromagnetic waves. Typical devices included quarter-wave plates to achieve the conversion from linearly polarized to circularly polarized states. The core characteristic of these devices was that the dielectric constant of the material was in tensor form, with diagonal quantities being anisotropic and off-diagonal quantities being zero. This allowed for the application of a phase difference in the two orthogonal directions of linearly polarized light decomposition, thus achieving polarization manipulation.
[0004] While using dielectric anisotropic materials to convert linearly polarized light to circularly polarized light still presents a limitation on the polarization angle of the incident linearly polarized light, requiring consistent amplitude of the electromagnetic wave in two orthogonal directions. Chiral materials overcome this limitation. Compared to dielectric anisotropic materials, chiral materials have a characteristic where the off-diagonal portion of the dielectric tensor is no longer zero. This results in different responses to left-handed and right-handed circularly polarized waves. By decomposing linearly polarized light into left-handed and right-handed circularly polarized waves, and utilizing the circular dichroism (selective reflection, absorption, or transmission) of chiral materials for different directions of circular polarization, the conversion from linear to circular polarization can be achieved without limiting the incident polarization angle, offering a natural advantage in expanding application scenarios.
[0005] Natural materials face challenges such as the degree of anisotropy in dielectric diagonal and the small amount of off-diagonal dielectric material, resulting in large device size and inflexible adjustment of application wavelengths. The emergence of metasurfaces has driven a revolutionary breakthrough in polarization manipulation technology. By arranging anisotropic nanostructures (such as metal gratings and dielectric pillars) at the subwavelength scale, the amplitude, phase, and polarization state of light can be flexibly controlled, achieving functions that traditional devices cannot accomplish.
[0006] However, previous designs had the following problems: the chiral metasurfaces exhibited low circular dichroism, failing to achieve high selectivity for left-handed and right-handed circularly polarized waves, thus affecting polarization control. Furthermore, the emergence of circular dichroism depended on strong magnetoelectric coupling, resulting in a very narrow bandwidth for generating strong chirality, even limited to a single frequency point, which imposed limitations on application scenarios. Summary of the Invention
[0007] To address the aforementioned issues and resolve problems encountered in the conversion of electromagnetic waves from linear to circular polarization in the infrared band, such as limitations on the incident polarization angle, narrow operating bandwidth, and low circular dichroism of natural materials, this invention proposes a broadband infrared polarization-modulating metasurface based on a chiral structure design. Due to the presence of the chiral structure, the conversion from linear to circular polarization is no longer limited by the incident polarization angle. Furthermore, the metasurface design offers several advantages over natural materials, such as: higher degree of freedom in structural design adjustment, easier adjustment and expansion of the operating bandwidth; strong interaction between the subwavelength structure and electromagnetic waves, stimulating strong chirality; and subwavelength structural dimensions, resulting in smaller and more compact polarization-modulating devices compared to those made from natural materials.
[0008] To achieve the above objectives, the specific technical solution of the present invention is as follows:
[0009] A broadband infrared polarization-tunable metasurface based on chiral structure design is composed of periodic unit cells. The unit cell structure is a two-layer structure, including a base layer and a chiral structure layer.
[0010] The substrate layer is made of silicon dioxide and has a square planar shape with a side length of a unit period P. The chiral structure layer is made of silicon and is composed of two cuboids of the same height but different lengths and widths. The geometric centers of the two cuboids overlap and are located at the center of the substrate layer.
[0011] The base layer establishes a spatial coordinate system with one corner of a square as the origin, one side as the x-axis, the other side as the y-axis, and the z-axis perpendicular to the base layer and pointing upwards. The center coordinates of the base layer are (P / 2, P / 2, 0), and the geometric center coordinates of the two cuboids are (P / 2, P / 2, H / 2), where H is the height of the cuboid. From a top-down view, one cuboid of the chiral structure layer is parallel to the y-axis, and the other is rotated by an angle α around its geometric center in the xy plane, where angle α satisfies 0° < α < 90°.
[0012] Furthermore, the chiral structure layer exhibits high dielectric strength and low loss characteristics in the mid-infrared band. The two cuboids of the chiral structure, with the same height but different lengths and widths, have a length of l. x1 l x2 Width is l y1 l y2 The parameters P and l were adjusted using simulation software. x1 l x2 l y1 l y2H and α were used to adjust the circular dichroism of the chiral structure. Periodic boundaries were set for the above unit structure using simulation software, and left-handed circularly polarized waves, right-handed circularly polarized waves, and linearly polarized waves with different polarization angles were incident perpendicularly. The reflectivity, transmittance, and ellipticity of the reflected waves were calculated.
[0013] In summary, this invention employs two intersecting cuboid Si strips to construct a chiral unit structure, which is periodically arranged on a SiO2 substrate to form a chiral metasurface array. Circularly polarized waves with different rotational directions are incident perpendicularly to observe the circular dichroism of the chiral metasurface array. Linearly polarized waves with different polarization angles are also incident perpendicularly, and the ellipticity of the reflected waves is calculated to verify the linear-to-circular polarization conversion capability of the metasurface. The infrared polarization-modulated metasurface of this invention can optimize the circular dichroism performance of the chiral metasurface and control and expand the bandwidth of polarization conversion by adjusting parameters such as the unit period, the length, width, and height of the two cuboids, and the relative rotation angle, thus exhibiting multi-degree-of-freedom adjustment characteristics. Because a chiral structure is used for polarization control, the control effect is no longer sensitive to the incident polarization angle, greatly expanding the application range. Furthermore, this metasurface has only two layers: the substrate and the chiral structure, which, while ensuring broadband circular dichroism, has the advantage of structural simplicity compared to other multilayer chiral metasurfaces. Attached Figure Description
[0014] Figure 1 The diagram shows (a) a side view, (b) a top view, and (c) and (d) schematic diagrams of the metasurface unit structure of the present invention after arrangement.
[0015] Figure 2 For the example, when a circularly polarized wave is incident perpendicularly, the metasurface shows (a) reflectivity and (b) transmittance for left-handed and right-handed circularly polarized waves, respectively.
[0016] Figure 3 The electric and magnetic field vector distributions of the metasurface in the example are shown in (a) and (b) for the electric field distribution when x-polarized waves are incident perpendicularly, with λ=3.3μm and λ=4.5μm; (c) and (d) for the magnetic field distribution when x-polarized waves are incident perpendicularly, with λ=3.3μm and λ=4.5μm; (e) and (f) for the electric field distribution when y-polarized waves are incident perpendicularly, with λ=3.3μm and λ=4.5μm.
[0017] Figure 4 For example, the chiral structural parameters control the circular dichroism: (a) represents the width l of the cube of revolution. y2 The effect of circular dichroism on bandwidth, (b) represents the length l of the cube of revolution. x2 The effect on circular dichroism and bandwidth.
[0018] Figure 5 This illustrates the effect of rotation angle α on circular dichroism and bandwidth;
[0019] Figure 6 After final optimization, the metasurface exhibits (a) reflectivity and (b) transmittance for left-handed and right-handed circularly polarized waves when a circularly polarized wave is incident perpendicularly in the embodiment.
[0020] Figure 7 (a) Electric field distribution, (b) Magnetic field distribution, and (c) Calculated chiral density in the xy plane of the optimized chiral unit structure under right-hand circularly polarized wave incident at different wavelengths;
[0021] Figure 8 (a) Electric field intensity distribution and (b) metasurface far-field effect on the yz plane of the chiral unit structure of the embodiment under the incident light of left-handed and right-handed circularly polarized waves with λ=4.4μm;
[0022] Figure 9 The ellipticity of the reflected wave is calculated for example under perpendicular incidence of linearly polarized waves at different polarization angles. Detailed Implementation
[0023] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.
[0024] This embodiment provides a broadband infrared polarization-tunable metasurface based on a chiral structure design; it is composed of periodic unit cells. The unit cell structure has two layers, including a base layer and a chiral structure layer, as shown below. Figure 1 As shown in (a).
[0025] The substrate layer is silicon dioxide (SiO2), which is transparent in the mid-infrared band. Its real dielectric part ε1 is approximately 1.4, and its imaginary dielectric part ε2 tends to 0. The substrate layer has a square planar shape with a side length equal to the unit period P. In this embodiment, the unit period P is 2700 nm. A coordinate system is established with one corner of the square substrate as the origin, one side as the x-axis, the other side as the y-axis, and the z-axis perpendicular to the substrate layer and pointing upwards.
[0026] The chiral structure layer is silicon (Si), which exhibits high dielectric strength and low loss in the mid-infrared band. Its real dielectric part ε1 is approximately 11.9, and its imaginary dielectric part ε2 approaches 0. The chiral structure is as follows... Figure 1 As shown in (b), it is formed by the intersection of two cuboids of the same height but different lengths and widths: the length and width of the two cuboids are l and l respectively. x1 l y1 l x2 l y2 The height of all are H. In this embodiment, l x1 =500nm, l y1=2700nm, H=950nm; the geometric centers of the two cuboids overlap and are located at the center of the square base layer (geometric center coordinates: x=P / 2, y=P / 2, z=H / 2); top view Figure 1 (b) In this case, one cuboid is parallel to the y-axis, and the other cuboid is rotated by an angle α around the geometric center in the xy plane, where the angle α satisfies 0° < α < 90°.
[0027] Figure 1 (c) and (d) are schematic diagrams of a metasurface composed of periodic units. A circularly polarized wave is incident perpendicularly along the negative z-axis, and the reflectivity of the metasurface for left-handed and right-handed circularly polarized waves is measured. The difference between the two is the circular dichroism of reflection: CD = R. LCP -R RCP .
[0028] When the angle α is not 0° or 90°, the mirror symmetry of the structure is broken, and it cannot be reunited with its own mirror image through translation or rotation; that is, the structure exhibits geometric chirality. When a circularly polarized wave is incident, the circular dichroism will no longer be 0, and it can be adjusted by rotating the angle α.
[0029] When l x2 =1700nm, l y2 When the wavelength is 600 nm and a = 20°, the circular dichroism of the reflection is: Figure 2 As shown in (a), Figure 2 (b) represents the transmission circular dichroism of the unit cell structure. The peaks occur at wavelengths λ=3.3μm and λ=4.5μm.
[0030] l x2 =1700nm, l y2 At a wavelength of 600 nm and an angle of α = 20°, the peaks of circular dichroism appear at wavelengths λ = 3.3 μm and λ = 4.5 μm. The circularly polarized wave is decomposed into x-polarized and y-polarized waves, which are then incident perpendicularly onto the structure. Field monitors are placed at the corresponding wavelengths. The distribution of electric and magnetic fields within the unit structure is used to analyze the cause of the circular dichroism. The electric and magnetic field vectors within the structure are observed, such as... Figure 3 As shown in (a) and (b), when an x-polarized wave is incident, regardless of whether the wavelength is λ = 3.3 μm or λ = 4.5 μm, an electric field is generated in a cuboid parallel to the y-axis. This circulating electric field will excite an in-plane magnetic moment in the x-direction, as shown in (a) and (b). Figure 3 As shown in (c) and (d). When a y-polarized wave is incident, the electric field is concentrated along the length of the side of the rotating cuboid, as shown in... Figure 3 As shown in (e) and (f), an electric moment is formed in the xy plane, and it has a parallel component with the magnetic moment in the plane. Using the chiral density formula:
[0031] C = (-ω / 2)Im(D*·B)
[0032] Where ω is the frequency, D* is the conjugate of the electric displacement vector, B is the magnetic induction vector, and the Im operator represents taking the imaginary part.
[0033] Because the in-plane electric moment excited by the incident y-polarized wave has parallel components to the in-plane magnetic moment excited by the incident x-polarized wave, the chiral density will no longer be zero. Since the circularly polarized wave can be considered as a superposition of the x-polarized and y-polarized waves, this explains why the structure exhibits circular dichroism at wavelengths λ=3.3μm and λ=4.5μm under circularly polarized wave incidence.
[0034] Note Figure 3 In (e), the in-plane electric moment is concentrated in the short arm of the rotating cuboid at λ = 3.3 μm, while... Figure 3 (f) When λ = 4.5 μm, the in-plane electric moment is concentrated in the long arm of the rotating cuboid. The excitation wavelength of the electric moment depends on the structural dimensions (arm length), which can be adjusted by changing the length and width of the rotating cuboid (l... x2 l y2 This allows for a smooth transition in the arm length of the rotating cuboid from short to long, enabling the excitation of in-plane electric moments within a wide bandwidth and constructing wideband circular dichroism.
[0035] Figure 4 (a) In the chiral structure of the embodiment, when a = 20°, the rotating cuboid, l y2 The effect of changing from 600nm to 1100nm on circular dichroism and bandwidth. Figure 4 (b) In the chiral structure of the embodiment, when a = 20°, the rotation of the cuboid l x2 The effect of changing from 1700nm to 2000nm on circular dichroism and bandwidth. Figure 5 For when l x2 =960nm, l y2 Circular dichroism when the rotation angle α changes from 20° to 80° at 1980nm.
[0036] After further optimization of the structural parameters, the final structural dimensions of this embodiment were determined to be a=20°, l x2 =960nm, l y2 =1980nm, the structure's reflectivity and transmittance for left-handed and right-handed circularly polarized waves are as follows: Figure 6 As shown in (a) and (b), the circumchromatic bandwidth (CD>0.6) is 1.1 μm, and the relative bandwidth is 26.5%.
[0037] Figure 7The electric field distribution, magnetic field distribution, and calculated chiral density of the chiral unit structure in the xy plane under right-hand circularly polarized wave incident at different wavelengths are shown for this structural size. The electric field distribution shows that the electric moment moves from the short arm to the long arm as the incident wavelength increases.
[0038] Figure 8 Given a=20°, l x2 =960nm, l y2 At 1980 nm, the examples illustrate the electric field intensity distribution on the yz plane of the unit cell structure and the far-field effect of the metasurface formed by the periodic arrangement of unit cell structures when circularly polarized waves of different rotation directions are incident. Combined with... Figure 6 The given reflectivity and transmittance reflect the actual effect of the designed chiral metasurface: high transmittance for left-handed circularly polarized waves and high reflectance for right-handed circularly polarized waves.
[0039] In the embodiment where linearly polarized waves with different polarization angles are incident perpendicularly, the ellipticity of the reflected wave is calculated using the following formula:
[0040] η=2Mag(S 11 )Mag(S 21 )Sin(Ph(S 11 )-Ph(S 21 )) / (Mag(S 11 )Mag(S 11 )+Mag(S 21 )Mag(S 21 ))
[0041] Where S 11 S is the common polarization reflection coefficient. 21 The cross-polarization reflection coefficient is represented by the Mag operator for amplitude and the Ph operator for phase.
[0042] In the embodiment where linearly polarized waves with different polarization angles are incident perpendicularly, the ellipticity calculation results of the reflected waves are as follows: Figure 9 As shown, even as the polarization angle φ changes from 0° to 180°, the ellipticity of the reflected wave remains at a high level (η>0.6), which is an advantage brought by the chiral structure. Furthermore, the polarization conversion bandwidth is 926 nm, with a relative bandwidth of 22.2%. This is because by adjusting the multi-degree-of-freedom parameters of the metasurface structure, strong magnetoelectric coupling can be excited at multiple wavelengths, thus generating broadband strong chirality.
[0043] As can be seen from the above embodiments, the broadband infrared polarization-tunable metasurface based on chiral structure provided by this invention has been proven feasible. The polarization-tunable metasurface of this invention utilizes a chiral structure to overcome the problem of polarization angle sensitivity in polarization conversion, and can achieve a broadband effect simply by adjusting structural parameters, thus solving the narrow-band problem in current polarization conversion. Furthermore, the metasurface proposed in this invention has the characteristics of simple structure, multiple degrees of freedom in control, and common materials, effectively accommodating the diversity of application scenarios.
Claims
1. A broadband infrared polarization-tunable metasurface based on a chiral structure design, characterized in that: It is composed of periodic units, and the unit structure is a two-layer structure, including a basal layer and a chiral structure layer. The base layer is made of silicon dioxide and has a square planar shape with a side length of a unit period P; the chiral structure layer is made of silicon and is composed of two cuboids of the same height but different lengths and widths intersecting each other, with the geometric centers of the two cuboids overlapping and located at the center of the base layer. The base layer establishes a spatial coordinate system with one corner of a square as the origin, one side as the x-axis, the other side as the y-axis, and the z-axis perpendicular to the base layer and pointing upwards. The center coordinates of the base layer are (P / 2, P / 2, 0), and the geometric center coordinates of the two cuboids are (P / 2, P / 2, H / 2), where H is the height of the cuboid. From a top-down perspective, one cuboid of the chiral structure layer is parallel to the y-axis, and the other is rotated by an angle α around its geometric center in the xy plane, where the angle α satisfies 0° < α < 90°.
2. The broadband infrared polarization-tunable metasurface based on chiral structure design as described in claim 1, characterized in that: By adjusting the unit period P and the length l of the two cuboids x1 l x2 Width l y1 l y2 By adjusting the height H and rotation angle α, the circular dichroism properties and polarization conversion bandwidth of the chiral structure can be controlled.
3. The broadband infrared polarization-tunable metasurface based on chiral structure design as described in claim 1 or 2, characterized in that: The metasurface has a unit cell period of P = 2700 nm and a fixed cuboid parameter of l. x1 =500nm, l y1 =2700nm, H=950nm, rotation parameters are α=20°, l x2 =960nm, l y2 =1980nm.
4. The broadband infrared polarization-tunable metasurface based on chiral structure design as described in claim 1, characterized in that: The metasurface achieves a circular dichroism bandwidth of 1.1 μm and a relative bandwidth of 26.5% in the mid-infrared band; the conversion bandwidth from linearly polarized to circularly polarized waves is 926 nm and the relative bandwidth is 22.2%, and the conversion effect is independent of the incident polarization angle.