Infrared broadband absorption metasurface structure

Abstract

The invention belongs to the field of micro-nano photonics and infrared sensing, and relates to an infrared broadband absorption metasurface structure. A metal layer-dielectric layer-metal layer structural design is adopted, an upper metal layer and a lower metal layer are made of platinum, a dielectric layer is made of traditional material silicon, array arrangement is achieved by combining subunits of different sizes, the infrared broadband absorption metasurface based on a square beveled corner structure is designed, and the average absorption rate in an atmosphere window wave band of 8-13 microns exceeds 60%. The bottom metal layer and the dielectric layer are of a uniform layered structure, the top metal layer is formed by beveling a corner of a square, and the ratio of the corner cut to the side length of the square is related to the absorption wavelength. The light field is regulated and controlled based on the parameter change of the micro-nano structure, the wavelength selectivity and modulation performance are good, the selected material is good in chemical stability, and practicability and process compatibility are achieved.

Description

Technical Field

[0001] The present invention belongs to the fields of micro-nano photonics and infrared sensing, and more specifically, relates to an infrared broadband absorption metasurface structure. Background Art

[0002] Infrared absorption materials have important application values in the fields of infrared sensing, thermal radiation management, spectral detection, etc. Especially in the 8-13μm atmospheric window band, their performance directly affects the infrared signal feature regulation ability of the target object. Traditional infrared absorption structures are mostly designed based on metal thin films, semiconductor heterojunctions or multi-layer dielectric films, and achieve selective absorption of specific wavelengths through plasmon resonance, exciton effect or interference cancellation mechanism. However, each of the above solutions has its own defects. Metal thin films rely on surface plasmon resonance absorption, but the full width at half maximum of its absorption peak is narrow and it is difficult to cover the wide spectrum requirements; although multi-layer dielectric films can expand the bandwidth through thickness regulation, the refractive index mismatch between layers easily leads to interface scattering loss and the average absorption rate is low. In addition, some of the existing structures face problems such as poor material thermal stability and low processing tolerance, which seriously restrict their engineering applications in complex environments.

[0003] In recent years, research has tried to excite multi-mode resonance through asymmetric metasurfaces such as L-shaped and cross-shaped structures. However, the electromagnetic coupling effect between units is weak, and the absorption peaks still show discrete characteristics after superposition, and the requirements for micro-nano processing accuracy are high. Using non-traditional process materials, it is difficult to prepare on a large scale. To address the above problems, there is an urgent need for an innovative design that combines broadband efficient absorption, material stability and process compatibility to break through the bottleneck of existing technologies. Summary of the Invention

[0004] The present invention proposes an infrared broadband absorption metasurface structure, which belongs to the MIM composite structure metasurface. By introducing an asymmetric structure with sub-units having missing corners, a multi-mode coupling effect is achieved, effectively enhancing the absorption ability of the resonant units in different frequency bands. Further, the parameters of the supercell structure that composes the infrared broadband absorption metasurface structure are finely adjusted to enhance the coupling and superposition effect between the supercell structures, forming a continuous and smooth broadband absorption response. Thereby, the technical problems of broadband efficient absorption, poor material stability and poor process compatibility faced in the prior art are solved.

[0005] According to the purpose of the present invention, there is provided an infrared broadband absorption metasurface structure, which is formed by periodically arranging supercell structures. The supercell structure specifically includes: a bottom platinum metal layer, a dielectric layer covering the bottom platinum metal layer, and four sub-units a, b, c, and d arranged on the dielectric layer; and the materials of the four sub-units a, b, c, and d are platinum, and the material of the dielectric layer is silicon; both the bottom platinum metal layer and the dielectric layer are square.

[0006] The four sub-units a, b, c, and d are arranged in a square matrix. The arrangement order is that a is located in the upper left corner, b is located in the upper right corner, c is located in the lower left corner, and d is located in the lower right corner; among the four sub-units a, b, c, and d, the distance between the centers of two adjacent sub-units is equal;

[0007] Among the four sub-units a, b, c, and d, at least one sub-unit is a pentagon formed by a square with a cut corner. The cut corner is cut along two adjacent sides of the square, and the other sub-units are squares without cut corners; let the length of the side that is not cut off in the sub-unit of the square with a cut corner be the side length of the sub-unit of the square with a cut corner, then the side lengths of at least two sub-units are different;

[0008] When 1-3 sub-units among the four sub-units a, b, c, and d are squares with cut corners, each side that is not cut off in the sub-unit of the square with a cut corner and each side of the other sub-units that are squares without cut corners are parallel or perpendicular to each side of the bottom platinum metal layer;

[0009] When all of the four sub-units a, b, c, and d are squares with cut corners, each side that is not cut off in the sub-unit of the square with a cut corner is parallel or perpendicular to each side of the bottom platinum metal layer.

[0010] Preferably, when two or more of the four sub-units a, b, c, and d have a cut corner, each cut corner is in the upper left corner of the corresponding sub-unit, or each cut corner is in the upper right corner of the corresponding sub-unit, or each cut corner is in the lower left corner of the corresponding sub-unit, or each cut corner is in the lower right corner of the corresponding sub-unit.

[0011] Preferably, the shape of the cut corner is an isosceles right triangle.

[0012] Preferably, the waist length of the isosceles right triangle is less than or equal to 60% of the side length of the sub-unit where the cut corner is located.

[0013] Preferably, the side lengths of the four sub-units a, b, c, and d are independently selected from 0.6-2 μm.

[0014] Preferably, the bottom platinum metal layer and the dielectric layer are square, and the side length is 2-5 μm.

[0015] Preferably, the thickness of the bottom platinum metal layer is 50-1000 nm.

[0016] Preferably, the thickness of the dielectric layer is 100-300 nm.

[0017] Preferably, the four sub-units a, b, c, and d have the same thickness, all of which are 50-300 nm.

[0018] Preferably, the average absorption rate of the infrared broadband absorption metasurface structure exceeds 60% in the 8-13 μm atmospheric window band, and is lower than 20% in the 5-8 μm band and 13-20 μm band.

[0019] Generally speaking, compared with the prior art, the above technical solutions conceived by the present invention mainly have the following technical advantages:

[0020] (1) The average absorption rate of the metasurface structure with broadband absorption in the infrared band of the present invention exceeds 60% in the 8-13 μm atmospheric window band, and is lower than 20% in the 5-8 μm band and 13-20 μm band.

[0021] (2) By adjusting the side length and chamfer angle, the present invention regulates the resonance frequency and absorption bandwidth, realizing flexible optical regulation and application.

[0022] (3) The metal platinum and dielectric material silicon selected in the present invention are both traditional process materials, with mature processing technology; they have stable chemical properties, good oxidation resistance, and long-term stability; they have acid resistance, are not easily corroded by etchant during the process, and have good process compatibility; and they have moderate hardness, improving the mechanical stability of the system.

[0023] (4) The method of the present invention has a simple structure and low process difficulty. The bottom metal and dielectric layer are uniform material layers, and the top metal layer adopts a square chamfer structure. Description of the Drawings

[0024] Figure 1 It is a schematic structural diagram provided by the present invention.

[0025] Figure 2 It is the influence of the square side length on the absorption rate of the sub-unit without chamfer.

[0026] Figure 3 It is the influence of the ratio of different chamfer lengths to the square side length on the absorption rate of the sub-unit.

[0027] Figure 4 It is the absorption rate of the metasurface when near-infrared light is incident perpendicularly.

[0028] Figure 5 It is the surface electric field corresponding to the resonance wavelength of the four units.

[0029] Wherein: 1 - dielectric layer, 2 - bottom platinum metal layer. Detailed Embodiments

[0030] In order to make the objectives, technical solutions and advantages of the present invention more clear and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0031] A mid-infrared broadband absorption metasurface structure of the present invention, wherein the infrared broadband absorption metasurface structure is formed by periodically arranging supercell structures. The supercell structure specifically includes: a bottom platinum metal layer, a dielectric layer covering the bottom platinum metal layer, and four sub-units a, b, c, and d arranged on the dielectric layer; and the materials of the four sub-units a, b, c, and d are platinum, and the material of the dielectric layer is silicon.

[0032] The four sub-units a, b, c, and d are arranged in a square matrix, and the arrangement order is that a is located in the upper left corner, b is located in the upper right corner, c is located in the lower left corner, and d is located in the lower right corner; among the four sub-units a, b, c, and d, the distance between the centers of two adjacent sub-units is equal; each side of the four sub-units a, b, c, and d is parallel or perpendicular to each other.

[0033] The four sub-units a, b, c, and d are squares, at least two of the four sub-units have different side lengths, and at least one of the four sub-units a, b, c, and d has a cut corner, and the cut corner is cut along two adjacent sides of the square.

[0034] Through the simulation of a single sub-unit, adjust the size of a single sub-unit, the thickness of each layer, and the arrangement method, and calculate the absorption spectrum of mid-infrared light under normal incidence in different cases.

[0035] In some embodiments, the supercell structure is periodically arranged along the x and y directions, the single-period size is 3 μm, the bottom platinum thickness is 160 nm, the silicon layer thickness is 200 nm, and the top platinum thickness is 100 nm.

[0036] In some embodiments, the shapes of the sub-units a, b, c, and d are all squares, and the sub-units a, c, and d are cut off at a corner in the same direction, and the lengths of the two sides of the cut corner are the same.

[0037] In some embodiments, the side length of the square of the sub-unit a is 1.45 μm, and the ratio of the cut corner length to the square side length is 0.4;

[0038] The side length of the square of the sub-unit b is 0.9 μm, and there is no cut corner;

[0039] The side length of the square of the sub-unit c is 1 μm, and the ratio of the cut corner length to the square side length is 0.3;

[0040] The side length of the square of sub-unit d is 1.15 μm, and the ratio of the chamfer length to the side length of the square is 0.4;

[0041] In some embodiments, the absorption rates of the four sub-units a, b, c, and d at their corresponding resonant wavelengths are greater than 80%, and the average absorption rate of the metasurface for light in the range of 8-13 μm is greater than 60%.

[0042] According to the basic law of electromagnetic wave transmission, when a beam of light is incident on the sub-unit structure, the energy will be divided into three parts: transmitted light, reflected light, and absorbed by the structure. According to the law of conservation of energy, the sum of the absorption rate A(ω) of the structure, the reflectance R(ω), and the transmittance T(ω) of the structure is 1. Therefore, the absorption rate A(ω) of the structure is:

[0043] A(ω) = 1 - R(ω) - T(ω)

[0044] The thickness of the bottom layer metal platinum of the present invention is 160 nm, which is much larger than the skin depth of platinum in the mid-infrared band, and the transmittance T(ω) is zero. The above formula can be simplified to:

[0045] A(ω) = 1 - R(ω)

[0046] To maximize the absorption rate of the structure, the reflectance R(ω) should be reduced. By optimizing and adjusting the size and material of the unit structure, the impedance at the incident interface is matched with the impedance of free space:

[0047] The present invention selects metal platinum, which has a small real part of the dielectric constant and a large imaginary part of the dielectric constant, thereby reducing the reflectance of the incident light and attenuating the electromagnetic wave incident into the material.

[0048] The square sub-unit structure can achieve high absorption rate in a narrow band through single-mode resonance and interference effects. By cutting off a corner of the square, the C4 symmetry is broken, and multimodal absorption can be formed within one sub-unit. Although the absorption rate will decrease slightly, the absorption peak can be broadened to cover a wider absorption range.

[0049] Scan the absorption rate spectra of different sizes of the unit structure, including side length, chamfer size, thickness of each layer, etc., and require that the absorption rate of each sub-unit at the corresponding resonant frequency is greater than 80%. Combine the absorption spectra of the unit sizes to combine unit structures of different sizes to achieve broadband absorption coverage. Select four sub-units to form a supercell of a periodic structure. At the same time, combine the electromagnetic coupling effect between the units to enhance the overall absorption intensity, and fine-tune the parameters of each unit so that the absorption peaks are partially misaligned and superimposed within the target frequency spectrum, thereby forming a continuous and efficient broadband response.

[0050] The finite-difference time-domain (FDTD) method is used to simulate the sub-unit structure and the metasurface. The incident light is defined as a plane wave with a wavelength of 5 - 20 μm incident normally. The boundary conditions in the horizontal direction are both set to be periodic, and the boundary conditions in the vertical direction are set to be a perfectly matched layer to simulate the absorption of the metasurface when the electromagnetic wave is incident normally.

[0051] Through optimizing the structure design, a metasurface with broadband absorption in the infrared band is obtained. The average absorption rate is less than 20% in the wavelength ranges of 5 - 8 μm and 13 - 20 μm, and the average absorption rate is greater than 60% in the wavelength range of 8 - 13 μm.

[0052] The following are specific embodiments

[0053] Embodiment 1

[0054] As Figure 1 shown, the infrared broadband absorption metasurface structure is formed by the periodic arrangement of supercell structures. The supercell structure specifically includes: a bottom platinum metal layer 2, a dielectric layer 1 covering the bottom platinum metal layer, and four sub-units a, b, c, and d arranged on the dielectric layer; and the materials of the four sub-units a, b, c, and d are platinum, and the material of the dielectric layer is silicon;

[0055] This mid-infrared broadband absorption metasurface structure is a supercell metasurface composed of four sub-units a, b, c, and d with different sizes arranged in a planar square array. The distance between the centers of the sub-units is 1.5 μm. The supercells are arranged periodically in the x and y directions. In a supercell structure, the side lengths of the bottom metal layer and the dielectric layer are 3 μm. The thicknesses of the four sub-units a, b, c, and d are 100 nm, the thickness of the bottom metal layer is 160 nm, and the thickness of the middle dielectric layer is 200 nm. The specific parameters of its single-period structure are shown in the following table:

[0056] Table 1 Single-period structure parameters

[0057]

[0058] In this method, the finite-difference time-domain (FDTD) method is used to simulate the sub-units and supercells. The incident light is defined as a plane wave with a wavelength of 5 - 20 μm incident normally. The boundary conditions in the horizontal direction are both set to be periodic, and the boundary conditions in the vertical direction are set to be a perfectly matched layer to simulate the absorption of the metasurface when the electromagnetic wave is incident normally.

[0059] Figure 2 It is the absorption rate curve corresponding to different side lengths of the square without chamfer. The absorption peak shows a red-shift trend as the side length of the square increases. In this method, four sub-units with different side lengths are selected to cover the entire absorption range of 8 - 13 μm.

[0060] Figure 3Absorption rate curves for different ratios of the cut-off angle length to the side length of the square when the side length of the sub-unit is 1.2 μm. As the cut-off angle increases, the absorption peak decreases and broadens at the same time. In this method, the absorption curve is flattened by adjusting the cut-off angles of the four sub-units, so that the absorption rate in the wavelength range of 8-13 μm is basically above 60%. The absorption rate of the finally formed metasurface is as Figure 4 shown.

[0061] Figure 5 The upper surface electric field profile of the metasurface when light with the resonant wavelength corresponding to the four sub-units is incident. The incident light energy has been normalized. At the resonant wavelength corresponding to each sub-unit, the electric field energy is concentrated around the sub-unit, enhancing the loss of the electric field by the silicon layer, thereby achieving a high absorption rate.

[0062] It is easy for those skilled in the art to understand that the above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. An infrared broadband absorption metasurface structure, characterized in that, The infrared broadband absorption metasurface structure is formed by the periodic arrangement of supercell structures. The supercell structure specifically includes: a bottom platinum metal layer, a dielectric layer covering the bottom platinum metal layer, and four sub-units a, b, c, and d arranged on the dielectric layer. The materials of the four sub-units a, b, c, and d are platinum, and the material of the dielectric layer is silicon. Both the bottom platinum metal layer and the dielectric layer are square. The four sub-units a, b, c, and d are arranged in a square matrix. The arrangement order is that a is located in the upper left corner, b is located in the upper right corner, c is located in the lower left corner, and d is located in the lower right corner. Among the four sub-units a, b, c, and d, the distance between the centers of two adjacent sub-units is equal. At least one of the four sub-units a, b, c, and d is a pentagon formed by a square with one cut corner. The cut corner is cut along two adjacent sides of the square, and the other sub-units are squares without cut corners. Let the length of the side that is not cut off in the cut-corner square sub-unit be the side length of the cut-corner square sub-unit. Then at least two sub-units have different side lengths. When 1-3 of the four sub-units a, b, c, and d are cut-corner squares, each side that is not cut off in the cut-corner square sub-unit and each side of the other square sub-units without cut corners are parallel or perpendicular to each side of the bottom platinum metal layer. When all of the four sub-units a, b, c, and d are cut-corner squares, each side that is not cut off in the cut-corner square sub-unit is parallel or perpendicular to each side of the bottom platinum metal layer.

2. The infrared broadband absorption metasurface structure according to claim 1, characterized in that, When two or more of the four sub-units a, b, c, and d have a cut corner, each cut corner is in the upper left corner of the corresponding sub-unit, or each cut corner is in the upper right corner of the corresponding sub-unit, or each cut corner is in the lower left corner of the corresponding sub-unit, or each cut corner is in the lower right corner of the corresponding sub-unit.

3. The infrared broadband absorption metasurface structure according to claim 1, wherein The shape of the cut corner is an isosceles right triangle.

4. The infrared broadband absorption metasurface structure according to claim 3, wherein, The waist length of the isosceles right triangle is less than or equal to 60% of the side length of the sub-unit where the cut corner is located.

5. The infrared broadband absorption metasurface structure according to claim 1, characterized in that, The side lengths of the four sub-units a, b, c, and d are independently selected from 0.6 - 2 μm.

6. The infrared broadband absorption metasurface structure according to claim 1, characterized in that, The bottom platinum metal layer and the dielectric layer are square, with a side length of 2 - 5 μm.

7. The infrared broadband absorption metasurface structure according to claim 1, characterized in that, The thickness of the bottom platinum metal layer is 50 - 1000 nm.

8. The infrared broadband absorption metasurface structure according to claim 1, wherein The thickness of the dielectric layer is 100 - 300 nm.

9. The infrared broadband absorption metasurface structure according to claim 1, characterized in that, The four sub-units a, b, c, and d have the same thickness, which is 50 - 300 nm.

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