Realization of base film integrated strong laser film based on continuous gradient

Abstract

The application belongs to the technical field of high-power laser films, and is a high-threshold and high-stability laser film based on continuous gradual change of refractive index and integrated with a base film, and the film structure is S / L1L2L3 / A, wherein S represents a substrate, L1 represents a transition layer between the substrate and L2, L2 represents a function implementation layer of continuous gradual change of refractive index, L3 represents an impedance adaptation layer between L2 and an incident medium, and A represents the incident medium. The high-strength laser film based on continuous gradual change of refractive index and integrated with a base film provided by the application is subjected to defect flattening treatment, L1 and L2 are prepared by a double-zone co-sputtering method, the refractive index is continuously changed from the substrate to the incident medium, and different optical function requirements are achieved. The above integrated film effectively solves the stress mismatching problem caused by the difference in thermal expansion coefficients between the substrate and the film layer and between different layers in the film layer, eliminates the abrupt interface between materials, avoids the jump characteristics of the discrete interface of the traditional film, and improves the laser damage threshold and stability of the film.

Description

TECHNICAL FIELD

[0001] The present application relates to high laser damage threshold, high stability laser film, in particular, a laser film which is integrated with the substrate and the incident medium with continuous and gradual change of refractive index. BACKGROUND

[0002] In high power laser systems, the application of optical film is very important. The traditional processing method of fused quartz substrate is through forming, grinding and mechanical and chemical polishing processes, which takes a long time to achieve a surface roughness of less than 1 nm of super-smooth surface, and the subsequent surface and subsurface defects of optical elements are prone to occur, and some longitudinal micro-cracks may extend to the interior of the substrate, usually up to hundreds of microns, which causes the interface of the base film to be discrete and the surface damage threshold of the optical element to be greatly reduced. The traditional optical film is usually composed of multiple layers of materials with different refractive indexes to achieve the required anti-reflection, high reflection and other optical properties. However, such a multi-layer film structure has some significant disadvantages. Due to the differences in material properties, there are material property mismatches in 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 laser is incident on the film layer, the film layer will be thermally distorted, which will affect the optical performance of the film layer. Therefore, the surface and subsurface defects of the substrate and the mismatch of material properties between the film layers during the coating process are the key factors inducing the damage of the laser film.

[0003] The refractive index gradient film eliminates the interface due to its own characteristics, increases the design sensitivity of the film, and can achieve optical performance that is difficult to achieve by traditional layered dielectric film, such as better adhesion, stress release, reduced light scattering and improved laser damage threshold of the film. However, the preparation of the refractive index gradient film requires high precision and is difficult to design and prepare. Current preparation methods include double-source alternating deposition (continuous change of one of the film materials or simultaneous change of the deposition rate of two film materials), magnetron sputtering method (precise control of the composition ratio of sputtering gas and reaction gas to change the refractive index), and oblique angle deposition method.

[0004] A method for preparing a graded refractive index film by atomic layer deposition is proposed by Weiyao Wei et al. (Prior art CN 107132604 A) 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 due to the limitation of the refractive index that can be obtained, there is still a material mismatch at the interface inside the film layer. Moreover, atomic layer deposition is a chemical method, which is easy to introduce impurities into the film layer, thereby affecting the laser damage threshold of the film. In 2021, Yang Jinhu et al. proposed a graded refractive index antireflection film and its preparation method and application. Each group of antireflection film layers is alternately stacked by refractive index limiting layers and refractive index gradient layers, and the refractive index of each group of antireflection film layers is precisely controlled by adjusting the thickness of the refractive index gradient layer. However, it has not yet achieved an integrated film that continuously and gradually changes the refractive index of the film layer from the substrate to the incident medium. SUMMARY

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

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

[0007] A strong laser film based on continuous gradient to realize base film integration, the structure of which is S / L1 L2 L3 / A. The structure of the strong laser film from bottom to top is substrate S, transition layer L1, function implementation layer L2, impedance matching layer L3 and 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 implementation layer L2 is composed of high refractive index material with refractive index n H , low refractive index material with refractive index n L , the starting refractive index n s , and the terminal refractive index n e .

[0010] The relationship between the refractive index of the function implementation 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 the high refractive index n Hand a low refractive index material n L The average refractive index n p is the refractive index difference between the peak and valley of the L2 layer, f(x) represents the refractive index modulation function, which is a Gaussian, sinusoidal or linear function, expressed as follows,

[0013]

[0014] Where k and b are coefficients, N is the thickness of a single sinusoidal period, and λ represents the central wavelength.

[0015] The transition layer L1 has a monotonous linear distribution of refractive index distribution starting from n0 to the starting refractive index n s of the L2 layer, where n0 < n s The impedance matching layer L3 has a monotonous linear distribution of refractive index distribution starting from the terminal refractive index n e of the L2 layer to n1 as the end point, where n1 < n e ;

[0016] The strong laser film has a continuous and gradual change in refractive index distribution from the substrate S to the integrated film L2 to the incident medium A.

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

[0018] The functional implementation layer L2 has a high refractive index material selected from one of Al2O3, Nb2O5, Ta2O5, and HfO2, and a low refractive index material of SiO2.

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

[0020] The transition layer L1 is composed of a mixture of the substrate material and the bottom layer material of L2.

[0021] The impedance matching layer L3 is composed of a mixture of the top layer material of L2 and the incident medium.

[0022] The application also provides a preparation method of the strong laser film based on the continuous and gradual change to realize the integration of the substrate film, which comprises the following steps:

[0023] Step 1: planarization treatment of substrate defects

[0024] The substrate S after ultrasonic cleaning is subjected to fusion polishing by a continuous CO2 laser, or is subjected to multiple cycles of deposition and etching by ion beam etching technology to remove the defects on the surface and subsurface of the substrate.

[0025] Step 2: design the transmission spectrum of the functional implementation layer L2 and determine the refractive index distribution

[0026] The function implementation layer L2 to be designed is subjected to Fourier transform to obtain a refractive index distribution with periodic variation, or the thin film is discretized by a characteristic matrix method to determine discrete film layer number i (i≤1000) and discrete film layer thickness d (10 nm≤d≤50 nm), and it is approximately considered that each thin layer has a uniform refractive index, and a traditional optical thin film calculation method is used to evaluate the reflectivity and transmissivity and other optical properties of the function implementation layer L2 with a continuous refractive index distribution to obtain a refractive index distribution function n(x) of the function implementation layer L2 gradient refractive index film layer and a thickness of the function implementation layer L2;

[0027] Step 3: Preparation of the transition layer L1

[0028] The transition layer L1 is prepared by a double-zone co-sputtering method, the area of the region where the combined target material is sputtered is uniformly moved from the low-refractive-index target material to the high-refractive-index target material, the refractive index distribution of the transition layer L1 is linearly distributed with n0 as a starting point and n s as an end point;

[0029] Step 4: Preparation of the function implementation L2 layer

[0030] The function implementation L2 layer is prepared by a double-zone co-sputtering method, the positions of the high-refractive-index and low-refractive-index combined targets are continuously changed during the film plating process, the mixing ratio of the materials is adjusted by precisely controlling the area of the region where the combined target material is sputtered and the moving rate to prepare a mixed thin film;

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

[0032] The high-energy ion bombardment combined material target is used to realize the change of the low-refractive-index material region ratio from 100% to 0% according to different gradient rules, the gradient change of the low-refractive-index material to the high-refractive-index material is controlled, and the corresponding relationship between the refractive index of the mixed thin film material and the sputtering region of the target is determined. The change relationship of the refractive index distribution with respect to the film layer thickness is converted into the change relationship of the refractive index with respect to the target moving rate and the film plating time;

[0033] Step 4.2: Control of the material gradient change

[0034] The L2 layer is prepared by a double-zone co-sputtering method according to the designed refractive index distribution, the influence of the gradient rule on the film layer composition, optical properties, microstructure and the like is analyzed, and the optical constants such as the refractive index and extinction coefficient and the growth rate at different sputtering positions are calculated by using the spectral characteristics;

[0035] Step 5: Preparation of the impedance adaptation layer L3

[0036] The impedance adaptation layer L3 is prepared by a tilt deposition or double-zone co-sputtering method, and the refractive index distribution of the L3 layer is linearly distributed with the terminal refractive index n eThe microstructure shape of the L3 layer can be a sharp cone structure such as a circular cone or a prismatic cone.

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

[0038] The present application provides a high-threshold and high-stability laser film based on base film integration, which realizes an integrated film without interface and continuous refractive index gradient between the base and the incident medium through the flattening treatment of the base defects and the double-zone co-sputtering preparation method, effectively solves the stress mismatch and thermal distortion problems caused by the difference in the thermal expansion coefficient between the base and the film layer and between different layers in the film layer, and ensures that the laser film has high laser damage threshold and high optical performance stability under high-power and high-energy laser incidence. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 It is a schematic diagram of a double-zone co-sputtering integrated film preparation method for mixing high and low refractive index materials.

[0040] Figure 2 It is a schematic diagram of a base film integrated strong laser film structure based on continuous refractive index gradient.

[0041] Figure 3 It is a refractive index distribution graph of the high-threshold and high-stability laser film based on base film integration in Example 1 with optical thickness.

[0042] Figure 4 It is a transmittance graph of the high-threshold and high-stability laser film based on base film integration in Example 1 with wavelength variation.

[0043] Figure 5 It is a transmittance graph of the traditional discrete film in Example 1 after optimization according to the same optimization target with wavelength variation.

[0044] Figure 6 It is a refractive index distribution graph of the high-threshold and high-stability laser film based on base film integration in Example 2 with optical thickness.

[0045] Figure 7 It is a transmittance graph of the high-threshold and high-stability laser film based on base film integration in Example 2 with wavelength variation.

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

[0047] The present application will be further described below with reference to the accompanying drawings Figure 1 , 2 The present application will be further described below with reference to the accompanying drawings Figure 1A schematic diagram of a preparation method of a two-zone co-sputtering integrated thin film of mixed high and low refractive index materials. Figure 2 A schematic diagram of a structure of a strong laser thin film integrated with a base film based on a continuous gradual change in refractive index, from bottom to top, the structure is in turn a substrate, a transition layer L1 representing the substrate and the function-implementation layer, a function-implementation layer L2 representing the gradual change in refractive index, an impedance matching layer L3 representing the function-implementation layer and the incident medium, and an incident medium A.

[0048] Example 1

[0049] A strong laser thin film integrated with a base film based on a continuous gradual change in refractive index is shown in Figure 2 The substrate is fused quartz, and the substrate after ultrasonic cleaning is subjected to fusion polishing by a continuous CO2 laser. The high and low refractive index materials of the film layer are Al2O3 and SiO2, and the incident medium is air. The transmission spectrum is Fourier transformed to obtain that the function-implementation layer L2 is composed of six layers of alternating Al2O3 and SiO2, and the mixed material of Al2O3 and SiO2 is used between the high and low refractive index material layers to realize the transition of refractive index. The refractive index distribution of the L1 layer transitions from 1.44 of the substrate to the initial refractive index 1.62 of the L2 layer, and the thickness of the L1 layer is 10 nm. The refractive index distribution of the L3 layer transitions from the initial refractive index 1.46 of the L2 layer to the refractive index 1 of A, and the thickness of the L3 layer is 152 nm. The refractive index distribution of the high threshold high stability laser thin film integrated with the base film is shown in Figure 3 The total thickness of the thin film is 1414 nm, and the transmittance changes with wavelength as shown in Figure 4 The L1 layer and the L2 layer are plated by a two-zone co-sputtering method, and the Al2O3 and SiO2 combined material target is selected. Before plating the thin film, the combined material target is bombarded by high-energy ions for 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 corresponding target area position of the refractive index distribution of the thin film is obtained by comparing the film thickness corresponding to the refractive index distribution inflection points 1.62, 1.44, etc. According to the plating rate of the equipment, the change relationship of the refractive index with respect to the film thickness is finally converted into the change relationship of the refractive index with respect to the plating time. The L3 layer is prepared by a SiO2 inclined deposition method, which combines the traditional vacuum deposition method with controllable substrate rotation, and realizes the transition of the refractive index of air and SiO2 by accurately adjusting the porosity.

[0050] Figure 5 For the optimized traditional thin film without a transition layer, an impedance matching layer and a non-gradual change in Example 1 according to the same optimization target, the transmittance changes with wavelength as shown in the figure, it can be seen that at the same optimization target, the wavelength transmittance can reach Figure 4The refractive index of the integrated laser thin film with a continuously graded base film is measured, but it generates many high-transmittance oscillations near the target wavelength, which will reduce the optical stability of the film in subsequent fabrication.

[0051] Example 2:

[0052] A high-power laser thin film with integrated substrate based on continuous gradient refractive index Figure 2 As shown, the substrate is fused silica, and the substrate is melt-polished after ultrasonic cleaning using a continuous CO2 laser. The high and low refractive index materials of the film are HfO2 and SiO2, and the incident medium is quartz glass. The design process is as in Example 1. The refractive index of the functional layer L2 exhibits a Gaussian-modulated sinusoidal variation with the film thickness. The refractive index distribution of the L1 layer transitions from 1.44 on the substrate to the initial refractive index of 1.75 in the L2 layer, and the thickness of the L1 layer is 630 nm. The refractive index distribution of the L3 layer transitions from the final refractive index of 1.75 in the L2 layer to the refractive index of 1.44 in the A layer, and the thickness of the L3 layer is 630 nm. The variation of the refractive index distribution with the physical thickness of the high threshold, high stability laser thin film based on substrate-film integration is shown below. Figure 6 As shown, the total thickness of the film is 12143 nm, and its transmittance varies with wavelength as follows: Figure 7 As shown. Layers L1 and L2 were prepared using a dual-zone co-sputtering method, following the procedure in Example 1. Through multiple tests, the proportion of SiO2 material within the ion beam sputtering range was adjusted from 100% to 0%, and the refractive index of the mixed material at different corresponding positions was recorded. The film thickness corresponding to the inflection points of the sinusoidal function in the refractive index distribution (1.46, 1.75, 1.78, 1.81, etc.) in the design was compared to obtain the corresponding target region positions during deposition. Based on the equipment's deposition rate, the relationship between refractive index and film thickness was ultimately transformed into a relationship between refractive index and deposition time. Layer L3 was also prepared using the dual-zone co-sputtering method, transitioning the refractive index distribution from the terminal refractive index of L2 (1.75) to the refractive index of A (1.44).

[0053] Figure 8 In Example 2, the transmittance of a traditional non-gradient thin film without a transition layer or impedance matching layer is optimized according to the same optimization objective, as shown in the wavelength variation graph. It can be seen that at the target wavelength of 1064 nm, conventional discrete thin film wavelength transmittance optimization methods generally struggle to control a narrower bandwidth range, and also generate many oscillations near the target wavelength, leading to instability in subsequent fabrication.

Claims

1. A strong laser film integrated with a base film based on continuous grading, characterized in that, The structure is S / L1L2L3 / A, and the structure of the strong 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 functional implementation layer L2 is composed of a high refractive index material with a refractive index n H , and a low refractive index material with a refractive index n L , the initial refractive index is n s , and the final refractive index is n e . The relationship between the refractive index of the functional implementation layer L2 and the layer thickness x of L2 is expressed in the following form: where n(x) represents the refractive index profile as a function of film thickness x, n a is the average refractive index of the high refractive index n H and low refractive index material n L , n p is the refractive index difference between the peak and valley of the L2 layer, and f(x) represents the refractive index modulation function, which is a Gaussian or linear function, expressed in the following form: Wherein, k and b are coefficients, N is the thickness of a single sinusoidal wave period, and λ represents the central wavelength; The transition layer L1 has a refractive index distribution which is monotonically linear from a starting value n0 to an end value n s , where n0 < n s , and the impedance adaptation layer L3 has a refractive index distribution which is monotonically linear from a starting value n e , where n e ; The refractive index distribution of the strong laser film is always continuously graded from the substrate S to the function realization layer L2 and then to the incident medium A.

2. The strong laser film based on continuous grading to realize base film integration according to claim 1, characterized in that, The substrate S is fused quartz or K9 glass.

3. The strong laser film based on continuous grading to realize base film integration according to claim 1, characterized in that, The high-refractive-index material of the function realization layer L2 is one of Al2O3, Nb2O5, Ta2O5 or HfO2, and the low-refractive-index material is SiO2.

4. The strong laser film based on continuous grading to realize base film integration according to claim 1, characterized in that, The incident medium A is air or fused quartz.

5. A method for manufacturing a strong laser film based on a continuous gradient to realize a base film integration, according to any one of claims 1 to 4, characterized in that, The method comprises the following steps: Step 1: planarization treatment of substrate defects The substrate S is fused and polished by a continuous CO2 laser after ultrasonic cleaning, or the substrate surface and subsurface defects are removed through multiple cycles of deposition and etching by ion beam etching technology; Step 2: design of function realization layer L2 transmission spectrum and determination of refractive index distribution The function realization layer L2 transmission spectrum to be designed is Fourier transformed to obtain a refractive index distribution with periodic variation, or the thin film is discretized by a characteristic matrix method to determine the discrete film layer number i, i≤1000, and the discrete film layer thickness d, 10nm≤d≤50nm, and it is approximately considered that each thin layer has a uniform refractive index, the reflectivity and transmissivity of the function realization layer L2 with a continuous refractive index distribution are evaluated by using a traditional optical thin film calculation method to obtain the refractive index distribution function n(x) of the function realization layer L2 graded refractive index film layer and the thickness of the function realization layer L2; Step 3: preparation of transition layer L1 The transition layer L1 is prepared by a double-zone co-sputtering method. The area of the sputtered region of the combined target is uniformly moved from the low-refractive-index target to the high-refractive-index target, so that the refractive index distribution of the transition layer L1 has a starting point of n0 and an end point of n s , which is linearly distributed. Step 4: preparation of function realization layer L2 The function realization layer L2 is prepared by a double-zone co-sputtering method, the positions of the high-refractive-index and low-refractive-index combined targets are continuously changed during the film deposition, the mixing ratio of the materials is adjusted by accurately controlling the sputtered area and moving rate of the combined targets to prepare a mixed thin film; Step 4.1: establishment of the relationship between the refractive index and the sputtering rate The combined material target is bombarded by high-energy ions, the proportion of the low-refractive-index material region is changed from 100% to 0% according to different gradient rules, the gradient grading of the low-refractive-index material to the high-refractive-index material is controlled, and the corresponding relationship between the refractive index of the mixed thin film material and the sputtering area of the target is determined; The relationship between the refractive index distribution and the film layer thickness is converted into the relationship between the refractive index and the sputtering position and the film deposition time; Step 4.2: control of material gradient grading The L2 layer is prepared by a double-zone co-sputtering method according to the designed refractive index distribution, the influence of the gradient rule on the film layer composition, optical properties and microstructure is analyzed, and the refractive index, extinction coefficient and growth rate at different sputtering positions are calculated by using spectral characteristics; Step 5: preparation of impedance matching layer L3 The impedance adaptation layer L3 is prepared by means of oblique deposition or two-zone co-sputtering, so that the refractive index distribution of the L3 layer takes the terminal refractive index n e of the L2 layer as the starting point and the refractive index n1 of the incident medium as the terminal point, and the microstructure shape of the L3 layer is a circular cone or a prismatic cone.

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