An ultrathin film based on asymmetric dimer unit structure and preparation method and application thereof

Abstract

The present application relates to a kind of metasurface based on asymmetric dimer unit structure and its preparation method and application, the metasurface includes substrate and dimer unit in array distribution on the substrate;With the plane of the substrate as xy axis plane, x axis is perpendicular to y axis;The dimer unit is in array distribution along x axis and y axis;The dimer unit includes first microstructure and second microstructure;The shape and size of the first microstructure and second microstructure are different, wherein the cross section parallel to substrate in the first microstructure is curved shape, the second microstructure includes at least two isolated sub-microstructure;The distance of the upper surface of the first microstructure from the substrate is recorded as L1, the distance of the upper surface of the second microstructure from the substrate is recorded as L2, and the L1 is greater than L2.The present application can change the quality factor at resonance wavelength by designing the structure of metasurface, and can also change the wavefront energy distribution at resonance wavelength position, to realize near-infrared waveband local electric field and magnetic field energy separation.

Description

Technical Field

[0001] This invention relates to the field of optical materials technology, and in particular to a metasurface based on an asymmetric dimer unit structure, its preparation method, and its application. Background Technology

[0002] Optical metasurfaces, as planar metasurface materials, achieve precise control over parameters such as amplitude, phase, and polarization of light through the periodic arrangement of subwavelength metallic or dielectric structures on a two-dimensional plane. This controllability breaks through the boundaries of traditional light-matter interactions, providing possibilities for the fabrication of novel optical devices. Optical metasurfaces are artificially designed metasurface materials that can purposefully control light through customized arrangement of subwavelength nanostructures, thereby realizing customized functional metasurfaces, including the separate control of local electric and magnetic field energy, and the enhancement of ultra-high electric fields. In recent years, metasurfaces have become a research hotspot due to their superior electromagnetic field control capabilities, subwavelength-scale local field enhancement, thinness, and integration.

[0003] CN108803088A discloses a transflective-transmitter converter based on metasurface-based optical polarization control, comprising an upper nanodimer array and a lower substrate. The substrate plane is an xy-axis plane, with the x-axis perpendicular to the y-axis. Two semiconductor cylindrical nanostructures of identical material are arranged at a fixed distance to form a nanodimer. The line connecting the centers of the two cylinders in the nanodimer is the direction of the dimer axis, i.e., the x-axis direction. The nanodimer array on the substrate plane is a periodically arranged array of nanodimers along the x and y axes. The ratio of the length of the entire nanodimer array along the y-axis to the length along the x-axis is less than 1. The nanodimer array is arranged along the x and y axes with a fixed period, and the period setting meets the requirements of nanoresonator density. That is, CN108803088A discloses a symmetrical structure. The resonance peaks of symmetrical structures often overlap (e.g., the resonance frequencies of electric dipoles and magnetic dipoles are close), resulting in the inability to effectively separate the electric and magnetic field energies within the target frequency band.

[0004] The main challenge in separating electric and magnetic fields is the inherent coupling between the electric and magnetic fields of light, governed by Maxwell's equations. One current approach involves using mirrors to form standing-wave nodes, effectively suppressing the electric field enhancement to near zero. However, forming these nodes is extremely difficult in practical applications, hindering widespread adoption. Another technique for spatial separation involves generating a laser beam with an azimuth polarization angle. This beam eliminates the electric field along the optical axis while maximizing the axial magnetic field, achieving a separation effect three times greater than the previous method. However, obtaining such a laser beam is costly and technically challenging, limiting its widespread application. Furthermore, a coaxial dielectric tube structure can separate the electric and magnetic fields, coupling anapole states within the tube structure to achieve a magnetic field enhancement factor of 13.8 and an electric field enhancement factor of 4.16. However, this method faces significant fabrication challenges, preventing the production of optical-grade dielectric tube structures and thus limiting its large-scale application.

[0005] In summary, there is a need to develop a metasurface structure and its preparation method. By designing the structure of the metasurface, the electric and magnetic field separation rates of the metasurface can be made high, and the metasurface can be prepared by a low-cost and easy-to-process method. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a metasurface based on an asymmetric dimer unit structure. By designing the structure of the metasurface, the cross-section of the first microstructure parallel to the substrate in the dimer unit is curved, the second microstructure includes at least two isolated sub-microstructures, and the sizes of the first and second microstructures are different, as are the distances between the upper surface of the first microstructure and the substrate and the upper surface of the second microstructure and the substrate. This allows the magnetic field energy and electric field energy to be localized at different microstructure locations, thereby improving the electric and magnetic field separation rate of the metasurface.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a metasurface based on an asymmetric dimer unit structure, the metasurface comprising a substrate and dimer units arranged in an array on the substrate;

[0009] The plane of the substrate is taken as the xy-axis plane, with the x-axis perpendicular to the y-axis; the dimer units are arranged in an array along the x-axis and y-axis;

[0010] The dimer unit includes a first microstructure and a second microstructure;

[0011] The first microstructure and the second microstructure have different shapes and sizes. The cross-section of the first microstructure parallel to the substrate is curved. The second microstructure includes at least two isolated sub-microstructures. The distance between the upper surface of the first microstructure and the substrate is denoted as L1, and the distance between the upper surface of the second microstructure and the substrate is denoted as L2. L1 is greater than L2.

[0012] This invention designs the structure of dimer units in a metasurface by introducing asymmetric first and second microstructures into the dimer unit structure, and creating gaps between the sub-microstructures of the second microstructure to induce bound-states-in-the-continuum (BIC) resonance, which is dominated by electric quadruples (EQ). In this EQ-dominated BIC resonance mode, energy is locally distributed along the dimer unit structure. The asymmetric dimer unit can modulate the light wavefront at the resonance wavelength, thereby achieving the effect of energy localization at different spatial locations. Specifically, because the first and second microstructures have different shapes and sizes, their equivalent refractive indices differ within a dimer unit structure, leading to differences in the localization effect of energy and causing electromagnetic separation. Simultaneously, the presence of sharp corners at the apex of the second microstructure generates a tip effect, allowing electric field energy to accumulate at the sharpest points with higher curvature, while magnetic field energy, more dependent on the toroidal shape, accumulates at the first microstructure, achieving better separation and improving the separation rate. Furthermore, the second microstructure includes at least two isolated sub-microstructures. Due to the gaps between the sub-microstructures, electric field energy is coupled between them, which can improve the effect of local electric field enhancement and further increase the localization effect of electric field energy at the second microstructure, ultimately improving the separation rate. At the same time, the distance between the upper surface of the first microstructure and the substrate is greater than the distance between the upper surface of the second microstructure and the substrate, which can increase the asymmetry of the dimer unit to enhance the separation effect of electric and magnetic fields.

[0013] As a preferred embodiment of the present invention, the dimer unit includes a first microstructure and a second microstructure in the x-axis direction.

[0014] Preferably, the acute angle formed by the line connecting the geometric center of the first microstructure and the geometric center of the second microstructure intersecting the x-axis is 0.4-1.3°, for example, it can be 0.4°, 0.6°, 0.8°, 1.0°, 1.2° or 1.3°, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0015] Preferably, the distance between the geometric center of the first microstructure and the geometric center of the second microstructure is 640-680nm, for example, it can be 640nm, 650nm, 660nm, 670nm or 680nm, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0016] Preferably, the second microstructure comprises three independent sub-microstructures formed by two gaps.

[0017] In this invention, the two gaps between the three sub-microstructures are symmetrically distributed.

[0018] Preferably, the two gaps have the same width.

[0019] Preferably, the width of the gap is 16-20nm, for example, it can be 16nm, 17nm, 18nm, 19nm or 20nm, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0020] The present invention further preferably limits the width of the gap, which can reduce the difficulty of fabricating the sub-microstructure and the gap, reduce the leakage of electric field energy, and enable the electric field energy to be localized in the second microstructure, thereby increasing the electric field and magnetic field separation efficiency of the metasurface.

[0021] Preferably, L1 is 10-30 nm larger than L2, for example, it can be 10 nm, 15 nm, 20 nm, 25 nm or 30 nm, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0022] The present invention further preferably limits the difference between the upper surface of the first microstructure and the second microstructure and the substrate, so that in the EQ-dominated BIC resonance mode, the electric field energy is locally distributed along the dimer unit structure to enhance the electric and magnetic field separation efficiency of the metasurface. This not only has a high BIC resonance effect, but also avoids the multipole generated by the dominant dimer unit from transforming into two opposite electric dipoles. Therefore, the energy distributed locally along the dimer unit structure will not be guided to the two ends of the dimer unit, thus maintaining a high separation efficiency.

[0023] As a preferred embodiment of the present invention, on the substrate, the number of dimer units arranged in the x-axis direction is equal to the number of dimer units arranged in the y-axis direction.

[0024] Preferably, in the x-axis direction, the number of dimer units arranged on the substrate is 30-50, for example, 30, 35, 40, 45 or 50, but not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0025] Preferably, the period of the dimer unit in the x-axis direction is the same as the period of the dimer unit in the y-axis direction.

[0026] Preferably, the period of the dimer unit in the x-axis direction is 1280-1360nm, for example, it can be 1280nm, 1300nm, 1320nm, 1340nm or 1360nm, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0027] As a preferred embodiment of the present invention, the sub-microstructure has a sharp angle in its cross-section parallel to the substrate.

[0028] In this invention, the sharp angle is the angle formed by the intersection of two lines.

[0029] Preferably, the shape of the first microstructure includes an elliptical cylinder.

[0030] Preferably, the contour shape formed by the sub-microstructure and the gap in the second microstructure is a hexagonal prism.

[0031] The present invention further preferably uses an elliptical cylinder for the shape of the first microstructure and a hexagonal cylinder for the contour formed by the sub-microstructure and the gap in the second structure. This is because a hexagonal cylinder has sharp vertices, which can produce a tip effect. Electric field energy tends to concentrate in regions with higher curvature. Therefore, a hexagonal cylinder with sharp vertices can achieve greater focusing of electric field energy in space. Magnetic field energy tends to focus near relatively ring-shaped structures. Therefore, designing the first microstructure as an elliptical cylinder eliminates the sharp vertices that "compete" with the second microstructure for energy. Furthermore, compared to a cylindrical structure that also lacks sharp vertices, the elliptical cylinder structure, due to the difference between its major and minor axes, tends to have an outer ring-shaped induced current in the plane perpendicular to the substrate. This results in a much higher ability to focus magnetic field energy along its own major axis in the plane than a cylindrical structure. Therefore, by limiting the shape of the first microstructure and the shape of the contour formed by the sub-microstructure and the gap in the second microstructure, the present invention can achieve separation of electric and magnetic field energy in space, so that magnetic field energy is mainly localized at the location of the elliptical cylinder and electric field energy is mainly localized at the location of the hexagonal cylinder, thereby improving the electric and magnetic field separation rate of the metasurface.

[0032] Preferably, the acute angle formed by the intersection of the line containing the semi-major axis of the elliptical cylinder and the y-axis is denoted as θ1, and the acute angle formed by the intersection of the line containing the major axis of symmetry of the hexagonal prism and the y-axis is denoted as θ2, where θ1 and θ2 are the same; and the extension of the semi-major axis of the elliptical cylinder intersects the extension of the major axis of symmetry of the hexagonal prism.

[0033] Preferably, the acute angle formed by the intersection of the line containing the semi-major axis of the elliptical cylinder and the y-axis is 20-30°, for example, it can be 20°, 22°, 24°, 26°, 28° or 30°, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0034] The present invention further preferably defines the acute angle formed by the intersection of the line containing the semi-major axis of the elliptical cylinder and the y-axis and the acute angle formed by the intersection of the line containing the major axis of symmetry of the hexagonal cylinder and the y-axis, so as to create a difference in the local energy capacity of the first microstructure and the second microstructure, thereby increasing the quality factor at the resonance wavelength and changing the wavefront energy distribution at the resonance wavelength position, so as to achieve local electric and magnetic field energy separation in the near-infrared band.

[0035] As a preferred technical solution of the present invention, the semi-major axis of the elliptical cylinder is 300-400nm, for example, it can be 300nm, 320nm, 340nm, 360nm, 380nm or 400nm, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0036] Preferably, the semi-minor axis of the elliptical cylinder is 100-120nm, for example, it can be 100nm, 105nm, 110nm, 115nm or 120nm, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0037] Preferably, L1 is 220-240nm, for example, it can be 220nm, 225nm, 230nm, 235nm or 240nm, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0038] As a preferred technical solution of the present invention, the long axis of symmetry of the hexagonal prism is 320-340nm, for example, it can be 320nm, 325nm, 330nm, 335nm or 340nm, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0039] Preferably, the short axis of symmetry of the hexagonal prism is 120-140nm, for example, it can be 120nm, 125nm, 130nm, 135nm or 140nm, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0040] Preferably, L2 is 210-230nm, for example, it can be 210nm, 215nm, 220nm, 225nm or 230nm, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0041] As a preferred embodiment of the present invention, the substrate material includes silicon-on-insulator (SOI).

[0042] The present invention further optimizes the material of the substrate, which can significantly improve the electric and magnetic field separation rate of the metasurface. This is because the top silicon layer of SOI is monocrystalline silicon with a crystal orientation of

[100] . Compared with amorphous silicon, monocrystalline silicon has a lower k-value in the near-infrared band, less absorption of electromagnetic waves, and can utilize more energy, thereby achieving a higher separation rate.

[0043] Preferably, the material of the dimer unit includes any one or a combination of at least two of silicon nitride, amorphous silicon, or SOI, wherein typical but non-limiting combinations include: a combination of silicon nitride and amorphous silicon, a combination of silicon nitride and SOI, a combination of amorphous silicon and SOI, a combination of silicon nitride, amorphous silicon, and SOI, preferably SOI.

[0044] Preferably, the substrate comprises a base silicon, an insulating silicon oxide layer, and a top silicon layer stacked sequentially.

[0045] Preferably, the top silicon layer is located close to the dimer units distributed in the array.

[0046] Preferably, the thickness of the top silicon layer is ≥240nm, for example, it can be 240nm, 245nm, 250nm, 255nm or 260nm, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0047] In a second aspect, the present invention provides a method for preparing the metasurface according to the first aspect, the method comprising the following steps:

[0048] (1) A metal film mark is formed on the surface of the substrate to obtain a substrate with a metal film mark;

[0049] (2) A groove is formed on the surface of the substrate with metal film markings as described in step (1), the position of the groove being the same as the position of the second microstructure, to obtain a substrate with grooves;

[0050] (3) A dimer unit is formed on the substrate surface with grooves described in step (2) to obtain the metasurface.

[0051] This invention achieves the fabrication of metasurface structures through electron beam exposure and plasma etching processes. The fabrication conditions are simple, fast, convenient, and low-cost, and the sample size is adjustable, enabling large-area, large-scale production.

[0052] As a preferred technical solution of the present invention, the step of forming the metal film mark in step (1) includes obtaining a substrate containing the first photoresist mark by using a first photoresist and a first electron beam exposure process, sputtering a metal film on the surface of the substrate containing the first photoresist mark, and using a lift-off process to peel the metal film off from the first photoresist to obtain a substrate with the metal film mark.

[0053] Preferably, step (2) of forming the groove includes obtaining a first adhesive layer with a first pattern on the surface of the substrate with metal film markings using a second photoresist and a second electron beam exposure process. The first pattern has a groove, and the position of the groove in the first adhesive layer is the same as the position of the second microstructure. Plasma is used to perform a first etching on the substrate exposed at the groove portion of the first adhesive layer to form a groove structure corresponding to the position of the second microstructure. The second photoresist is then removed to obtain a substrate with a groove.

[0054] Preferably, step (3) of forming the dimer unit includes forming a second adhesive layer with a second pattern on the surface of the substrate with grooves using a third photoresist and a third electron beam exposure process. The second pattern has grooves, and the position of the grooves in the second adhesive layer is the same as the position of the first microstructure and the second microstructure. Plasma is used to perform a second etching on the substrate exposed at the groove portion of the second adhesive layer, and the third photoresist is removed to obtain the metasurface with the dimer unit structure.

[0055] Preferably, the preparation method further includes cleaning the substrate before step (1).

[0056] Preferably, the cleaning process includes ultrasonic cleaning.

[0057] Preferably, the solution used for the cleaning process includes acetone and / or isopropanol.

[0058] Preferably, the sputtering process in step (1) includes magnetron sputtering and / or electron beam evaporation.

[0059] Preferably, before sputtering the metal film in step (1), the substrate containing the first photoresist mark is further immersed in a first developing solution.

[0060] Preferably, step (2) further includes immersing the substrate in a second developer and a first fixer sequentially between the second electron beam exposure process and the first etching process.

[0061] Preferably, step (3) further includes immersing the substrate in a third developer and a second fixer sequentially between the third electron beam exposure process and the second etching.

[0062] Preferably, the material of the metal film includes aluminum and / or gold.

[0063] Preferably, the first photoresist comprises polymethyl methacrylate (PMMA) and / or ultraviolet photoresist.

[0064] Preferably, the second photoresist and the third photoresist each independently comprise electron beam positive photoresist and / or electron beam negative photoresist.

[0065] Preferably, the first developer comprises isopropanol and / or an alkaline solution containing tetramethylammonium hydroxide.

[0066] Preferably, the second and third developing solutions each independently comprise any one or a combination of at least two of tetramethylammonium hydroxide, n-hexylbenzene, or amyl acetate, wherein typical but non-limiting combinations include: a combination of tetramethylammonium hydroxide and n-hexylbenzene, a combination of tetramethylammonium hydroxide and amyl acetate, a combination of n-hexylbenzene and amyl acetate, and a combination of tetramethylammonium hydroxide, n-hexylbenzene, and amyl acetate.

[0067] Preferably, the first fixing solution and the second fixing solution each independently comprise deionized water and / or isopropanol.

[0068] Preferably, the atmospheres for the first etching and the second etching each independently comprise a mixture of sulfur hexafluoride and oxygen.

[0069] Preferably, the volume ratio of sulfur hexafluoride to oxygen in the mixed gas is (3-4):1, for example, it can be 3:1, 3.2:1, 3.4:1, 3.6:1, 3.8:1 or 4:1, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0070] Preferably, the etching depth of the first etching in step (2) is 10-30nm, for example, it can be 10nm, 15nm, 20nm, 25nm or 30nm, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0071] Preferably, the etching depth of the second etching in step (3) is 220-240nm, for example, it can be 220nm, 225nm, 230nm, 235nm or 240nm, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0072] Preferably, the preparation method includes the following steps:

[0073] (1) Clean the substrate;

[0074] (2) Coating the surface of the substrate with a first photoresist to make overlay marks, transferring the overlay marks onto the first photoresist using a first electron beam exposure process to obtain a substrate containing the first photoresist marks, and immersing the substrate containing the first photoresist marks in a first developing solution;

[0075] (3) Sputter a metal film onto the surface of the substrate containing the first photoresist mark, and use a lift-off process to peel the metal film off from the first photoresist to obtain a substrate with a metal film mark;

[0076] (4) A second photoresist is coated on the surface of the substrate, and a first adhesive layer with a first pattern is obtained on the surface of the substrate with metal film markings using a second electron beam exposure process. The first pattern has grooves, and the position of the grooves in the first adhesive layer is the same as the position of the second microstructure. The substrate is then immersed in a second developer and a first fixer in sequence. The substrate exposed by the grooves in the first adhesive layer is then etched by plasma to form a groove structure with a depth of 10-30 nm corresponding to the position of the second microstructure. The second photoresist is then removed to obtain a substrate with grooves.

[0077] (5) The third photoresist is coated on the surface of the substrate, and a second adhesive layer with a second pattern is formed on the surface of the substrate with grooves using a third electron beam exposure process. The second pattern has grooves, and the position of the grooves in the second adhesive layer is the same as the position of the first microstructure and the second microstructure. The substrate is then immersed in the third developer and the second fixer in sequence. The substrate exposed by the grooves in the second adhesive layer is then etched by plasma to remove the third photoresist, thereby obtaining the metasurface with a dimer unit structure.

[0078] Thirdly, the present invention provides an application of the metasurface described in the first aspect in biomedical imaging, magnetic particle detection, or magneto-optical spectroscopy.

[0079] The metasurface in this invention has an ultra-high separation rate in electric and magnetic field separation, which can enhance the magnetic field response in the interaction between light and matter and suppress the electric field response, thus significantly improving the quality of magnetic resonance imaging. It has broad application prospects in biomedical imaging, magnetic particle detection, or magneto-optical spectroscopy.

[0080] Compared with the prior art, the present invention has at least the following beneficial effects:

[0081] (1) The present invention designs the structure of the metasurface so that the shape and size of the first microstructure and the second microstructure in the dimer unit are different, so that the magnetic field energy is mainly localized at the position of the first microstructure and the electric field energy is mainly localized at the position of the second microstructure, thereby improving the electric field and magnetic field separation rate of the metasurface and making the electric field separation rate reach more than 4.9 times and the magnetic field separation rate reach more than 189 times.

[0082] (2) The present invention realizes the preparation of metasurface structure through electron beam exposure and plasma etching process. The preparation conditions are simple, fast and convenient, and low in cost, and can realize large-area, large-scale production preparation. Attached Figure Description

[0083] Figure 1 This is a schematic diagram of the metasurface structure provided in Embodiment 1 of the present invention.

[0084] Figure 2 This is the present invention. Figure 1 A three-dimensional schematic diagram of the dimer unit within the dashed box.

[0085] Figure 3 This is the present invention. Figure 1 Top view of the dimer unit within the dashed box.

[0086] Figure 4 This is the present invention. Figure 1 Front view of the dimer unit within the dashed box.

[0087] Figure 5 This is a schematic diagram of the structure of the substrate provided in Embodiment 1 of the present invention.

[0088] Figure 6 This is a front view of a substrate coated with a first photoresist during the metasurface preparation process provided in Embodiment 1 of the present invention.

[0089] Figure 7 This is a front view of the substrate after immersion in isopropanol developer during the metasurface preparation process provided in Embodiment 1 of the present invention.

[0090] Figure 8 This is a front view of the substrate coated with an aluminum film during the metasurface preparation process provided in Embodiment 1 of the present invention.

[0091] Figure 9This is a front view of a substrate with aluminum film markings during the metasurface preparation process provided in Embodiment 1 of the present invention.

[0092] Figure 10 This is a front view of a substrate coated with a second photoresist during the metasurface preparation process provided in Embodiment 1 of the present invention.

[0093] Figure 11 This is a front view of the substrate after immersion in isopropanol, the first fixing solution, during the metasurface preparation process provided in Embodiment 1 of the present invention.

[0094] Figure 12 This is a front view of the substrate with grooves obtained after the first etching during the metasurface preparation process provided in Embodiment 1 of the present invention.

[0095] Figure 13 This is a front view of the substrate with grooves obtained after removing the second photoresist during the metasurface preparation process provided in Embodiment 1 of the present invention.

[0096] Figure 14 This is a front view of a substrate coated with a third photoresist during the metasurface preparation process provided in Embodiment 1 of the present invention.

[0097] Figure 15 This is a front view of the substrate after immersion in deionized water as the second fixing solution during the metasurface preparation process provided in Embodiment 1 of the present invention.

[0098] Figure 16 This is a front view of the substrate with grooves obtained after the second etching during the metasurface preparation process provided in Embodiment 1 of the present invention.

[0099] Figure 17 This is a front view of the metasurface with a dimer unit structure obtained after removing the third photoresist during the metasurface preparation process provided in Embodiment 1 of the present invention.

[0100] Figure 18 This is a physical image of the metasurface provided in Embodiment 1 of the present invention.

[0101] Figure 19 This is a SEM image of the metasurface provided in Embodiment 1 of the present invention.

[0102] Figure 20 These are the transmittance test diagrams of the metasurfaces provided in Embodiments 1, 6, and 7 of this invention.

[0103] Figure 21 This is a schematic diagram illustrating the separation rate as a function of the gap width w in this invention.

[0104] Figure 22 This is a schematic diagram illustrating how the optical performance varies with angles θ1 and θ2 in this invention.

[0105] Wherein, 1-first microstructure; 2-second microstructure; 3-substrate silicon; 4-insulating layer silicon oxide; 5-top silicon; 6-PMMA layer; 7-aluminum film layer; 8-electron beam positive photoresist layer; 9-electron beam negative photoresist layer; L1-distance between the upper surface of the elliptical cylinder and the substrate; L2-distance between the upper surface of the hexagonal cylinder and the substrate; Px-period of the dimer unit in the x-axis direction; Py-period of the dimer unit in the y-axis direction; w-width of the gap; d - The distance between the geometric centers of the elliptical cylinder and the hexagonal cylinder; θ1 - The acute angle formed by the intersection of the line containing the semi-major axis of the elliptical cylinder and the y-axis; θ2 - The acute angle formed by the intersection of the line containing the semi-major axis of the hexagonal cylinder and the y-axis; a1 - The semi-major axis of the elliptical cylinder; b1 - The semi-minor axis of the elliptical cylinder; a2 - The major axis of symmetry of the hexagonal cylinder; b2 - The minor axis of symmetry of the hexagonal cylinder; s - The ratio of the size difference between the first microstructure and the second microstructure, s=2×a1 / a2. Detailed Implementation

[0106] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments. However, the following examples are merely simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention is determined by the claims.

[0107] Example 1

[0108] This embodiment provides a metasurface based on an asymmetric dimer unit structure, the structure of which is as follows: Figure 1 As shown, it includes a substrate and dimer units; the plane of the substrate is the xy-axis plane, with the x-axis perpendicular to the y-axis; the dimer units are arrayed along the x-axis and y-axis, and the number of dimer units arrayed on the substrate in both the x-axis and y-axis directions is 40.

[0109] The substrate and the dimer units are both made of SOI. The substrate includes a substrate silicon 3, an insulating silicon oxide layer 4 and a top silicon layer 5 stacked in sequence. The top silicon layer is close to the arrayed dimer units and has a thickness of 250 nm.

[0110] The structural schematic diagram of the dimer unit is shown below. Figure 2 As shown, the dimer unit includes a first microstructure 1 and a second microstructure 2 in the x-axis direction, both of which are elliptical cylinders. The first microstructure is elliptical cylinders, and the distance L1 between the upper surface of the elliptical cylinder and the substrate is 230 nm. The second microstructure 2 includes three independent sub-microstructures formed by two gaps. The contour shape formed by the three sub-microstructures and the gaps is a hexagonal cylinder, and the distance L2 between the upper surface of the hexagonal cylinder and the substrate is 220 nm.

[0111] The top view and front view of the dimer unit are respectively as follows: Figure 3 and Figure 4 As shown, the period Px of the dimer unit in the x-axis direction and the period Py in the y-axis direction are both 1300 nm. The distance d between the geometric centers of the elliptical cylinder and the hexagonal cylinder is 660 nm, and the acute angle formed by the line connecting their geometric centers and the x-axis is 0.43°. The semi-major axis a1 of the elliptical cylinder is 350 nm, and the semi-minor axis b1 is 110 nm. The major axis of symmetry a2 of the hexagonal cylinder is 330 nm, and the minor axis of symmetry b2 is... The size difference between the first microstructure and the second microstructure is 130nm, and the gap width is 20nm. That is, the ratio of the size difference between the first microstructure and the second microstructure is s = 2 × a1 / a2 = 2.12. The acute angle formed by the intersection of the line containing the semi-major axis of the elliptical cylinder and the y-axis is denoted as θ1, and the acute angle formed by the intersection of the line containing the major axis of symmetry of the hexagonal cylinder and the y-axis is denoted as θ2. Both θ1 and θ2 are 25°. The extension of the semi-major axis of the elliptical cylinder intersects the extension of the major axis of symmetry of the hexagonal cylinder.

[0112] This embodiment also provides a method for preparing the metasurface, the method comprising the following steps:

[0113] (1) The substrate was ultrasonically cleaned in acetone for 25 minutes to obtain the following result: Figure 5 The substrate shown;

[0114] (2) A PMMA layer 6 is coated on the surface of the substrate for overlay marking, resulting in the following: Figure 6 The substrate shown was subjected to a first electron beam lithography process to transfer overlay marks onto PMMA, resulting in a PMMA-labeled substrate. The PMMA-labeled substrate was then immersed in isopropanol to obtain the desired product. Figure 7 The substrate shown;

[0115] (3) An aluminum film layer 7 is sputtered onto the surface of the substrate containing the PMMA mark using a magnetron sputtering process to obtain the following: Figure 8 The substrate shown is prepared by using a lift-off process to peel the aluminum film from the PMMA, resulting in the following: Figure 9 The substrate shown has aluminum foil markings;

[0116] (4) An electron beam positive photoresist (ZEP) layer 8 is coated on the surface of the substrate to obtain the following: Figure 10 The substrate shown is subjected to a second electron beam lithography process to obtain a first adhesive layer with a first pattern on the surface of the substrate marked with aluminum film. The first pattern contains grooves, and the positions of the grooves in the first adhesive layer are the same as the positions of the second microstructures. The substrate is then sequentially immersed in n-hexylbenzene for 30 seconds and isopropanol for 1 minute to obtain the following result. Figure 11The substrate shown is then subjected to a first etching process using plasma to etch the exposed groove portion of the first adhesive layer, forming a groove structure with a depth of 10 nm corresponding to the position of the second microstructure, resulting in the following... Figure 12 The substrate shown is subjected to electron beam positive photoresist removal to obtain the following result: Figure 13 The base shown has a groove;

[0117] (5) An electron beam negative photoresist (Hydrogen Silsesquioxane, HSQ) layer 9 is coated on the surface of the substrate to obtain the following: Figure 14 The substrate shown is subjected to a third electron beam lithography process to form a second adhesive layer with a second pattern on the surface of the substrate with grooves. The second pattern contains grooves, and the positions of the grooves in the second adhesive layer are the same as the positions of the first and second microstructures. The substrate is then sequentially immersed in tetramethylammonium hydroxide for 30 seconds and deionized water for 1 minute to obtain the following result. Figure 15 The substrate shown is then subjected to a second etching process using plasma to expose the grooves in the second adhesive layer. The depth of the second etching is 230 nm, resulting in the substrate as shown. Figure 16 The substrate shown is used, and the electron beam negative photoresist is removed to obtain the following: Figure 17 The metasurface shown has a dimer unit structure; a physical image of the metasurface is shown below. Figure 18 As shown, the SEM image of the metasurface is as follows: Figure 19 As shown.

[0118] The atmosphere for both the first and second etching processes is a mixture of sulfur hexafluoride and oxygen in a volume ratio of 3.5:1.

[0119] Example 2

[0120] This embodiment provides a metasurface based on an asymmetric dimer unit structure. The metasurface includes a substrate and dimer units. The plane of the substrate is the xy-axis plane, with the x-axis perpendicular to the y-axis. The dimer units are arrayed along the x-axis and y-axis. On the substrate, the number of dimer units arrayed in both the x-axis and y-axis directions is 50.

[0121] The substrate is made of SOI, the dimer unit is made of silicon nitride, and the substrate includes a substrate silicon, an insulating silicon oxide layer and a top silicon layer stacked in sequence. The top silicon layer is close to the arrayed dimer units and has a thickness of 240 nm.

[0122] The dimer unit includes a first microstructure and a second microstructure in the x-axis direction, both of which are elliptical cylinders. The first microstructure is elliptical cylinders, and the distance L1 between the upper surface of the elliptical cylinder and the substrate is 240 nm. The second microstructure includes three independent sub-microstructures formed by two gaps. The contour shape formed by the three sub-microstructures and the gaps is a hexagonal cylinder, and the distance L2 between the upper surface of the hexagonal cylinder and the substrate is 210 nm.

[0123] The period of the dimer unit in the x-axis direction and the period in the y-axis direction are both 1280nm. The distance between the geometric centers of the elliptical cylinder and the hexagonal cylinder is 640nm, and the acute angle formed by the line connecting the two geometric centers and the x-axis is 1.3°. The semi-major axis of the elliptical cylinder is 300nm and the semi-minor axis is 100nm. The major axis of symmetry of the hexagonal cylinder is 320nm, the minor axis of symmetry is 120nm, and the width of the gap is 16nm. That is, the ratio of the size difference between the first microstructure and the second microstructure is s = 2 × a1 / a2 = 1.88. The acute angle formed by the line containing the semi-major axis of the elliptical cylinder and the y-axis is denoted as θ1, and the acute angle formed by the line containing the major axis of symmetry of the hexagonal cylinder and the y-axis is denoted as θ2. Both θ1 and θ2 are 30°, and the extension of the semi-major axis of the elliptical cylinder intersects the extension of the major axis of symmetry of the hexagonal cylinder.

[0124] This embodiment also provides a method for preparing the metasurface, the method comprising the following steps:

[0125] (1) Place the substrate in isopropanol for ultrasonic cleaning for 20 min;

[0126] (2) Coating ultraviolet photoresist (S1813) on the surface of the substrate to make overlay marks, transferring the overlay marks onto the ultraviolet photoresist using the first electron beam exposure process to obtain a substrate containing ultraviolet photoresist marks, and immersing the substrate containing ultraviolet photoresist marks in an alkaline solution (NMD developer) containing tetramethylammonium hydroxide.

[0127] (3) A gold film is sputtered on the surface of the substrate containing ultraviolet photoresist markings using an electron beam evaporation process, and the gold film is peeled off from the ultraviolet photoresist using a lift-off process to obtain a substrate with gold film markings.

[0128] (4) Electron beam negative photoresist (HSQ) is coated on the surface of the substrate. A first adhesive layer with a first pattern is obtained on the surface of the substrate with gold film marking using a second electron beam exposure process. The first pattern has grooves. The position of the grooves in the first adhesive layer is the same as the position of the second microstructure. The substrate is then immersed in tetramethylammonium hydroxide for 30s and deionized water for 1min in sequence. The substrate exposed by the grooves in the first adhesive layer is then etched by plasma to form a groove structure with a depth of 30nm corresponding to the position of the second microstructure. The electron beam negative photoresist is removed to obtain a substrate with grooves.

[0129] (5) Electron beam positive photoresist (ZEP) is coated on the surface of the substrate. A second photoresist layer with a second pattern is formed on the surface of the substrate with grooves using a third electron beam exposure process. The second pattern has grooves. The position of the grooves in the second photoresist layer is the same as the position of the first microstructure and the second microstructure. The substrate is then immersed in amyl acetate for 30 seconds and isopropanol for 1 minute. The substrate exposed by the grooves in the second photoresist layer is then etched by plasma. The depth of the second etching is 240 nm. The electron beam positive photoresist is then removed to obtain the metasurface with a dimer unit structure.

[0130] The atmosphere for both the first and second etching processes is a mixture of sulfur hexafluoride and oxygen in a volume ratio of 3:1.

[0131] Example 3

[0132] This embodiment provides a metasurface based on an asymmetric dimer unit structure. The metasurface includes a substrate and dimer units. The plane of the substrate is the xy-axis plane, with the x-axis perpendicular to the y-axis. The dimer units are arrayed along the x-axis and y-axis. On the substrate, the number of dimer units arrayed in both the x-axis and y-axis directions is 30.

[0133] The substrate is made of SOI, the dimer unit is made of amorphous silicon, and the substrate includes a substrate silicon, an insulating silicon oxide layer and a top silicon layer stacked sequentially. The top silicon layer is close to the arrayed dimer units and has a thickness of 260 nm.

[0134] The dimer unit includes a first microstructure and a second microstructure in the x-axis direction, both of which are elliptical cylinders. The first microstructure is elliptical cylinders, and the distance L1 between the upper surface of the elliptical cylinder and the substrate is 220 nm. The second microstructure includes three independent sub-microstructures formed by two gaps. The contour shape formed by the three sub-microstructures and the gaps is a hexagonal cylinder, and the distance L2 between the upper surface of the hexagonal cylinder and the substrate is 210 nm.

[0135] The period of the dimer unit in the x-axis direction and the period in the y-axis direction are both 1360nm. The distance between the geometric centers of the elliptical cylinder and the hexagonal cylinder is 680nm, and the acute angle formed by the line connecting the two geometric centers and the x-axis is 0.4°. The semi-major axis of the elliptical cylinder is 400nm and the semi-minor axis is 120nm. The major axis of symmetry of the hexagonal cylinder is 340nm, the minor axis of symmetry is 140nm, and the width of the gap is 18nm. That is, the ratio of the size difference between the first microstructure and the second microstructure is s = 2 × a1 / a2 = 2.35. The acute angle formed by the line containing the semi-major axis of the elliptical cylinder and the y-axis is denoted as θ1, and the acute angle formed by the line containing the major axis of symmetry of the hexagonal cylinder and the y-axis is denoted as θ2. Both θ1 and θ2 are 20°, and the extension of the semi-major axis of the elliptical cylinder intersects the extension of the major axis of symmetry of the hexagonal cylinder.

[0136] This embodiment also provides a method for preparing the metasurface, the method comprising the following steps:

[0137] (1) Place the substrate in isopropanol for ultrasonic cleaning for 30 min;

[0138] (2) PMMA is coated on the surface of the substrate for overlay marking, and the overlay marking is transferred to PMMA using a first electron beam exposure process to obtain a substrate containing PMMA marking, and the substrate containing PMMA marking is immersed in isopropanol;

[0139] (3) Aluminum film is sputtered onto the surface of the substrate containing PMMA markings using magnetron sputtering process, and the aluminum film is peeled off from the PMMA using lift-off process to obtain a substrate with aluminum film markings;

[0140] (4) Electron beam positive photoresist (ZEP) is coated on the surface of the substrate. A first adhesive layer with a first pattern is obtained on the surface of the substrate with aluminum film markings using a second electron beam exposure process. The first pattern has grooves. The position of the grooves in the first adhesive layer is the same as the position of the second microstructure. The substrate is then immersed in n-hexylbenzene for 30s and isopropanol for 1min in sequence. The substrate exposed by the grooves in the first adhesive layer is then etched by plasma to form a groove structure with a depth of 10nm corresponding to the position of the second microstructure. The electron beam positive photoresist is removed to obtain a substrate with grooves.

[0141] (5) The electron beam positive photoresist is coated on the surface of the substrate, and a second adhesive layer with a second pattern is formed on the surface of the substrate with grooves using a third electron beam exposure process. The second pattern has grooves, and the position of the grooves in the second adhesive layer is the same as the position of the first microstructure and the second microstructure. The substrate is then immersed in amyl acetate for 30s and isopropanol for 1min in sequence. The substrate exposed by the grooves in the second adhesive layer is then etched by plasma. The depth of the second etching is 220nm, and the electron beam positive photoresist is removed to obtain the metasurface with a dimer unit structure.

[0142] The atmosphere for both the first and second etching processes is a mixture of sulfur hexafluoride and oxygen in a volume ratio of 4:1.

[0143] Example 4

[0144] This embodiment provides a metasurface based on an asymmetric dimer unit structure. The only difference from Embodiment 1 is that the substrate material is replaced with silicon dioxide instead of SOI.

[0145] Example 5

[0146] This embodiment provides a metasurface based on an asymmetric dimer unit structure. The only difference from Embodiment 1 is that the distance L1 between the upper surface of the elliptical cylinder and the substrate is adjusted to 260 nm, that is, the distance L1 between the upper surface of the elliptical cylinder and the substrate is 40 nm greater than the distance L2 between the upper surface of the hexagonal cylinder and the substrate. All other aspects are the same as in Embodiment 1.

[0147] Example 6

[0148] This embodiment provides a metasurface based on an asymmetric dimer unit structure. The only difference from Embodiment 1 is that the width of the gap between the three sub-microstructures is adjusted from 20 nm to 30 nm, while the rest is the same as Embodiment 1.

[0149] Example 7

[0150] This embodiment provides a metasurface based on an asymmetric dimer unit structure. The only difference from Embodiment 1 is that the width of the gap between the three sub-microstructures is adjusted from 20 nm to 11 nm, while the rest is the same as Embodiment 1.

[0151] The transmittance of the metasurfaces in Examples 1, 6, and 7 was tested, and the results are as follows: Figure 20 As shown.

[0152] Meanwhile, to better understand the impact of gap width on metasurface transmittance, metasurfaces with gap widths of 5 nm, 8 nm, 16 nm, 25 nm, and 35 nm were prepared based on Examples 1, 6, and 7, and their separation rates were tested. The results are as follows: Figure 21 As shown.

[0153] Figure 20 and Figure 21 The effects of the gap width w on the transmission spectrum and separation efficiency of the structure are described respectively. Figure 20 As shown, changing the value of w within the provided range allows for fine-tuning of the resonance position, but the overall linewidth of the curve remains essentially unchanged. Therefore, the overall Q-factor changes smoothly, and the energy binding efficiency remains at a high level. Under the same plane wave excitation, the total energy that can be bound in the structure remains essentially unchanged. However, changing the value of w alters the coupling effect between the three sub-microstructures of the second microstructure. This leads to a change in the local electric field capability of the second microstructure, thereby affecting the separation rate of the electric and magnetic fields in the structure's spatial domain. Figure 21 As shown, when the gap is too large, the coupling effect between the sub-microstructures is weakened, and the effect of improving the localization of electric field energy cannot be achieved. The magnetic field energy is not strongly dependent on the first microstructure, which leads to a significant reduction in the separation rate of electric and magnetic fields in the structure space. The highest separation rate can be obtained at approximately w = 20 nm.

[0154] Example 8

[0155] This embodiment provides a metasurface based on an asymmetric dimer unit structure. The only difference from Embodiment 1 is that the acute angle θ1 formed by the intersection of the line containing the semi-major axis of the elliptical cylinder and the y-axis and the acute angle θ2 formed by the intersection of the line containing the major axis of symmetry of the hexagonal cylinder and the y-axis are both adjusted to 10°. All other aspects are the same as in Embodiment 1.

[0156] To better investigate the effect of the acute angle formed by the intersection of the line containing the semi-major axis of the elliptical cylinder and the y-axis on the optical properties of metasurfaces, metasurfaces with θ1=θ2=0°, 20°, and 30° were prepared based on Examples 1 and 8, respectively. Their optical properties were then tested, and the results were fitted to obtain... Figure 22 .

[0157] from Figure 22 It can be seen that when the angles are θ1 = θ2 = 0°, the structure's transmission spectrum exhibits a symmetric protected BIC, at which point the resonance linewidth is zero and cannot be observed. When the angles are not 0°, the original symmetric protected BIC couples with the far-field radiation mode, and the resulting energy leakage is transformed into an observable quasi-BIC, meaning the resonance linewidth is no longer zero. This indicates that angles θ1 and θ2 are the primary causes of this resonance.

[0158] Example 9

[0159] This embodiment provides a metasurface based on an asymmetric dimer unit structure. The only difference from Embodiment 1 is that the acute angle θ1 formed by the intersection of the line containing the semi-major axis of the elliptical cylinder and the y-axis and the acute angle θ2 formed by the intersection of the line containing the major axis of symmetry of the hexagonal cylinder and the y-axis are both adjusted to 40°. All other aspects are the same as in Embodiment 1.

[0160] Example 10

[0161] This embodiment provides a metasurface based on an asymmetric dimer unit structure. The only difference from Embodiment 1 is that the shape of the first microstructure is cylindrical, the distance between the upper surface of the cylinder and the substrate is 230 nm, and the radius of the cylinder is 110 nm. All other aspects are the same as in Embodiment 1.

[0162] Comparative Example 1

[0163] This comparative example provides a metasurface based on an asymmetric dimer unit structure. The only difference between this example and Example 1 is that, except that the shape and size of the second microstructure are the same as those of the first microstructure, everything else is the same as in Example 1.

[0164] This comparative example also provides a method for preparing the metasurface, which differs from Example 1 only in that the preparation method does not include step (4), but is otherwise the same as Example 1.

[0165] Comparative Example 2

[0166] This comparative example provides a metasurface based on an asymmetric dimer unit structure. The only difference between this example and Example 1 is that, except that the shape and size of the first microstructure are the same as those of the second microstructure, everything else is the same as in Example 1.

[0167] This comparative example also provides a method for preparing the metasurface, which differs from Example 1 only in that the preparation method does not include step (4), but is otherwise the same as Example 1.

[0168] Comparative Example 3

[0169] This comparative example provides a metasurface based on an asymmetric dimer unit structure. The only difference from Example 1 is that, except that the sub-microstructures of the second microstructure are arranged continuously, i.e. there are no gaps between the sub-microstructures, everything else is the same as Example 1.

[0170] Comparative Example 4

[0171] This comparative example provides a metasurface based on an asymmetric dimer unit structure. The only difference from Example 1 is that the second microstructure is cylindrical with a diameter of 330 nm and the distance L2 between the upper surface of the cylinder and the substrate is 220 nm. All other aspects are the same as in Example 1.

[0172] Comparative Example 5

[0173] This comparative example provides a metasurface based on an asymmetric dimer unit structure. The only difference from Example 1 is that the distance L2 between the upper surface of the hexagonal prism and the substrate is adjusted to 230 nm, that is, the distance L1 between the upper surface of the elliptical prism and the substrate and the distance L2 between the upper surface of the hexagonal prism and the substrate are equal. All other aspects are the same as in Example 1.

[0174] The electric and magnetic field intensity distributions on the metasurface were measured using a scanning near-field optical microscope (SNOM) under normal incidence of a plane wave with wavelength λ. The electric and magnetic field information of the metasurface at the first and second microstructures under the incident wavelength λ was collected separately, and the magnitudes of the electric and magnetic fields at the first and second microstructures were compared. The separation rate was calculated using Emax / Emin and Hmax / Hmin, where Emax is the maximum local electric field enhancement factor, Emin is the minimum local electric field enhancement factor, Hmax is the maximum local magnetic field enhancement factor, and Hmin is the minimum local magnetic field enhancement factor. The separation rate data are shown in Table 1.

[0175] Table 1

[0176]

[0177]

[0178] The test results show that:

[0179] (1) As can be seen from Examples 1 to 3, the present invention designs the structure of the metasurface so that the shape and size of the first microstructure and the second microstructure in the dimer unit are different, so that the magnetic field energy is mainly localized in the position of the first microstructure and the electric field energy is mainly localized in the position of the second microstructure, thereby improving the electric field and magnetic field separation rate of the metasurface and making the electric field separation rate reach more than 4.9 times and the magnetic field separation rate reach more than 189 times.

[0180] (2) As can be seen from Examples 1 and 4, the substrate material in the metasurface of Example 1 is SOI, and its Emax / Emin is 5.4 and Hmax / Hmin is 244.2; the substrate material in the metasurface of Example 4 is silicon dioxide, and its Emax / Emin is 3.2 and Hmax / Hmin is 67.5. This shows that by further limiting the substrate material, the present invention can significantly improve the electric and magnetic field separation rate of the metasurface. This is because the top silicon layer of SOI is single-crystal silicon with a crystal orientation of

[100] . Compared with amorphous silicon, single-crystal silicon has a lower k value in the near-infrared band, less absorption of electromagnetic waves, and can utilize more energy, thereby achieving a higher separation rate.

[0181] (3) As can be seen from Examples 1 and 5, in Example 1, the distance L1 between the upper surface of the elliptical cylinder and the substrate is 10 nm larger than the distance L2 between the upper surface of the hexagonal cylinder and the substrate, and the Emax / Emin of its metasurface is 5.4 and the Hmax / Hmin is 244.2; while in Example 5, L1 is 40 nm larger than L2, and the Emax / Emin of its metasurface is 4.3 and the Hmax / Hmin is 152.6. This shows that the present invention further limits the difference between L1 and L2, so that the electric field energy is locally distributed along the dimer unit structure in the EQ-dominated BIC resonance mode, thereby enhancing the electric and magnetic field separation efficiency of the metasurface.

[0182] (4) As can be seen from Examples 1 and 6-7, in Example 1, the width of the gap between the three sub-microstructures is 20 nm, and the Emax / Emin of its metasurface is 5.4 and the Hmax / Hmin is 244.2; while in Example 6, the width of the gap between the three sub-microstructures is 30 nm, and the Emax / Emin of its metasurface is 4.2 and the Hmax / Hmin is 43.2; in Example 7, the width of the gap between the three sub-microstructures is 11 nm, and the Emax / Emin of its metasurface is 4.9 and the Hmax / Hmin is 44.1. This shows that by further limiting the width of the gap between the sub-microstructures, the electric field energy can be localized in the second microstructure, thereby increasing the electric and magnetic field separation efficiency of the metasurface.

[0183] (5) As can be seen from Examples 1 and 8-9, in Example 1, the acute angle θ1 formed by the intersection of the semi-major axis of the elliptical cylinder and the y-axis and the acute angle θ2 formed by the intersection of the major axis of symmetry of the hexagonal cylinder and the y-axis are both 25°, and the Emax / Emin of the metasurface is 5.4 and the Hmax / Hmin is 244.2; while in Example 8, θ1 and θ2 are both 10°, and the Emax / Emin of the metasurface is 4.8 and the Hmax / Hmin is 43.8; in Example 9, θ1 and θ2 are both 40°, and the Emax / Emin of the metasurface is 2.4 and the Hmax / Hmin is 22.2. This shows that by further limiting the angles of θ1 and θ2, the present invention can increase the quality factor at the resonance wavelength and change the wavefront energy distribution at the resonance wavelength position to achieve local electric and magnetic field energy separation in the near-infrared band.

[0184] (6) As can be seen from Examples 1 and 10, the shape of the first microstructure in Example 1 is an elliptical cylinder, and the Emax / Emin of its metasurface is 5.4 and the Hmax / Hmin is 244.2; while the shape of the first microstructure in Example 10 is a cylinder, and the Emax / Emin of its metasurface is 4.1 and the Hmax / Hmin is 153.8. This shows that by further defining the shape of the first microstructure, the present invention, due to the difference between the major and minor axes of the elliptical cylinder structure, has a tendency to have an outer ring induced current in the plane perpendicular to the substrate. This results in its ability to localize magnetic field energy along its own major axis in the plane being much higher than that of the cylindrical structure, so that the magnetic field energy is mainly localized at the location of the elliptical cylinder, thereby improving the electric and magnetic field separation rate of the metasurface.

[0185] (7) As can be seen from Example 1 and Comparative Examples 1-2, the present invention designs the structure of the dimer unit so that the shape and size of the first microstructure and the second microstructure in the dimer unit are different, so that the dimer unit forms an asymmetric structure. Therefore, the equivalent refractive index of the first microstructure and the second microstructure in a dimer unit structure is different, which in turn produces a difference in the localization effect of energy and causes electromagnetic separation.

[0186] (8) As can be seen from Example 1 and Comparative Examples 3-4, the present invention defines the second microstructure as including at least two isolated sub-microstructures. Since there are gaps between the sub-microstructures, the electric field energy can be coupled between the sub-microstructures, which can improve the effect of local electric field enhancement and further increase the local effect of electric field energy at the second microstructure, ultimately achieving an improvement in the separation rate.

[0187] (9) As can be seen from Example 1 and Comparative Example 5, the present invention can increase the asymmetry of the dimer unit by limiting the distance between the upper surface of the first microstructure and the substrate to be greater than the distance between the upper surface of the second microstructure and the substrate, thereby increasing the separation effect of electric and magnetic fields.

[0188] In summary, this invention designs the structure of the metasurface so that the first and second microstructures in the dimer unit have different shapes and sizes, so that the magnetic field energy is mainly localized at the position of the first microstructure and the electric field energy is mainly localized at the position of the second microstructure, thereby improving the electric and magnetic field separation rate of the metasurface.

[0189] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A metasurface based on an asymmetric dimer unit structure, characterized in that, The metasurface includes a substrate and dimer units arranged in an array on the substrate; The plane of the substrate is taken as the xy-axis plane, with the x-axis perpendicular to the y-axis; the dimer units are arranged in an array along the x-axis and y-axis; The dimer unit includes a first microstructure and a second microstructure; The first microstructure and the second microstructure have different shapes and sizes, wherein the cross-section of the first microstructure parallel to the substrate is curved, and the second microstructure includes at least two isolated sub-microstructures; The distance between the upper surface of the first microstructure and the substrate is denoted as L1, and the distance between the upper surface of the second microstructure and the substrate is denoted as L2, wherein L1 is greater than L2; The distance between the upper surface of the first microstructure and the substrate is different from the distance between the upper surface of the second microstructure and the substrate, so that the magnetic field energy and electric field energy are localized at different microstructure locations, thereby improving the electric and magnetic field separation rate of the metasurface. The shape of the first microstructure includes an elliptical cylinder; The contour shape formed by the sub-microstructures and gaps in the second microstructure is a hexagonal prism.

2. The metasurface according to claim 1, characterized in that, The dimer unit includes a first microstructure and a second microstructure in the x-axis direction.

3. The metasurface according to claim 1, characterized in that, The acute angle formed by the line connecting the geometric center of the first microstructure and the geometric center of the second microstructure intersecting the x-axis is 0.4-1.3°.

4. The metasurface according to claim 1, characterized in that, The distance between the geometric center of the first microstructure and the geometric center of the second microstructure is 640-680 nm.

5. The metasurface according to claim 1, characterized in that, The second microstructure comprises three independent sub-microstructures formed by two gaps.

6. The metasurface according to claim 5, characterized in that, The two gaps have the same width.

7. The metasurface according to claim 5, characterized in that, The width of the gap is 16-20 nm.

8. The metasurface according to claim 1, characterized in that, The L1 is 10-30 nm larger than the L2.

9. The metasurface according to claim 1, characterized in that, On the substrate, the number of dimer units arranged in the x-axis direction is equal to the number of dimer units arranged in the y-axis direction.

10. The metasurface according to claim 1, characterized in that, In the x-axis direction, the number of dimer units arranged on the substrate is 30-50.

11. The metasurface according to claim 1, characterized in that, The period of the dimer unit in the x-axis direction is the same as the period of the dimer unit in the y-axis direction.

12. The metasurface according to claim 1, characterized in that, The period of the dimer unit in the x-axis direction is 1280-1360 nm.

13. The metasurface according to claim 1, characterized in that, The sub-microstructure has a sharp angle in its cross-section, which is parallel to the substrate.

14. The metasurface according to claim 1, characterized in that, The acute angle formed by the intersection of the line containing the semi-major axis of the elliptical cylinder and the y-axis is denoted as θ1, and the acute angle formed by the intersection of the line containing the major axis of symmetry of the hexagonal prism and the y-axis is denoted as θ2. θ1 and θ2 are the same; and the extension of the semi-major axis of the elliptical cylinder intersects the extension of the major axis of symmetry of the hexagonal prism.

15. The metasurface according to claim 1, characterized in that, The acute angle formed by the intersection of the line containing the semi-major axis of the elliptical cylinder and the y-axis is 20-30°.

16. The metasurface according to claim 1, characterized in that, The semi-major axis of the elliptical cylinder is 300-400 nm.

17. The metasurface according to claim 1, characterized in that, The semi-minor axis of the elliptical cylinder is 100-120 nm.

18. The metasurface according to claim 1, characterized in that, The L1 is 220-240nm.

19. The metasurface according to claim 1, characterized in that, The long axis of symmetry of the hexagonal prism is 320-340 nm.

20. The metasurface according to claim 1, characterized in that, The short axis of symmetry of the hexagonal prism is 120-140 nm.

21. The metasurface according to claim 1, characterized in that, The L2 is 210-230nm.

22. The metasurface according to claim 1, characterized in that, The substrate material includes SOI.

23. The metasurface according to claim 1, characterized in that, The material of the dimer unit includes any one or a combination of at least two of silicon nitride, amorphous silicon, or SOI.

24. The metasurface according to claim 23, characterized in that, The material of the dimer unit is SOI.

25. The metasurface according to claim 1, characterized in that, The substrate comprises a base silicon, an insulating silicon oxide layer, and a top silicon layer, which are stacked sequentially.

26. The metasurface according to claim 25, characterized in that, The top silicon layer is located near the dimer units distributed in the array.

27. The metasurface according to claim 25, characterized in that, The thickness of the top silicon layer is ≥240nm.

28. A method for preparing a metasurface according to any one of claims 1-27, characterized in that, The preparation method includes the following steps: (1) A metal film mark is formed on the surface of the substrate to obtain a substrate with a metal film mark; (2) A groove is formed on the surface of the substrate with metal film markings as described in step (1), the position of the groove being the same as the position of the second microstructure, to obtain a substrate with grooves; (3) A dimer unit is formed on the substrate surface with grooves described in step (2) to obtain the metasurface.

29. The preparation method according to claim 28, characterized in that, The step of forming the metal film mark in step (1) includes obtaining a substrate containing the first photoresist mark using a first photoresist and a first electron beam exposure process, sputtering a metal film on the surface of the substrate containing the first photoresist mark, and using a lift-off process to peel the metal film off from the first photoresist to obtain a substrate with the metal film mark.

30. The preparation method according to claim 28, characterized in that, Step (2) involves forming a groove by using a second photoresist and a second electron beam exposure process to obtain a first adhesive layer with a first pattern on the surface of the substrate with metal film markings. The first pattern has a groove, and the position of the groove in the first adhesive layer is the same as the position of the second microstructure. Plasma is used to perform a first etching on the substrate exposed by the groove in the first adhesive layer to form a groove structure corresponding to the position of the second microstructure. The second photoresist is then removed to obtain a substrate with a groove.

31. The preparation method according to claim 28, characterized in that, Step (3) involves forming a second adhesive layer with a second pattern on the surface of the substrate with grooves using a third photoresist and a third electron beam exposure process. The second pattern has grooves, and the position of the grooves in the second adhesive layer is the same as the position of the first microstructure and the second microstructure. Plasma is used to perform a second etching on the substrate exposed at the groove portion of the second adhesive layer, and the third photoresist is removed to obtain the metasurface with the structure of the dimer unit.

32. The preparation method according to claim 30, characterized in that, Step (2) The etching depth of the first etching is 10-30 nm.

33. The preparation method according to claim 31, characterized in that, Step (3) The etching depth of the second etching is 220-240 nm.

34. The application of a metasurface according to any one of claims 1-27 in biomedical imaging, magnetic particle detection or magneto-optical spectroscopy.

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