Yttrium barium fluoride optical thin film, preparation method thereof and middle and far infrared optical structure

Abstract

The invention provides a barium yttrium fluoride optical thin film, a preparation method thereof and a middle and far infrared optical structure. The preparation method of the yttrium barium fluoride optical thin film comprises the following steps that an optical substrate and a yttrium barium fluoride compound are provided, the yttrium barium fluoride compound is a composite material which is obtained through quenching treatment after yttrium fluoride and barium fluoride are subjected to a high-temperature solid-phase reaction, and the grain size of the yttrium barium fluoride compound is in the submicron level; the metastable state crystal phase structure is a cubic phase with an Fm-3m space group and / or a tetragonal phase with a lattice distortion P4 / nmm space group; the optical substrate is heated to 150 DEG C to 200 DEG C; and depositing the barium yttrium fluoride optical thin film on an optical substrate by using an electron gun evaporation method and taking the barium yttrium fluoride compound as an evaporation source. The yttrium barium fluoride optical thin film can still keep a compact and continuous structure morphology without microcrack distribution under the condition that the single-layer film thickness reaches 2-6 microns, and has very good optical transparency, thermal stability and mechanical integrity.

Description

Technical Field

[0001] The present invention relates to the technical field of optical thin films, and particularly to a yttrium barium fluoride optical thin film, a preparation method thereof, and a mid-infrared optical structure. Background Art

[0002] In a mid-infrared optical system, in order to achieve a high transmittance output of high-power laser or a high signal-to-noise ratio imaging effect of a thermal imaging system, it is usually necessary to deposit an optical thin film with a low refractive index on the surface of an optical element to construct a single-layer or multi-layer film structure with an anti-reflection function. According to the optical film system design principle, the thickness of a single-layer anti-reflection film needs to satisfy that the optical path is one-fourth of the working wavelength. Therefore, when working in the 10.6 μm band, the physical thickness of the required thin film usually reaches 2 μm - 4 μm.

[0003] However, for traditional fluoride optical materials, due to their low mechanical strength and large differences in thermal expansion coefficients with common optical substrates such as ZnSe, Ge, and Si, when depositing a single-layer thin film with a thickness exceeding 1 μm, stress accumulation is extremely likely to occur during the film growth process, thereby triggering structural damage phenomena such as cracks, peeling, or warping, and it is difficult to pass environmental reliability tests such as high and low temperature cycling or thermal shock, which limits their practical applications in thick film application scenarios.

[0004] In addition, in mid-infrared laser systems with a relatively high laser power density, such as CO 2 lasers, some existing technologies attempt to use a multi-layer interference film structure formed by alternately depositing high refractive index materials such as ZnS and low refractive index fluoride materials to improve the transmittance. However, due to the multi-layer film system having more interface absorption positions and significant interlayer energy stacking effects, local temperature rise and micro-area thermal damage often occur, ultimately resulting in optical failure of the thin film or shortening of the service life. If a low-absorption fluoride material such as YbF 3 is used as a single-layer anti-reflection film, although a relatively high transmittance can be theoretically achieved, its thermodynamic stability and thick film forming ability are still insufficient, and it is difficult to achieve stable deposition with a thickness greater than 2 μm without sacrificing film formation integrity and environmental resistance. Summary of the Invention

[0005] The present invention provides a low refractive index optical thin film material applicable to the mid-infrared band and capable of stably depositing a thickness of more than 2 μm, and a preparation method thereof. This material has excellent thermal stability, low stress, low absorption, and good interface matching ability with the substrate to meet the composite performance requirements of high damage threshold, high transmittance, and thick film reliability of a laser system, and can be applied to high-power mid-infrared laser devices and optical systems.

[0006] According to one aspect of the present invention, a preparation method of a yttrium barium fluoride optical thin film is provided, including the following steps: Provide an optical substrate and a yttrium barium fluoride composite, wherein the yttrium barium fluoride composite is a composite material having a metastable crystal phase structure obtained by a high-temperature solid-phase reaction of yttrium fluoride and barium fluoride and then quenching, and the grain size of the yttrium barium fluoride composite is submicron-level, and the metastable crystal phase structure is a cubic phase with a Fm-3m space group and / or a tetragonal phase with a lattice-distorted P4 / nmm space group; Heating the optical substrate to 150° C.-200° C.; The yttrium barium fluoride optical film is deposited on the optical substrate by using an electron gun evaporation method and taking the yttrium barium fluoride complex as an evaporation source.

[0007] Optionally, the preparation method of the yttrium barium fluoride complex comprises the following steps: Yttrium fluoride and barium fluoride are mixed in a mass ratio of 6:4-4:6, and a high-temperature solid phase reaction is carried out at 700° C.-900° C. in an atmosphere rich in fluoride ions and under the protection of an inert gas to obtain a sub-molten composite reaction body; The sub-molten composite reaction body is solidified into a yttrium barium fluoride frit at a cooling rate of 200° C. / s-600° C. / s by a rapid quenching process; The yttrium-barium fluoride frit is crushed, cleaned and dried to obtain the yttrium-barium fluoride composite.

[0008] Optionally, the particles formed after the yttrium barium fluoride frit is crushed have an equiaxed or blocky morphology, and the particle size of the particles is 0.5 mm-2 mm.

[0009] Optionally, the method of using an electron gun evaporation method to deposit a yttrium barium fluoride optical film on the optical substrate using the yttrium barium fluoride composite as an evaporation source comprises the following steps: The yttrium barium fluoride complex is evaporated at a temperature range of 1150° C. to 1250° C. at an evaporation rate of 0.1 nm / s to 0.3 nm / s by electron beam full surface scanning; The yttrium barium fluoride complex is then evaporated continuously at a temperature range of 1260° C. to 1350° C. at an evaporation rate of 0.5 nm / s to 1.5 nm / s, thereby depositing a yttrium barium fluoride optical film on the optical substrate.

[0010] Optionally, before heating the optical substrate to 150° C.-200° C., the following steps are further included: A barium yttrium fluoride seed film with an amorphous structure is deposited on the optical substrate, and the barium yttrium fluoride seed film has a thickness of 5nm-50nm.

[0011] Optionally, depositing a barium yttrium fluoride seed film of an amorphous structure on the optical substrate comprises the following steps: Using the electron gun evaporation method, with the yttrium barium fluoride complex as the evaporation source, in the temperature range from room temperature to 100 °C, evaporate the yttrium barium fluoride complex at an evaporation rate of 0.05 nm / s - 0.2 nm / s and by means of full-surface electron beam scanning to obtain the yttrium barium fluoride seed film.

[0012] According to another aspect of the present invention, there is provided a yttrium barium fluoride optical thin film, which is prepared by using the preparation method as described above. The yttrium barium fluoride optical thin film has a single-layer structure and a thickness of 2 μm - 6 μm.

[0013] According to still another aspect of the present invention, there is provided a mid-infrared and far-infrared optical structure, including an optical substrate and the yttrium barium fluoride optical thin film as described above.

[0014] Optionally, the material of the optical substrate is ZnSe, Ge, CaF 2 or a silicon-based material.

[0015] According to the solution of the present application, the yttrium barium fluoride optical thin film is deposited on the surface of the optical substrate. This thin film is deposited by the electron gun evaporation method from a yttrium barium fluoride complex with a metastable crystal phase structure formed by high-temperature solid-phase - quenching treatment. The particle size of the yttrium barium fluoride complex is controlled at the sub-micron level, effectively balancing the evaporation rate, thermal stability and particle denseness, and having excellent compositional consistency and equilibrium of thermal escape behavior during the subsequent electron beam evaporation process, avoiding problems such as sputtering, component separation and local overheating of particles. The cubic phase with the Fm-3m space group and / or the tetragonal phase with the P4 / nmm space group with lattice distortion are rich in lattice distortion and structural defects inside, such as stacking faults, dislocations and non-ideal packing arrangements. These lattice distortions and structural defects can effectively absorb or transfer stress during the film growth process, inhibit the concentrated accumulation of macroscopic stress, and prevent the formation of through-crack microcracks. During the electron beam evaporation deposition process, the evaporation driving force of the metastable crystal phase structure of the yttrium barium fluoride complex is moderate, the evaporation rate is stably controlled, and the escape rates of each component (barium fluoride and yttrium fluoride) are close, which is beneficial to maintaining the consistency of the film refractive index and optical uniformity, and avoiding problems such as local devitrification, enhanced scattering or refractive index fluctuations caused by phase separation or grain growth. In addition, in the prior art, fluoride materials (such as YF 3 、LaF 3 、MgF 2(0) During the evaporation process, problems such as large differences in component vapor pressure, poor thermal stability, and uneven evaporation are likely to occur, often accompanied by splashing, phase decomposition, or partial dissociation phenomena. To compensate for the structural and compositional instabilities during the evaporation of the above materials, the prior art usually needs to heat the substrate temperature to above 200°C to promote film adsorption and structural densification. However, this high-temperature process will exacerbate the accumulation of thermal stress between the film and the substrate and also pose limitations on some heat-sensitive substrate materials. The yttrium barium fluoride composite material used in the embodiments of the present invention has a metastable crystal phase structure and a submicron-sized particle size distribution, and can achieve stable and uniform evaporation deposition at a relatively low temperature (150°C - 200°C), with both compositional consistency and structural densification, thereby effectively reducing the dependence on high-temperature substrates and realizing a milder and more compatible film-forming process. The finally formed yttrium barium fluoride optical thin film can still maintain a dense, continuous, and microcrack-free structural morphology under the condition that the single-layer film thickness reaches 2μm - 6μm, and has very good optical transparency, thermal stability, and mechanical integrity. The yttrium barium fluoride optical thin film is particularly suitable for low-refractive-index optical thin film materials in mid- to far-infrared band laser systems or infrared imaging windows, and can be used to construct an anti-reflection film structure, which is suitable for both single-layer antireflection film design and can also be used as a low-refractive-index sublayer in a multi-layer interference film system, thereby effectively reducing the optical interface reflection loss, improving the system transmittance, and maintaining a high laser damage threshold and film layer stability under high-power laser irradiation conditions.

[0016] The above description is only an overview of the technical solution of the present invention. In order to be able to understand the technical means of the present invention more clearly and implement it according to the content of the specification, the following describes the preferred embodiments of the present invention in detail as follows. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Shows a schematic flow chart of a method for preparing a yttrium barium fluoride optical thin film according to an embodiment of the present invention; Figure 2 Shows a schematic flow chart of a method for preparing a yttrium barium fluoride composite according to an embodiment of the present invention; Figure 3 Shows a differential scanning calorimetry (DSC) test curve of a sub-molten composite reaction body according to an embodiment of the present invention; Figure 4 Shows a comparison diagram of X-ray diffraction (XRD) patterns of samples obtained in embodiments and control examples of the present invention; Figure 5 Shows an appearance image of a yttrium barium fluoride frit at a macroscopic scale according to an embodiment of the present invention; Figure 6 Shows a scanning electron microscope image of a yttrium barium fluoride composite with an equiaxed particle morphology according to an embodiment of the present invention; Figure 7Shows the cubic phase structure diagram of the Fm-3m space group according to an embodiment of the present invention; Figure 8 Shows the tetragonal phase structure diagram of the P4 / nmm space group with lattice distortion according to an embodiment of the present invention; Figure 9 Shows the X-ray diffraction (XRD) pattern of the yttrium barium fluoride composite with a biphasic coexistence structure according to an embodiment of the present invention; Figure 10 Shows a schematic method flow chart for depositing a yttrium barium fluoride optical thin film on an optical substrate by an electron gun evaporation method according to an embodiment of the present invention; Figure 11 Shows a cross-sectional scanning electron microscope image of the yttrium barium fluoride optical thin film according to an embodiment of the present invention. Detailed implementation manners

[0018] The technical solutions of the present invention will be further described below in conjunction with specific embodiments.

[0019] Figure 1 Shows a schematic flow chart of a method for preparing a yttrium barium fluoride optical thin film according to an embodiment of the present invention. As Figure 1 shown, the preparation method includes: Step S100, providing an optical substrate and a yttrium barium fluoride composite. The yttrium barium fluoride composite is a composite material with a metastable crystal phase structure obtained by subjecting yttrium fluoride and barium fluoride to a high-temperature solid-phase reaction followed by quenching treatment. The grain size of the yttrium barium fluoride composite is submicron-level, and the metastable crystal phase structure is a cubic phase with the Fm-3m space group and / or a tetragonal phase with the P4 / nmm space group having lattice distortion; Step S200, heating the optical substrate to 150°C - 200°C; Step S300, using the electron gun evaporation method, with the yttrium barium fluoride composite as the evaporation source, depositing a yttrium barium fluoride optical thin film on the optical substrate.

[0020] According to the solution of the embodiment of the present invention, the yttrium barium fluoride optical thin film is deposited on the surface of an optical substrate. This thin film is formed by depositing a yttrium barium fluoride composite with a metastable crystal phase structure formed by high-temperature solid-phase quenching treatment through an electron gun evaporation method. The particle size of the yttrium barium fluoride composite is controlled at the sub-micron level, effectively balancing the evaporation rate, thermal stability, and particle compactness, and having excellent compositional consistency and equilibrium of thermal escape behavior during the subsequent electron beam evaporation process, avoiding problems such as sputtering, component separation, and local overheating of particles. The cubic phase with the Fm-3m space group and / or the tetragonal phase with the P4 / nmm space group with lattice distortion are rich in lattice distortion and structural defects inside, such as stacking faults, dislocations, and non-ideal packing arrangements. These lattice distortions and structural defects can effectively absorb or transfer stress during the film growth process, inhibit the concentration and accumulation of macroscopic stress, and prevent the formation of through microcracks. During the electron beam evaporation deposition process, the evaporation driving force of the metastable crystal phase structure of the yttrium barium fluoride composite is moderate, the evaporation rate is stably controlled, and the escape rates of each component (barium fluoride and yttrium fluoride) are close, which is conducive to maintaining the consistency of the film refractive index and optical uniformity, and avoiding problems such as local devitrification, enhanced scattering, or refractive index fluctuations caused by phase separation or grain growth. In addition, in the prior art, fluoride materials (such as YF 3 、LaF 3 、MgF 2 ) are prone to problems such as large differences in component vapor pressure, poor thermal stability, and uneven evaporation during the evaporation process, often accompanied by sputtering, phase decomposition, or partial dissociation phenomena. To make up for the structural and compositional instability during the evaporation of the above materials, the prior art usually needs to heat the substrate temperature to above 200 °C to promote film adsorption and structural densification. However, this high-temperature process will exacerbate the accumulation of film-substrate thermal stress and also pose limitations to some thermosensitive substrate materials. The yttrium barium fluoride composite material adopted in the embodiment of the present invention has a metastable crystal phase structure and a sub-micron particle size distribution, and can achieve stable and uniform evaporation deposition at a lower temperature (150 °C - 200 °C), with both compositional consistency and structural densification, thereby effectively reducing the dependence on high-temperature substrates and realizing a milder and more compatible film-forming process. The finally formed yttrium barium fluoride optical thin film can still maintain a dense, continuous, and microcrack-free structural morphology under the condition that the single-layer film thickness reaches 2 μm - 6 μm, and has very good optical transparency, thermal stability, and mechanical integrity. This yttrium barium fluoride optical thin film is particularly suitable for low-refractive-index optical thin film materials in mid- and far-infrared band laser systems or infrared imaging windows, and can be used to construct an anti-reflection film structure, which is suitable for both single-layer antireflection film design and can also be used as a low-refractive-index sublayer in a multi-layer interference film system, thereby effectively reducing the optical interface reflection loss, improving the system transmittance, and maintaining a high laser damage threshold and film stability under high-power laser irradiation conditions.

[0021] In step S100, the material of the optical substrate can be, for example, ZnSe, Ge, CaF 2 or an optical substrate such as a silicon-based material suitable for the mid-infrared and far-infrared bands.

[0022] Figure 2 FIG. shows a schematic flow chart of a method for preparing a yttrium barium fluoride composite according to an embodiment of the present invention. As Figure 2 shown, the preparation method includes: Step S101, mixing yttrium fluoride and barium fluoride in a mass ratio of 6:4 - 4:6, and performing a high-temperature solid-phase reaction at 700°C - 900°C under the protection of an atmosphere rich in fluoride ions and an inert gas to obtain a sub-molten composite reaction body; Step S102, using a rapid quenching process to solidify the sub-molten composite reaction body into a yttrium barium fluoride melt block at a cooling rate of 200°C / s - 600°C / s; Step S103, performing crushing, cleaning, and drying treatments on the yttrium barium fluoride melt block to obtain a yttrium barium fluoride composite.

[0023] In this step S101, the mass ratio of the yttrium fluoride and the barium fluoride can be, for example, 3:2, 1:1, 5:4, or 2:3, or any other value within 6:4 - 4:6. This ratio range not only meets the stoichiometric balance required for the formation of the yttrium barium fluoride crystal but also helps to regulate the structural stability and evaporation behavior of the composite phase. Within this ratio range, by appropriately "enriching barium" or "enriching yttrium", the occupancy ratio of different ions in the crystal and the degree of lattice binding can be adjusted, thereby affecting the ability of the material to form a metastable crystal phase structure during the cooling process of the high-temperature solid-phase reaction, improving the compositional homogeneity and grain boundary integrity, and suppressing the phase separation tendency of barium fluoride or yttrium fluoride to precipitate alone. At the same time, it improves the grain size and grain boundary distribution, which is beneficial to the denseness and integrity of the subsequent film layer. This mass ratio regulation can also effectively alleviate the uneven component escape phenomenon caused by the difference in vapor pressure between barium fluoride and yttrium fluoride during the evaporation process, making the escape rates of barium and yttrium elements more consistent, thereby improving the homogeneity of the evaporation components and the film formation stability.

[0024] Setting an atmosphere rich in fluoride ions and supplemented with inert gas protection during the high-temperature solid-phase treatment can effectively inhibit the decomposition and volatilization loss of fluorides under high-temperature conditions, preventing the escape of fluorine elements from causing deviation of the stoichiometric ratio and incomplete crystal structure. That is to say, the fluoride ion atmosphere helps to form a fluorination equilibrium environment on the material surface, reducing the driving force of the pyrolysis reaction of yttrium fluoride and barium fluoride in the reaction system, and reducing the risk of yttrium fluoride and barium fluoride converting to oxides such as yttrium oxide and barium oxide at high temperatures. This atmosphere rich in fluoride ions is preferably a mixed atmosphere of HF and NH 4 released by the thermal decomposition of ammonium fluoride (NH 3 F), and can also be HF / Ar or HF / N2 Low-concentration hydrogen fluoride mixed gases such as these, in which the volume fraction of HF is 0.5% - 5%. At the same time, the inert gas, as a non-reactive carrier gas, can isolate external oxygen molecules from entering the high-temperature region, avoiding interference from oxidation reactions to the formation of the complex crystal phase. This inert gas can be, for example, high-purity argon or high-purity nitrogen. The setting of the above atmosphere is conducive to the formation of a stable yttrium barium fluoride crystal structure, improving the homogeneity of the material composition and the structural integrity during the high-temperature solid-phase reaction process.

[0025] The temperature of this high-temperature solid-phase reaction can be, for example, 700 °C, 750 °C, 800 °C or 900 °C, or any other value within 700 °C - 900 °C. Although the temperature range of 700 °C - 900 °C is lower than the melting points of yttrium fluoride and barium fluoride respectively, based on the thermodynamic behavior of the binary fluoride system, this temperature range can effectively stimulate the interfacial ion diffusion and interaction between the two, enabling the formation of local sub-molten regions at the grain boundaries or particle contact areas, thereby promoting the formation of a sub-molten composite reaction body with uniform composition and stable structure. This treatment process relies on the non-oxidizing and low-fluoride-evolution reaction environment constructed by the combined action of the fluorine-rich atmosphere and the inert gas, which can further reduce the reaction barrier and stabilize the reaction path, avoiding adverse phenomena such as fluoride loss, oxidation or phase decomposition of the material. By controlling this temperature range, the synergistic effect of the solid-solid reaction and the local liquid-phase assistance mechanism can be achieved, thereby obtaining a sub-molten composite reaction body with reaction activity and structural controllability, providing a basis for subsequent quenching to form a yttrium barium fluoride composite with a metastable crystal phase structure. Here, the "sub-molten composite reaction body" refers to an intermediate composite material structure in a state that has not completely melted but has undergone significant physical and chemical reactions, whose state is between the solid phase and the complete liquid phase, without the characteristics of an overall liquid phase, but interfacial ion diffusion, preliminary lattice reconstruction and defect enrichment, the generation of short-range ordered structures induced by interaction, and local liquid-phase assistance behavior occur at the grain boundaries, particle contact areas or local micro-regions.

[0026] Figure 3 shows a differential scanning calorimetry (DSC) test curve of the sub-molten composite reaction body according to an embodiment of the present invention. In this embodiment, the mass ratio of yttrium fluoride to barium fluoride is 6:4, the fluorine-ion-rich atmosphere is a hydrogen fluoride mixed gas of 3% HF and Ar by volume fraction, the inert gas is high-purity argon, a high-temperature solid-phase reaction is carried out at 900 °C for 2 h, and the product formed after the reaction is ground and used for the DSC test. From Figure 3It can be seen that a broad and gentle endothermic peak appears at about 850 °C (corresponding to the gray dotted line in the figure). This endothermic behavior does not exhibit the characteristics of a typical melting phase transition, and the peak shape is relatively gentle, indicating that the material does not undergo an overall liquid-phase transformation in this temperature range. This endothermic peak reflects the ion diffusion, preliminary reaction, and local structure reorganization behaviors that mainly occur at the particle contact interface during the heating process of the material, belonging to a typical sub-melting reaction process. Therefore, it can be reasonably inferred from this endothermic behavior that the sub-melting composite reaction body formed in step S100 of the embodiment of the present invention is in a metastable thermodynamic state. This structure has not experienced an overall liquid-phase transformation, but ion diffusion and local reaction processes have occurred in the particle interface region.

[0027] In this step S102, the rapid quenching process refers to instantaneously cooling the sub-melting composite reaction body at a cooling rate of 200 °C / s - 600 °C / s, so that it rapidly solidifies within an extremely short time to form a yttrium barium fluoride frit with a uniform structure. This cooling rate is significantly higher than that of conventional natural cooling or slow cooling processes and belongs to a high-gradient non-equilibrium solidification path in the field of heat treatment. The rapid quenching process plays a very crucial role. On the one hand, it can inhibit the long-distance ion rearrangement and the formation of thermodynamically stable crystalline phases during the solidification process of the material, thereby inducing the formation of a metastable crystalline phase structure with slightly distorted lattice, defect enrichment, and high free energy state characteristics. On the other hand, rapid cooling can limit grain growth, enabling the grain size inside the frit to be controlled at the sub-micron level, which is beneficial to the thermal uniformity and film-forming consistency during the subsequent evaporation process. In addition, this quenching process can also avoid phenomena such as fluorine volatilization, component segregation, and phase decomposition caused by too long high-temperature residence time, ensuring that the finally obtained yttrium barium fluoride frit has good structural stability and compositional uniformity.

[0028] It should be explained that although the sub-melting composite reaction body has not reached the completely liquefied state, ion diffusion and preliminary reactions have locally occurred at the particle contact interface or grain boundary region to form a metastable intermediate with uniform composition and structural activity. This intermediate is in a thermodynamically unstable or metastable state. If slow cooling is adopted, it is extremely easy to drive the system to transform into a stable crystalline phase, resulting in problems such as particle size growth, phase separation, or crystal coarsening, thereby damaging the microstructure uniformity and optical property consistency of the subsequent film layer. The introduction of the rapid quenching process is precisely the dynamic freezing and structural capture of the above-mentioned intermediate phase. Its high cooling rate can effectively "lock" the non-equilibrium structural characteristics in the sub-melting composite reaction body, inhibit long-range diffusion and the formation of stable phases, so that the finally solidified yttrium barium fluoride frit has metastable crystalline phase characteristics and sub-micron grain size. The two-step process collaboratively controls the heat treatment and cooling paths, ensuring that the material realizes component homogenization, grain refinement, and structural compatibility during the subsequent evaporation process while maintaining reaction activity and structural stability, thereby providing a key basis for the realization of optical uniformity, denseness, and low-stress performance in thick film deposition.

[0029] To verify the importance of the rapid quenching process, the inventors designed and conducted a comparative experiment. In the comparative experiment, the only difference between the control example and the embodiment of the present invention was the cooling rate. The cooling rate in the control example was 30 °C / s, and the cooling rate in the embodiment of the present invention was 400 °C / s. In addition, in the embodiment of the present invention, the mass ratio of yttrium fluoride to barium fluoride was 6:4, the atmosphere rich in fluoride ions was a hydrogen fluoride mixed gas of HF and Ar with a volume fraction of 3%, the inert gas was high-purity argon, and a high-temperature solid-phase reaction was carried out at 900 °C for 2 h. Figure 4 Figure 1 shows a comparison chart of the X-ray diffraction (XRD) patterns of the samples obtained in the embodiment of the present invention and the control example. The sample was the obtained yttrium barium fluoride frit. Figure 4 As can be seen, in the XRD diffraction peaks of the sample in the embodiment of the present invention, the main peak positions are at angles of 28°, 47°, and 56°, belonging to the cubic phase of the Fm-3m space group. The XRD diffraction peaks of this embodiment are relatively broad and gentle and the intensity is relatively low, indicating that its grain size is small and there are lattice distortions and defect enrichments, having the characteristics of a metastable crystal phase. The diffraction peaks of the control example sample are significantly sharp and have high intensity, indicating that its grains grow fully, the crystallinity is high, a thermodynamically stable crystal phase is formed, the crystals are thick, and phase separation may occur. The above results confirm that the rapid quenching process plays a key role in obtaining a metastable crystal phase structure and a submicron-sized grain size.

[0030] The formed yttrium barium fluoride frit has a disc-shaped or flat block-shaped structure, and its physical size is 10 mm - 40 mm in diameter and 5 mm - 20 mm in thickness. Figure 5 Figure 2 shows the appearance image of the yttrium barium fluoride frit at the macroscopic scale according to an embodiment of the present invention. Figure 5 In this embodiment, the mass ratio of yttrium fluoride to barium fluoride is 6:4, the atmosphere rich in fluoride ions is a hydrogen fluoride mixed gas of HF and Ar with a volume fraction of 3%, the inert gas is high-purity argon, a high-temperature solid-phase reaction is carried out at 900 °C for 2 h, and the cooling rate is 400 °C / s. As Figure 5 can be seen, the yttrium barium fluoride frit as a whole presents an approximate disc-shaped appearance, the surface is relatively flat, the edge contour is clear, and it has a regular geometric morphology. And it can be observed that there are no obvious cracks, warping or pore distributions in the yttrium barium fluoride frit, indicating that the high-temperature solid-phase - quenching path control scheme of the embodiment of the present invention has good crystallization continuity and bulk phase homogeneity.

[0031] In step S103, the crushing process can be performed by conventional mechanical methods in the prior art, such as using a ceramic ball mill, a vibrating crusher or an impact grinding device for preliminary crushing to form particles with equiaxed or blocky morphology. Preferably, the melt is crushed to a particle size within the range of 0.5 mm to 2 mm, so as to maintain the uniformity of heat conduction and the integrity of the evaporation surface during the subsequent electron beam evaporation process. This particle size range can ensure the dense accumulation of the evaporation source and avoid uneven evaporation rate caused by excessive gaps between particles.

[0032] In an embodiment of the present invention, in order to remove the atmospheric reaction byproducts, oxidative impurities or dust pollution introduced during the mechanical crushing process that may remain on the surface of the frit, an organic solvent such as anhydrous ethanol, acetone or deionized water is used for multiple ultrasonic cleanings. Preferably, a multi-stage washing process is adopted, and each round of 5-10 minutes of ultrasonic cleaning is allowed to settle until the supernatant is clarified to ensure the purity of the material and the low outgassing characteristics during the subsequent vacuum evaporation process. During the drying process, the cleaned particles are placed in a vacuum oven and subjected to low-temperature drying treatment in the temperature range of 40°C-80°C. The drying time is preferably 6-12 hours, depending on the total amount of particles and the type of residual solvent. This drying step can effectively remove residual solution and moisture in the pores of the particles, and prevent gas release, splashing or film defects during electron beam evaporation.

[0033] The above-mentioned "equiaxed" particles refer to particles with approximately equal length, width and height, close to spherical or polyhedral shapes. When electron beam evaporation is performed under high vacuum conditions, they have the advantages of good thermal uniformity, low risk of stress concentration, and good consistency of thermal expansion before evaporation. Compared with flake or needle-shaped particles, equiaxed particles are less prone to local overheating or cracking, and can effectively prevent unstable behaviors such as material splashing and bursting during evaporation. Figure 6 A scanning electron microscope image of a yttrium barium fluoride composite having an equiaxed particle morphology according to an embodiment of the present invention is shown. Figure 6 In the embodiment of the invention, the mass ratio of yttrium fluoride to barium fluoride is 6:4, the atmosphere rich in fluoride ions is a mixture of HF and Ar with a volume fraction of 3%, the inert gas is high-purity argon, a high-temperature solid phase reaction is carried out at 900°C for 2h, the cooling rate is 400°C / s, a ceramic ball mill is used for crushing, anhydrous ethanol is used for ultrasonic cleaning, and the cleaned particles are placed in a vacuum oven and dried at 60°C for 8h. Figure 6 It can be seen that the particles of the yttrium barium fluoride composite are equiaxed, the aspect ratio of the particles is close, and the surface is smooth and uniform, without obvious cracks or defects. This shows that the crushed and processed yttrium barium fluoride composite maintains good physical stability at the macro and micro scales, and it is measured that the size range of the particles is controlled between 0.5mm and 2mm, which meets the requirements of the subsequent electron beam evaporation process.

[0034] The above-mentioned "blocky" particles refer to a type of particles with an irregular polyhedral morphology, a relatively thick overall contour, and certain volume and mass distribution balance characteristics. This morphology has many process compatibility advantages in a high-vacuum electron beam evaporation environment. First, due to the relatively high mass concentration and large heat capacity of blocky particles, a more stable heat conduction path can be achieved during electron beam irradiation, thereby forming a more thermally stable evaporation interface on the surface of the evaporation source, reducing the risk of uneven local melting or material splashing. Second, blocky particles have good filling and packing characteristics, and a relatively dense and structurally continuous filling layer can be constructed in the evaporation boat or crucible, avoiding problems such as high porosity and poor thermal contact that are prone to occur during the accumulation of fine powder, thereby improving the overall heat uniformity of the material and the integrity of the evaporation surface. Compared with flaky or cracked particles, blocky particles are not easily warped or peeled due to local heating of the thin layer at the initial stage of evaporation, and have higher morphological stability during the electron beam energy loading process, which is beneficial to the controllability of the evaporation rate and the continuity of the film-forming process.

[0035] In this step S100, the metastable crystal phase structure of the yttrium barium fluoride composite is a cubic phase with the Fm-3m space group and / or a tetragonal phase with the P4 / nmm space group with lattice distortion. Figure 7 shows a cubic phase structure diagram of the Fm-3m space group according to an embodiment of the present invention. In Figure 7 the shown embodiment, yttrium fluoride and barium fluoride are mixed in a mass ratio of 6:4. The obtained mixture is subjected to a high-temperature solid-phase reaction at 900 °C for 2 hours in a mixed atmosphere formed by HF and Ar with a volume fraction of 3% under the protection of high-purity argon. After the reaction, rapid quenching is carried out at a cooling rate of 400 °C / s. The obtained solid ingot is crushed by a ceramic ball mill and ultrasonically cleaned with anhydrous ethanol. The cleaned sample is placed in a vacuum oven and dried at 60 °C for 8 hours, and finally a yttrium barium fluoride composite with an Fm-3m space group structure is obtained. Figure 7 in, the blue balls represent Ba 2+ , the green balls represent Y 3+ , and the red balls represent F - . It can be seen from Figure 7 that Ba 2+ occupies 8 corner points of the unit cell, forming a stable framework in the crystal. Y 3+ is located at the body center of the unit cell and is the core cation of the structure, dominating the coordination direction of F - . F - is mainly distributed on the face center of the unit cell or approximate coordination positions, and forms a YF 3+ or YF 5 2- or YF 6 3- octahedral or tetragonal bipyramid coordination structure.

[0036] Figure 8 Shows the tetragonal phase structure diagram of the P4 / nmm space group with lattice distortion according to an embodiment of the present invention. In Figure 8 the shown embodiment, yttrium fluoride and barium fluoride are mixed according to a mass ratio of 5:5. The mixture is in a mixed atmosphere formed by 1% HF and N 2 under the environment of high-purity nitrogen as an inert protective gas, and a high-temperature solid-phase reaction is carried out at 750 °C for 1 hour. After the reaction is completed, rapid quenching is carried out at a cooling rate of 600 °C / s. The obtained solid ingot is crushed by a ceramic ball mill, and then the obtained particles are ultrasonically cleaned with absolute ethanol. The cleaned sample is placed in a vacuum oven and dried at 60 °C for 8 hours, and finally a yttrium barium fluoride composite with a P4 / nmm space group structure is obtained. Figure 8 Among them, the blue spheres represent Ba 2+ ions, occupying the corner positions of the unit cell. The light green spheres are Y 3+ ions, occupying the center position of the unit cell. The red and dark green spheres are F - ions, which are distributed near the center and near the boundary of the unit cell respectively. Some F - ions not only participate in the local coordination with the central Y 3+ (such as YF 6 3- or distorted YF 5 2- polyhedra), but also periodically appear at the corner points and boundaries of the unit cell. These ions form bridging bonds with multiple metal ions in the structure, and this structure reflects the multiple coordination characteristics and spatial periodic distribution characteristics of F - ions in the crystal, indicating that the crystal structure is a metastable crystal phase structure under non-thermodynamic stable conditions.

[0037] Preferably, the metastable crystal phase structure of the obtained yttrium barium fluoride composite is a two-phase coexistence structure of a cubic phase with an Fm-3m space group and a tetragonal phase with a P4 / nmm space group with lattice distortion. In this preferred embodiment, the mass ratio of yttrium fluoride and barium fluoride is controlled to be 1:1 ± 5%, the temperature of the high-temperature solid-phase reaction is 780 °C - 850 °C, the fluorine-rich atmosphere is selected as HF or NH 4 F, plus Ar or N 2 with a purity of more than 95%, the holding time is 20 min - 45 min. The cooling rate is 300 °C / s - 450 °C / s, the ingot size is 10 mm - 20 mm, and the grain size after crushing is 300 nm - 600 nm.

[0038] Figure 9Shows the X-ray diffraction (XRD) pattern of a yttrium barium fluoride composite with a biphasic coexistence structure according to an embodiment of the present invention. In Figure 9 In the illustrated embodiment, yttrium fluoride and barium fluoride are mixed evenly in a mass ratio of 1:1 (allowing a deviation of no more than ±5%) and placed in a quartz crucible. In a fluorine-rich atmosphere composed of a mixed gas of 2% by volume of HF and N 2 (where the purity of N 2 is not less than 99.999%), a high-temperature solid-phase reaction is carried out. The reaction temperature is set at 820 °C, and the holding reaction time is 1 h. After the reaction is completed, the obtained sub-molten product is rapidly quenched at a cooling rate of 350 °C / s to obtain a yttrium barium fluoride melt block with a size of about 15 mm. The melt block is put into a ceramic ball mill for mechanical crushing. The obtained particles are ultrasonically cleaned in an anhydrous ethanol medium for 15 minutes and dried at 60 °C under vacuum conditions for 8 hours. Finally, a sample of yttrium barium fluoride composite particles with a grain size in the range of 300 nm - 600 nm is obtained. From Figure 9 It can be observed that there are multiple groups of diffraction peak position splitting or shoulder peak characteristics, such as between 28° - 29°, 47° - 48°, and 56° - 57.5°. This indicates that the sample contains more than a single crystal phase structure, but rather a coexistence of a cubic phase with the Fm-3m space group and a distorted tetragonal phase with the P4 / nmm space group. The presence of broadened peaks and weak peaks reflects fine grains (sub-micron scale), lattice distortion, or enrichment of structural defects, which is in line with the characteristics of a metastable biphasic coexistence structure. This proves that the metastable crystal phase structure of the yttrium barium fluoride composite is a biphasic coexistence structure of a cubic phase with the Fm-3m space group and a tetragonal phase with a lattice-distorted P4 / nmm space group.

[0039] In this preferred embodiment, the cubic phase of the Fm-3m space group has high crystal symmetry and low internal stress characteristics, which can provide a low-stress region with stable structure for the film layer. The distorted tetragonal phase of the P4 / nmm space group has slight lattice distortion and defect enrichment characteristics, with certain structural flexibility and stress relaxation ability. The two cooperate and nest at the microscale, which can construct a multi-scale stress relaxation path, effectively disperse the internal stress generated in the film layer under deposition and thermal load conditions, reduce the formation probability of through microcracks, and thus contribute to the dense film formation and crack resistance stability of the optical thin film structure with a film thickness greater than 2 μm without ion-assisted deposition conditions. At the same time, this coexisting two-phase structure can significantly improve the consistency of component volatilization during the evaporation process, reduce the refractive index fluctuation caused by the difference in vapor pressure, and improve the film layer composition distribution and optical uniformity. In addition, under the condition of high-power laser irradiation, the distorted tetragonal phase grains can effectively inhibit the expansion of thermally induced microdefects through their lattice elasticity and dislocation energy absorption, and improve the laser damage threshold and environmental adaptability of the optical thin film. Further, the coexisting two-phase structure can provide heterogeneous sites with different lattice constants and crystallization rates at the initial stage of film nucleation and growth, optimize the thin film microstructure, and promote the surface flatness and thickness control ability of the film layer. Therefore, this coexisting two-phase structure has significant technical advantages in aspects such as stress regulation, thick film deposition, evaporation stability, and high-energy laser adaptability.

[0040] In this step S200, the optical substrate is heated to 150°C - 200°C. The purpose is to stimulate the initial adsorption, surface diffusion, and nucleation processes between the active species of yttrium barium fluoride and the substrate surface during the thin film deposition process without introducing high-temperature thermal stress, so as to effectively construct high adhesion and dense film-substrate interface. The present invention improves the vapor stability and particle energy distribution uniformity of the evaporation source material at low temperatures by introducing a yttrium barium fluoride composite with a metastable crystal phase structure and submicron particle size, and then enables the film layer to achieve continuous and dense film formation behavior in the range of 150°C - 200°C. This temperature range is preferably 150°C, 180°C, or 200°C, or any other value in the range of 150°C - 200°C. Compared with the substrate temperature above 200°C commonly used in the prior art, this temperature range significantly reduces the risk of accumulation of film-substrate thermal expansion mismatch stress, and is particularly suitable for mid- and far-infrared optical systems with heat-sensitive materials such as infrared-grade silicon-based, Ge, CaF 2 etc. as substrates. In addition, the lower deposition temperature helps to form a quasi-amorphous or small-grain state structure in the initial stage of the thin film, inhibits the formation of grain boundary dislocations and pores, and improves the overall structural integrity and stability of the film layer. The finally formed yttrium barium fluoride optical thin film can still maintain excellent optical uniformity and density even under the condition of a thickness of 2 μm - 6 μm, with no microcrack distribution in the film layer structure, stable transmission performance, and excellent laser damage threshold and thermal cycle reliability.

[0041] Figure 10 FIG. shows a schematic process flow diagram of depositing a yttrium barium fluoride optical thin film on an optical substrate by using an electron gun evaporation method according to an embodiment of the present invention. As Figure 10 shown, the step S300 includes: Step S301, evaporating the yttrium barium fluoride composite at an evaporation rate of 0.1 nm / s - 0.3 nm / s in a temperature range of 1150°C - 1250°C and by means of full-surface scanning of an electron beam; Step S302, continuously evaporating the yttrium barium fluoride composite at an evaporation rate of 0.5 nm / s - 1.5 nm / s in a temperature range of 1260°C - 1350°C, so as to deposit and form a yttrium barium fluoride optical thin film on the optical substrate.

[0042] In step S300 of the embodiment of the present invention, by dividing the electron gun evaporation process of the yttrium barium fluoride composite into two stages, that is, implementing the initial deposition step S301 at an evaporation rate of 0.1 nm / s–0.3 nm / s in a temperature range of 1150°C - 1250°C, and implementing the main deposition step S302 at an evaporation rate of 0.5 nm / s–1.5 nm / s in a temperature range of 1260°C - 1350°C, it is possible to realize the coordinated optimization of nucleation quality, film layer uniformity and deposition efficiency during the whole deposition process. In the initial deposition stage, the directional adsorption and diffusion ability of active particles on the substrate surface can be significantly improved by means of low-temperature and low-speed deposition, promoting the formation of a uniform and continuous nucleation layer, thereby enhancing the film-substrate adhesion and inhibiting the aggregation of large grains. In the main deposition stage, stable evaporation output and efficient film formation of the yttrium barium fluoride composite are realized by means of increasing the temperature and speed, so as to quickly obtain a dense film layer with a thickness of 2 μm - 6 μm on the basis of ensuring the film formation quality. This step-by-step evaporation process can effectively avoid the risks of grain coarsening, stress concentration, refractive index non-uniformity and devitrification easily generated in traditional single-stage deposition, ensuring the structural integrity and optical uniformity of the film layer under the environment of high-power laser irradiation and multiple thermal and cold shocks.

[0043] In the initial deposition step of step S301, the temperature can be, for example, 1150 °C, 1200 °C, 1230 °C or 1250 °C, or any other value within 1150 °C - 1250 °C. The evaporation rate can be, for example, 0.1 nm / s, 0.2 nm / s or 0.3 nm / s, or any other value within 0.1 nm / s - 0.3 nm / s. This temperature range is in the critical region of effective evaporation of the material, which can ensure the stable escape of the evaporation source components without violent decomposition or compositional segregation, and form a physical adsorption and local reconstruction process on the substrate surface. At the same time, this evaporation rate range can significantly extend the residence and diffusion time of the active particles on the substrate surface, facilitating the formation of a continuous and uniform nucleation layer, enhancing the nucleation density and interface compactness, effectively suppressing grain coarsening and local thickness mutation of the film layer, thereby establishing a stable initial structure template and providing structural support for the subsequent high-temperature and high-efficiency deposition process. The parameter settings of this initial deposition can significantly improve the film adhesion performance and structural integrity, and reduce the risk of thermal stress accumulation and film-substrate mismatch caused by inappropriate initial evaporation temperature and rate.

[0044] In the main deposition step of step S302, the temperature can be, for example, 1260 °C, 1280 °C, 1300 °C, 1320 °C, 1330 °C or 1350 °C, or any other value within 1260 °C - 1350 °C. The evaporation rate can be, for example, 0.5 nm / s, 1 nm / s or 1.5 nm / s, or any other value within 0.5 nm / s - 1.5 nm / s. This temperature range can significantly improve the vapor output capacity of the composite, ensure the synchronous release of each component during evaporation, thereby reducing the refractive index fluctuation and component separation phenomenon caused by the vapor pressure difference. At the same time, it avoids the risk of structural instability caused by high-temperature decomposition or excessive fluorine escape. Controlling the evaporation rate within 0.5 nm / s - 1.5 nm / s can achieve rapid film formation on the premise of ensuring the continuity and compactness of the film layer, meet the deposition requirements of low refractive index films with a thickness of 2 μm - 6 μm in the mid-infrared optical system, and suppress problems such as pores, thermal stress accumulation or non-uniform grain growth caused by too fast stacking rate. The main deposition parameter settings help to achieve the structural uniformity, optical stability and batch-to-batch repeatability of the yttrium barium fluoride optical film while maintaining high evaporation efficiency.

[0045] Figure 11 Shows a cross-sectional scanning electron microscope image of a yttrium barium fluoride optical film according to an embodiment of the present invention. In Figure 11 the shown embodiment, an infrared-grade silicon substrate with a size of 25.4 mm × 1 mm is selected as the optical substrate, and it is heated to 180 °C and kept warm for 10 minutes before evaporation to promote the surface adsorption and directional diffusion of active particles during the subsequent deposition process. The yttrium barium fluoride composite evaporation source used is Figure 9The grains of the metastable biphasic coexistence structure shown have a grain size controlled at about 400 nm. The evaporation process is implemented by electron gun evaporation, and the specific parameters are as follows: In step S301, the evaporation temperature is set at 1230 °C, the evaporation rate is 0.2 nm / s, the electron beam uses a spiral scanning mode to uniformly heat the evaporation source surface, and the deposition time is about 20 minutes; in step S302, the evaporation temperature is raised to 1300 °C, the evaporation rate is set at 1.0 nm / s, and the deposition continues for 90 minutes. During the entire deposition process, the base pressure of the cavity is maintained below 3×10 -5 Pa, no ion assistance is used, the substrate is kept at a constant temperature of 180 °C, and after the film layer deposition is completed, the substrate is taken out after natural cooling to room temperature. The thickness of the deposited film layer is about 4 μm. As can be seen from Figure 11 , the deposited yttrium barium fluoride optical film layer is located on the surface of the lower optical substrate, has good interface continuity and overall structural compactness, and no obvious defects such as pores, peeling or cracking are observed. The film layer thickness shown in the figure is about 4 μm. In addition, the interface between the film layer and the lower substrate is flat, and no obvious interface voids or desorption phenomena are seen.

[0046] In a preferred embodiment, to further improve the nucleation uniformity and structural compactness of the yttrium barium fluoride optical film, the following steps are also included before step S200: depositing an amorphous yttrium barium fluoride seed film on the optical substrate, and the thickness of the yttrium barium fluoride seed film is 5 nm - 50 nm. The deposition method of the yttrium barium fluoride seed film is: using the electron gun evaporation method, with the yttrium barium fluoride complex as the evaporation source, in the temperature range from room temperature to 100 °C, evaporating the yttrium barium fluoride complex at an evaporation rate of 0.05 nm / s - 0.2 nm / s and by the full surface scanning mode of the electron beam to obtain the yttrium barium fluoride seed film.

[0047] According to the solution of the embodiment of the present invention, introducing the amorphous yttrium barium fluoride seed film can effectively improve the nucleation density and uniformity of the main film layer in step S300. The seed film forms a uniform and dense amorphous buffer layer through a low-temperature and low-speed evaporation process. Its amorphous characteristics can shield the substrate surface defects and lattice orientation interference, enhance the adsorption and lateral diffusion ability of the deposited particles on the substrate surface, and thus promote the formation of a continuous and uniform initial crystal nucleus structure of the main film layer. At the same time, the seed film shows a good intermediate transition effect in terms of structure and thermal expansion characteristics, can effectively relieve the interface stress concentration caused by the film-substrate thermal expansion mismatch during the subsequent deposition stage of step 300, improve the adhesion stability of the main film layer, and reduce the risks of cracks, voids and interface peeling. Further, the thickness of the seed film is controlled within the range of 5 nm - 50 nm, and the overall structure is extremely thin, and hardly has an adverse impact on the transmittance, phase delay or interference performance of the final optical film layer. Thus, the introduction of the amorphous seed film significantly improves the optical uniformity, thick film integrity and deposition process stability of the main film layer.

[0048] According to another aspect of the present invention, a yttrium barium fluoride optical thin film is provided. The yttrium barium fluoride optical thin film is prepared by using the preparation method as described above. The yttrium barium fluoride optical thin film has a single-layer structure and a thickness of 2 μm - 6 μm.

[0049] According to the solution of the embodiment of the present invention, the film layer of the obtained yttrium barium fluoride optical thin film still maintains the morphological characteristics of being highly dense, continuous, and without micro-cracks within the thickness range of 2 μm to 6 μm. Moreover, the yttrium barium fluoride optical thin film has mechanical properties of being hard, firm, and having excellent thermal stability. In practical applications, it can withstand a heat treatment environment of up to 300 °C for a long time without any destructive behaviors such as film layer cracking, peeling, or falling off, fully meeting the harsh requirements of the mid-infrared and far-infrared high-energy systems for environmental adaptability and thermal stability.

[0050] In addition, the thin film exhibits extremely low absorption and scattering losses in the wavelength range of 0.4 μm to 12 μm and has excellent optical transparency. The test results show that its refractive index is in the range of 1.2 to 1.35, providing a key low-refractive-index material guarantee for realizing the anti-reflection design with low reflectivity. It is particularly suitable for the anti-reflection film layer design in mid-infrared and far-infrared windows, infrared detector packaging, 10.6 μm laser systems, and infrared imaging optical components. Overall, the yttrium barium fluoride optical thin film prepared in the embodiment of the present invention shows comprehensive advantages superior to traditional fluoride coating materials in terms of optical performance, mechanical strength, thermal stability, and thick film reliability.

[0051] According to still another aspect of the present invention, a mid-infrared and far-infrared optical structure is further provided, which includes an optical substrate and the yttrium barium fluoride optical thin film as described above. In one embodiment, the material of the optical substrate is ZnSe, Ge, CaF 2 or a silicon-based material.

[0052] The key and importance of the process steps of this application are elaborated below through specific examples and comparative examples.

[0053] Example 1: Example 1 of the present invention provides a preparation method of a yttrium barium fluoride optical thin film, and applies the yttrium barium fluoride optical thin film to a mid-infrared and far-infrared optical structure as an anti-reflection film. The preparation method of the yttrium barium fluoride optical thin film includes: Step 1, mixing yttrium fluoride and barium fluoride in a mass ratio of 1:1.05, and under the protection of NH 4 F and an inert gas of Ar with a purity of more than 95%, performing a high-temperature solid-phase reaction at 850 °C for 30 min to obtain a sub-molten composite reaction body; Step 2, adopting a rapid quenching process to solidify the sub-molten composite reaction body into a yttrium barium fluoride melt block at a cooling rate of 400 °C / s; Step 3: Crush, wash, and dry the yttrium barium fluoride frit to obtain a yttrium barium fluoride composite; Step 4: Heat the optical substrate to 180 °C. The optical substrate is made of ZnSe material; Step 5: Evaporate the yttrium barium fluoride composite at a temperature of 1200 °C at an evaporation rate of 0.2 nm / s by means of full-surface electron beam scanning; Step 6: Then, continue to evaporate the yttrium barium fluoride composite at a temperature of 1300 °C at an evaporation rate of 0.8 nm / s, thereby depositing a yttrium barium fluoride optical thin film on the optical substrate.

[0054] Example 2: The difference between this Example 2 and Example 1 of the present invention is only that Step 5 is omitted in this Example 2.

[0055] Comparative Example 1: The difference between this Comparative Example and Example 1 of the present invention is only that the temperature setting of the high-temperature solid-phase reaction in Step 1 is different. Among them, the temperature of the high-temperature solid-phase reaction in Comparative Example 1 is 1400 °C.

[0056] Comparative Example 2: The difference between this Comparative Example and Example 1 of the present invention is only that the cooling rate in Step 2 is different. Among them, in Comparative Example 2, the rapid quenching process in Step 2 is replaced by cooling and solidifying the sub-molten composite reaction body at a cooling rate of 80 °C / s to obtain a yttrium barium fluoride frit.

[0057] Comparative Example 3: The difference between this Comparative Example and Example 1 of the present invention is only that the heating temperature of the optical substrate in Step 4 is different. The heating temperature of the optical substrate in Comparative Example 3 is 220 °C.

[0058] The following Table 1 shows the comparison results of the key properties of the yttrium barium fluoride optical thin films prepared in the examples and comparative examples under different process parameters.

[0059]

[0060] In Table 1 above, the film thickness represents the deposition thickness of a single-layer thin film, and the refractive index is the value measured under the operating conditions at a wavelength of 10.6 μm. During the test, the film thickness was measured using an ellipsometer, with the test wavelength range being 2 - 12 μm. The ellipsometry data was fitted using the Cauchy model to evaluate the average thickness of the yttrium barium fluoride thin film in the deposition area, and the uniformity was confirmed by multi-point sampling. For samples with a film thickness greater than 3 μm, a step profiler was used for cross-sectional verification. The refractive index was measured by reflectance using variable-angle spectroscopic ellipsometry (VASE) and the real part of the refractive index at a wavelength of 10.6 μm was obtained by fitting with an optical model. The test environment was carried out in a dry nitrogen atmosphere to eliminate water vapor interference. The laser damage threshold was tested according to the ISO 21254-1:2011 standard, using a CO 2 pulse laser with a wavelength of 10.6 μm and a pulse width of 8 ns. An increasing energy density was applied to the surface of the sample, and the energy density value at which the first observable damage occurred was recorded. The test was carried out on at least 10 non-overlapping areas on the surface of the sample, and the average value was taken. The number of cracks in the film was imaged on the surface of the thin film at a magnification of 5000× using a scanning electron microscope. The number of cracks was counted within randomly selected fields of view of more than five 100 μm 2 and the average value was taken. The measurement of the transmittance improvement was carried out by measuring the change in the transmittance of the sample at a wavelength of 10.6 μm using a Fourier transform infrared spectrometer covering the mid- and far-infrared range. The transmittance improvement value was the difference between the transmittance of the sample after coating the thin film and the original transmittance of the substrate. Baseline calibration was carried out under the same light source and optical path system conditions before and after testing each group of samples.

[0061] The optical thin film obtained in Example 1 had a relatively high film thickness (5.8 μm), a lower refractive index (1.25 @10.6 μm), a higher laser damage threshold (8.2 J / cm²), and no cracks in the film were detected within a range of 100 μm², with a transmittance improvement of 5.4%. Although the overall structure of the thin film in Example 2 remained dense, a thick film of nearly 5 μm (4.9 μm) was still achieved, with a slightly increased refractive index (1.29), a laser damage threshold reduced to 7.5 J / cm², and a small number of cracks (1 crack / 100 μm²) appeared, and the transmittance improvement also decreased to 4.2%. This set of data fully demonstrates that not dividing the electron gun evaporation process of the yttrium barium fluoride composite into two stages, that is, only implementing the main deposition step without implementing the initial deposition step, will to a certain extent affect the nucleation density of the main film layer, the uniformity of the film-forming interface transition zone, and the structural integrity of the subsequent film layer.

[0062] In contrast, in Comparative Example 1, under the condition that the temperature of the high-temperature solid-phase reaction was set at 1400 °C, although the melting was sufficient, the film thickness of the formed thin film was significantly reduced (1.7 μm), the refractive index increased to 1.38, the laser damage threshold was only 3.6 J / cm², and there were as many as 10 cracks per 100 μm² in the film, indicating that the high-temperature treatment led to abnormal grain growth, phase decomposition or stress concentration, seriously weakening the film quality. In Comparative Example 2, although the thin film formed at a cooling rate of 80 °C / s was slightly improved, the film thickness was still insufficient, and the refractive index and the number of cracks were not fundamentally improved. In Comparative Example 3, under the condition that the substrate temperature was raised to 220 °C, although the film adhesion was enhanced, the risk of film thermal stress accumulation increased, resulting in a decrease in the laser damage threshold.

[0063] In summary, by synergistically controlling the raw material ratio, the temperature of the high-temperature solid-phase reaction, the rapid quenching cooling rate, the segmented evaporation process, and the appropriate substrate heating temperature, an effective balance can be achieved among multiple performance dimensions such as the structural compactness, low internal stress, thick film continuity, and optical uniformity of the yttrium barium fluoride thin film. The obtained optical thin film exhibits excellent film integrity and a high laser damage threshold, and is particularly suitable for the design of high-performance antireflection films in mid- and far-infrared optical structures.

[0064] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0065] The above-described embodiments only represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A method for preparing a barium yttrium fluoride optical film, characterized in that: The steps include: Provide an optical substrate and a yttrium barium fluoride composite, wherein the yttrium barium fluoride composite is a composite material having a metastable crystal phase structure obtained by a high-temperature solid-phase reaction of yttrium fluoride and barium fluoride and then quenching, and the grain size of the yttrium barium fluoride composite is submicron-level, and the metastable crystal phase structure is a cubic phase with a Fm-3m space group and / or a tetragonal phase with a lattice-distorted P4 / nmm space group; Heating the optical substrate to 150° C.-200° C.; The yttrium barium fluoride optical film is deposited on the optical substrate by using an electron gun evaporation method and taking the yttrium barium fluoride complex as an evaporation source.

2. The preparation method according to claim 1, characterized in that: The preparation method of the yttrium barium fluoride complex comprises the following steps: Yttrium fluoride and barium fluoride are mixed in a mass ratio of 6:4-4:6, and a high-temperature solid phase reaction is carried out at 700° C.-900° C. in an atmosphere rich in fluoride ions and under the protection of an inert gas to obtain a sub-molten composite reaction body; The sub-molten composite reaction body is solidified into a yttrium barium fluoride frit at a cooling rate of 200° C. / s-600° C. / s by a rapid quenching process; The yttrium-barium fluoride frit is crushed, cleaned and dried to obtain the yttrium-barium fluoride composite.

3. The preparation method according to claim 2, characterized in that: The particles formed after the yttrium barium fluoride frit is crushed have an irregular granular morphology, and the particle size of the particles is 0.5mm-2mm.

4. The preparation method according to claim 3, characterized in that: The method of using an electron gun evaporation method to deposit a yttrium barium fluoride optical film on the optical substrate using the yttrium barium fluoride composite as an evaporation source comprises the following steps: The yttrium barium fluoride complex is evaporated at a temperature range of 1150° C. to 1250° C. at an evaporation rate of 0.1 nm / s to 0.3 nm / s by electron beam full surface scanning; The yttrium barium fluoride complex is then evaporated continuously at a temperature range of 1260° C. to 1350° C. at an evaporation rate of 0.5 nm / s to 1.5 nm / s, thereby depositing a yttrium barium fluoride optical film on the optical substrate.

5. The preparation method according to any one of claims 1 to 4, characterized in that: Before heating the optical substrate to 150° C.-200° C., the method further comprises the following steps: A barium yttrium fluoride seed film with an amorphous structure is deposited on the optical substrate, and the barium yttrium fluoride seed film has a thickness of 5nm-50nm.

6. The preparation method according to claim 5, characterized in that: The method of depositing a barium yttrium fluoride seed film with an amorphous structure on the optical substrate comprises the following steps: The yttrium barium fluoride seed film is obtained by using an electron gun evaporation method, with the yttrium barium fluoride complex as an evaporation source, at a temperature range of room temperature to 100° C., at an evaporation rate of 0.05 nm / s-0.2 nm / s and by electron beam full-surface scanning.

7. A barium yttrium fluoride optical film, characterized in that: The barium yttrium fluoride optical film is prepared by the preparation method according to any one of claims 1 to 6. The barium yttrium fluoride optical film is a single-layer structure with a thickness of 2 μm to 6 μm.

8. A mid-to-far infrared optical structure, characterized in that: The optical film comprises an optical substrate and the barium yttrium fluoride optical film as claimed in claim 7.

9. The mid-to-far infrared optical structure according to claim 8, characterized in that: The material of the optical substrate is ZnSe, Ge, CaF2 or silicon-based material.

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