Neutral density optical filter with high spectral flatness and preparation method and application thereof

Abstract

The invention discloses a neutral density optical filter with hyperspectral flatness and a preparation method and application thereof, the optical filter sequentially comprises a transparent substrate, a metal nickel film layer, an oxide layer and a nano protection layer from bottom to top, the oxide layer is formed by natural oxidation of the metal nickel film layer, and the nano protection layer is selected from any one of an Al2O3 layer, an SiO2 layer, a ZnS layer and an HfO2 layer. The pure metal nickel film has good light transmittance and spectrum flatness, and different optical density levels from ND0.3 to ND16.0 can be obtained by adjusting the thickness of the nickel film. The nickel oxide layer is formed on the surface of the metal nickel film through natural oxidation, so that the process consistency is improved; and the nanometer protection layer is deposited on the surface of the nickel oxide layer, so that the high transmittance and the spectrum flatness of the optical filter can be guaranteed. The prepared neutral density optical filter has flat transmittance in a broadband, the maximum fluctuation does not exceed 2%, the physical stability is good, and the neutral density optical filter can be used in high-temperature and high-light-intensity environments and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical thin film materials, and specifically relates to a neutral density filter with high spectral flatness, a preparation method and application thereof, and more particularly to a neutral density filter based on nanoscale nickel thin film, a preparation method and application thereof. Background Art

[0002] Neutral density filters (ND filters for short) are optical components that uniformly attenuate light intensity while keeping the spectral composition essentially unchanged. They are widely used in optical measurement, laser systems, photography, videography, microscopy, and other fields. Traditional neutral density filters are mostly made of absorptive filter materials (such as optical glass doped with absorbers) or metal-dielectric multilayer film structures. Among them: although absorptive filters are simple to make, their spectral flatness is poor, especially in the ultraviolet or near-infrared regions, where "light leakage" often occurs. Although multilayer film filters have highly controllable transmission performance, their preparation process is complex, the process requirements are high, the cost is high, and the film structure is complex and the stability is poor.

[0003] In recent years, with the advancement of nanofilm fabrication technology, researchers have begun exploring the possibility of using single metal thin films to create neutral density filters. However, a technical solution for metallic neutral density filters that offers simple fabrication, stable performance, mass production, and high spectral flatness is still lacking. Therefore, developing a new neutral density filter with a simple fabrication process, excellent performance, spectral flatness, and stable UV / IR transmission to meet the needs of wide-spectrum light attenuation control in precision optical systems is of great practical value.

[0004] At present, there is still much room for improvement in the structural stability, environmental durability and large-area consistency of nano-metal thin film neutral density filters. There is an urgent need to develop a new type of neutral density filter that can significantly improve its performance and stability while maintaining a simple process. Summary of the Invention

[0005] The present invention aims to address at least one of the technical problems existing in the aforementioned prior art. To this end, the present invention provides a neutral density filter with high spectral flatness, as well as a preparation method and application thereof. The neutral density filter has a simple structure, stable performance, controllable process, and high spectral flatness.

[0006] In order to solve the above technical problems, the first aspect of the present invention provides a neutral density filter, which includes, from bottom to top, a transparent substrate, a metal nickel thin film layer, an oxide layer and a nano protective layer, wherein the oxide layer is formed by natural oxidation of the metal nickel thin film layer, and the nano protective layer is selected from any one of an Al2O3 layer, a SiO2 layer, a ZnS layer, and a HfO2 layer.

[0007] Specifically, the neutral density filter of the present invention is composed of a transparent substrate, a metal nickel thin film layer deposited thereon, a naturally formed nickel oxide layer, and an outer nano protective layer. Studies have found that pure metal nickel thin film has good light transmittance and spectral flatness. By adjusting the thickness of the nickel film, different optical density levels from ND0.3 to ND16.0 can be obtained. However, if the nickel film is in direct contact with the air, it will undergo continuous oxidation and water absorption, resulting in an overall decrease in the light transmittance of the filter and a significant deterioration in the spectral flatness. Therefore, the present invention forms a nickel oxide layer by natural oxidation on the surface of the metal nickel film to provide process consistency; at the same time, a nano-Al2O3 layer, a SiO2 layer, a ZnS layer or a HfO2 layer is deposited on the surface of the nickel oxide layer to further enhance the durability and oxidation resistance of the metal nickel film, thereby ensuring the high transmittance and spectral flatness of the filter.

[0008] In some embodiments of the present invention, the oxide layer is a NiO and Ni(OH)2 layer, which is formed by natural oxidation of a metal nickel thin film layer exposed to air. The ratio of NiO and Ni(OH)2 in the oxide layer is affected by the temperature and humidity of the air, as well as the exposure time.

[0009] In some embodiments of the present invention, the thickness of the oxide layer is 0.5-2 nm.

[0010] In some embodiments of the present invention, the nano-protective layer has a thickness of 1-5 nm and can be formed by electron beam evaporation or atomic layer deposition (ALD). The nano-protective layer is primarily used to enhance the oxidative resistance and wet-heat stability of the neutral density filter, thereby improving its durability and optical consistency in high humidity, high light intensity, and acidic and alkaline environments.

[0011] In some embodiments of the present invention, the thickness of the metal nickel thin film layer is 2-40 nm. The ND value of the neutral density filter can be controlled by adjusting the thickness of the metal nickel thin film layer. The neutral density filter structure of the metal nickel thin film layer thickness is stable and is suitable for high light intensity, high humidity and wide temperature range environments.

[0012] In some embodiments of the present invention, the transparent substrate is selected from any one of quartz glass, sapphire glass, and transparent polymer materials.

[0013] A second aspect of the present invention provides a method for preparing the above-mentioned neutral density filter, comprising the following steps:

[0014] (1) depositing metallic nickel on the surface of a transparent substrate using a thermal evaporation process under vacuum conditions to form a metallic nickel thin film layer;

[0015] (2) exposing the metal nickel film layer to air for natural oxidation to form an oxide layer;

[0016] (3) Al2O3, SiO2, ZnS or HfO2 are deposited on the surface of the oxide layer to form a nano protective layer to obtain the neutral density filter.

[0017] In some embodiments of the present invention, in step (1), the vacuum condition is a vacuum degree of less than 5×10 -4 Pa.

[0018] In some embodiments of the present invention, in step (1), the rate of depositing nickel metal is 0.1-0.3 nm / min. Studies have found that low vacuum and extremely low nickel metal deposition rate are conducive to ensuring a dense and uniform film layer, thereby obtaining a nickel metal film with high spectral flatness.

[0019] In some embodiments of the present invention, in step (2), the natural oxidation time is 12-24 hours, and the natural oxidation time is adjusted according to application requirements and air temperature and humidity.

[0020] In some embodiments of the present invention, in step (1), the transparent substrate is subjected to an oxygen plasma treatment before the metallic nickel is deposited on the surface thereof. The oxygen plasma treatment is performed at a power of 50-100 W for 1-10 minutes. Oxygen plasma treatment of the transparent substrate is beneficial for improving the adhesion and uniformity of the metallic nickel film layer, thereby improving the spectral flatness of the filter.

[0021] In some embodiments of the present invention, in step (1), the transparent substrate further includes ultrasonic cleaning and drying steps before being subjected to oxygen plasma treatment.

[0022] In some embodiments of the present invention, the ultrasonic cleaning is performed in three steps using acetone, ethanol and deionized water, each cleaning lasting 10-15 minutes.

[0023] In some embodiments of the present invention, the drying is performed by blowing with pure nitrogen or performing thermal drying to remove trace moisture.

[0024] A third aspect of the present invention provides applications of the neutral density filter in the fields of optical measurement, laser systems, photography, or microscopy.

[0025] Compared with the prior art, the above technical solution of the present invention has at least the following technical effects or advantages:

[0026] (1) The neutral density filter of the present invention is composed of a transparent substrate, a metal nickel thin film layer deposited thereon, a naturally formed nickel oxide layer, and an outer nano protective layer. Among them: the pure metal nickel thin film has good light transmittance and spectral flatness. By adjusting the thickness of the nickel film, different optical density levels from ND0.3 to ND16.0 can be obtained. Natural oxidation on the surface of the metal nickel film to form a nickel oxide layer is beneficial to improving the consistency of the process; at the same time, a nano-Al2O3 layer, a SiO2 layer, a ZnS layer, or a HfO2 layer is deposited on the surface of the nickel oxide layer, which can further improve the durability and oxidation resistance of the metal nickel thin film, thereby ensuring the high transmittance and spectral flatness of the filter.

[0027] (2) The neutral density filter of the present invention has a simple structure, a highly controllable preparation process, and a low cost. The prepared neutral density filter has a flat transmittance within a wide wavelength band (350-1000nm), with a maximum fluctuation of no more than 2%, which is superior to existing neutral density filters on the market. At the same time, the required ND value can be precisely controlled by the film thickness to meet the needs of different occasions. The finished product has good physical stability and can be used in high temperature, high light intensity and other environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Graphs showing transmission spectra of neutral density filters prepared in Examples 1-11;

[0029] Figure 2 Graph showing the variation trend of the transmittance of the neutral density filters prepared in Examples 1-11 at different wavelengths as a function of the thickness of the metal nickel film;

[0030] Figure 3 This is a performance comparison chart of the neutral density filter prepared in Example 1 and the commercially available filters of Comparative Examples 6-9;

[0031] Figure 4 This is a performance comparison chart of the neutral density filters prepared in Example 1 and Comparative Example 1;

[0032] Figure 5 This is a performance comparison chart of the neutral density filters prepared in Example 1 and Comparative Example 2;

[0033] Figure 6 This is a performance comparison chart of the neutral density filters prepared in Example 1 and Comparative Example 3;

[0034] Figure 7 This is a performance comparison chart of the neutral density filters prepared in Example 1 and Comparative Example 4;

[0035] Figure 8 The performance comparison chart of the neutral density filters prepared in Example 1 and Comparative Example 5 is shown. DETAILED DESCRIPTION

[0036] The present invention is described in detail below with reference to the examples to facilitate understanding of the present invention by those skilled in the art. It is necessary to point out that the examples are only used to further illustrate the present invention and are not to be construed as limiting the scope of protection of the present invention. Non-essential improvements and adjustments made to the present invention by those skilled in the art based on the above-mentioned invention should still fall within the scope of protection of the present invention. At the same time, the raw materials mentioned below that are not described in detail are all commercially available products; the process steps or preparation methods that are not mentioned in detail are all process steps or preparation methods known to those skilled in the art.

[0037] Example 1

[0038] A method for preparing a neutral density filter comprises the following steps:

[0039] (1) Pretreatment and cleaning of quartz glass substrate

[0040] An optical-grade quartz glass sheet with dimensions of 25 mm (length × width × thickness = 25 mm × 25 mm × 1 mm) was selected as the transparent substrate. The transparent substrate was placed in an ultrasonic cleaning tank and first cleaned with acetone solution for 10 minutes. Then, an anhydrous ethanol solution was used for ultrasonic cleaning for 10 minutes to remove residual polar organic matter. Finally, ultrasonic cleaning was performed in deionized water for 15 minutes to completely remove any residue. The transparent substrate was then blown dry with high-purity nitrogen.

[0041] The dried glass substrate is placed in a plasma cleaning machine and activated with oxygen plasma at a power of 50-100W for 2-3 minutes to enhance the adhesion and uniformity of the metal nickel film.

[0042] (2) Vacuum thermal evaporation deposition of nickel thin film

[0043] The transparent substrate treated in step (1) was placed on the sample holder of the evaporation chamber, ensuring that the substrate was perpendicular to the evaporation source and at a distance of 20-30 cm. High-purity nickel metal particles (≥99.995%) were added to the evaporation chamber as the evaporation source material, and the vacuum pump group was started to evacuate the evaporation chamber to a high vacuum with a vacuum degree of 4.0×10 -4 Pa; start the heating device (tungsten boat or molybdenum boat), gradually raise the temperature to the nickel evaporation temperature, use a quartz crystal oscillator to monitor the film thickness and deposition rate in real time, control the deposition rate to 01-0.3nm / min, and ensure the density and uniformity of the film layer; according to the required ND value, accurately control the thickness of the nickel film between 2.1nm, and the deposition time depends on the target thickness; after the metal nickel film deposition is completed, naturally cool the sample to room temperature, and then break the vacuum to take out the sample.

[0044] (3) Natural oxidation of the nickel film layer

[0045] The sample obtained in step (2) was exposed to air (humidity 25° C., humidity 50-60%) for natural oxidation for 24 hours to form NiO and Ni(OH) 2 oxide layers.

[0046] (4) Surface nano-protective layer deposition

[0047] The electron beam evaporation process is used to deposit nano-Al2O3 on the surface of the oxide layer to form a 1-5 nm nano-protective layer to obtain the neutral density filter of this embodiment.

[0048] (5) Closure and preservation

[0049] Package the neutral density filters with dust-free packaging materials in a clean environment and store them in a dry box to avoid direct exposure to strong light, hot and humid environments.

[0050] Example 2-11

[0051] The only difference between Examples 2-11 and Example 1 is that the thickness of the nickel film in step (2) of the preparation method of the neutral density filter is different, which are 3.8nm, 6.5nm, 7.1nm, 11.5nm, 15.6nm, 20.3nm, 23.1nm, 26.6nm, 30.2nm, and 32.9nm respectively.

[0052] Comparative Example 1

[0053] The difference between Comparative Example 1 and Example 1 is that the preparation method of the neutral density filter does not contain step (3), that is, the metal nickel film obtained in step (2) is not naturally oxidized, and a nano-Al2O3 protective layer is directly deposited on its surface.

[0054] Comparative Example 2

[0055] The difference between Comparative Example 2 and Example 1 is that the preparation method of the neutral density filter does not contain step (4), that is, the nano-Al2O3 protective layer is not deposited on the surface of the oxide layer.

[0056] Comparative Example 3

[0057] The difference between Comparative Example 3 and Example 1 is that in the preparation method of the neutral density filter, nano-ZnO is used in step (4) to replace the nano-Al2O3 protective layer.

[0058] Comparative Example 4

[0059] The difference between Comparative Example 4 and Example 1 is that in the preparation method of the neutral density filter, in step (1), the dried glass substrate is not placed in a plasma cleaning machine for oxygen plasma activation treatment.

[0060] Comparative Example 5

[0061] The difference between Comparative Example 5 and Example 1 is that in the preparation method of the neutral density filter, the vacuum degree in step (2) is 7×10 -4 Pa, and the deposition rate of metallic nickel is 2 nm / min.

[0062] Comparative Example 6

[0063] The commercially available Firecrest neutral density filter has a structure of metal + multi-layer dielectric + metal reflection / absorption layer, and achieves neutral attenuation and spectral control through multi-layer coating.

[0064] Comparative Example 7

[0065] The commercially available Schott NG3 filter achieves uniform attenuation of light intensity by doping absorbers (such as transition metal oxides) into optical glass, and is a doped absorption optical glass.

[0066] Comparative Example 8

[0067] The commercially available Zomei Pro ND0.9ⅡMC filter is Zomei's second-generation neutral density filter. It uses an optical glass substrate and a thin metal oxide material, and achieves a neutral attenuation effect of approximately ND 0.9 (3 blocks of light) through multi-layer nano-coating (MC).

[0068] Comparative Example 9

[0069] The Zomei Pro ND8 filter, an early Zomei product, is marketed as ND8 (corresponding to an ND value of approximately 0.9). It achieves light attenuation through a single or limited metal oxide coating. While this filter exhibits a moderate degree of neutral attenuation in the visible light band, it suffers from poor spectral flatness, significant color cast, and unstable transmission in the ultraviolet and infrared bands. Furthermore, due to the lack of effective anti-reflection and protective coatings, it is susceptible to environmental degradation over time.

[0070] Performance Testing

[0071] The transmittance of the neutral density filters prepared in Examples 1-11 was tested and the optical density values ​​were calculated. The results are as follows: Figure 1-2 As shown. Wherein: Optical density value (ND value) is calculated by formula ND = -log 10 (T), where T represents the average transmittance (in units of percentages), and the ND value corresponding to each film thickness sample is calculated.

[0072] Depend on Figure 1As can be seen, the neutral density filters prepared in Examples 1-11 exhibit flat transmittance across a wide wavelength range of 350-1000nm, with a maximum fluctuation of no more than 2%. When the film thickness is 32.9nm, the sample exhibits a maximum transmittance of 7.4% and a minimum transmittance of 5.9% between 350-1000nm, demonstrating excellent spectral flatness. Furthermore, at 246nm in the ultraviolet region, the transmittance remains as high as 3.3%, demonstrating excellent UV response.

[0073] Figure 3 The spectral flatness comparison chart of the commercially available neutral density filters of Example 1 (This work) and Comparative Examples 6-9 is shown in FIG. Figure 3 It can be seen that the neutral density filter prepared in Example 1 exhibits better spectral flatness and lower infrared leakage risk in the visible and ultraviolet bands, and its performance indicators are better than those of commercially available filters in all aspects.

[0074] Figure 4-8 The transmittance comparison diagrams of the neutral density filters prepared in Example 1 and Comparative Examples 1-5 after being placed in air for 100 days are shown. Figure 4-8 It can be seen that the spectral flatness of the neutral density filters in Comparative Examples 1-5 is significantly worse than that in Example 1 because the neutral density filters do not contain NiO and Ni(OH)2 oxide layers, do not contain nano-Al2O3 protective layers, use nano-ZnO instead of nano-Al2O3 protective layers, and the glass substrate is not subjected to plasma activation treatment, and the vacuum degree and speed of metal nickel deposition are higher. This indicates that the spectral flatness of the neutral density filter of the present invention is closely related to its layer structure and preparation process.

[0075] In the case of the neutral density filter of Comparative Example 2 without nanocoating, due to oxidation and deterioration of the film surface, the transmittance is overall lower than that of Example 1 of the nickel film with nanocoating, and the spectral flatness is significantly worsened, indicating that the nanocoating effectively prevents the continuous oxidation and water absorption of the nickel film.

[0076] Since Comparative Example 5 uses a higher vacuum degree and a faster deposition rate than Example 1, the flatness of the neutral density filter obtained is also significantly worse than that of Example 1. This shows that a low vacuum degree and a slow metal nickel deposition rate are more conducive to ensuring the density and uniformity of the film layer, thereby obtaining higher spectral flatness.

[0077] For those skilled in the art to which the present invention belongs, a number of simple deductions or substitutions can be made without departing from the concept of the present invention, without having to resort to creative work. Therefore, based on the disclosure of the present invention, simple improvements made by those skilled in the art to the present invention should be within the scope of protection of the present invention. The above embodiments are preferred embodiments of the present invention, and all processes similar to the present invention and equivalent changes made should fall within the scope of protection of the present invention.

Claims

1. A neutral density filter, characterized in that: From bottom to top, it includes a transparent substrate, a metal nickel film layer, an oxide layer and a nano protective layer. The oxide layer is formed by natural oxidation of the metal nickel film layer. The nano protective layer is selected from any one of an Al2O3 layer, a SiO2 layer, a ZnS layer and a HfO2 layer.

2. The neutral density filter according to claim 1, wherein The oxide layer is a NiO and Ni(OH)2 layer; and / or the oxide layer has a thickness of 0.5-2 nm.

3. The neutral density filter according to claim 1, wherein The thickness of the nano protective layer is 1-5 nm.

4. The neutral density filter according to claim 1, wherein The thickness of the metal nickel film layer is 2-40 nm.

5. The neutral density filter according to claim 1, wherein: The transparent substrate is selected from any one of quartz glass, sapphire glass, and transparent polymer materials.

6. A method for preparing a neutral density filter according to any one of claims 1 to 5, characterized in that: The following steps are involved: (1) depositing metallic nickel on the surface of a transparent substrate using a thermal evaporation process under vacuum conditions to form a metallic nickel thin film layer; (2) exposing the metal nickel film layer to air for natural oxidation to form an oxide layer; (3) Al2O3, SiO2, ZnS or HfO2 are deposited on the surface of the oxide layer to form a nano protective layer to obtain the neutral density filter.

7. The method for preparing a neutral density filter according to claim 6, wherein: In step (1), the vacuum degree of the vacuum condition is lower than 5×10 -4 Pa; and / or, the deposition rate of metallic nickel is 0.1-0.3 nm / min.

8. The method for preparing a neutral density filter according to claim 6, wherein: In step (2), the natural oxidation time is 12-24 hours.

9. The method for preparing a neutral density filter according to claim 6, wherein: In step (1), the transparent substrate is further subjected to oxygen plasma treatment before metal nickel is deposited on the surface thereof. The process conditions of the oxygen plasma treatment are: treatment at a power of 50-100 W for 10-25 minutes.

10. Use of the neutral density filter according to any one of claims 1 to 5 in the fields of optical measurement, laser systems, photography or microscopy.

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