Metasurface based on asymmetric dimer unit structure as well as preparation method and application of metasurface

Abstract

The invention relates to a metasurface based on an asymmetric dimer unit structure and a preparation method and application of the metasurface. The metasurface comprises a substrate and dimer units distributed on the substrate in an array mode. The plane of the substrate is an xy-axis plane, and the x-axis is perpendicular to the y-axis; the dimer units are distributed in an array along an x axis and a y axis; the dimer unit comprises a first microstructure and a second microstructure; the first microstructure and the second microstructure are different in shape and size, the cross section, parallel to the substrate, in the first microstructure is in a curved shape, and the second microstructure comprises at least two isolated sub-microstructures; the distance between the upper surface of the first microstructure and the substrate is recorded as L1, the distance between the upper surface of the second microstructure and the substrate is recorded as L2, and L1 is larger than L2. By designing the structure of the metasurface, the mass factor at the resonant wavelength can be changed, and meanwhile, the wavefront energy distribution at the resonant wavelength can be changed, so that near-infrared band local electric field and magnetic field energy separation is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of optical materials, and in particular to a super surface based on an asymmetric dimer unit structure, and a preparation method and application thereof. Background Art

[0002] Optical metasurfaces, as a type of planar metasurface material, achieve precise control of light parameters such as amplitude, phase, and polarization through periodically arranged subwavelength metal or dielectric structures on a two-dimensional plane. This control capability breaks through the boundaries of traditional light-matter interactions and provides possibilities for the preparation of new 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 separation and control of local electric and magnetic field energy, and the enhancement of ultra-high electric fields. In recent years, metasurfaces have become a hot topic of research due to their excellent electromagnetic field control capabilities, subwavelength-scale local field enhancement, thinness, and integration.

[0003] CN108803088A discloses a transflective integrated converter based on optical polarization control of a metasurface, including an upper nano dimer array and a lower substrate, wherein the substrate plane is an xy axis plane, the x axis is perpendicular to the y axis, and two semiconductor cylindrical nano structures of exactly the same structural material are arranged at a fixed distance to form a nano dimer, and the direction of the connecting line of the centers of the two cylinders in the nano dimer is the direction of the dimer axis, i.e., the x axis direction; the nano dimer array on the substrate plane is a nano dimer array periodically arranged along the x axis and the y axis. The ratio of the length in the y axis direction to the length in the x axis direction of the entire nano dimer array is less than 1. The nano dimer array is arranged in a fixed period along the x axis and the y axis, respectively, and the period setting meets the requirements of the density of the nano resonator. That is, CN108803088A discloses a symmetrical structure, and the resonance peaks of the symmetrical structure often overlap (such as the resonance frequencies of the electric dipole and the magnetic dipole are close), resulting in the inability to effectively separate the electric field and magnetic field energy in the target frequency band.

[0004] The main problem that needs to be solved in electric field and magnetic field separation is that the electric field and magnetic field of light are inherently coupled and controlled by Maxwell's equations. At present, one method is to use a reflector to form a standing wave node to achieve electric field and magnetic field separation, which can suppress the electric field enhancement effect to near zero. However, the formation of standing wave nodes is extremely difficult in practical applications and cannot be promoted. Another technology to achieve electric field and magnetic field spatial separation is to form a laser beam with an azimuthal polarization angle. The electric field of the laser beam disappears on the optical axis and the axial magnetic field reaches the maximum, so a 3-fold electric field and magnetic field separation effect can be obtained. However, the equipment cost of obtaining such a laser beam is high, and the technical difficulty is high, making it difficult to be widely used. In addition, there is also a method of achieving electric field and magnetic field separation through a coaxial dielectric tube structure, coupling in the tube structure to obtain an Anapole state, and obtaining a magnetic field enhancement factor of 13.8 and an electric field enhancement factor of 4.16. Due to the great difficulty of processing technology, this method cannot produce a dielectric tube structure in the optical frequency band, so it cannot be applied on a large scale.

[0005] In summary, it is necessary to develop a metasurface structure and a preparation method thereof, by designing the structure of the metasurface so that the electric field and magnetic field separation rate of the metasurface is high, and the metasurface is prepared by a low-cost and easy-to-process preparation method. Summary of the invention

[0006] In order to solve the above technical problems, the present 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 in the dimer unit parallel to the substrate is curved, the second microstructure includes at least two isolated sub-microstructures, and the sizes of the first microstructure and the second microstructure are different, and 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 the electric field energy are respectively localized at different microstructure positions, so as to improve the electric field and magnetic field separation rate of the metasurface.

[0007] To achieve this object, the present invention adopts the following technical solutions:

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

[0009] The plane of the substrate is taken as the xy axis plane, the x axis is perpendicular to the y axis; the dimer units are distributed in an array along the x axis and the 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, wherein the cross section of the first microstructure parallel to the substrate is a curved shape, the second microstructure includes at least two isolated sub-microstructures, the distance between the upper surface of the first microstructure and the substrate is recorded as L1, and the distance between the upper surface of the second microstructure and the substrate is recorded as L2, and L1 is greater than L2.

[0012] The present invention designs the structure of the dimer unit in the super surface, introduces an asymmetric first microstructure and a second microstructure in the dimer unit structure, and forms a gap between the sub-microstructures of the second microstructure, inducing a continuum bound state (Bound-states In the Continuum, BIC) resonance, which is dominated by an electric quadruple (ElectricQuadruple, EQ). In the BIC resonance mode dominated by the EQ, energy is locally distributed along the dimer unit structure, and the asymmetric dimer unit can regulate the light wavefront at the resonance wavelength position, thereby realizing the effect of energy localization at different positions in space, specifically manifested as the first microstructure and the second microstructure are different in shape and size, therefore, the equivalent refractive index of the position where the first microstructure and the second microstructure are located in a dimer unit structure is different, and then the local effect of energy is different to cause electromagnetic separation, and at the same time, due to the existence of the sharp corner of the second microstructure top, a tip effect can be produced, and the electric field energy can be gathered at a sharp point with a higher curvature, while the magnetic field energy is more dependent on the curved shape of the ring-like, gathered at the first microstructure, achieving a better separation effect, and improving the separation rate. Furthermore, the second microstructure includes at least two isolated sub-microstructures. Due to the gaps between the sub-microstructures, the electric field energy is coupled between the sub-microstructures, which can improve the effect of local electric field enhancement, further increase the local effect of the electric field energy at the second microstructure, and ultimately achieve an increase in 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 increase the separation effect of the electric and magnetic fields.

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

[0014] Preferably, the acute angle formed by the line between the geometric center of the first microstructure and the geometric center of the second microstructure and 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 is not limited to the listed values, and other unlisted values ​​within the above numerical 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 is not limited to the listed values, and other unlisted values ​​within the above numerical range are also applicable.

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

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

[0018] Preferably, the widths of the two gaps are the same.

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

[0020] The present invention further preferably limits the width of the gap, which can reduce the difficulty of preparing 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 may be 10 nm, 15 nm, 20 nm, 25 nm or 30 nm, but it is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0022] The present invention further preferably limits the difference in distance between the upper surface of the first microstructure and the upper surface of 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 field and magnetic field separation efficiency of the metasurface. It not only has a higher BIC resonance effect, but also can avoid the multipole of the BIC resonance generated by the dominant dimer unit from being transformed from an electric quadrupole to two opposite electric dipoles, so that the energy locally distributed along the dimer unit structure will not be guided to the two ends of the dimer unit, and a higher separation efficiency can be maintained.

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

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

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

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

[0027] As a preferred technical solution of the present invention, the cross section of the sub-microstructure parallel to the substrate has a sharp corner.

[0028] The sharp angle in the present invention is the angle formed by the intersection of two lines.

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

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

[0031] The present invention further prefers that the shape of the first microstructure is an elliptical column, and the contour shape formed by the sub-microstructure and the gap in the second structure is a hexagonal column. This is because the hexagonal column has a sharp vertex and can produce a tip effect, and the electric field energy tends to be concentrated in the area with higher curvature, so the hexagonal column with a sharp vertex can achieve a larger electric field energy focusing in space; the magnetic field energy tends to focus near the relatively annular structure, so the first microstructure is designed as an elliptical column and there is no sharp vertex to "compete" for energy with the second microstructure. Further, compared with the cylindrical structure that also does not have a sharp vertex, the elliptical column structure has a tendency of an outer annular induced current in the plane perpendicular to the direction of the substrate due to the difference between the major and minor axes, which leads to its ability to focus magnetic field energy in the plane along its own major axis direction much higher than the cylindrical structure. Therefore, the present invention can achieve the separation of electric field and magnetic field energy in space 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, so that the magnetic field energy is mainly localized at the location of the elliptical column, and the electric field energy is mainly localized at the location of the hexagonal column, so as to improve the electric field and magnetic field separation rate of the super surface.

[0032] Preferably, the acute angle formed by the intersection of the straight line where the semi-major axis of the elliptical column is located and the y-axis is recorded as θ1, and the acute angle formed by the intersection of the straight line where the long symmetry axis of the hexagonal column is located and the y-axis is recorded as θ2, and θ1 and θ2 are the same; and the extension line of the semi-major axis of the elliptical column intersects with the extension line of the long symmetry axis of the hexagonal column.

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

[0034] The present invention further preferably limits the acute angle formed by the intersection of the straight line where the semi-major axis of the elliptical column is located and the y-axis, and the acute angle formed by the intersection of the straight line where the long symmetry axis of the hexagonal column is located and the y-axis, so as to produce a difference in the local energy capacity of the first microstructure and the second microstructure, so as to increase the quality factor at the resonant wavelength, and at the same time be able to change the wavefront energy distribution at the resonant 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, and other unlisted values ​​within the above numerical range are also applicable.

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

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

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

[0039] Preferably, the short symmetry axis of the hexagonal column is 120-140 nm, for example, 120 nm, 125 nm, 130 nm, 135 nm or 140 nm, but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

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

[0041] As a preferred technical solution of the present invention, the material of the substrate includes Silicon-On-Insulator (SOI).

[0042] The present invention further preferably limits the material of the substrate, which can significantly improve the electric field and magnetic field separation rate of the metasurface. This is because the SOI top layer silicon 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, absorbs less electromagnetic waves, and can utilize more energy, thereby achieving a higher separation rate.

[0043] Preferably, the material of the dimer unit includes any one of silicon nitride, amorphous silicon or SOI or a combination of at least two of them, 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 base silicon, insulating layer silicon oxide and top layer silicon which are stacked in sequence.

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

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

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

[0048] (1) forming a metal film mark on the surface of a substrate to obtain a substrate having a metal film mark;

[0049] (2) forming a groove on the surface of the substrate having the metal film mark in step (1), wherein the position of the groove is the same as the position of the second microstructure, thereby obtaining a substrate having a groove;

[0050] (3) forming a dimer unit on the surface of the substrate having the groove in step (2) to obtain the super surface.

[0051] The present invention realizes the preparation of super surface structure through electron beam exposure and plasma etching process. The preparation conditions are simple, easy, fast, convenient and low-cost. The sample size is adjustable, and large-scale production and preparation can be realized.

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

[0053] Preferably, the step of forming the groove in step (2) includes using a second photoresist and a second electron beam exposure process to obtain a first glue layer having a first pattern on the surface of the substrate having the metal film mark, wherein the first pattern has a groove, and the position of the groove in the first glue layer is the same as the position of the second microstructure, and plasma is used to perform a first etching on the substrate where the groove portion in the first glue layer is exposed to form a groove structure corresponding to the position of the second microstructure, and the second photoresist is removed to obtain a substrate having a groove.

[0054] Preferably, the step of forming a dimer unit in step (3) includes using a third photoresist and a third electron beam exposure process to form a second glue layer having a second pattern on the surface of the substrate having grooves, wherein the second pattern has grooves, and the positions of the grooves in the second glue layer are the same as the positions of the first microstructure and the second microstructure, and plasma is used to perform a second etching on the substrate exposed at the groove portion in the second glue layer, and the third photoresist is removed to obtain the super surface having a dimer unit structure.

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

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

[0057] Preferably, the cleaning solution comprises acetone and / or isopropyl alcohol.

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

[0059] Preferably, before the sputtering of the metal film in step (1), the step further includes immersing the substrate containing the first photoresist mark in a first developer.

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

[0061] Preferably, the step (3) further includes immersing the substrate in a third developer and a second fixer in sequence 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 includes polymethyl methacrylate (PMMA) and / or ultraviolet photoresist.

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

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

[0066] Preferably, the second developer and the third developer each independently include any one of tetramethylammonium hydroxide, n-hexylbenzene or amyl acetate, or a combination of at least two of them, 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 isopropyl alcohol.

[0068] Preferably, the atmospheres of the first etching and the second etching each independently include a mixed gas 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 is not limited to the listed values, and other unlisted values ​​within the above numerical range are also applicable.

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

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

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

[0073] (1) Cleaning the substrate;

[0074] (2) coating a first photoresist on the surface of the substrate to perform overlay marking, transferring the overlay marking to the first photoresist by a first electron beam exposure process to obtain a substrate containing the first photoresist marking, and immersing the substrate containing the first photoresist marking in a first developer;

[0075] (3) sputtering a metal film on the surface of the substrate having the first photoresist mark, and using a lift-off process to peel off the metal film and the first photoresist to obtain a substrate having the metal film mark;

[0076] (4) coating a second photoresist on the surface of the substrate, and obtaining a first glue layer having a first pattern on the surface of the substrate having the metal film mark by a second electron beam exposure process, wherein the first pattern has a groove, and the position of the groove in the first glue layer is the same as the position of the second microstructure, and the substrate is immersed in a second developer and a first fixer in sequence, and then a first etching is performed on the substrate where the groove in the first glue layer is exposed by plasma to form a groove structure with a depth of 10-30 nm corresponding to the position of the second microstructure, and the second photoresist is removed to obtain a substrate having a groove;

[0077] (5) coating the third photoresist on the surface of the substrate, forming a second glue layer having a second pattern on the surface of the substrate having grooves by a third electron beam exposure process, wherein the second pattern has grooves, and the positions of the grooves in the second glue layer are the same as the positions of the first microstructure and the second microstructure, and immersing the substrate in a third developing solution and a second fixing solution in sequence, and then performing a second etching on the exposed groove portion of the second glue layer by plasma, and removing the third photoresist, thereby obtaining the super surface having a dimer unit structure.

[0078] In a third aspect, the present invention provides an application of the metasurface according to the first aspect in biomedical imaging, magnetic particle detection or magneto-optical spectroscopy.

[0079] The supersurface in the present invention has an ultra-high separation rate in electric and magnetic field separation, can enhance the magnetic field response in the interaction between light and matter, suppress the electric field response therein, can significantly improve the quality of magnetic resonance imaging, and 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 shapes and sizes 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, so as to improve the electric field and magnetic field separation rate of the metasurface, and make 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 super surface structure through electron beam exposure and plasma etching process. The preparation conditions are simple, easy, fast, convenient and low-cost, and can realize large-area and large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0083] Figure 1 It is a schematic diagram of the structure of the super surface provided in Example 1 of the present invention.

[0084] Figure 2 The present invention Figure 1 Schematic diagram of the dimer unit within the dashed box.

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

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

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

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

[0089] Figure 7 This is a front view of the substrate after being immersed in isopropyl alcohol developer during the super surface preparation process provided in Example 1 of the present invention.

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

[0091] Fig. 9This is a front view of a substrate having an aluminum film mark during the supersurface preparation process provided in Example 1 of the present invention.

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

[0093] Fig.11 This is a front view of the substrate after being immersed in the first fixing solution isopropyl alcohol in the process of preparing the super surface provided in Example 1 of the present invention.

[0094] Fig.12 This is a front view of a substrate with grooves obtained after the first etching in the process of preparing the super surface provided in Example 1 of the present invention.

[0095] Fig.13 It is a front view of a substrate with grooves obtained after removing the second photoresist in the process of preparing the super surface provided in Example 1 of the present invention.

[0096] Fig.14 It is a front view of a substrate coated with a third photoresist during the supersurface preparation process provided in Example 1 of the present invention.

[0097] Fig.15 This is a front view of the substrate after being immersed in the second fixing solution deionized water in the supersurface preparation process provided in Example 1 of the present invention.

[0098] Fig.16 This is a front view of a substrate with grooves obtained after the second etching in the super surface preparation process provided in Example 1 of the present invention.

[0099] Fig.17 This is a front view of a supersurface having a dimer unit structure obtained after removing the third photoresist during the supersurface preparation process provided in Example 1 of the present invention.

[0100] Fig.18 This is a physical picture of the super surface provided in Example 1 of the present invention.

[0101] Fig.19 This is a SEM image of the supersurface provided in Example 1 of the present invention.

[0102] Fig. 20 This is a transmittance test chart of the metasurface provided in Examples 1, 6 and 7 of the present invention.

[0103] Fig.21 Schematic diagram of the separation rate versus gap width w in the present invention.

[0104] Fig. 22 It is a schematic diagram of the optical performance according to the change of the angles θ1 and θ2 in the present invention.

[0105] Among them, 1-first microstructure; 2-second microstructure; 3-base 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-the distance between the upper surface of the elliptical column and the base; L2-the distance between the upper surface of the hexagonal column and the base; Px-the period of the dimer unit in the x-axis direction; Py-the period of the dimer unit in the y-axis direction; w-the width of the gap; d -The distance between the geometric center of the elliptical column and the geometric center of the hexagonal column; θ1-the acute angle formed by the intersection of the straight line where the semi-major axis of the elliptical column lies and the y-axis, θ2-the acute angle formed by the intersection of the straight line where the semi-major axis of the hexagonal column lies and the y-axis, a1-the semi-major axis of the elliptical column; b1-the semi-minor axis of the elliptical column; a2-the major symmetry axis of the hexagonal column; b2-the minor symmetry axis of the hexagonal column; s-the ratio of the size difference between the first microstructure and the second microstructure, s=2×a1 / a2. DETAILED DESCRIPTION

[0106] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and through specific implementation methods. However, the following examples are only 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 shall be subject to the claims.

[0107] Example 1

[0108] This embodiment provides a super surface based on an asymmetric dimer unit structure. The structure of the super surface is as follows: Figure 1 As shown, it includes a substrate and a dimer unit; the plane of the substrate is the xy axis plane, and the x axis is perpendicular to the y axis; the dimer units are distributed in an array along the x axis and the y axis, and on the substrate, the number of the dimer units displayed and distributed in the x axis and y axis directions is 40.

[0109] The materials of the substrate and the dimer unit are both SOI, and the substrate includes a base silicon 3, an insulating silicon oxide 4 and a top silicon 5 which are stacked in sequence, wherein the top silicon is close to the array-distributed dimer unit and has a thickness of 250nm.

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

[0111] The top view and the front view of the dimer unit are respectively as shown in 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 1300nm, the distance d between the geometric center of the elliptical column and the geometric center of the hexagonal column is 660nm, and the acute angle formed by the line between the two geometric centers and the x-axis is 0.43°, wherein the semi-major axis a1 of the elliptical column is 350nm, the semi-minor axis b1 is 110nm, the major symmetry axis a2 of the hexagonal column is 330nm, and the minor symmetry axis b2 is The size of the first microstructure is 130nm, and the width of the gap 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, and the acute angle formed by the intersection of the straight line where the semi-major axis of the elliptical column is located and the y-axis is recorded as θ1, and the acute angle formed by the intersection of the straight line where the long symmetry axis of the hexagonal column is located and the y-axis is recorded as θ2, θ1 and θ2 are both 25°, and the extension line of the semi-major axis of the elliptical column intersects with the extension line of the long symmetry axis of the hexagonal column.

[0112] This embodiment also provides a method for preparing the super surface, and the preparation method comprises the following steps:

[0113] (1) The substrate was ultrasonically cleaned in acetone for 25 min to obtain Figure 5 The substrate shown;

[0114] (2) A PMMA layer 6 is coated on the surface of the substrate for overlay marking to obtain the following Figure 6 The substrate shown in FIG. 1 is used to transfer the overlay mark to PMMA using a first electron beam exposure process to obtain a substrate containing PMMA marks, and the substrate containing PMMA marks is immersed in isopropanol to obtain a substrate as shown in FIG. Figure 7 The substrate shown;

[0115] (3) Using a magnetron sputtering process, an aluminum film layer 7 is sputtered on the surface of the substrate containing the PMMA mark to obtain a Figure 8 The substrate shown in FIG. 1 is subjected to a lift-off process to peel off the aluminum film and the PMMA to obtain a substrate as shown in FIG. Fig. 9 The substrate shown has an aluminum film marking;

[0116] (4) coating the surface of the substrate with an electron beam positive photoresist (ZEP) layer 8 to obtain a Fig.10 The substrate shown in FIG. 1 is used to obtain a first adhesive layer having a first pattern on the surface of the substrate having the aluminum film mark by a second electron beam exposure process, wherein 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, and the substrate is sequentially immersed in n-hexylbenzene for 30 seconds and isopropanol for 1 minute to obtain a first adhesive layer having a first pattern. Fig.11The substrate shown in FIG. 1 is then subjected to a first etching process on the exposed substrate at the groove portion in the first glue layer by plasma to form a groove structure with a depth of 10 nm corresponding to the position of the second microstructure, and the substrate is obtained as shown in FIG. Fig.12 The substrate shown in FIG. 1 is then stripped of the electron beam positive photoresist to obtain a substrate as shown in FIG. Fig.13 The substrate shown has a groove;

[0117] (5) coating the surface of the substrate with an electron beam negative photoresist (Hydrogen Silsesquioxane, HSQ) layer 9 to obtain Fig.14 The substrate shown in FIG. 1 is used to form a second adhesive layer having a second pattern on the surface of the substrate having grooves by a third electron beam exposure process, wherein the second pattern has grooves, and the positions of the grooves in the second adhesive layer are the same as the positions of the first microstructure and the second microstructure, and the substrate is sequentially immersed in tetramethylammonium hydroxide for 30 seconds and deionized water for 1 minute to obtain a substrate as shown in FIG. Fig.15 The substrate shown in FIG. 1 is then subjected to a second etching by plasma on the exposed substrate at the groove portion in the second glue layer. The depth of the second etching is 230 nm, and the substrate shown in FIG. Fig.16 The substrate shown in FIG. 1 is then removed and the electron beam negative photoresist is removed to obtain a substrate as shown in FIG. Fig.17 The super surface having a dimer unit structure is shown in FIG. 1 ; the physical image of the super surface is shown in FIG. Fig.18 As shown, the SEM image of the super surface is as follows Fig.19 shown.

[0118] The atmospheres of the first etching and the second etching are both a mixed gas 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, wherein the metasurface includes a substrate and a dimer unit; the plane of the substrate is the xy axis plane, and the x axis is perpendicular to the y axis; the dimer units are distributed in an array along the x axis and the y axis, and on the substrate, the number of dimer units displayed and distributed in the x axis and y axis directions is 50.

[0121] The material of the substrate is SOI, the material of the dimer unit is silicon nitride, the substrate includes base silicon, insulating layer silicon oxide and top silicon stacked in sequence, wherein the top silicon is close to the array-distributed dimer units, and the thickness of the top silicon is 240nm.

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

[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 center of the elliptical column and the geometric center of the hexagonal column is 640nm, and the acute angle formed by the line between the two geometric centers and the x-axis is 1.3°, wherein the semi-major axis of the elliptical column is 300nm, the semi-minor axis is 100nm, the major symmetry axis of the hexagonal column is 320nm, the minor symmetry axis 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 s=2×a1 / a2=1.88, and the acute angle formed by the intersection of the straight line where the semi-major axis of the elliptical column and the y-axis is θ1, and the acute angle formed by the intersection of the straight line where the major symmetry axis of the hexagonal column and the y-axis is θ2, θ1 and θ2 are both 30°, and the extension line of the semi-major axis of the elliptical column intersects with the extension line of the major symmetry axis of the hexagonal column.

[0124] This embodiment also provides a method for preparing the super surface, and the preparation method comprises the following steps:

[0125] (1) The substrate was ultrasonically cleaned in isopropanol for 20 min;

[0126] (2) coating a UV photoresist on the surface of the substrate (S1813) to perform overlay marking, transferring the overlay marking to the UV photoresist by a first electron beam exposure process to obtain a substrate containing the UV photoresist marking, and immersing the substrate containing the UV photoresist marking in an alkaline solution (NMD developer) containing tetramethylammonium hydroxide;

[0127] (3) sputtering a gold film on the surface of the substrate having the ultraviolet photoresist mark by using an electron beam evaporation process, and peeling the gold film from the ultraviolet photoresist by using a lift-off process to obtain a substrate having the gold film mark;

[0128] (4) coating an electron beam negative photoresist (HSQ) on the surface of the substrate, and obtaining a first glue layer having a first pattern on the surface of the substrate having the gold film mark by a second electron beam exposure process, wherein the first pattern has a groove, and the position of the groove in the first glue layer is the same as the position of the second microstructure, and the substrate is sequentially immersed in tetramethylammonium hydroxide for 30 seconds and deionized water for 1 minute, and then a first etching is performed on the substrate where the groove in the first glue layer is exposed by plasma to form a groove structure with a depth of 30 nm corresponding to the position of the second microstructure, and the electron beam negative photoresist is removed to obtain a substrate having a groove;

[0129] (5) coating an electron beam positive photoresist (ZEP) on the surface of the substrate, forming a second glue layer having a second pattern on the surface of the substrate having the grooves by a third electron beam exposure process, wherein the second pattern has grooves, and the positions of the grooves in the second glue layer are the same as the positions of the first microstructure and the second microstructure, and immersing the substrate in amyl acetate for 30 seconds and isopropanol for 1 minute, and then performing a second etching on the substrate exposed at the groove position in the second glue layer by plasma, wherein the depth of the second etching is 240 nm, and removing the electron beam positive photoresist, thereby obtaining the metasurface having a dimer unit structure;

[0130] The atmospheres of the first etching and the second etching are both a mixed gas of sulfur hexafluoride and oxygen in a volume ratio of 3:1.

[0131] Example 3

[0132] The present embodiment provides a metasurface based on an asymmetric dimer unit structure, the metasurface comprising a substrate and a dimer unit; the plane of the substrate is the xy axis plane, the x axis is perpendicular to the y axis; the dimer units are distributed in an array along the x axis and the y axis, and on the substrate, the number of dimer units displayed and distributed in the x axis and y axis directions is 30.

[0133] The material of the substrate is SOI, the material of the dimer unit is amorphous silicon, the substrate includes base silicon, insulating layer silicon oxide and top silicon stacked in sequence, wherein the top silicon is close to the array-distributed dimer unit, and the thickness of the top silicon is 260nm.

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

[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 center of the elliptical column and the geometric center of the hexagonal column is 680nm, and the acute angle formed by the line between the two geometric centers and the x-axis is 0.4°, wherein the semi-major axis of the elliptical column is 400nm, the semi-minor axis is 120nm, the major symmetry axis of the hexagonal column is 340nm, the minor symmetry axis 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 s=2×a1 / a2=2.35, and the acute angle formed by the intersection of the straight line where the semi-major axis of the elliptical column and the y-axis is θ1, and the acute angle formed by the intersection of the straight line where the major symmetry axis of the hexagonal column and the y-axis is θ2, θ1 and θ2 are both 20°, and the extension line of the semi-major axis of the elliptical column intersects with the extension line of the major symmetry axis of the hexagonal column.

[0136] This embodiment also provides a method for preparing the super surface, and the preparation method comprises the following steps:

[0137] (1) The substrate was ultrasonically cleaned in isopropanol for 30 min;

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

[0139] (3) sputtering an aluminum film on the surface of the substrate containing the PMMA mark by using a magnetron sputtering process, and peeling the aluminum film from the PMMA by using a lift-off process to obtain a substrate with an aluminum film mark;

[0140] (4) coating an electron beam positive photoresist (ZEP) on the surface of the substrate, and obtaining a first glue layer having a first pattern on the surface of the substrate having the aluminum film mark by a second electron beam exposure process, wherein the first pattern has a groove, and the position of the groove in the first glue layer is the same as the position of the second microstructure, and the substrate is sequentially immersed in n-hexylbenzene for 30 seconds and in isopropanol for 1 minute, and then a first etching is performed on the substrate where the groove in the first glue layer is exposed by plasma to form a groove structure with a depth of 10 nm corresponding to the position of the second microstructure, and the electron beam positive photoresist is removed to obtain a substrate having a groove;

[0141] (5) coating the electron beam positive photoresist on the surface of the substrate, forming a second glue layer having a second pattern on the surface of the substrate having the groove by a third electron beam exposure process, wherein the second pattern has a groove, and the position of the groove in the second glue layer is the same as the position of the first microstructure and the second microstructure, and immersing the substrate in amyl acetate for 30 seconds and isopropanol for 1 minute, and then performing a second etching on the exposed groove portion of the second glue layer by plasma, wherein the depth of the second etching is 220 nm, and removing the electron beam positive photoresist, thereby obtaining the super surface having a dimer unit structure;

[0142] The atmospheres of the first etching and the second etching are both a mixed gas 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, which is the same as Embodiment 1 except that the material of the substrate is replaced by silicon dioxide instead of SOI.

[0145] Example 5

[0146] The present embodiment provides a metasurface based on an asymmetric dimer unit structure, which is different from Embodiment 1 only in that, except 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, the rest is the same as Embodiment 1.

[0147] Example 6

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

[0149] Example 7

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

[0151] The transmittance of the metasurfaces of Example 1, Example 6 and Example 7 was tested, and the results are as follows: Fig. 20 shown.

[0152] At the same time, in order to better understand the effect of the gap width on the metasurface transmittance, metasurfaces with gap widths of 5 nm, 8 nm, 16 nm, 25 nm and 35 nm were prepared on the basis of Examples 1, 6 and 7, and their separation rates were tested. The results are as follows: Fig.21 shown.

[0153] Fig. 20 and Fig.21 The influence of the gap width w on the structure transmission spectrum and separation rate are described respectively. Fig. 20 As shown, by changing the size of w within the provided range, the resonance position can be fine-tuned, but the overall line width of the curve remains basically unchanged, so the overall Q factor changes smoothly, and the energy binding efficiency is maintained at a high level. Under the same plane wave excitation, the total energy that can be bound in the structure remains basically unchanged. However, changing the size of w will cause the coupling effect between the three sub-microstructures of the second microstructure to change, which will cause the local electric field capacity of the second microstructure to change, and then the electric and magnetic field separation rate of the structure space will be affected. Fig.21 As shown, when the gap is too large, the coupling effect between the sub-microstructures is weakened, and the effect of enhancing the localization of the electric field energy cannot be achieved. The magnetic field energy is not strongly dependent on the first microstructure, which leads to a significant decrease in the electric and magnetic field separation rate in the structural space. The highest separation rate can be obtained at about w = 20nm.

[0154] Example 8

[0155] This embodiment provides a metasurface based on an asymmetric dimer unit structure, which is different from Embodiment 1 only in that, except for the acute angle θ1 formed by the intersection of the straight line where the semi-major axis of the elliptical column and the y-axis and the acute angle θ2 formed by the intersection of the straight line where the long symmetry axis of the hexagonal column and the y-axis, both are adjusted to 10°, the rest are the same as Embodiment 1.

[0156] In order to better explore the influence of the acute angle formed by the intersection of the straight line where the semi-major axis of the elliptical cylinder is located and the y-axis on the optical performance of the metasurface, on the basis of Example 1 and Example 8, metasurfaces with θ1=θ2=0°, 20° and 30° were prepared respectively, and their optical performances were tested, and the results were fitted to obtain Fig. 22 .

[0157] from Fig. 22 It can be seen that when the angle is θ1=θ2=0°, there is a symmetrically protected BIC in the structure transmission spectrum, and the resonance line width is zero and cannot be observed; when the angle is not 0°, the original symmetrically protected BIC couples with the far-field radiation mode, and the resulting energy leakage is converted into an observable quasi-BIC, and the resonance line width is no longer zero. This shows that angles θ1 and θ2 are the main reasons for the generation of this resonance.

[0158] Example 9

[0159] This embodiment provides a metasurface based on an asymmetric dimer unit structure, which is different from Embodiment 1 only in that, except for the acute angle θ1 formed by the intersection of the straight line where the semi-major axis of the elliptical column and the y-axis and the acute angle θ2 formed by the intersection of the straight line where the long symmetry axis of the hexagonal column and the y-axis, both are adjusted to 40°, the rest are the same as Embodiment 1.

[0160] Example 10

[0161] This embodiment provides a metasurface based on an asymmetric dimer unit structure, which is different from Embodiment 1 only in that, except 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, the rest is the same as Embodiment 1.

[0162] Comparative Example 1

[0163] This comparative example provides a metasurface based on an asymmetric dimer unit structure, which is different from Example 1 only in that, except that the shape and size of the second microstructure are the same as those of the first microstructure, the rest are the same as Example 1.

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

[0165] Comparative Example 2

[0166] This comparative example provides a metasurface based on an asymmetric dimer unit structure, which is different from Example 1 only in that, except that the shape and size of the first microstructure are the same as those of the second microstructure, the rest are the same as Example 1.

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

[0168] Comparative Example 3

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

[0170] Comparative Example 4

[0171] This comparative example provides a supersurface based on an asymmetric dimer unit structure, which is the same as Example 1 except that the shape of the second microstructure is cylindrical, the diameter of the cylinder is 330nm, and the distance L2 between the upper surface of the cylinder and the substrate is 220nm.

[0172] Comparative Example 5

[0173] This comparative example provides a metasurface based on an asymmetric dimer unit structure, which is different from Example 1 only in that, except that the distance L2 between the upper surface of the hexagonal column and the substrate is adjusted to 230 nm, that is, the distance L1 between the upper surface of the elliptical column and the substrate is equal to the distance L2 between the upper surface of the hexagonal column and the substrate, the rest is the same as Example 1.

[0174] A scanning near-field optical microscope (SNOM) was used to test the information on the distribution of electric and magnetic field intensity on the metasurface under the condition of normal incidence of a plane wave with a wavelength of λ. The electric and magnetic field information of the metasurface at the first microstructure and the second microstructure under the condition of incident wavelength λ were collected respectively, and the electric and magnetic field sizes at the first microstructure and the second microstructure 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 data results of the separation rate are shown in Table 1.

[0175] Table 1

[0176]

[0177]

[0178] The test results show that:

[0179] (1) It can be seen from Examples 1 to 3 that the present invention designs the structure of the metasurface so that the shapes and sizes 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, so as to improve the electric field and magnetic field separation rate of the metasurface, and make the electric field separation rate reach more than 4.9 times, and the magnetic field separation rate reach more than 189 times.

[0180] (2) It can be seen from Examples 1 and 4 that the material of the substrate in the supersurface of Example 1 is SOI, and the Emax / Emin of the supersurface is 5.4 and Hmax / Hmin is 244.2; the material of the substrate in the supersurface of Example 4 is silicon dioxide, and the Emax / Emin of the supersurface is 3.2 and Hmax / Hmin is 67.5, which shows that the present invention can significantly improve the electric field and magnetic field separation rate of the supersurface by further limiting the material of the substrate. This is because the SOI top silicon 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, absorbs less electromagnetic waves, and can utilize more energy, thereby achieving a higher separation rate.

[0181] (3) It can be seen from Example 1 and Example 5 that 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 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.

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

[0183] (5) It can be seen from Example 1 and Examples 8-9 that the acute angle θ1 formed by the intersection of the straight line where the semi-major axis of the elliptical column and the y-axis and the acute angle θ2 formed by the intersection of the straight line where the long symmetry axis of the hexagonal column and the y-axis in Example 1 are both 25°, and the Emax / Emin of its 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 its 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 its metasurface is 2.4, and the Hmax / Hmin is 22.2. This shows that the present invention can increase the quality factor at the resonant wavelength by further limiting the angles of θ1 and θ2, and at the same time can change the wavefront energy distribution at the resonant wavelength position to achieve local electric and magnetic field energy separation in the near-infrared band.

[0184] (6) It can be seen from Example 1 and Example 10 that the shape of the first microstructure in Example 1 is an elliptical cylinder, and its supersurface Emax / Emin is 5.4 and Hmax / Hmin is 244.2; while the shape of the first microstructure in Example 10 is a cylindrical cylinder, and its supersurface Emax / Emin is 4.1 and Hmax / Hmin is 153.8. This shows that the present invention further limits the shape of the first microstructure. Due to the difference between the major and minor axes of the elliptical cylinder structure, it has a tendency of external annular induced current in the plane perpendicular to the substrate, which results in its ability to localize magnetic field energy along its own major axis in the plane 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 field and magnetic field separation rate of the supersurface.

[0185] (7) It can be seen from Example 1 and Comparative Examples 1-2 that the present invention designs the structure of the dimer unit so that the shapes and sizes 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 positions of the first microstructure and the second microstructure in a dimer unit structure is different, which in turn produces a difference in the local effect of energy to cause electromagnetic separation.

[0186] (8) It can be seen from Example 1 and Comparative Examples 3-4 that the present invention, by limiting the second microstructure to include at least two isolated sub-microstructures, can enhance the local electric field enhancement effect by allowing the electric field energy to couple between the sub-microstructures due to the presence of gaps between the sub-microstructures, thereby further increasing the local effect of the electric field energy at the second microstructure, and ultimately achieving an improvement in the separation rate.

[0187] (9) It can be seen from Example 1 and Comparative Example 5 that 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 larger than the distance between the upper surface of the second microstructure and the substrate, thereby increasing the separation effect of the electric field and the magnetic field.

[0188] In summary, the present invention designs the structure of the metasurface so that the shapes and sizes 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, so as to improve the electric field and magnetic field separation rate of the metasurface.

[0189] The applicant declares that the above is only a specific implementation mode 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 thought of by those skilled in the art within the technical scope disclosed by the present invention shall fall within the protection scope and disclosure scope of the present invention.

Claims

1. A metasurface based on an asymmetric dimer unit structure, characterized in that: The super surface comprises a substrate and dimer units distributed in an array on the substrate; The plane of the substrate is taken as the xy axis plane, the x axis is perpendicular to the y axis; the dimer units are distributed in an array along the x axis and the y axis; The dimer unit includes a first microstructure and a second microstructure; The first microstructure and the second microstructure are different in shape and size, wherein a cross section of the first microstructure parallel to the substrate is a curved shape, 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 recorded as L1, and the distance between the upper surface of the second microstructure and the substrate is recorded as L2, and L1 is greater than L2.

2. The supersurface according to claim 1, characterized in that The dimer unit includes one first microstructure and one second microstructure in the x-axis direction; Preferably, the acute angle formed by the line between the geometric center of the first microstructure and the geometric center of the second microstructure and the x-axis is 0.4-1.3°; Preferably, the distance between the geometric center of the first microstructure and the geometric center of the second microstructure is 640-680 nm; Preferably, the second microstructure includes three independent sub-microstructures formed by dividing two gaps; Preferably, the widths of the two gaps are the same; Preferably, the width of the gap is 16-20 nm; Preferably, L1 is 10-30 nm larger than L2.

3. The supersurface according to claim 1 or 2, characterized in that: On the substrate, the number of dimer units distributed in the x-axis direction is equal to the number of dimer units distributed in the y-axis direction; Preferably, in the x-axis direction, the number of dimer units displayed and distributed on the substrate is 30-50; 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; Preferably, the period of the dimer unit in the x-axis direction is 1280-1360 nm.

4. The supersurface according to any one of claims 1 to 3, characterized in that: The cross section of the sub-microstructure parallel to the substrate has a sharp corner; Preferably, the shape of the first microstructure comprises an elliptical column; Preferably, the outline shape formed by the sub-microstructure and the gap in the second microstructure is a hexagonal column; Preferably, the acute angle formed by the intersection of the straight line where the semi-major axis of the elliptical column is located and the y-axis is recorded as θ1, and the acute angle formed by the intersection of the straight line where the long symmetry axis of the hexagonal column is located and the y-axis is recorded as θ2, and θ1 and θ2 are the same; and the extension line of the semi-major axis of the elliptical column intersects the extension line of the long symmetry axis of the hexagonal column; Preferably, the acute angle formed by the intersection of the straight line where the semi-major axis of the elliptical cylinder lies and the y-axis is 20-30°.

5. The supersurface according to claim 4, characterized in that The semi-major axis of the elliptical cylinder is 300-400nm; Preferably, the semi-minor axis of the elliptical cylinder is 100-120 nm; Preferably, the L1 is 220-240 nm.

6. The supersurface according to claim 4 or 5, characterized in that: The long symmetry axis of the hexagonal column is 320-340 nm; Preferably, the short symmetry axis of the hexagonal column is 120-140 nm; Preferably, L2 is 210-230nm.

7. The supersurface according to any one of claims 1 to 6, characterized in that: The material of the substrate includes SOI; Preferably, the material of the dimer unit includes any one of silicon nitride, amorphous silicon or SOI or a combination of at least two thereof, preferably SOI; Preferably, the substrate comprises a base silicon, an insulating layer silicon oxide and a top layer silicon which are stacked in sequence; Preferably, the top layer of silicon is close to the dimer units distributed in the array; Preferably, the thickness of the top silicon layer is ≥240 nm.

8. A method for preparing a supersurface according to any one of claims 1 to 7, characterized in that: The preparation method comprises the following steps: (1) forming a metal film mark on the surface of a substrate to obtain a substrate having a metal film mark; (2) forming a groove on the surface of the substrate having the metal film mark in step (1), wherein the position of the groove is the same as the position of the second microstructure, thereby obtaining a substrate having a groove; (3) forming a dimer unit on the surface of the substrate having the groove in step (2) to obtain the super surface.

9. The preparation method according to claim 8, characterized in that: The step of forming the metal film mark in step (1) comprises 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 off the metal film from the first photoresist to obtain a substrate with the metal film mark; Preferably, the step of forming the groove in step (2) comprises obtaining a first glue layer having a first pattern on the surface of the substrate having the metal film mark by using a second photoresist and a second electron beam exposure process, wherein the first pattern has a groove, and the position of the groove in the first glue layer is the same as the position of the second microstructure, and performing a first etching on the substrate where the groove in the first glue layer is exposed by plasma to form a groove structure corresponding to the position of the second microstructure, and removing the second photoresist to obtain a substrate having a groove; Preferably, the step of forming the dimer unit in step (3) comprises forming a second glue layer having a second pattern on the surface of the substrate having the grooves by using a third photoresist and a third electron beam exposure process, wherein the second pattern has grooves, and the positions of the grooves in the second glue layer are the same as the positions of the first microstructure and the second microstructure, and performing a second etching on the substrate exposed at the groove position in the second glue layer by using plasma, and removing the third photoresist, so as to obtain the super surface having the dimer unit structure; Preferably, the etching depth of the first etching in step (2) is 10-30 nm; Preferably, the etching depth of the second etching in step (3) is 220-240 nm.

10. An application of the metasurface according to any one of claims 1 to 7 in biomedical imaging, magnetic particle detection or magneto-optical spectroscopy.

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