An inorganic radiative cooling visible window with excellent spectral selectivity

Abstract

The present invention relates to the technical field of optical radiation refrigeration, and specifically relates to an inorganic radiation refrigeration visible window with excellent spectral selectivity. Its structure is composed of a top super-surface, an intermediate dielectric layer, and a lower periodic film layer structure. The top super-surface is an AZO super-surface layer, the intermediate dielectric layer is a TiO2 dielectric layer, and the lower periodic film layer structure is a (TiO2 / Ag / TiO2) n periodic film layer structure, where n represents the number of periods. The present invention has good transmittance and reflectance in the visible light band and the near-infrared band respectively. The average absorption rate reaches 86.4% within the "atmospheric window" of 8μm - 13μm, and also shows good emission performance in the 3μm - 5μm atmospheric window. It is insensitive to polarization and can still maintain a high absorption rate at a large incident angle of 70°. It has good thermal emission performance and can emit infrared radiation to achieve the effect of radiation refrigeration.

Description

Technical Field

[0001] The present invention relates to the technical field of optical radiation cooling, and specifically to an inorganic radiation cooling visible window with excellent spectral selectivity. Background Art

[0002] With the global shortage of resources and the increasing energy consumption, energy conservation has become a top priority. To reduce the cooling energy consumption, it is required that automotive windows and building glasses have both high solar reflectivity and good heat dissipation functions, and prevent excessive heat loss due to indoor heat emission in winter.

[0003] In the atmosphere, the transmission coefficients are different for different wavelength bands, and the wavelength bands with high transmission coefficients are called "atmospheric windows", such as: as Figure 1 shown, mainly including 0.3 - 2.5 μm, 3 - 5 μm, 8 - 13 μm, etc. 8 - 13 μm is an important wavelength band for blackbody radiation at room temperature (300K). "Passive radiative cooling" is to radiate the heat of the earth into outer space through the atmospheric transparent windows, 3 - 5 μm and 8 - 13 μm, so as to achieve the purpose of cooling without increasing the external energy consumption. The temperature of the object undergoing radiative cooling is often lower than the ambient temperature. Radiative cooling phenomena can also be observed in nature, such as frosting. Frosting occurs because of the radiative cooling of the ground, so that although the ambient temperature is higher than 0°C, the ground temperature is lower than 0°C due to radiative cooling, resulting in frosting. Radiative cooling can be widely applied to the radiative cooling of solar cells, buildings, textiles, and water collection, etc. This is of great significance for energy utilization, green energy conservation, environmental protection, etc.

[0004] In the prior art, CN110774673B discloses a radiative cooling transmissive film, which includes a selective emission layer and a selective reflection layer arranged in sequence from outside to inside; the selective reflection layer includes a metal layer and a first non-metal layer, and the first non-metal layer is arranged on the incident light side of the metal layer. Further, in the above technical solution, a second non-metal layer is included, and the second non-metal layer is arranged on the other side of the incident light of the metal layer. By the superposition of the metal layer, the first non-metal layer and the second non-metal layer and appropriate film layer design, the transmission state of incident light is changed, that is, the function of transmitting visible light and reflecting near-infrared light is realized through the optical interference of multiple layers of films. It is particularly emphasized that "the thicknesses of the metal layer, the first non-metal layer, and the second non-metal layer are all between 0.1 nm and 50 nm. The material of the metal layer is one or more of Ag, Al, and Cu; the material of the first non-metal layer and / or the second non-metal layer is one or more of ITO, GTO, ATO, MoO3, WO3, AZO, TiO2, SiO2, Ti, ZnO, Al2O3, NiCr, and MgF2." The radiative cooling transmissive film recorded in this technical solution has an ultraviolet light blocking rate of more than 90%, a near-infrared light reflectivity of more than 30%, and a visible light transmittance of more than 50%. The film layer of the selective reflection layer can be designed according to the requirements for visible light in the application place, so that the visible light transmittance can be selected between 50% and 80%.

[0005] In this application solution, it is emphasized that in the band of "7-14 μm", the emissivity is more than 85%. However, no in-depth research has been done on the radiation in other bands. In addition, a selective emission layer containing organic substances such as polymer resin is used in this document. The polymer emission layer mainly emits by the intrinsic properties of the material, and has the deficiency that the emission frequency band and intensity cannot be designed. Research shows that if the radiation cooling structure emission frequency band coincides with the atmospheric window, better radiation cooling effect can be obtained. At the same time, its polymer emission layer is prone to aging under direct sunlight, affecting its service life. Therefore, it is very necessary to develop an inorganic radiative cooling visible window with excellent spectral selectivity. Summary of the Invention

[0006] The present invention provides a radiative cooling thin film, which can solve the radiation problems in the 3-5 μm band in addition to the 8-13 μm band.

[0007] The technical solution of this application is as follows. A radiative cooling thin film has a structure composed of a top layer, an intermediate dielectric layer, and a lower layer with a periodic structure. The top layer is an AZO metasurface layer, the intermediate dielectric layer is a TiO2 dielectric layer, and the lower layer with a periodic structure is a periodic structure layer of (TiO2 / Ag / TiO2) n .

[0008] The length of the AZO metasurface layer is 1.1 μm, the period of the metasurface is 1.3 μm, and the thickness of the AZO metasurface is 0.04 μm.

[0009] The thickness of the TiO2 dielectric layer is 1.4 μm.

[0010] The thickness of TiO2 in the periodic structure (TiO2 / Ag / TiO2) layer ranges from 0.015 μm to 0.03 μm, and the thickness of Ag ranges from 0.005 μm to 0.02 μm.

[0011] n is from 1 to 4.

[0012] The present invention provides a thin film model composed of an AZO metasurface, a TiO2 dielectric film, and (TiO2 / Ag / TiO2). n It can be applied to the radiative cooling of architectural glass and automotive glass, and it is required to have high transmittance in the visible light band range, high reflectivity in the near-infrared band range, and high emissivity (absorbance) in the mid-infrared, mainly in the "atmospheric window" (3 μm - 5 μm, 8 μm - 13 μm). The finite-difference time-domain method is used to study the influence of the thicknesses of Ag and TiO2 in the reflective layer (TiO2 / Ag / TiO2) and the number of film layers (TiO2 / Ag / TiO2) on the performance of the model in each band. The influence of polarization and incident angle on the spectral performance of the model is also studied. Finally, it is found that the model has good transmittance and reflectivity in the visible light band and near-infrared band respectively, the average absorbance reaches 86.4% in the "atmospheric window" of 8 μm - 13 μm, and it also shows good emission performance in the 3 μm - 5 μm atmospheric window. It is insensitive to polarization and can still maintain a high absorbance at a large incident angle of 70°. It has good thermal emission performance and can emit infrared radiation to the universe to achieve the effect of radiative cooling. Description of the Drawings

[0013] Figure 1 It is the curve of the transmittance of the "atmospheric window" changing with wavelength;

[0014] Figure 2 It is the schematic structural diagram of the model based on radiative cooling described in the present invention;

[0015] Figure 3(a) is the transmission spectrum of the model in the visible light range when the Ag film thickness changes;

[0016] Figure 3(b) is the reflection spectrum of the model in the near-infrared range when the Ag film thickness changes;

[0017] Figure 3(c) is the absorption spectrum of the model in the mid-infrared range when the Ag film thickness changes

[0018] Figure 4(a) shows the transmission spectrum of the model in the visible light range when the thickness of the TiO2 film changes;

[0019] Figure 4(b) shows the reflection spectrum of the model in the near-infrared range when the thickness of the TiO2 film changes;

[0020] Figure 4(c) shows the absorption spectrum of the model in the mid-infrared range when the thickness of the TiO2 film changes

[0021] Figure 5(a) shows the transmission spectrum of the model in the visible light range when the number of underlying film layer structures changes;

[0022] Figure 5(b) shows the reflection spectrum of the model in the near-infrared range when the number of underlying film layer structures changes;

[0023] Figure 5(c) shows the absorption spectrum of the model in the mid-infrared range when the number of underlying film layer structures changes;

[0024] Figure 6(a) shows the model curve composed of the AZO metasurface, dielectric layers of different materials, and the composite film Figure 1 ;

[0025] Figure 6(b) shows the model curve composed of the AZO metasurface, dielectric layers of different materials, and the composite film Figure 2 ;

[0026] Figure 6(c) shows the model curve diagram three composed of the AZO metasurface, dielectric layers of different materials, and the composite film;

[0027] Figure 6(d) shows the model curve diagram four composed of the AZO metasurface, dielectric layers of different materials, and the composite film;

[0028] Figure 7(a) shows the model curve composed of the AZO metasurface, dielectric layers of different materials, and the Ag reflective layer Figure 1 ;

[0029] Figure 7(b) shows the model curve composed of the AZO metasurface, dielectric layers of different materials, and the Ag reflective layer Figure 2 ;

[0030] Figure 8(a) shows the model curve composed of the AZO metasurface, dielectric layers of different materials, and the AZO reflective layer Figure 1 ;

[0031] Figure 8(b) shows the model curve composed of the AZO metasurface, dielectric layers of different materials, and the AZO reflective layer Figure 2 ;

[0032] Figure 8(c) shows the model curve diagram three composed of the AZO metasurface, dielectric layers of different materials, and the AZO reflective layer;

[0033] Figure 8(d) is the fourth model curve graph composed of an AZO metasurface, dielectric layers of different materials, and an AZO reflective layer;

[0034] Figure 9 They are the OSR absorption spectra corresponding to TE-polarized and TM-polarized electromagnetic waves under normal incidence;

[0035] Figure 10(a) shows the variation of the absorption rate of the model with the incident angle in the TE mode;

[0036] Figure 10(b) shows the variation of the absorption rate of the model with the incident angle in the TM mode. Detailed implementation manners

[0037] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0038] The basic structure of the model described in the present invention is as Figure 2 shown. It is composed of a top-layer transparent conductive oxide (aluminum-doped zinc oxide) AZO metasurface, a TiO2 dielectric layer, and a (TiO2 / Ag / TiO2) n reflective layer. Compared with other common noble metals, Ag has higher visible light transmittance and lower infrared transmittance. Therefore, Ag is selected as the metal film layer; by utilizing the high refractive index of TiO2 to reduce the reflection of the silver layer, and aiming at problems such as the silver film being easily oxidized, eroded, and having poor adhesion, it is arranged on both sides of the silver.

[0039] Embodiment 1: An inorganic radiative cooling visible window with excellent spectral selectivity, the structure of which is composed of a top-layer metasurface, an intermediate dielectric layer, and a bottom layer with a periodic structure. The top layer is an AZO metasurface layer, the intermediate dielectric layer is a TiO2 dielectric layer, and the lower layer with a periodic structure is a (TiO2 / Ag / TiO2) n periodic structure layer. Wherein, n represents the number of periods of the periodic structure layer.

[0040] Among them, the length of the AZO metasurface takes a value of 1.1 μm, the period of the AZO metasurface takes a value of 1.3 μm, the thickness of the AZO metasurface takes a value of 0.04 μm, the thickness of the TiO2 dielectric layer is 1.4 μm, the thickness of Ag in (TiO2 / Ag / TiO2) takes a value of 0.01 μm, the thickness of TiO2 takes a value of 0.025 μm, the thicknesses of the two layers of TiO2 are the same, and n is 3.

[0041] Example 2. An inorganic radiative cooling visible window with excellent spectral selectivity, the structure of which consists of a top layer, an intermediate dielectric layer, and a lower layer with a periodic structure. The top layer is an AZO metasurface layer, the intermediate dielectric layer is a TiO2 dielectric layer, and the lower layer with a periodic structure is (TiO2 / Ag / TiO2). n The periodic structure layer.

[0042] Among them, the length of the AZO metasurface is 1.1 μm, the period of the AZO metasurface is 1.3 μm, the thickness of the AZO metasurface is 0.04 μm, the thickness of the TiO2 dielectric layer is 1.4 μm, the thickness of Ag in (TiO2 / Ag / TiO2) is 0.005 μm, the thickness of TiO2 is 0.025 μm, the thicknesses of the two TiO2 layers are the same, and n is 3.

[0043] Example 3. An inorganic radiative cooling visible window with excellent spectral selectivity, the structure of which consists of a top layer, an intermediate dielectric layer, and a lower layer with a periodic structure. The top layer is an AZO metasurface layer, the intermediate dielectric layer is a TiO2 dielectric layer, and the lower layer with a periodic structure is (TiO2 / Ag / TiO2). n The periodic structure layer.

[0044] Among them, the length of the AZO metasurface is 1.1 μm, the period of the AZO metasurface is 1.3 μm, the thickness of the AZO metasurface is 0.04 μm, the thickness of the TiO2 dielectric layer is 1.4 μm, the thickness of Ag in (TiO2 / Ag / TiO2) is 0.015 μm, the thickness of TiO2 is 0.025 μm, the thicknesses of the two TiO2 layers are the same, and n is 3.

[0045] Example 4. An inorganic radiative cooling visible window with excellent spectral selectivity, the structure of which consists of a top layer, an intermediate dielectric layer, and a lower layer with a periodic structure. The top layer is an AZO metasurface layer, the intermediate dielectric layer is a TiO2 dielectric layer, and the lower layer with a periodic structure is (TiO2 / Ag / TiO2). n The periodic structure layer.

[0046] Among them, the length of the AZO metasurface is 1.1 μm, the period of the AZO metasurface is 1.3 μm, the thickness of the AZO metasurface is 0.04 μm, the thickness of the TiO2 dielectric layer is 1.4 μm, the thickness of Ag in (TiO2 / Ag / TiO2) is 0.02 μm, the thickness of TiO2 is 0.025 μm, the thicknesses of the two TiO2 layers are the same, and n is 3.

[0047] Example 5. An inorganic radiative cooling visible window with excellent spectral selectivity, whose structure consists of a top layer, an intermediate dielectric layer, and a lower layer with a periodic structure. The top layer is an AZO metasurface layer, the intermediate dielectric layer is a TiO2 dielectric layer, and the lower layer with a periodic structure is (TiO2 / Ag / TiO2). n The periodic structure layer.

[0048] Among them, the length of the AZO metasurface is 1.1 μm, the period of the AZO metasurface is 1.3 μm, the thickness of the AZO metasurface is 0.04 μm, the thickness of the TiO2 dielectric layer is 1.4 μm, the thickness of Ag in (TiO2 / Ag / TiO2) is 0.01 μm, the thickness of TiO2 is 0.015 μm, the thicknesses of the two TiO2 layers are the same, and n is 3. n The thickness of Ag in (TiO2 / Ag / TiO2) is 0.01 μm, the thickness of TiO2 is 0.015 μm, the thicknesses of the two TiO2 layers are the same, and n is 3.

[0049] Example 6. An inorganic radiative cooling visible window with excellent spectral selectivity, whose structure consists of a top layer, an intermediate dielectric layer, and a lower layer with a periodic structure. The top layer is an AZO metasurface layer, the intermediate dielectric layer is a TiO2 dielectric layer, and the lower layer with a periodic structure is (TiO2 / Ag / TiO2). n The periodic structure layer.

[0050] Among them, the length of the AZO metasurface is 1.1 μm, the period of the AZO metasurface is 1.3 μm, the thickness of the AZO metasurface is 0.04 μm, the thickness of the TiO2 dielectric layer is 1.4 μm, the thickness of Ag in (TiO2 / Ag / TiO2) is 0.01 μm, the thickness of TiO2 is 0.02 μm, the thicknesses of the two TiO2 layers are the same, and n is 3.

[0051] Example 7. An inorganic radiative cooling visible window with excellent spectral selectivity, whose structure consists of a top layer, an intermediate dielectric layer, and a lower layer with a periodic structure. The top layer is an AZO metasurface layer, the intermediate dielectric layer is a TiO2 dielectric layer, and the lower layer with a periodic structure is (TiO2 / Ag / TiO2). n The periodic structure layer.

[0052] Among them, the length of the AZO metasurface is 1.1 μm, the period of the AZO metasurface is 1.3 μm, the thickness of the AZO metasurface is 0.04 μm, the thickness of the TiO2 dielectric layer is 1.4 μm, the thickness of Ag in (TiO2 / Ag / TiO2) is 0.01 μm, the thickness of TiO2 is 0.03 μm, the thicknesses of the two TiO2 layers are the same, and n is 3.

[0053] Example 8. An inorganic radiative cooling visible window with excellent spectral selectivity, whose structure consists of a top layer, an intermediate dielectric layer, and a lower layer with a periodic structure. The top layer is an AZO metasurface layer, the intermediate dielectric layer is a TiO2 dielectric layer, and the lower layer with a periodic structure is (TiO2 / Ag / TiO2). n The periodic structure layer.

[0054] Among them, the length of the AZO metasurface is 1.1 μm, the period of the AZO metasurface is 1.3 μm, the thickness of the AZO metasurface is 0.04 μm, the thickness of the TiO2 dielectric layer is 1.4 μm, the thickness of Ag in (TiO2 / Ag / TiO2) is 0.01 μm, the thickness of TiO2 is 0.025 μm, the thicknesses of the two TiO2 layers are the same, and n is 1.

[0055] Example 9. An inorganic radiative cooling visible window with excellent spectral selectivity, whose structure consists of a top layer, an intermediate dielectric layer, and a lower layer with a periodic structure. The top layer is an AZO metasurface layer, the intermediate dielectric layer is a TiO2 dielectric layer, and the lower layer with a periodic structure is (TiO2 / Ag / TiO2). n The periodic structure layer.

[0056] Among them, the length of the AZO metasurface is 1.1 μm, the period of the AZO metasurface is 1.3 μm, the thickness of the AZO metasurface is 0.04 μm, the thickness of the TiO2 dielectric layer is 1.4 μm, the thickness of Ag in (TiO2 / Ag / TiO2) is 0.01 μm, the thickness of TiO2 is 0.025 μm, the thicknesses of the two TiO2 layers are the same, and n is 2.

[0057] Example 10. An inorganic radiative cooling visible window with excellent spectral selectivity, whose structure consists of a top layer, an intermediate dielectric layer, and a lower layer with a periodic structure. The top layer is an AZO metasurface layer, the intermediate dielectric layer is a TiO2 dielectric layer, and the lower layer with a periodic structure is (TiO2 / Ag / TiO2). n The periodic structure layer.

[0058] Among them, the length of the AZO metasurface is 1.1 μm, the period of the AZO metasurface is 1.3 μm, the thickness of the AZO metasurface is 0.04 μm, the thickness of the TiO2 dielectric layer is 1.4 μm, the thickness of Ag in (TiO2 / Ag / TiO2) is 0.01 μm, the thickness of TiO2 is 0.025 μm, the thicknesses of the two TiO2 layers are the same, and n is 4.

[0059] In this invention, FDTD Solutions is used to simulate and analyze the transmittance of the model in the visible light range, the reflectance in the near-infrared band, and the absorptance in the mid-infrared spectral range. The incident light source is a plane wave, which is incident perpendicularly to the model surface along the z-axis, and the electric field is polarized along the x-axis. The step size of the grid is set to 0.05 μm in the x-y direction and 0.005 μm in the z direction; since this structure is still symmetric in the x and y directions, Anti-symmetric and Symmetric boundary conditions are adopted in the x and y directions respectively, and the PML boundary condition is adopted in the z direction.

[0060] The AZO metasurface adopts a square structure. When electromagnetic waves are incident on the square AZO metasurface, the electric field in any direction can cause the unit to resonate, which can well improve the polarization sensitivity of the structure; choosing a suitable metal reflection layer can reflect the solar spectrum. Under the action of the external electromagnetic field, the metal reflection layer and the AZO metasurface can form a closed current loop composed of currents flowing in opposite directions and displacement currents in the intermediate dielectric layer. The induced magnetic flux in the closed current loop continuously changes under the action of the external electromagnetic field and generates an induced electromotive force. According to the right-hand rule, the current in the closed loop will generate a magnetic moment, and then the induced magnetic field and the external magnetic field act together to generate magnetic resonance. When electromagnetic waves are incident, the electromagnetic resonances generated on the surface are superimposed on each other, absorbing the incident electromagnetic waves. By changing parameters such as the metasurface structure and the thickness of the dielectric layer, impedance matching is achieved, thereby broadening the absorption band and increasing the peak value of the absorption peak.

[0061] Influence of structural parameters on the performance of the thin film described in this invention

[0062] 1. Influence of Ag thickness on the performance of the thin film

[0063] This section discusses the influence of the change in Ag film thickness on the visible light transmittance, near-infrared reflectance, and mid-infrared absorption (emission) rate of the model. When the length of the AZO metasurface is 1.1 μm, the period of the AZO metasurface is 1.3 μm, the thickness of the AZO metasurface is 0.04 μm, the thickness of the TiO2 dielectric layer is 1.4 μm, and the thickness of TiO2 in (TiO2 / Ag / TiO2) is 0.025 μm.

[0064] Set different Ag film thicknesses to 0.005 μm, 0.01 μm, 0.015 μm, and 0.02 μm respectively, and use FDTD Solutions to simulate and analyze them. The comparison diagrams of the curves of the visible light transmittance, near-infrared reflectance, and mid-infrared absorption (emission) rate changing with wavelength are shown in Figure 3(a) 、 3(b) 、3(c).

[0065] With the increase of the Ag film thickness: As can be seen from Fig. 3(a), the transmittance of the model in the visible light band decreases significantly with the increase of the Ag film thickness;

[0066] As can be seen from Fig. 3(b), when the Ag thickness is 0.005μm, the reflectivity has a large difference compared with 0.01μm, 0.015μm, and 0.02μm. This shows that when the Ag film thickness is relatively thin, the model cannot reflect the near-infrared spectrum well, and the reflectivity is poor at this time. The reflectivity of the model in the near-infrared band increases with the increase of the Ag film thickness. When the Ag film thickness is greater than 0.015μm, the reflectivity of the model in the near-infrared no longer changes significantly with the change of the thickness;

[0067] As can be seen from Fig. 3(c), the absorption rate of the model in the mid-infrared band increases with the increase of the Ag film thickness. When the Ag film thickness is greater than 0.005μm, the absorption rate in the mid-infrared no longer changes significantly with the change of the thickness and remains basically the same. To sum up, in this paper, the Ag film thickness is selected as 0.01μm. At this time, the model has relatively good transmittance and reflectivity in the visible light band and the near-infrared band, respectively. Moreover, the average absorption rate in the "atmospheric window" of 8μm - 13μm reaches 86.4%, and it has good thermal emission performance, which can emit infrared radiation into the universe to achieve the effect of radiative cooling.

[0068] 2. Influence of TiO2 Thickness on Film Properties

[0069] This section discusses the influence of the change of the TiO2 film thickness in the reflective layer (TiO2 / Ag / TiO2) on the visible light transmittance, near-infrared reflectivity, and mid-infrared absorption (emission) rate of the film. When the length of the AZO metasurface is 1.1μm, the period of the AZO metasurface is 1.3μm, the thickness of the AZO metasurface is 0.04μm, the thickness of the TiO2 dielectric layer is 1.4μm, and the Ag film thickness in (TiO2 / Ag / TiO2) is 0.01μm. The TiO2 film thicknesses of the reflective layer are set to 0.015μm, 0.02μm, 0.025μm, and 0.03μm respectively, and FDTD Solutions is used to simulate and analyze them to obtain the curve comparison diagrams of the visible light transmittance, near-infrared reflectivity, and mid-infrared internal absorption (emission) rate changing with the wavelength, as shown in Figure 4(a) 、 4(b) 、4(c).

[0070] With the increase of the TiO2 film thickness: As can be seen from Figure 4(a), when the TiO2 thickness increases from 0.015 μm to 0.02 μm, the transmittance of the model in the visible light band increases significantly. When the TiO2 thickness increases from 0.02 μm to 0.03 μm, the transmittance of the model in the visible light band does not increase or decrease with the increase of the TiO2 thickness, but rather shows an upward or downward state accordingly;

[0071] As can be seen from Figure 4(b), the near-infrared reflectance of the model first decreases, then increases, and then decreases. The change trend of the visible light transmittance is similar. It does not show a linear change with the increase of the TiO2 thickness. When the TiO2 thickness is 0.015 μm and 0.025 μm, the two reflectances are basically the same, and the corresponding near-infrared reflectance is significantly higher than that when the TiO2 film thickness is 0.02 μm and 0.03 μm;

[0072] As can be seen from Figure 4(c), the mid-infrared absorption rate does not change significantly with the increase of the TiO2 film thickness.

[0073] In summary, the present invention selects the TiO2 film thickness of 0.025 μm. At this time, the model has relatively good transmittance and reflectance in the visible light and near-infrared, respectively. Moreover, the average absorption rate in the "atmospheric window" of 8 μm - 13 μm reaches 86.4%, and it has good thermal emission performance, and can achieve the purpose of radiative cooling by emitting infrared radiation into the universe.

[0074] 3. Influence of the number of film layer structures on the model performance

[0075] First, (TiO2 / Ag / TiO2) is defined as a film layer structure. When the length of the AZO metasurface is 1.1 μm, the period of the AZO metasurface is 1.3 μm, the thickness of the AZO metasurface is 0.04 μm, the thickness of the TiO2 dielectric layer is 1.4 μm, and the thickness of Ag in (TiO2 / Ag / TiO2) n , is 0.01 μm, and the thickness of TiO2 is 0.025 μm. This section discusses the influence of the number of film layer structures on the visible light transmittance, near-infrared reflectance, and mid-infrared absorption (emission) rate of the model. The number of film layer structures is set to 1, 2, 3, and 4 respectively, and FDTD Solutions is used to perform simulation analysis on it. The comparison diagrams of the curves of the transmittance in the visible light, near-infrared reflectance, and mid-infrared absorption (emission) rate changing with the wavelength are as Figure 5(a) 、 5(b) 、shown in Figure 5(c).

[0076] With the increase of the number of film layer structures: As can be seen from Figure 5(a), the visible light transmittance of the model gradually decreases with the increase of the number of film layer structures, and the transmission peak gradually becomes narrower with the increase of the number of film layer structures, but the change is small;

[0077] As can be seen from Fig. 5(b), the near-infrared reflectivity of the model gradually increases with the increase in the number of film layer structures. This is because the increase in the total thickness of the metal layer will enhance its reflection ability accordingly. After calculation, when the number of film layer structures is 1, 2, 3, and 4, the average reflectivities in the near-infrared band are 45%, 67.2%, 75.6%, and 77.6% respectively. It can be seen that when the number of film layer structures is 1, the near-infrared reflection ability is relatively low. When the number of film layer structures is greater than 3, the reflectivity no longer increases significantly;

[0078] As can be seen from Fig. 5(c), the mid-infrared absorptivity of the model does not change significantly with the increase in the number of film layer structures. To sum up, when the number of film layer structures is 3, the model has relatively good transmittance and reflectivity in the visible light and near-infrared regions respectively, and the average absorptivity within the "atmospheric window" of 8 μm - 13 μm reaches 86.4%, having good thermal emission performance and being able to emit infrared radiation into the universe to achieve the purpose of radiative cooling.

[0079] 4. Determination of the intermediate dielectric layer

[0080] As Figure 6(a) , 6(b) , 6(c), and 6(d) show, they are the model curve graphs composed of the AZO metasurface, dielectric layers of different materials, and the reflection layer. Among them, the intermediate dielectric layer uses SiO2 with a thickness of 1.4 μm, TiO2 with a thickness of 1.4 μm, and Al2O 3。 The reflection layer uses Ag with a thickness of 200 nm, ZAO thin film, and (TiO2 / Ag / TiO2) n periodic structure. Among them, Figure 6(a) , 6(b) is the absorption spectrum, Figure 6(c) , 6(d) are the reflection spectrum and transmission spectrum respectively.

[0081] From Figure 6(a) , 6(b) , 6(c), and 6(d), it can be seen that compared with SiO2 and Al2O3, when TiO2 is used as the dielectric layer, the range of its high emission (absorption) is in relatively good agreement with the atmospheric window (3 μm - 5 μm, 8 μm - 13 μm) band, and its near-infrared reflection and visible light transmission are not much different from those of the other two materials. Therefore, TiO2 can be selected as the dielectric layer.

[0082] As Figure 7(a) , 7(b) show, they are the model curve graphs composed of the AZO metasurface, dielectric layers of different materials, and the Ag reflection layer. Among them, the thickness of the Ag film in the reflection layer is set to 200 nm, so the transmittance of the model can be regarded as zero. Figure 7(a) ,7(b) They respectively represent the absorption spectrum and the reflection spectrum. As can be seen from Figure 7, when the dielectric layer is TiO2, the solar spectrum reflection performance is significantly better than that of other dielectric layers; in the mid-infrared band, the absorption bands corresponding to the three dielectric layer materials are different, among which when TiO2 is the dielectric layer, it perfectly coincides with the atmospheric window, and the other two dielectric layers have a wider absorption band in the mid-infrared range.

[0083] Such as Figure 8(a) , 8(b) As shown in 8(a), 8(b), 8(c), and 8(d), it is a model curve graph composed of an AZO metasurface, dielectric layers of different materials, and an AZO reflection layer. Among them, Figure 8(a) , 8(b) are the reflection spectrum and the transmission spectrum, Figure 8(c) , 8(d) are the absorption spectra at different wavelengths.

[0084] In the visible light range, the model has a higher transmittance when the dielectric layer uses SiO2 and Al2O3; when the dielectric layer is TiO2, the near-infrared reflectance of the model is higher than that when the dielectric layer is SiO2 and Al2O3, but the reflectance of all three materials does not exceed 0.6; in the mid-infrared band, the absorption bands of the three dielectric layers are different, among which the absorption band of the TiO2 dielectric layer perfectly coincides with the atmospheric window band, but the absorption bands of the other two materials are wider.

[0085] At the same time, compared with the reflection layers Ag and ZAO, using (TiO2 / Ag / TiO2) n can simultaneously obtain better visible light transmittance and near-infrared reflectance. For this reason, the middle dielectric layer uses 1.4μm TiO2, and the reflection layer uses the periodic structure (TiO2 / Ag / TiO2) n .

[0086] 5. Influence of polarization and incident angle on the performance of its model

[0087] The structural model designed by the solution of the present invention has multiple functions of visible light transmission, near-infrared reflection, and mid-infrared absorption. However, in actual design, the influence of polarization and incident angle on its performance also needs to be considered. For this reason, the present invention respectively studies the influence of polarization and incident angle on its performance. The incident wave is sequentially set as TE and TM polarization.

[0088] Such as Figure 9 shown, it is the model absorption spectrum corresponding to TE and TM polarization under normal incidence. As can be seen from Figure 9 , the absorption spectra of the model under TE and TM polarization are basically the same. This is because the square AZO metasurface unit structure at the top of the designed model has good symmetry, which can well improve the sensitivity of the model structure to polarization. It can be concluded therefrom that the model designed by this solution is insensitive to polarization.

[0089] On the basis of the optimized structural parameters mentioned above, the spectral performance of the model is simulated for different incident angles in TE and TM modes respectively. First, before simulating and calculating using the FDTD Solutions software, change the plane wave type to BFAST (Broadband Fixed Angle Source Technology) in the Source General tab. It can be used for periodic structures when they are incident by a broadband light source at a certain angle, and it can provide broadband simulation results at a fixed incident angle. Then change the boundary conditions in the x and y directions to Bloch boundary conditions, and keep the PML boundary conditions unchanged in the z direction. Finally, obtain the influence on the model performance by changing the angle.

[0090] As shown in Figures 10(a) and 10(b), Figures 10(a) and 10(b) respectively show the influence of the incident angle on the spectral performance of the model in TE and TM modes. In the TE mode, as can be seen from Figure 10(a), when the incident angle gradually increases from 0° to 10°, 40°, and 70°, in the range of 3μm - 5μm, the absorption peak blue-shifts, the absorption rate does not change significantly, but the absorption peak gradually becomes narrower with the increase of the angle; in the range of 8μm - 13μm, the absorption peak blue-shifts. In the TM mode, as can be seen from Figure 10(b), when the incident angle gradually increases from 0° to 10°, 40°, and 70°, in the range of 3μm - 5μm, the absorption peak blue-shifts, the peak of the absorption peak decreases accordingly, the absorption band becomes narrower, and in the range of 8μm - 13μm, the absorption peak blue-shifts. However, in both TE and TM modes, with the increase of the incident angle, the model still has high emission (high absorption) in the atmospheric window (8μm - 13μm) range. In summary, the model designed by the present invention has the characteristics of polarization insensitivity and large-angle absorption.

[0091] The finally obtained thin film composed of the top transparent conductive oxide AZO metasurface, TiO2, (TiO2 / Ag / TiO2) 3 can be used as the visual window of the present invention applied to radiative cooling.

[0092] When the length of the AZO metasurface is 1.1 μm, the period of the metasurface is 1.3 μm, the thickness of the AZO metasurface is 0.04 μm, the thickness of the TiO2 dielectric layer is 1.4 μm, the thicknesses of Ag and TiO2 in the (TiO2 / Ag / TiO2) reflective layer are 0.01 μm and 0.025 μm respectively, and the number of film layer structures is 3, this structural model has high emission (absorption) performance in the atmospheric windows (3 μm - 5 μm, 8 μm - 13 μm), high reflectivity in the near-infrared band, and high transmittance in the visible light. The average absorptivity within the "atmospheric window" of 8 μm - 13 μm reaches 86.4%, and it is insensitive to polarization, and can still maintain high thermal emission (high absorptivity) under large-angle incidence.

[0093] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An inorganic radiative cooling visible window with excellent spectral selectivity, characterized in that, The structure is composed of a top super-surface, an intermediate dielectric layer, and a lower periodic film layer structure. The top super-surface is an AZO super-surface layer, the intermediate dielectric layer is a TiO2 dielectric layer, and the lower periodic film layer structure is a (TiO2 / Ag / TiO2) n periodic film layer structure, where n represents the number of periods; Periodic film structure (TiO2 / Ag / TiO2) n In this structure, the number of periods n is 3, the thickness of TiO2 is 0.025 μm, and the thickness of Ag is 0.01 μm; the thickness of the TiO2 dielectric layer is 1.4 μm; The length of the AZO metasurface layer is 1.1 μm, the period is 1.3 μm, and the thickness is 0.04 μm.

Images