A design method for atmospheric window infrared broadband absorbing metamaterial structure

Abstract

The present invention belongs to the field of infrared artificial electromagnetic metamaterials and radiation cooling, and more specifically relates to a design method for regulating a specific band infrared broadband absorbing metamaterial structure. The present invention uses a metal film bottom structure to block transmission, uses impedance matching to reduce reflection, and achieves strong absorption of 3-5μm near-infrared bands and 8-13μm medium-wave infrared electromagnetic waves in the atmospheric window through a serrated multi-layer dual-medium periodic structure, while high reflection in other bands of the non-atmospheric window. From bottom to top, it is a basic unit composed of a metal base plate and a multi-layer serrated dual-medium structure. The metal base plate is composed of a metal film, the upper medium of the dual-medium structure is a metal film, and the lower medium is a Ge or P semiconductor film. The basic unit is periodically unfolded on an x-y two-dimensional plane. The present invention can improve the absorbing ability of the atmospheric window material, the reflection ability of the non-atmospheric window material, and the radiation cooling ability of the material, meeting the needs of energy saving and environmental protection.

Description

Technical Field

[0001] The invention belongs to the field of infrared artificial electromagnetic metamaterials and radiation refrigeration, and relates to an infrared broadband absorbing structure for radiative refrigeration and a design method thereof. Background Art

[0002] Metamaterials are composite materials with sub-wavelength structures composed of artificially constructed basic "units". They can exceed the limitations of natural material intrinsic parameters and achieve physical properties and functions that natural materials do not have. These materials modified by artificial structures are collectively referred to as "metamaterials". The macroscopic material properties of metamaterials are determined by their structure rather than by the intrinsic properties of the material. Therefore, the birth of metamaterials has introduced a new design concept to the material world, changing the previous practice of what kind of material is used to make what kind of object in nature, and creating a reverse design method, that is, manufacturing materials with corresponding functions according to the application needs of electromagnetic waves. It has great historical significance in the field of material science.

[0003] Kirchhoff's law shows that under thermal equilibrium conditions, the ratio of any object's radiation emittance to its absorption ratio of black body radiation is always equal to the radiation emittance of the black body at the same temperature. The blackness of the object, i.e. the emissivity, is equal to the absorptivity of the object at the same temperature. The greater the ability of an object to absorb radiation, the greater its ability to emit radiation. Therefore, a good absorber is also a good emitter.

[0004] Infrared absorbers can radiate energy to the cold outer space through the infrared transparent windows of the atmosphere, thereby achieving passive radiation cooling. The infrared transparent windows of the atmosphere are from 3 to 5μm, and 8 to 13μm. Infrared absorbers with such spectral selective absorption can also be used as imaging and detectors, with the advantage of low background noise. Therefore, broadening the selective band of the absorber to cover the two main transparent windows of the atmosphere is of great significance to promoting the application of absorbers in the field of radiation cooling.

[0005] Existing radiation cooling materials have defects such as narrow bandwidth, weak absorption rate, and poor radiation cooling effect. Summary of the invention

[0006] In view of the above technical problems, the present invention provides a broadband absorbing metamaterial for radiative cooling and a design method thereof, which can enable the absorbing metamaterial to have broadband strong absorption in a specific band and high reflection in other bands.

[0007] To achieve this goal, we designed a broadband metamaterial absorber that absorbs electromagnetic waves in the 3-5μm near-infrared band and 8-13μm mid-wave infrared band in the atmospheric window, while also having high reflection in other bands outside the atmospheric window.

[0008] The infrared selective band metamaterial absorber is composed of a periodic unit structure, each unit of which is a metal base plate and a pyramid-shaped multi-layer double film from bottom to top, and the multi-layer double film is formed by the alternating stacking of semiconductor dielectric layer materials and metal films.

[0009] The basic unit structure is periodically unfolded in the xy plane with a period of 0.5 to 5 μm.

[0010] The metal base plate is composed of metal films, such as gold, silver, copper, aluminum, etc., and its thickness is greater than the skin depth of infrared electromagnetic waves in metal, which is greater than 50nm.

[0011] In the multi-layer double-film structure, the upper medium is a metal film with a thickness of 10nm to 100nm, the lower medium is a semiconductor film such as Ge, P, etc. with a thickness of 10 to 100nm, and the number of layers of the double-medium structure is 13 to 30.

[0012] The dual dielectric structure is etched into a pyramid shape, and the cross-section in the xz plane is a trapezoid, the width of the top layer of the trapezoid is 0.2μm to 4μm, and the width of the bottom layer of the trapezoid is 0.3μm to 4.5μm.

[0013] The metal film underlying structure is used to block transmission, impedance matching is used to reduce reflection, and the serrated multi-layer dual-medium periodic structure is used to achieve absorption of 3-5μm near-infrared band and 8-13μm medium-wave infrared electromagnetic waves in the atmospheric window, while high reflection is achieved in other bands outside the atmospheric window.

[0014] According to absorption A=1-transmission T-reflection R, in order to reduce transmission, a metal base plate is added to the bottom layer. The base plate is composed of a metal film and its thickness is greater than the penetration depth of the infrared band, so that transmission T=0; in order to reduce the reflection of the atmospheric window, the wave impedance Z of the structure in this band must match the wave impedance Z of the external environment. The wave impedance Z of the structure outside the atmospheric window does not match the wave impedance of the external environment:

[0015] Step (1) uses the time domain difference method to perform numerical simulation. The incident light is a plane wave incident along the -z direction. The mode is a transverse electric field TE polarized wave. Due to the symmetry of the absorbing material in the direction perpendicular to the light propagation direction, a transverse electromagnetic field TM polarized wave is used to verify the result, which is consistent with the TE result.

[0016] Step (2) scanning parameters or using a genetic algorithm or a particle swarm algorithm to find parameters at which the absorption of the absorbing material in the atmospheric window of 3 μm to 5 μm and 8 μm to 13 μm is greater than 90%;

[0017] Step (3) scans the parameters, observes the absorption of the absorbing material in the range of 3 μm to 5 μm and 8 μm to 13 μm, and the reflection in other bands, and finds the curve with the strongest absorption and the corresponding structural parameters.

[0018] Step (4) calculates the absorption of the absorbing material at 3 μm to 5 μm and 8 μm to 13 μm at different incident angles of light, and the reflection of other bands, and finds the structural parameters with the strongest absorption and insensitive to the incident angle. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic diagram of a periodic unit of the absorbing structure.

[0020] Figure 2 Schematic diagram of the wave absorbing structure.

[0021] Figure 3 This is the infrared absorption, reflection and transmission diagram of Example 1.

[0022] Figure 4 This is the infrared absorption, reflection and transmission diagram of Example 2. DETAILED DESCRIPTION

[0023] Infrared selective band metamaterial absorbers are composed of periodic unit structures, such as Figure 1 As shown, Figure 1 This is a cross-section of a periodic unit of the designed absorber in the xz plane. Each unit is composed of a metal base plate 3 and a multi-layer double film from bottom to top. The multi-layer double film is composed of a dielectric layer material 1 and a metal film 2 stacked alternately.

[0024] Figure 2 The 3D top view of the designed absorber shows that the basic unit is similar to a pyramid structure and is periodically unfolded in the xy two-dimensional plane.

[0025] In Example 1, the basic unit is periodically expanded in the xy two-dimensional plane, and the period of the periodic unit in the xy plane is 2.4 μm.

[0026] Figure 2 The metal base plate 3 is gold with a thickness of 100 nm.

[0027] Figure 2 In the double-film structure, the upper dielectric 2 is gold with a thickness of 25 nm, the lower dielectric 1 is Ge thin film with a thickness of 55 nm, and the number of layers of the multilayer double-dielectric structure is 16.

[0028] The designed absorber has a trapezoidal shape in the xz cross section, the width of the bottom layer of the trapezoid is 1.84 μm, and the width of the top layer is 0.8 μm.

[0029] Figure 3 This is the infrared absorption reflection transmission diagram of Example 1, which has an absorption of more than 50% in the mid-wave infrared of 3.3 to 5.1 μm and the far-wave infrared of 6.8 to 15.3 μm.

[0030] In Example 2, the basic unit is periodically developed in the xy two-dimensional plane, and the period of the periodic unit in the xy plane is 3 μm. The metal base plate 3 is silver and has a thickness of 100 nm.

[0031] In the double-film structure, the upper dielectric layer is a metal thin film gold 2 with a thickness of 25 nm, the lower dielectric layer 1 is a P thin film with a thickness of 45 nm, and the number of layers of the multi-layer double-dielectric structure is 20.

[0032] The designed absorber has a trapezoidal shape in the xz cross section, the width of the bottom layer of the trapezoid is 1.84 μm, and the width of the top layer is 0.7 μm.

[0033] Figure 4 This is the infrared absorption reflection transmission diagram of Example 2, which has an absorption of more than 50% in the mid-wave infrared of 3.8 to 6 μm and the far-wave infrared of 7.1 to 18 μm.

Claims

1. A design method for an atmospheric window infrared broadband absorbing metamaterial structure, characterized in that: The basic unit consists of a metal base plate and a multi-layer zigzag dual-medium structure from bottom to top. The basic unit is periodically unfolded on the xy two-dimensional plane: The metal bottom plate is composed of a metal film, the thickness of which is greater than the skin depth of infrared electromagnetic waves in metal, at least greater than 50nm; the upper medium in the dual-medium structure is a metal film, the thickness of which is 10nm to 100nm, the lower medium is a Ge or P semiconductor film, the thickness of which is 10 to 100nm, the number of layers of the dual-medium structure is 13 to 50, and the width and length of the basic unit of the dual-medium structure are 1μm to 3μm; The metal base plate material is metal gold, silver, copper or aluminum; The basic unit structure is periodically unfolded in the xy plane with a period of 0.5 to 5 μm. The dual dielectric structure is etched into a pyramid shape, and the cross-section in the xz plane is a trapezoid. The width of the top layer of the trapezoid is 0.2 μm to 4 μm, and the width of the bottom layer of the trapezoid is 0.3 μm to 4.5 μm. The metal film bottom structure is used to block transmission, impedance matching is used to reduce reflection, and the serrated multilayer dual-medium periodic structure is used to achieve absorption of 3-5μm near-infrared band and 8-13μm mid-wave infrared electromagnetic waves in the atmospheric window, while other bands outside the atmospheric window are highly reflected; According to absorption A=1-transmission T-reflection R, in order to reduce transmission, a metal base plate is added to the bottom layer. The base plate is composed of a metal film and its thickness is greater than the penetration depth of the infrared band, so that transmission T=0; in order to reduce the reflection of the atmospheric window, the wave impedance Z of the absorbing metamaterial structure in this band must match the wave impedance Z of the external environment, and the wave impedance Z of the non-atmospheric window absorbing metamaterial structure does not match the wave impedance of the external environment: Step (1) uses the time domain difference method to perform numerical simulation. The incident light is a plane wave incident along the -z direction. The mode is a transverse electric field TE polarized wave. Due to the symmetry of the absorbing material in the direction perpendicular to the light propagation direction, the transverse electromagnetic field TM polarized wave is used to verify the result, which is consistent with the TE result. Step (2) scanning parameters, using genetic algorithm and particle swarm algorithm to find parameters that make the absorption of the absorbing material greater than 90% in the atmospheric window of 3 μm to 5 μm and 8 μm to 13 μm; Step (3) scanning parameters, observing the absorption of the absorbing material in the range of 3 μm to 5 μm and 8 μm to 13 μm, and the reflection in other bands, and finding the curve with the strongest absorption and the corresponding structural parameters; Step (4) calculates the absorption of the absorbing material at 3 μm to 5 μm and 8 μm to 13 μm at different incident angles of light, and the reflection of other bands, and finds the structural parameters with the strongest absorption and insensitive to the incident angle.

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