Basefilm integrated intense laser thin film based on continuous gradual change

Abstract

The invention belongs to the technical field of high-power laser films, and relates to a high-threshold high-stability laser film for realizing base-film integration based on continuous gradual change of refractive index, the film structure is S / L1L2L3 / A, S represents a substrate, L1 represents a transition layer between the substrate and L2, L2 represents a function realization layer with continuous gradual change of refractive index, L3 represents an impedance adaptation layer between L2 and an incident medium, and A represents an impedance adaptation layer between L2 and an incident medium. A represents an incident medium. According to the base-film integrated intense laser thin film based on continuous gradual change of the refractive index, planarization treatment is carried out on defects of the base, a double-region co-sputtering preparation method is adopted for the L1 layer and the L2 layer, continuous gradual change of the refractive index from the base to an incident medium is achieved, and different optical function requirements are met. According to the integrated film, the problem of stress mismatching caused by thermal expansion coefficient difference between the substrate and the film layer and between different layers in the film layer is effectively solved, a sudden change interface between materials is eliminated, the jump characteristic of a discrete interface of a traditional film is avoided, and the laser damage threshold and stability of the film are improved.

Description

Technical Field

[0001] The invention relates to a high laser damage threshold and high stability laser film, in particular to a laser film integrated with a substrate and an incident medium with a continuously gradual change in refractive index. Background Art

[0002] In high-power laser systems, the application of optical thin films is crucial. The traditional processing method of fused quartz substrates undergoes processes such as molding, grinding and mechanical chemical polishing. This method takes a long time to achieve an ultra-smooth surface with a surface roughness of less than 1nm and is prone to surface and sub-surface defects of optical components. Some longitudinal microcracks may extend into the interior of the substrate and can usually reach a depth of hundreds of microns, resulting in the discrete interface of the base film, which greatly reduces the surface damage threshold of the optical component. Traditional optical films are usually stacked with multiple layers of materials with different refractive indices to achieve the required optical properties such as anti-reflection and high reflection. However, such a multilayer film structure has some significant disadvantages. Due to the difference in the physical properties of the materials, there is a problem of mismatching of material properties inside the film layer and at the interface between the film layer and the substrate and the incident medium. Due to the mismatch of material properties, when the laser is incident on the film layer, it will cause thermal distortion of the film layer, thereby affecting the optical properties of the film layer. Therefore, the surface and sub-surface defects of the substrate and the mismatch of material properties between the film layers during the coating process are the key factors inducing laser film damage.

[0003] Due to its own characteristics, the refractive index gradient film eliminates the interface and increases the design sensitivity of the film. It can achieve optical properties that are difficult to achieve with traditional layered dielectric films, such as better adhesion, stress release, reduced light scattering, and increased laser damage threshold of the film. However, at the same time, the preparation of the refractive index gradient film requires relatively high precision, and the design and preparation are relatively difficult. At present, its preparation methods include dual-source alternating deposition (continuously changing the deposition rate of one of the film materials or changing the deposition rate of two film materials at the same time), magnetron sputtering (precisely controlling the change in the composition ratio of the sputtering gas to the reaction gas to achieve the change of the refractive index), oblique angle incidence deposition, etc.

[0004] Wei Yaowei et al. [Prior technology CN 107132604 A] proposed a method for preparing a gradient refractive index film using atomic layer deposition in an attempt to solve this problem. However, the film prepared by this method still has the problem of mismatch between the film layer and the substrate and the incident medium material. And because the available refractive index is limited, there is still material mismatch at the interface inside the film layer. Moreover, atomic layer deposition is a chemical method, and it is easy to introduce impurities into the film layer, thereby affecting the laser damage threshold of the film. In 2021, Yang Jinhui et al. proposed a gradient refractive index anti-reflection film and its preparation method and application. Each group of anti-reflection film layers is alternately stacked by refractive index limiting layers and refractive index gradient layers. The refractive index of each group of anti-reflection film layers is precisely controlled by adjusting the thickness of the refractive index gradient layer in the anti-reflection film layer. It has not yet achieved an integrated film in which the refractive index of the film layer continuously changes from the substrate to the incident medium. Summary of the invention

[0005] The purpose of the present invention is to overcome the stress mismatch problem, low damage threshold and thermal distortion caused by the difference in thermal expansion coefficient between the substrate and the film layer and between different layers in the film layer in the existing optical film, and provide a method to avoid the jump characteristics and unstable characteristics of the discrete interface of the traditional film through the integration of the base film. The invention ensures that the laser film has a high laser damage threshold and high optical performance stability under high-power and high-energy laser irradiation.

[0006] The technical solution of the present invention is as follows:

[0007] A high-power laser film with a base film integrated based on continuous gradient, the structure of which is S / L1 L2 L3 / A, wherein the structure of the high-power laser film from bottom to top is a base S, a transition layer L1, a function realization layer L2, an impedance matching layer L3 and an incident medium A,

[0008] The refractive index of the substrate S is n0, and the refractive index of the incident medium A is n1;

[0009] The function realization layer L2 is composed of a refractive index of n H High refractive index material, refractive index n L The low refractive index material is mixed, and the initial refractive index is n s , end refractive index n e ;

[0010] The relationship between the refractive index of the function realization layer L2 and the film thickness x of the L2 layer can be expressed as follows:

[0011]

[0012] where n(x) represents the refractive index distribution as a function of film thickness x, n a Is a high refractive index n Hand low refractive index materials L The average refractive index, n p is the refractive index difference between the peak and valley values ​​of the L2 layer, and f(x) represents the refractive index modulation function, which is a Gaussian, sinusoidal or linear function and is expressed in the following form,

[0013]

[0014] Where k and b are coefficients, N is the thickness of a single sine wave cycle, and λ represents the central wavelength;

[0015] The refractive index distribution of the transition layer L1 starts from n0 and ends at the starting refractive index n of the L2 layer. s The endpoint is monotonically linearly distributed, where n0 <n s The impedance adaptation layer L3 has a refractive index distribution based on the termination refractive index n of the L2 layer. e As the starting point, n1 is the end point, showing a monotonic linear distribution, where n1 <n e ;

[0016] The refractive index distribution of the high-intensity laser film changes continuously and gradually from the substrate S to the integrated film L2 and then to the incident medium A;

[0017] The substrate S is fused quartz or K9 glass;

[0018] Function realization layer L2, the high refractive index material is one of Al2O3, Nb2O5, Ta2O5, or HfO2, and the low refractive index material is SiO2;

[0019] The incident medium A is air, fused quartz or water;

[0020] The transition layer L1 is a mixture of the base material and the L2 bottom material;

[0021] The impedance matching layer L3 is formed by mixing the top layer material of L2 and the incident medium.

[0022] The present invention also provides a method for preparing a high-intensity laser thin film with a base film integrated based on continuous gradient, comprising the following steps:

[0023] Step 1: Flattening of substrate defects

[0024] The substrate S after ultrasonic cleaning is melt-polished by continuous CO2 laser, or the substrate surface and subsurface defects are removed by alternating deposition and etching cycles using ion beam etching technology;

[0025] Step 2: Design functions to realize the transmission spectrum of layer L2 and determine the refractive index distribution

[0026] Perform Fourier transformation on the transmission spectrum of the function realization layer L2 to be designed to obtain a refractive index distribution with periodic variation, or discretize the thin film by the characteristic matrix method to determine the number of discrete film layers i (i≤1000) and the thickness d (10nm≤d≤50nm) of the discrete film layer, approximately assuming that each thin layer has a uniform refractive index, and use the traditional optical thin film calculation method to evaluate the optical properties such as reflectivity and transmittance of the function realization layer L2 with a continuous refractive index distribution, and obtain the refractive index distribution function n(x) of the gradient refractive index film layer of the function realization layer L2 and the thickness of the function realization layer L2;

[0027] Step 3: Prepare transition layer L1

[0028] The transition layer L1 is prepared by a double-zone co-sputtering method, and the area of ​​the combined target material being sputtered is controlled to move uniformly from the low-refractive index target material to the high-refractive index target material, so that the refractive index distribution of the transition layer L1 starts from n0 and ends at the starting refractive index n of the L2 layer. s is the end point, showing a linear distribution;

[0029] Step 4: Prepare the functional implementation L2 layer

[0030] The L2 layer is prepared by a dual-zone co-sputtering method. During the coating process, the positions of the high and low refractive index combined targets are continuously changed. The mixing ratio of the materials is adjusted by precisely controlling the sputtering area and movement rate of the combined targets to prepare a mixed film.

[0031] Step 4.1: Establish the relationship between refractive index and sputtering rate

[0032] Use high-energy ions to bombard the composite material target, and change the ratio of the low refractive index material area from 100% to 0% according to different gradient rules, control the gradient of the low refractive index material to the high refractive index material, and determine the corresponding relationship between the refractive index of the mixed thin film material and the target sputtering area. Convert the relationship between the refractive index distribution and the film thickness into the relationship between the refractive index and the target movement rate and coating time;

[0033] Step 4.2: Controlling the material gradient

[0034] According to the designed refractive index distribution, the L2 layer was prepared by double-zone co-sputtering method, and the influence of the gradient law on the film composition, optical properties, microstructure, etc. was analyzed. The optical constants such as the refractive index and extinction coefficient at different sputtering positions and the growth rate were calculated using the spectral characteristics.

[0035] Step 5: Prepare the impedance adaptation layer L3

[0036] The impedance adaptation layer L3 is prepared by oblique deposition or dual-zone co-sputtering, so that the refractive index distribution of the L3 layer is the end refractive index n of the L2 layer. eThe refractive index of the incident medium is n1, which is a continuous distribution with n as the starting point and n2 as the end point. The microstructure shape of the L3 layer can be a pointed cone structure such as a cone or a pyramid.

[0037] Compared with the prior art, the technical effects of the present invention are as follows:

[0038] The invention provides a high-threshold and high-stability laser film based on base film integration. By flattening the base defects and preparing by double-zone co-sputtering, an integrated film with no interface between the base and the incident medium and a continuously gradient refractive index is realized, which effectively solves the stress mismatch and thermal distortion problems caused by the difference in thermal expansion coefficients between the base and the film layer and between different layers within the film layer. It ensures that the laser film has a high laser damage threshold and high optical performance stability under high-power and high-energy laser irradiation. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 Schematic diagram of the dual-zone co-sputtering integrated thin film preparation method for mixed high and low refractive index materials.

[0040] Figure 2 Schematic diagram of the structure of a base film-integrated high-intensity laser thin film based on continuous gradient of refractive index.

[0041] Figure 3 The distribution diagram of the refractive index versus optical thickness of the high-threshold and high-stability laser film achieved by integrating the base film in Example 1.

[0042] Figure 4 This is a graph showing how the transmittance of the high-threshold and high-stability laser film achieved by integrating the base film in Example 1 varies with wavelength.

[0043] Figure 5 This is a graph showing how the transmittance of the traditional discrete film in Example 1 changes with wavelength after being optimized according to the same optimization target.

[0044] Figure 6 The distribution diagram of the refractive index versus optical thickness of the high-threshold and high-stability laser film achieved by integrating the base film in Example 2.

[0045] Figure 7 This is a graph showing how the transmittance of a high-threshold, high-stability laser film achieved by integrating the base film in Example 2 varies with wavelength.

[0046] Figure 8 This is a graph showing the variation of transmittance with wavelength after the traditional discrete film in Example 2 is optimized according to the same optimization target. DETAILED DESCRIPTION

[0047] The following is combined with Figure 1 , 2 The present invention is further described, but the protection scope of the present invention should not be limited thereto. Figure 1Schematic diagram of the dual-zone co-sputtering integrated thin film preparation method for mixed high and low refractive index materials. Figure 2 This is a schematic diagram of the structure of a base film-integrated high-intensity laser film based on continuous gradient of refractive index provided by the present invention. The structures from bottom to top are a substrate, a transition layer L1 representing the substrate and the functional implementation layer, a functional implementation layer L2 representing the gradient of refractive index, an impedance matching layer L3 between the functional implementation layer and the incident medium, and the incident medium A.

[0048] Embodiment 1:

[0049] A high-intensity laser film with integrated base film based on continuous gradient of refractive index Figure 2 As shown, the substrate is made of molten quartz, and a continuous CO2 laser is used to melt-polish the substrate after ultrasonic cleaning. Al2O3 and SiO2 are used as the high and low refractive index materials of the film layer, and the incident medium is air. The transmission spectrum is Fourier transformed to obtain the functional realization layer L2, which is composed of six layers of alternating Al2O3 and SiO2, and a mixed material of Al2O3 and SiO2 is used between the high and low refractive index material layers to achieve refractive index transition. The refractive index distribution of the L1 layer transitions from 1.44 of the substrate to the starting refractive index of 1.62 of the L2 layer, and the thickness of the L1 layer is 10nm. The refractive index distribution of the L3 layer transitions from the starting refractive index of 1.46 of the L2 layer to the refractive index of 1 of A, and the thickness of the L3 layer is 152nm. The refractive index distribution of the high-threshold and high-stability laser film based on base film integration changes with the physical thickness of the film as shown in the figure. Figure 3 As shown, the total thickness of the film is 1414nm, and its transmittance varies with wavelength as shown in Figure 4 As shown. The L1 and L2 layers are prepared by a dual-zone co-sputtering method, and a composite material target of Al2O3 and SiO2 is selected. After multiple tests before film coating, the composite material target is bombarded with high-energy ions, the proportion of SiO2 material in the ion beam sputtering range is adjusted from 100% to 0%, and the refractive index of the mixed material at different corresponding positions is recorded. By comparing the film thickness corresponding to the inflection points of the refractive index distribution of 1.62 and 1.44 during design, the target area position corresponding to the refractive index distribution of the film during coating is obtained. According to the coating rate of the equipment, the relationship between the change of the refractive index and the film thickness is finally converted into the relationship between the change of the refractive index and the coating time. The L3 layer adopts the preparation method of SiO2 tilted deposition, combining the traditional vacuum deposition method with controllable substrate rotation, and realizing the refractive index transition of air and SiO2 by precisely controlling the porosity.

[0050] Figure 5 This is a graph showing the variation of transmittance with wavelength after the traditional non-transition layer, impedance matching layer and non-gradient film is optimized according to the same optimization target in Example 1. It can be seen that at the same optimization target, the wavelength transmittance performance can reach Figure 4The refractive index is continuously graded and the base film is integrated with the laser film. However, many high transmittance oscillations are generated near the target wavelength, which will reduce the optical stability of the film in subsequent preparation.

[0051] Embodiment 2:

[0052] A high-intensity laser film with integrated base film based on continuous gradient of refractive index Figure 2 As shown, the substrate is made of molten quartz, and a continuous CO2 laser is used to melt-polish the substrate after ultrasonic cleaning. HfO2 and SiO2 are selected as the high and low refractive index materials of the film layer, and the incident medium is quartz glass. The design process is as in Example 1, and the refractive index of the functional implementation layer L2 layer is selected to show a Gaussian modulated sinusoidal change with the thickness of the film layer. The refractive index distribution of the L1 layer transitions from 1.44 of the substrate to the starting refractive index of 1.75 of the L2 layer, and the thickness of the L1 layer is 630nm. The refractive index distribution of the L3 layer transitions from the ending refractive index of 1.75 of the L2 layer to the refractive index of 1.44 of A, and the thickness of the L3 layer is 630nm. The refractive index distribution of the high-threshold and high-stability laser thin film based on the integration of the base film changes with the physical thickness of the film as shown in the figure. Figure 6 As shown, the total thickness of the film is 12143nm, and its transmittance varies with wavelength as shown in Figure 7 As shown. The L1 layer and the L2 layer are plated by a dual-zone co-sputtering preparation method. The process is in accordance with Example 1. After multiple tests, the proportion of SiO2 material in the ion beam sputtering range is adjusted from 100% to 0%, and the refractive index of the mixed material at different corresponding positions is recorded. The film thickness corresponding to the inflection point of the sine function (1.46, 1.75, 1.78, 1.81, etc.) in the refractive index distribution during design is compared to obtain the corresponding target area position during plating. According to the coating rate of the equipment, the relationship between the change of the refractive index and the film thickness is finally converted into the relationship between the change of the refractive index and the coating time. The L3 layer is still prepared by the dual-zone co-sputtering method, so that the refractive index distribution transitions from the termination refractive index of 1.75 of the L2 layer to the refractive index of 1.44 of A.

[0053] Figure 8 This is a graph showing the variation of transmittance with wavelength after optimizing the traditional non-gradient film without transition layer, impedance matching layer according to the same optimization goal in Example 2. It can be seen that at the target wavelength of 1064nm, the general optimization method of wavelength transmittance of the traditional discrete film is difficult to control a narrower bandwidth range, and also generates many oscillations near the target wavelength, which will cause instability in subsequent preparation.

Claims

1. A high-intensity laser film with integrated base film based on continuous gradient, characterized in that: The structure is S / L1 L2 L3 / A. The structure of the high-intensity laser film from bottom to top is substrate S, transition layer L1, function realization layer L2, impedance matching layer L3 and incident medium A. The refractive index of the substrate S is n0, and the refractive index of the incident medium A is n1; The function realization layer L2 is composed of a layer with a refractive index of n H High refractive index material, refractive index n L The low refractive index material is mixed, and the initial refractive index is n s , end refractive index n e , the relationship between the refractive index of the function realization layer L2 and the film thickness x of the L2 layer can be expressed as follows: where n(x) represents the refractive index distribution as a function of film thickness x, n a Is a high refractive index n H and low refractive index materials L The average refractive index, n p is the refractive index difference between the peak and valley values ​​of the L2 layer, and f(x) represents the refractive index modulation function, which is a Gaussian, sinusoidal or linear function and is expressed in the following form: Where k and b are coefficients, N is the thickness of a single sine wave cycle, and λ represents the central wavelength; The refractive index distribution of the transition layer L1 starts from n0 and ends at the starting refractive index n of the L2 layer. s The endpoint is monotonically linearly distributed, where n0 <n s The impedance adaptation layer L3 has a refractive index distribution based on the termination refractive index n of the L2 layer. e As the starting point, n1 is the end point, showing a monotonic linear distribution, where n1 <n e ; The refractive index distribution of the high-intensity laser film changes continuously and gradually from the substrate S to the integrated film L2 and then to the incident medium A.

2. A high-intensity laser film with integrated base film based on continuous gradient as claimed in claim 1, characterized in that: The substrate S is fused quartz or K9 glass.

3. A high-intensity laser film with integrated base film based on continuous gradient as claimed in claim 1, characterized in that: The function realization layer L2, the high refractive index material is one of Al2O3, Nb2O5, Ta2O5, or HfO2, and the low refractive index material is SiO2.

4. A high-intensity laser film with integrated base film based on continuous gradient as claimed in claim 1, characterized in that: The incident medium A is air or fused quartz.

5. A method for preparing a high-power laser thin film with integrated base film based on continuous gradient as claimed in any one of claims 1 to 4, characterized in that: The steps include: Step 1: Flattening of substrate defects The substrate S after ultrasonic cleaning is melt-polished by continuous CO2 laser, or the substrate surface and subsurface defects are removed by alternating deposition and etching for multiple cycles by ion beam etching technology; Step 2: Design functions to realize the transmission spectrum of layer L2 and determine the refractive index distribution Perform Fourier transformation on the transmission spectrum of the function realization layer L2 to be designed to obtain a refractive index distribution with periodic variation, or discretize the thin film by the characteristic matrix method to determine the number of discrete film layers i (i≤1000) and the thickness d (10nm≤d≤50nm) of the discrete film layer, approximately assuming that each thin layer has a uniform refractive index, and use the traditional optical thin film calculation method to evaluate the optical properties such as reflectivity and transmittance of the function realization layer L2 with a continuous refractive index distribution, and obtain the refractive index distribution function n(x) of the gradient refractive index film layer of the function realization layer L2 and the thickness of the function realization layer L2; Step 3: Prepare transition layer L1 The transition layer L1 is prepared by a double-zone co-sputtering method, and the area of ​​the combined target material being sputtered is controlled to move uniformly from the low-refractive index target material to the high-refractive index target material, so that the refractive index distribution of the transition layer L1 starts from n0 and ends at the starting refractive index n of the L2 layer. s is the end point, showing a linear distribution; Step 4: Prepare the function implementation layer L2 The function realization layer L2 is prepared by a dual-zone co-sputtering method. During the coating process, the positions of the high and low refractive index combined targets are continuously changed. The mixing ratio of the materials is adjusted by precisely controlling the sputtering area and the moving rate of the combined targets to prepare a mixed film. Step 4.1: Establish the relationship between refractive index and sputtering rate Use high-energy ions to bombard the composite material target, and change the ratio of the low refractive index material area from 100% to 0% according to different gradient rules, control the gradient of the low refractive index material to the high refractive index material, and determine the corresponding relationship between the refractive index of the mixed thin film material and the target sputtering area. Convert the relationship between the refractive index distribution and the film thickness into the relationship between the refractive index and the target movement rate and coating time; Step 4.2: Controlling the material gradient According to the designed refractive index distribution, the L2 layer was prepared by double-zone co-sputtering method, and the influence of the gradient law on the film composition, optical properties, microstructure, etc. was analyzed. The optical constants such as the refractive index and extinction coefficient at different sputtering positions and the growth rate were calculated using the spectral characteristics. Step 5: Prepare the impedance adaptation layer L3 The impedance adaptation layer L3 is prepared by oblique deposition or dual-zone co-sputtering, so that the refractive index distribution of the L3 layer is the end refractive index n of the L2 layer. e The refractive index of the incident medium is n1, which is a continuous distribution with n as the starting point and n2 as the end point. The microstructure shape of the L3 layer can be a pointed cone structure such as a cone or a pyramid.

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