A method for preparing a refractive index tunable silica optical thin film

Abstract

The present application relates to the technical field of optical film, and provides a preparation method of a refractive index adjustable silicon dioxide optical film. In the present application, acid is used as a stabilizer, the Zeta potential of the alkaline silicon dioxide sol is changed from negative potential to positive potential by adding acid, and then the acid and the alkaline silicon dioxide sol are mixed, so that the stability of the acid-alkali mixed silicon dioxide sol is improved, and the service life is prolonged. Moreover, the silicon dioxide optical film obtained by coating the acid-alkali mixed silicon dioxide sol of the present application has a wide adjustable range of refractive index, and the adjustable range is 1.10-1.44. The silicon dioxide optical film can not only meet the preparation requirements of single-layer anti-reflection film on optical substrates with a refractive index of 1.21-2.07, but also can be used as a refractive index matching layer of a multi-layer film, so that the optical design range of the multi-layer film is greatly expanded.

Description

Technical Field

[0001] This invention relates to the field of optical thin film technology, and in particular to a method for preparing a silicon dioxide optical thin film with tunable refractive index. Background Technology

[0002] Optical antireflective coatings are widely used in optical lenses, photovoltaic glass, laser glass, and other fields. They reduce the reflection of incident light on the surface of optical substrates and improve light transmittance. From a film system design perspective, single-layer antireflective coatings are the simplest to design and fabricate. As long as the refractive index of the antireflective coating is the square root of the refractive index of the optical substrate, zero reflection can be achieved at a specific wavelength. However, the range of refractive indices that can be selected from natural materials is limited, making it difficult to meet the requirement of achieving zero reflection by fabricating only a single-layer thin film.

[0003] The sol-gel method for synthesizing silica nanomaterials and depositing them on optical substrates allows for the adjustment of the film's refractive index within a certain range, thus finding numerous applications in single-layer antireflective coatings for optical substrates. Using tetraethyl orthosilicate as a precursor and ethanol as a solvent, an alkaline sol composed of silica nanoparticles can be obtained with ammonia as a catalyst, while an acidic sol can be obtained with acids such as nitric acid or hydrochloric acid as catalysts. The refractive index of the alkaline sol film is generally around 1.20, while that of the acidic sol film is generally around 1.44. Mixing the alkaline and acidic sols in a certain proportion allows for an adjustable film refractive index between 1.20 and 1.44. However, the acid-base neutralization reaction that occurs when the alkaline and acidic sols are mixed severely affects the stability of the mixed sol, resulting in a very short lifespan, typically gelling within a few days. Even with reflux treatment to remove ammonia from alkaline sols, complete removal is difficult. While this can extend the lifespan of the mixed sol to several weeks, the dynamic changes over time make it challenging to control the consistency of the coating process. Therefore, improving the stability of mixed sols and extending their lifespan is a pressing issue in this field. Summary of the Invention

[0004] In view of this, the present invention provides a method for preparing a silica optical thin film with adjustable refractive index. The acid-base mixed silica sol provided by the present invention has good stability and long service life, and the silica optical thin film prepared using it has a wide adjustable refractive index range.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] A method for preparing an acid-base mixed silica sol includes the following steps:

[0007] An alkaline silica sol and a stabilizer are mixed to obtain a stabilized sol; the mass of the stabilizer is 0.1-5% of the mass of the alkaline silica sol; the stabilizer is an acid, and the concentration of the acid is 10-50 wt%.

[0008] The stabilized sol and the acidic silica sol are mixed to obtain the acid-base mixed silica sol; with the total mass of the alkaline silica sol and the acidic silica sol being 100%, the mass fraction of the acidic silica sol is greater than 0 and less than 100%.

[0009] Preferably, the acid includes one or more of sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, and acetic acid;

[0010] The concentration of the sulfuric acid is 10-30 wt%, the concentration of the nitric acid is 10-50 wt%, the concentration of the hydrochloric acid is 10-37 wt%, the concentration of the phosphoric acid is 10-50 wt%, and the concentration of the acetic acid is 10-50 wt%.

[0011] Preferably, the alkaline silica sol contains 1-10 wt% silica; the acidic silica sol contains 1-20 wt% silica.

[0012] Preferably, the preparation method of the alkaline silica sol includes the following steps:

[0013] The alkaline silica sol is obtained by mixing tetraethyl orthosilicate, a silane precursor, ethanol, ammonia, and water and then aging the mixture; the molar ratio of the silane precursor to the tetraethyl orthosilicate is 0 to 2, and is not 0.

[0014] Preferably, the silane precursor includes one or more of methyltriethoxysilane, dimethyldiethoxysilane, and hexamethyldisilazane; based on the total mass of the tetraethyl orthosilicate, silane precursor, ethanol, ammonia, and water as 100%, the mass fraction of the ammonia is 0.5–1 wt%; the mass fraction of the water is 2.5–3 wt%; and the concentration of the ammonia is 25–28 wt%.

[0015] Preferably, the preparation method of the acidic silica sol includes the following steps: mixing tetraethyl orthosilicate, ethanol, concentrated hydrochloric acid and water and then aging the mixture to obtain the acidic silica sol; the concentration of the concentrated hydrochloric acid is 37 wt%.

[0016] Based on the total mass of the tetraethyl orthosilicate, ethanol, concentrated hydrochloric acid, and water as 100%, the mass fraction of the tetraethyl orthosilicate is 5-70 wt%, the mass fraction of the concentrated hydrochloric acid is 0.01-0.1 wt%, the mass fraction of the water is 1.9-12.5 wt%, and the mass fraction of the ethanol is 20-93 wt%.

[0017] The present invention also provides an acid-base mixed silica sol prepared by the preparation method described above.

[0018] The present invention also provides a method for preparing a refractive index tunable silicon dioxide optical thin film, comprising the following steps: coating the acid-base mixed silicon dioxide sol described in the above scheme to obtain a refractive index tunable silicon dioxide optical thin film.

[0019] Preferably, the coating method is the dip-coating method.

[0020] The present invention also provides a refractive index tunable silicon dioxide optical thin film prepared by the preparation method described above, wherein the refractive index of the refractive index tunable silicon dioxide optical thin film is 1.10 to 1.44.

[0021] This invention provides a method for preparing an acid-base mixed silica sol, comprising the following steps: mixing an alkaline silica sol and a stabilizer to obtain a stabilized sol; the mass of the stabilizer is 0.1-5% of the mass of the alkaline silica sol; the stabilizer is an acid, and the concentration of the acid is 10-50 wt%; mixing the stabilized sol and the acidic silica sol to obtain the acid-base mixed silica sol; wherein, based on the total mass of the alkaline silica sol and the acidic silica sol being 100%, the mass fraction of the acidic silica sol is greater than 0 and less than 100%. This invention uses acid as a stabilizer. By adding acid, the zeta potential of the basic silica sol is changed from negative to positive. When acidic silica sol is subsequently added, since the acidic silica sol also has a positive potential, no aggregation occurs after mixing. Furthermore, the basic silica sol remains stable under both negative and positive potentials. Therefore, although an acid-base neutralization reaction occurs during the addition of the stabilizer, it only changes the electrolyte concentration and does not affect the sol stability. In summary, this invention improves the stability of acid-base mixed silica sol and extends its service life by adding a stabilizer. The results of the examples show that the spectral properties of the film obtained by coating the acid-base mixed silica sol of this invention remain essentially unchanged after being sealed at room temperature for more than 6 months.

[0022] Furthermore, in preparing the alkaline silica sol, the present invention uses two precursors, tetraethyl orthosilicate and silane. Compared with the alkaline silica sol that uses only tetraethyl orthosilicate as a precursor, the alkaline silica sol obtained by the present invention has a lower refractive index, down to 1.10, thereby broadening the adjustable range of refractive index of acid-base mixed silica sol.

[0023] This invention also provides a method for preparing a refractive index-tunable silica optical thin film, comprising the following steps: coating the acid-base mixed silica sol described above to obtain a refractive index-tunable silica optical thin film. The silica optical thin film prepared by the acid-base mixed silica sol of this invention has a wide adjustable refractive index range, from 1.10 to 1.44. This not only meets the requirements for preparing single-layer antireflective films on optical substrates with refractive indices from 1.21 to 2.07, but can also be used as a refractive index matching layer for multilayer films, greatly expanding the optical design range of multilayer films. Detailed Implementation

[0024] This invention provides a method for preparing an acid-base mixed silica sol, comprising the following steps:

[0025] An alkaline silica sol and a stabilizer are mixed to obtain a stabilized sol; the mass of the stabilizer is 0.1-5% of the mass of the alkaline silica sol; the stabilizer is an acid, and the concentration of the acid is 10-50 wt%.

[0026] The stabilized sol and the acidic silica sol are mixed to obtain the acid-base mixed silica sol; with the total mass of the alkaline silica sol and the acidic silica sol being 100%, the mass fraction of the acidic silica sol is greater than 0 and less than 100%.

[0027] This invention involves mixing an alkaline silica sol with a stabilizer to obtain a stabilized sol. In this invention, the silica content in the alkaline silica sol is 1–10 wt%; the mass of the stabilizer is 0.1–5% of the mass of the alkaline silica sol, preferably 1–4%; the stabilizer is an acid, preferably including one or more of sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, and acetic acid; the concentration of the sulfuric acid is preferably 10–30 wt%, more preferably 20–30 wt%; the concentration of the nitric acid is preferably 10–50 wt%, more preferably 20–50 wt%; the concentration of the hydrochloric acid is preferably 10–37 wt%, more preferably 15–37 wt%; the concentration of the phosphoric acid is preferably 10–50 wt%, more preferably 20–50 wt%; and the concentration of the acetic acid is preferably 10–50 wt%, more preferably 20–50 wt%. This invention does not require a special mixing method; any method well-known to those skilled in the art can be used.

[0028] In this invention, the method for preparing the alkaline silica sol preferably includes the following steps: mixing tetraethyl orthosilicate, a silane precursor, ethanol, ammonia and water and aging the mixture (referred to as the first aging) to obtain the alkaline silica sol; the molar ratio of the silane precursor to the tetraethyl orthosilicate is 0 to 2, and is not 0, more preferably 0.5 to 2.

[0029] In this invention, the silane precursor preferably includes one or more of methyltriethoxysilane, dimethyldiethoxysilane and hexamethyldisilazane, more preferably methyltriethoxysilane.

[0030] In this invention, based on the total mass of the tetraethyl orthosilicate, silane precursor, ethanol, ammonia, and water as 100%, the mass fraction of the ammonia is preferably 0.5–1 wt%, more preferably 0.6–0.8 wt%, and the mass fraction of the water is preferably 2.5–3 wt%. The concentration of the ammonia is preferably 25–28 wt%. In a specific embodiment of this invention, the ethanol is preferably anhydrous ethanol; the molar ratio of the anhydrous ethanol to the NH3·H2O in the ammonia is preferably 227–228:1; the molar ratio of the tetraethyl orthosilicate to the NH3·H2O in the ammonia is preferably 2–4.1:1; the molar ratio of the silane precursor to the NH3·H2O in the ammonia is preferably 2–4.1:1; and the molar ratio of the water to the NH3·H2O in the ammonia is preferably 19–20:1.

[0031] In a specific embodiment of the present invention, anhydrous ethanol, tetraethyl orthosilicate, methyltriethoxysilane, water, and ammonia are preferably added sequentially to a reaction vessel. After the addition is complete, the mixture is stirred at 20–30°C for 10–60 minutes, followed by aging. In the present invention, the temperature of the first aging is preferably 20–30°C, and the time is preferably 7 days; the first aging is preferably carried out under sealed conditions. In a specific embodiment of the present invention, after the first aging is completed, the resulting alkaline silica sol is further diluted with anhydrous ethanol to ensure that the silica content meets the requirements.

[0032] After obtaining the stabilized sol, the present invention mixes the stabilized sol and the acidic silica sol to obtain the acid-base mixed silica sol; based on the total mass of the alkaline silica sol and the acidic silica sol being 100%, the mass fraction of the acidic silica sol is greater than 0 and less than 100%, more preferably 0.1% to 99.9%. In a specific embodiment of the present invention, the ratio of the stabilized sol to the acidic silica sol can be determined according to the target refractive index.

[0033] In this invention, the silica content in the acidic silica sol is preferably 1-20 wt%, more preferably 3-10 wt%.

[0034] In this invention, the preferred method for preparing the acidic silica sol includes the following steps: mixing tetraethyl orthosilicate, ethanol, concentrated hydrochloric acid, and water, and then aging the mixture (referred to as the second aging) to obtain the acidic silica sol; this invention does not have special requirements for the concentrated hydrochloric acid, and commercially available concentrated hydrochloric acid well known in the art can be used. Specifically, the concentration of the concentrated hydrochloric acid is preferably 37 wt%, and the ethanol is preferably anhydrous ethanol; based on the total mass of the tetraethyl orthosilicate, ethanol, concentrated hydrochloric acid, and water as 100%, the mass fraction of the tetraethyl orthosilicate is preferably 5-70 wt%, more preferably 10-55 wt%. The mass fraction of concentrated hydrochloric acid is preferably 0.01–0.1 wt%, more preferably 0.02–0.08 wt%, the mass fraction of water is preferably 1.9–12.5 wt%, more preferably 2.5–10 wt%, and the mass fraction of ethanol is preferably 20–93 wt%, more preferably 30–90 wt%. In a specific embodiment of the present invention, the molar ratio of anhydrous ethanol to tetraethyl orthosilicate is preferably 37–38:1; the molar ratio of water to tetraethyl orthosilicate is preferably 2–3:1; and the molar ratio of hydrochloric acid to tetraethyl orthosilicate is preferably 0.003–0.004:1.

[0035] In a specific embodiment of the present invention, anhydrous ethanol, water, hydrochloric acid, and tetraethyl orthosilicate are preferably added to a reaction vessel in sequence, stirred at 20-30°C for 10-60 minutes, and then aged.

[0036] In this invention, the temperature of the second aging process is preferably 20-30°C, the time is preferably 3 days, and the second aging process is preferably carried out under sealed conditions.

[0037] This invention also provides an acid-base mixed silica sol prepared by the method described above. The acid-base mixed silica sol provided by this invention has good stability, long service life, and a wide range of adjustable refractive index for silica optical thin films prepared using it.

[0038] The present invention also provides a method for preparing a refractive index tunable silicon dioxide optical thin film, comprising the following steps: coating the acid-base mixed silicon dioxide sol described in the above scheme to obtain a refractive index tunable silicon dioxide optical thin film.

[0039] In this invention, the coating method is preferably the dip-coating method.

[0040] The present invention also provides a refractive index-tunable silica optical thin film prepared by the preparation method described above; the refractive index of the refractive index-tunable silica optical thin film is preferably 1.10 to 1.44; in the present invention, the refractive index of the silica optical thin film prepared by pure alkaline silica sol can be as low as 1.10, and the refractive index of the silica optical thin film prepared by pure acidic silica sol can be as high as 1.44. The refractive index of the silica optical thin film prepared by the acid-base mixed silica sol of the present invention is adjustable between 1.10 and 1.44, and the higher the proportion of acidic silica sol added, the higher the refractive index of the obtained silica optical thin film.

[0041] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0042] Example 1

[0043] Anhydrous ethanol, tetraethyl orthosilicate, methyltriethoxysilane, water, and ammonia (25 wt%) were added sequentially to a reaction vessel in a molar ratio of 227.89:2.03:4.06:19.72:1.00. The mixture was stirred at room temperature (20–30 °C) for 10 minutes, and then sealed and stored at room temperature for 7 days to obtain alkaline silica sol.

[0044] Alkaline silica sol stored for 7 days was mixed and diluted with anhydrous ethanol at a volume ratio of 1:1. After standing for 1 day, the film was coated by dip-coating method, and the refractive index of the film was measured to be 1.10.

[0045] Concentrated hydrochloric acid (37 wt%) was added to diluted alkaline silica sol at a concentration of 0.7 wt% of the sol's mass, causing its Zeta potential to become positive, thus obtaining a stabilized sol. The Zeta potential of the alkaline silica sol before the addition of concentrated hydrochloric acid was -11.4 mV, and after the addition, it changed to 1.1 mV.

[0046] Anhydrous ethanol, water, hydrochloric acid, and tetraethyl orthosilicate were mixed in a molar ratio of 37.39:2.87:3.85×10⁻⁶. -3 The solutions were added sequentially to the reaction vessel at a ratio of 1:1.00, stirred at room temperature (20–30°C) for 10 minutes, and then sealed and stored at room temperature for 3 days to obtain acidic silica sol. The acidic silica sol was then coated using the dip-coating method, and the refractive index of the resulting film was found to be 1.44.

[0047] The stabilized sol and acidic silica sol were mixed at a volume ratio of 5.5:1 to obtain acid-base mixed silica sol A. The acid-base mixed silica sol was then coated using a dip-coating method, and the resulting film had a refractive index of 1.21.

[0048] The stabilized sol and acidic silica sol were mixed in a volume ratio of 4:1 to obtain acid-base mixed silica sol B. The acid-base mixed silica sol was then coated using a dip-coating method, resulting in a film with a refractive index of 1.24.

[0049] The acid-base mixed silica sol A was sealed and left at room temperature for 6 months, and then coated again by dip-coating method. The refractive index of the resulting film was found to be 1.23.

[0050] Example 2

[0051] Anhydrous ethanol, tetraethyl orthosilicate, methyltriethoxysilane, water, and ammonia (25 wt%) were added sequentially to a reaction vessel in a molar ratio of 227.22:4.02:2.04:19.67:1.00. The mixture was stirred at room temperature (20–30 °C) for 10 minutes, and then sealed and stored at room temperature for 7 days to obtain alkaline silica sol.

[0052] Alkaline silica sol stored for 7 days was mixed and diluted with anhydrous ethanol at a volume ratio of 1:1. After standing for 1 day, the film was coated by dip-coating method, and the refractive index of the film was measured to be 1.15.

[0053] Concentrated hydrochloric acid (37 wt%) was added to diluted alkaline silica sol at a concentration of 0.7 wt% of the sol's mass, causing its Zeta potential to become positive, thus obtaining a stabilized sol. Testing showed that the Zeta potential of the alkaline silica sol before adding concentrated hydrochloric acid was -14.8 mV, and after adding the acid, it changed to 0.7 mV.

[0054] Anhydrous ethanol, water, hydrochloric acid, and tetraethyl orthosilicate were mixed in a molar ratio of 37.39:2.87:3.85×10⁻⁶. -3 The solutions were added sequentially to the reaction vessel at a ratio of 1:1.00, stirred at room temperature (20–30°C) for 10 minutes, and then sealed and stored at room temperature for 3 days to obtain acidic silica sol. The acidic silica sol was then coated using the dip-coating method, and the refractive index of the resulting film was found to be 1.44.

[0055] The stabilized sol and acidic silica sol were mixed at a volume ratio of 5.5:1 to obtain an acid-base mixed silica sol. The acid-base mixed silica sol was then coated using a dip-coating method, resulting in a film with a refractive index of 1.26.

[0056] The acid-base mixed silica sol was sealed and left at room temperature for 6 months, and then coated again by dip-coating method. The refractive index of the resulting film was found to be 1.28.

[0057] Example 3

[0058] Anhydrous ethanol, tetraethyl orthosilicate, methyltriethoxysilane, water, and ammonia (25 wt%) were added sequentially to a reaction vessel in a molar ratio of 227.22:4.02:2.04:19.67:1.00. The mixture was stirred at room temperature (20–30 °C) for 10 minutes, and then sealed and stored at room temperature for 7 days to obtain alkaline silica sol.

[0059] Alkaline silica sol stored for 7 days was mixed and diluted with anhydrous ethanol at a volume ratio of 1:1. After standing for 1 day, the film was coated by dip-coating method, and the refractive index of the film was measured to be 1.15.

[0060] Nitric acid (50 wt%) was added to diluted alkaline silica sol at a concentration of 0.5 wt% of the sol's mass, causing its zeta potential to become positive, thus stabilizing the sol. Testing showed that the zeta potential of the alkaline silica sol before adding nitric acid was -14.8 mV, and after adding nitric acid, the zeta potential changed to 0.6 mV.

[0061] Anhydrous ethanol, water, nitric acid, and tetraethyl orthosilicate were mixed in a molar ratio of 37.39:2.20:3.02×10⁻⁶. -3 The solutions were added sequentially to the reaction vessel at a ratio of 1:1.00, stirred at room temperature (20-30℃) for 10 minutes, and then sealed and stored at room temperature for 3 days to obtain an acidic sol. The acidic sol was then coated using the dip-coating method, and the refractive index of the film was found to be 1.44.

[0062] The stabilized sol and acidic silica sol were mixed at a volume ratio of 5.5:1 to obtain an acid-base mixed silica sol. The acid-base mixed silica sol was then coated using a dip-coating method, resulting in a film with a refractive index of 1.26.

[0063] The acid-base mixed silica sol was sealed and left at room temperature for 6 months, and then coated again by dip-coating method. The refractive index of the resulting film was found to be 1.28.

[0064] Comparative Example 1

[0065] Other conditions are the same as in Example 1, except that the addition of stabilizer is omitted. The diluted alkaline silica sol and acidic silica sol are directly mixed at a volume ratio of 5.5:1 to obtain an acid-base mixed silica sol. The acid-base mixed silica sol is coated by dip-coating method, and the refractive index of the resulting film is 1.19.

[0066] The resulting acid-base mixed silica sol forms a gel after being left at room temperature for 24 hours, becoming jelly-like and unable to be coated, resulting in a short service life.

[0067] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing an acid-base mixed silica sol, characterized in that, Includes the following steps: A stabilized sol is obtained by mixing an alkaline silica sol and a stabilizer; the mass of the stabilizer is 0.1-5% of the mass of the alkaline silica sol; the stabilizer is an acid with a concentration of 10-50 wt%. The stabilized sol and the acidic silica sol are mixed to obtain the acid-base mixed silica sol; with the total mass of the alkaline silica sol and the acidic silica sol being 100%, the mass fraction of the acidic silica sol is greater than 0 and less than 100%.

2. The preparation method according to claim 1, characterized in that, The acid includes one or more of sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, and acetic acid; The concentration of sulfuric acid is 10-30 wt%, the concentration of nitric acid is 10-50 wt%, the concentration of hydrochloric acid is 10-37 wt%, the concentration of phosphoric acid is 10-50 wt%, and the concentration of acetic acid is 10-50 wt%.

3. The preparation method according to claim 1, characterized in that, The alkaline silica sol contains 1-10 wt% silica; the acidic silica sol contains 1-20 wt% silica.

4. The preparation method according to claim 1, characterized in that, The preparation method of the alkaline silica sol includes the following steps: The alkaline silica sol is obtained by mixing tetraethyl orthosilicate, a silane precursor, ethanol, ammonia, and water and then aging the mixture; the molar ratio of the silane precursor to the tetraethyl orthosilicate is 0 to 2, and is not 0.

5. The preparation method according to claim 4, characterized in that, The silane precursor includes one or more of methyltriethoxysilane, dimethyldiethoxysilane, and hexamethyldisilazane; based on the total mass of the tetraethyl orthosilicate, silane precursor, ethanol, ammonia, and water as 100%, the mass fraction of the ammonia is 0.5-1 wt%, the mass fraction of the water is 2.5-3 wt%, and the concentration of the ammonia is 25-28 wt%.

6. The preparation method according to claim 1, characterized in that, The preparation method of the acidic silica sol includes the following steps: mixing tetraethyl orthosilicate, ethanol, concentrated hydrochloric acid and water and then aging the mixture to obtain the acidic silica sol; Based on the total mass of the tetraethyl orthosilicate, ethanol, concentrated hydrochloric acid and water as 100%, the mass fraction of the tetraethyl orthosilicate is 5~70wt%, the mass fraction of the concentrated hydrochloric acid is 0.01~0.1wt%, the mass fraction of the water is 1.9~12.5wt%, and the mass fraction of the ethanol is 20~93wt%.

7. The acid-base mixed silica sol prepared by the preparation method according to any one of claims 1 to 6.

8. A method for preparing a refractive index tunable silicon dioxide optical thin film, characterized in that, The process includes the following steps: coating the acid-base mixed silica sol as described in claim 7 to obtain a silica optical thin film with adjustable refractive index.

9. The preparation method according to claim 8, characterized in that, The coating method is the dip-coating method.

10. The refractive index tunable silicon dioxide optical thin film prepared by the preparation method according to claim 8 or 9, characterized in that, The refractive index of the tunable silica optical thin film is 1.10~1.44.