An ultralow reflectance optical element and a method of making the same

Abstract

This invention discloses an ultra-low reflectivity optical element and its fabrication method, comprising: S1, substrate pretreatment; S2, PVD vacuum evaporation sequentially depositing a SiO2 underlayer and an Al2O3 thin film; S3, hydrothermally treating the coated substrate in neutral pure water at 60-100℃ for 30-120 minutes to form a gradient refractive index porous structure in the Al2O3 thin film; S4, drying treatment. The resulting optical element comprises a substrate, a SiO2 underlayer, and an Al2O3 thin film layer. The Al2O3 thin film layer has a gradient refractive index porous structure with a continuous transition from high porosity at the surface to low porosity at the bottom, and an average reflectivity ≤0.1% in the wavelength range of 380-780nm. This invention features a simple and environmentally friendly process, is suitable for glass and plastic substrates, exhibits excellent film adhesion, and can be widely used in high-end optical equipment such as mobile phone lenses and automotive lenses.

Description

Technical Field

[0001] This invention relates to the field of optical equipment technology, and more specifically, to an ultra-low reflectivity optical element and its fabrication method. Background Technology

[0002] In optical imaging systems, reflections from lens surfaces can cause problems such as glare, ghosting, and reduced contrast. This is especially true in high-performance optical modules such as mobile phone lenses and automotive lenses, where the requirements for reflectivity are becoming increasingly stringent, with the aim of achieving ultra-low reflectivity levels of 0.1% or even lower. By fabricating a porous anti-reflective film with a gradient refractive index on the surface of optical components, reflectivity can be effectively reduced, thus improving image quality.

[0003] Chinese invention patent application CN101886247A discloses a method for preparing a glass-based porous alumina substrate. This method uses glass as the substrate and employs a vacuum thermal evaporation method to prepare a high-purity aluminum film. Then, a porous alumina layer is generated using a single or double anodizing method. Next, a pore-expansion process is used to optimize the pore size of the porous alumina layer. Finally, a high-temperature annealing treatment is applied to improve the overall transmittance of the glass-based porous alumina substrate. The resulting glass-based porous alumina substrate exhibits good mechanical strength, highly ordered alumina pores, and a visible light transmittance of approximately 75%. While this method successfully prepares a porous alumina structure, it still has the following shortcomings: the anodizing process is complex and faces environmental pressures; the 480℃ high-temperature heat treatment is not suitable for plastic lenses; and the visible light transmittance is only about 75%, corresponding to a relatively high reflectivity, making it difficult to meet the stringent requirements of high-end optical components for ultra-low reflectivity of 0.1%.

[0004] Therefore, there is an urgent need to develop a coating preparation method that is simple in process, environmentally friendly, applicable to glass and plastic substrates, and capable of stably achieving ultra-low reflectivity of 0.1% in the visible light band, in order to solve the above-mentioned technical problems existing in the prior art. Summary of the Invention

[0005] Therefore, it is necessary to provide an ultra-low reflectivity optical element and its fabrication method to address the aforementioned technical problems.

[0006] To address the aforementioned technical problems, the first aspect of this invention provides a method for fabricating an ultra-low reflectivity optical element, comprising the following steps: S1. Clean and surface-activate the optical substrate; S2. SiO2 underlayer and Al2O3 thin film are deposited sequentially on the pretreated substrate by PVD vacuum evaporation. S3. The substrate with SiO2 and Al2O3 films deposited is placed in neutral pure water at 60~100℃ for hydrothermal treatment for 30~120 minutes to form a porous structure with gradient refractive index in the Al2O3 film. S4. The hydrothermally treated components are dried to obtain ultra-low reflectivity optical components.

[0007] Furthermore, the thickness of the SiO2 underlayer is 50~100nm, and the deposition vacuum degree is maintained at 0.01~0.02Pa; the thickness of the Al2O3 film is 20~50nm, and the deposition vacuum degree is maintained at 0.012~0.025Pa.

[0008] Furthermore, in step S2, the substrate temperature for PVD vacuum evaporation is adjusted according to the substrate type: for glass substrates, it is controlled at 130~180℃, and for plastic substrates, it is controlled at 80~120℃.

[0009] Furthermore, the temperature of the hydrothermal treatment in step S3 is 60~70℃, and the time is 45~55 minutes.

[0010] Furthermore, pure water is kept circulating during the hydrothermal treatment process in step S3.

[0011] Furthermore, in step S4, after the surface moisture is dried by blowing with nitrogen, the product is dried in an oven at 80-120°C for 1-3 hours.

[0012] A second aspect of the present invention provides an ultra-low reflectivity optical element, which is prepared by any of the above-described methods, comprising: Optical substrate; SiO2 underlayer disposed on the substrate; An Al2O3 thin film layer is formed on the SiO2 substrate; After hydrothermal treatment, the Al2O3 thin film layer forms a gradient refractive index porous structure that continuously transitions from high porosity at the surface to low porosity at the bottom.

[0013] Furthermore, the optical element has an average reflectivity of ≤0.1% in the visible light wavelength range of 380~780nm.

[0014] Furthermore, the optical substrate is a glass lens or a plastic lens.

[0015] A third aspect of the present invention provides an optical device comprising the ultra-low reflectivity optical element described in any of the preceding claims.

[0016] Compared with the prior art, the present invention has the following beneficial effects: This invention discloses an ultra-low reflectivity optical element and its fabrication method. After sequentially depositing a SiO2 underlayer and an Al2O3 thin film via PVD vacuum evaporation, the Al2O3 film undergoes hydrothermal treatment in neutral pure water at 60-100°C for 30-120 minutes. This process creates a gradient refractive index porous structure in the Al2O3 film, with a continuous transition from high porosity at the surface to low porosity at the bottom, achieving an ultra-low reflectivity of ≤0.1% in the visible light band. By introducing the SiO2 underlayer as a stress buffer layer and nucleation site, the stress on the Al2O3 film is effectively alleviated. The mismatch in thermal expansion coefficients between the layer and the substrate prevents film cracking caused by stress concentration during hydrothermal treatment, while promoting the uniform growth of subsequent porous structures, enabling the film adhesion to reach the optimal standard of Grade 0. By adjusting the PVD deposition temperature to adapt to various substrates such as glass and plastics, the technical defects of existing anodizing high-temperature heat treatment that cannot be applied to plastic substrates are avoided. At the same time, the absence of electrolyte participation makes the process simple, environmentally friendly, and low-cost, which is conducive to large-scale industrial production and can be widely used in high-end optical equipment such as mobile phone lenses and automotive lenses. Attached Figure Description

[0017] Figure 1 A scanning electron microscope (SEM) cross-sectional image of the ultra-low reflectivity optical element prepared in Example 1 of the present invention; Figure 2 This is a scanning electron microscope (SEM) cross-sectional image of the ultra-low reflectivity optical element prepared in Example 2 of the present invention. Detailed Implementation

[0018] To enable those skilled in the art to better understand the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0019] This embodiment provides an ultra-low reflectivity optical element and its fabrication method, the specific steps of which include: S1. Substrate Pretreatment: Glass lenses are selected as the optical substrate and cleaned using an ultrasonic gradient method. Specifically, first, the substrate is ultrasonically cleaned for 5 minutes at 50°C in a 3% neutral degreasing cleaning solution, and this process is repeated twice; then, it is ultrasonically cleaned for 5 minutes in deionized water, and this process is repeated twice; finally, it is dried for later use.

[0020] S2, PVD Vacuum Evaporation Deposition: The pretreated glass substrate is placed in a vacuum coating machine, and after evacuating to a background vacuum level better than 0.002 Pa, a SiO2 layer with a thickness of 80 nm is deposited first at a substrate temperature of 150 °C at an evaporation rate of 3.0 Å per second, with the deposition vacuum level maintained at 0.012 Pa. Subsequently, while maintaining the substrate temperature at 150 °C, an Al2O3 film with a thickness of 30 nm is deposited on the SiO2 layer at the same evaporation rate of 3.0 Å per second, with the deposition vacuum level adjusted to 0.015 Pa.

[0021] S3. Hydrothermal Treatment: The deposited lens is immersed in neutral pure water at 65℃ for 50 minutes. Pure water circulation is maintained throughout the treatment to prevent contamination. Under these conditions, the Al2O3 film undergoes a moderate hydration reaction, forming a uniform gradient porous structure with a continuous transition from high porosity at the surface to low porosity at the bottom.

[0022] S4. Drying treatment: Take out the lens after hydrothermal treatment, blow the surface moisture with nitrogen, and dry it in an oven at 100°C for 2 hours to obtain the final ultra-low reflectivity optical element.

[0023] The ultra-low reflectivity optical element prepared in this embodiment has a film cross-sectional morphology as shown in the example. Figure 1 As shown, from Figure 1 It can be clearly observed that the Al2O3 film forms a uniform gradient porous structure that transitions continuously from high porosity on the surface to low porosity on the bottom.

[0024] The reflectance of the optical element prepared in this embodiment was tested in the visible light wavelength range of 380~780nm, and the test results are shown in Table 1. As can be seen from Table 1, the optical element prepared in this embodiment achieves an average reflectance of 0.027% in the visible light band on the glass, realizing an excellent ultra-low reflectance effect.

[0025] Table 1. Reflectance data of the optical element prepared in Example 1 in the wavelength range of 380~780nm. Example 2 This embodiment provides an ultra-low reflectivity optical element and its fabrication method, the specific steps of which include: S1. Substrate Pretreatment: Plastic lenses are selected as the optical substrate; in this embodiment, COP material is used. The substrate is cleaned using a centrifugal cleaner, then spun dry at high speed, and finally dried at 100°C for 2 hours.

[0026] S2, PVD Vacuum Evaporation Deposition: The pretreated plastic substrate is placed in a vacuum coating machine. After evacuating to a background vacuum level better than 0.002 Pa, the substrate temperature is first controlled at 100℃, and a 50 nm thick SiO2 layer is deposited at an evaporation rate of 3.0 Å per second, with the deposition vacuum level maintained at 0.01 Pa. Then, while keeping the substrate temperature at 100℃, an Al2O3 film with a thickness of 20 nm is deposited on the SiO2 layer at the same evaporation rate of 3.0 Å per second, with the deposition vacuum level adjusted to 0.012 Pa.

[0027] S3. Hydrothermal Treatment: The deposited lens is immersed in neutral pure water at 60℃ for 120 minutes. Pure water circulation is maintained throughout the treatment process to prevent contamination. Under these conditions, the extended treatment time compensates for the lower reaction rate at lower temperatures, allowing the Al2O3 film to form a fully developed gradient porous structure with uniform and dense pore distribution.

[0028] S4. Drying treatment: Take out the lens after hydrothermal treatment, blow the surface moisture with nitrogen, and dry it in an oven at 100°C for 2 hours to obtain the final ultra-low reflectivity optical element.

[0029] The ultra-low reflectivity optical element prepared in this embodiment has a film cross-sectional morphology as shown in the example. Figure 2 As shown, from Figure 2 It can be clearly observed that the Al2O3 film has formed a fully developed gradient porous structure with uniform and dense pore distribution.

[0030] The reflectance of the optical element prepared in this embodiment was tested in the visible light wavelength range of 380~780nm on irregular plastic. The test results are shown in Table 2. As can be seen from Table 2, the optical element prepared in this embodiment achieves an average reflectance of 0.058% in the visible light band, realizing an excellent ultra-low reflectance effect, proving that the method of the present invention is also applicable to plastic substrates.

[0031] Table 2. Reflectance data of the optical element prepared in Example 2 in the wavelength range of 380~780nm. Example 3 This embodiment provides an ultra-low reflectivity optical element and its fabrication method, the specific steps of which include: S1. Substrate pretreatment: Glass lens is selected as the optical substrate, and the cleaning steps are the same as step S1 in Example 1.

[0032] S2, PVD Vacuum Evaporation Deposition: The pretreated glass substrate is placed in a vacuum coating machine. After evacuating to a background vacuum level better than 0.002 Pa, a 100 nm thick SiO2 underlayer is deposited at a substrate temperature of 180 °C and an evaporation rate of 3.0 Å per second, with the deposition vacuum level maintained at 0.02 Pa. Subsequently, while maintaining the substrate temperature at 150 °C, an Al2O3 thin film with a thickness of 50 nm is deposited on the SiO2 layer at the same evaporation rate of 3.0 Å per second, with the deposition vacuum level adjusted to 0.022 Pa.

[0033] S3. Hydrothermal Treatment: The deposited lens is immersed in neutral pure water at 100℃ for 30 minutes. Pure water circulation is maintained throughout the process to prevent contamination. Under these conditions, the higher temperature accelerates the hydration reaction, allowing the Al2O3 film to form a porous structure with high surface porosity and a steep gradient in a shorter time.

[0034] S4. Drying treatment: Take out the lens after hydrothermal treatment, blow the surface moisture with nitrogen, and dry it in an oven at 80°C for 3 hours.

[0035] Example 4 This embodiment provides an ultra-low reflectivity optical element and its fabrication method, the specific steps of which include: S1. Substrate pretreatment: Glass lens is selected as the optical substrate, and the cleaning steps are the same as step S1 in Example 1.

[0036] S2, PVD Vacuum Evaporation Deposition: The pretreated glass substrate is placed in a vacuum coating machine. After evacuating to a background vacuum level better than 0.002 Pa, a 70 nm thick SiO2 underlayer is deposited at a substrate temperature of 130 °C and an evaporation rate of 3.0 Å per second, with the deposition vacuum level maintained at 0.015 Pa. Subsequently, while maintaining the substrate temperature at 150 °C, an Al2O3 thin film with a thickness of 40 nm is deposited on the SiO2 layer at the same evaporation rate of 3.0 Å per second, with the deposition vacuum level adjusted to 0.025 Pa.

[0037] S3. Hydrothermal Treatment: The deposited lens is immersed in neutral pure water at 80℃ for 90 minutes. Pure water circulation is maintained throughout the treatment to prevent contamination. Under these conditions, the Al2O3 film forms a porous structure with a gentle gradient and uniform pore distribution, balancing reaction efficiency and structural uniformity.

[0038] S4. Drying treatment: Take out the lens after hydrothermal treatment, blow the surface moisture with nitrogen, and dry it in an oven at 120°C for 1 hour.

[0039] Comparative Example 1 This comparative example provides a method for fabricating an ultra-low reflectivity optical element, which differs from Example 1 in that the hydrothermal treatment conditions are different. The specific steps include: S1. Substrate pretreatment: Same as step S1 in Example 1, but use glass lenses for cleaning and drying.

[0040] S2, PVD vacuum evaporation deposition: Same as step S2 in Example 1, deposit 80nm SiO2 and 30nm Al2O3.

[0041] S3. Hydrothermal Treatment: Immerse the deposited lenses in neutral pure water at 45℃ for 50 minutes. Maintain pure water circulation throughout the process to prevent contamination.

[0042] S4. Drying treatment: Same as step S4 in Example 1.

[0043] Due to the excessively low hydrothermal treatment temperature, the Al2O3 hydration reaction rate was extremely slow. As a result, the sample film prepared by this comparative method failed to form a sufficiently gradient porous structure, resulting in insufficient porosity and an inability to achieve an ultra-low reflectance target of less than 0.1%.

[0044] Comparative Example 2 This comparative example provides a method for fabricating an ultra-low reflectivity optical element, which differs from Example 1 in that the hydrothermal treatment conditions are different. The specific steps include: S1. Substrate pretreatment: Same as step S1 in Example 1, but use glass lenses for cleaning and drying.

[0045] S2, PVD vacuum evaporation deposition: Same as step S2 in Example 1, deposit 80nm SiO2 and 30nm Al2O3.

[0046] S3. Hydrothermal Treatment: Immerse the deposited lenses in neutral pure water at 120℃ for 50 minutes. Maintain pure water circulation throughout the process to prevent contamination.

[0047] S4. Drying treatment: Same as step S4 in Example 1.

[0048] Due to the excessively high hydrothermal treatment temperature, the Al2O3 film underwent an over-hydration reaction, resulting in the formation of a loose layer of hydrated alumina particles on the surface of the sample prepared by this comparative method. The film swelled, peeled, and even detached, exhibiting white hazy defects and severely reduced adhesion.

[0049] Comparative Example 3 This comparative example provides a method for fabricating an ultra-low reflectivity optical element, which differs from Example 1 in that the hydrothermal treatment conditions are different. The specific steps include: S1. Substrate pretreatment: Same as step S1 in Example 1, but use glass lenses for cleaning and drying.

[0050] S2, PVD vacuum evaporation deposition: Same as step S2 in Example 1, deposit 80nm SiO2 and 30nm Al2O3.

[0051] S3. Hydrothermal Treatment: Immerse the deposited lens in a hydrochloric acid solution with a pH of 3.0 at 65°C for 50 minutes. Maintain solution circulation during the treatment process to prevent contamination.

[0052] S4. Drying treatment: Same as step S4 in Example 1.

[0053] In this comparative example, under acidic conditions, the Al2O3 film underwent a chemical dissolution reaction rather than a controllable hydration reaction, resulting in a reduction in film thickness, structural damage, the appearance of etching pits on the surface, an abnormally high reflectivity, and film failure.

[0054] Comparative Example 4 This comparative example provides a method for fabricating an ultra-low reflectivity optical element, which differs from Example 1 in that the hydrothermal treatment conditions are different. The specific steps include: S1. Substrate pretreatment: Same as step S1 in Example 1, but use glass lenses for cleaning and drying.

[0055] S2, PVD vacuum evaporation deposition: Same as step S2 in Example 1, deposit 80nm SiO2 and 30nm Al2O3.

[0056] S3. Hydrothermal Treatment: Immerse the deposited lens in a sodium hydroxide aqueous solution with a pH of 10.0 at 65°C for 50 minutes. Maintain solution circulation during the treatment process to prevent contamination.

[0057] S4. Drying treatment: Same as step S4 in Example 1.

[0058] In this comparative example, under alkaline conditions, the Al2O3 film undergoes a violent reaction, generating soluble aluminates, which leads to rapid corrosion and peeling of the film, preventing the formation of a complete porous structure, exposing the substrate, and resulting in a reflectivity close to that of a bare substrate.

[0059] Comparative Example 5 This comparative example provides a method for fabricating an ultra-low reflectivity optical element. The difference from Example 1 is that the hydrothermal treatment is omitted. The specific steps include: S1. Substrate pretreatment: Same as step S1 in Example 1, but use glass lenses for cleaning and drying.

[0060] S2, PVD vacuum evaporation deposition: Same as step S2 in Example 1, deposit 80nm SiO2 and 30nm Al2O3.

[0061] S3. Hydrothermal treatment: The hydrothermal treatment step is omitted in this comparative example.

[0062] S4. Drying treatment: Same as step S4 in Example 1.

[0063] Because hydrothermal treatment was omitted in this comparative example, the Al2O3 film maintained a dense structure and failed to form a porous structure with a gradient refractive index. Therefore, the reflectivity was high and the ultra-low reflectivity could not be achieved.

[0064] Comparative Example 6 This comparative example provides a method for fabricating an ultra-low reflectivity optical element. The difference from Example 1 is that the SiO2 underlayer deposition step is omitted. The specific steps include: S1. Substrate pretreatment: Same as step S1 in Example 1, but use glass lenses for cleaning and drying.

[0065] S2, PVD vacuum evaporation deposition: The pretreated glass substrate was placed in a vacuum coating machine, and after evacuating to a background vacuum level better than 0.002 Pa, an Al2O3 film with a thickness of 30 nm was directly deposited at a substrate temperature of 150 °C at an evaporation rate of 3.0 Å per second, with the deposition vacuum level maintained at 0.018 Pa. The deposition of the SiO2 underlayer was omitted in this comparative example.

[0066] S3. Hydrothermal Treatment: Immerse the deposited lenses in neutral pure water at 65℃ for 50 minutes. Maintain pure water circulation throughout the process to prevent contamination.

[0067] S4. Drying treatment: Same as step S4 in Example 1.

[0068] In this comparative example, due to the lack of SiO2 as a stress buffer and nucleation site, the thermal expansion coefficients of the Al2O3 film and the glass substrate are mismatched. During the hydrothermal treatment, microcracks or even large-area cracks are generated in the film layer, the reflectivity is significantly increased, and the film adhesion is poor.

[0069] Verification Example The optical element samples prepared in Examples 1-4 and Comparative Examples 1-6 were evaluated to verify the technical effects of the present invention.

[0070] The average reflectance (R%) and average transmittance (T%) of the samples in the wavelength range of 380~780nm were tested using a Shimadzu UV-3600 spectrophotometer. Three points were selected at the center and three at the edge of each sample for testing, and the average value was taken. Adhesion was tested according to GB / T9286-1998 using a cross-cut adhesion test, with grades ranging from 0 to 5 (grade 0 being the best). Visual inspection of appearance defects was conducted under a 1000lx D65 light source. The test results are shown in Table 3.

[0071] Table 3. Performance test results of each embodiment and comparative sample As shown in Table 3, the optical elements prepared in Examples 1-4 of this invention all exhibit extremely low average reflectivity and extremely high average transmittance, with adhesion reaching the optimal standard of grade 0 and no defects in appearance. This indicates that the method of this invention can stably prepare ultra-low reflectivity optical elements with both excellent optical performance and good film quality. The mechanism is as follows: when the alumina film is placed in deionized water at 60-100°C, water penetrates into the interior of the film, inducing slight hydration and structural loosening of the surface and subsurface, causing some Al2O3 to be converted into hydrated alumina. This hydration process leads to the formation of micropores in local areas of the film and gradually forms a gradient structure from a dense layer to a transition layer and then to a porous layer on the film surface, thereby achieving a continuous transition of refractive index and an ultra-low reflection effect.

[0072] In contrast, the comparative samples exhibited performance defects to varying degrees: Comparative Example 1, treated with a low-temperature hydrothermal treatment at 45℃, resulted in insufficient reaction and significantly higher reflectivity than the example; Comparative Example 2, treated with a high-temperature hydrothermal treatment at 120℃, led to excessive reaction of the film layer, significantly reduced adhesion, and white fogging and detachment; Comparative Example 3, treated with an acidic medium, and Comparative Example 4, treated with an alkaline medium, resulted in chemical dissolution and severe corrosion of the Al2O3 film layer, respectively, leading to a significant increase in reflectivity, severe deterioration of adhesion, and extremely obvious appearance defects; Comparative Example 5, omitting the hydrothermal treatment, maintained a dense structure in the Al2O3 film, with reflectivity far exceeding that of the example; Comparative Example 6, omitting the SiO2 underlayer, resulted in large-area cracking of the film layer after hydrothermal treatment, significantly increased reflectivity, and extremely poor adhesion. These results demonstrate that the present invention, through the synergistic combination of the SiO2 underlayer and neutral pure hydrothermal treatment, can stably achieve an ultra-low reflectivity of ≤0.1% in the visible light band within an optimized process window of 60–100℃.

[0073] Obviously, the embodiments described above are merely some embodiments of this application, not all embodiments, and do not limit the patent scope of this application. This application can be implemented in many different forms; on the contrary, the purpose of providing these embodiments is to make the disclosure of this application more thorough and comprehensive. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing specific embodiments, or make equivalent substitutions for some of the technical features. Any equivalent structures made using the content of this application's specification, directly or indirectly applied to other related technical fields, are similarly within the patent protection scope of this application.

Claims

1. A method for fabricating an ultra-low reflectivity optical element, characterized in that, Includes the following steps: S1. Clean and surface-activate the optical substrate; S2. SiO2 underlayer and Al2O3 thin film are deposited sequentially on the pretreated substrate by PVD vacuum evaporation. S3. The substrate with SiO2 and Al2O3 films deposited is placed in neutral pure water at 60~100℃ for hydrothermal treatment for 30~120 minutes to form a porous structure with gradient refractive index in the Al2O3 film. S4. The hydrothermally treated components are dried to obtain ultra-low reflectivity optical components.

2. The preparation method according to claim 1, characterized in that, The thickness of the SiO2 underlayer is 50~100nm, and the deposition vacuum is maintained at 0.01~0.02Pa; the thickness of the Al2O3 film is 20~50nm, and the deposition vacuum is maintained at 0.012~0.025Pa.

3. The preparation method according to claim 1, characterized in that, In step S2, the substrate temperature for PVD vacuum evaporation is adjusted according to the substrate type: for glass substrates, it is controlled at 130~180℃, and for plastic substrates, it is controlled at 80~120℃.

4. The preparation method according to claim 1, characterized in that, The temperature of the hydrothermal treatment in step S3 is 60~70℃, and the time is 45~55 minutes.

5. The preparation method according to claim 1, characterized in that, In step S3, pure water is kept circulating during the hydrothermal treatment process.

6. The preparation method according to claim 1, characterized in that, The drying process described in step S4 involves blowing away surface moisture with nitrogen gas and then drying in an oven at 80-120°C for 1-3 hours.

7. An ultra-low reflectivity optical element, characterized in that, Prepared by the preparation method according to any one of claims 1 to 6, comprising: Optical substrate; SiO2 underlayer disposed on the substrate; An Al2O3 thin film layer is formed on the SiO2 substrate; After hydrothermal treatment, the Al2O3 thin film layer forms a gradient refractive index porous structure that continuously transitions from high porosity at the surface to low porosity at the bottom.

8. The ultra-low reflectivity optical element according to claim 7, characterized in that, The optical element has an average reflectivity of ≤0.1% in the visible light wavelength range of 380~780nm.

9. The ultra-low reflectivity optical element according to claim 7, characterized in that, The optical substrate is a glass lens or a plastic lens.

10. An optical device, characterized in that, Includes the ultra-low reflectivity optical element as described in any one of claims 7 to 9.

Images