Yttrium barium fluoride optical film and preparation method thereof, mid- and far-infrared optical structure

Abstract

The present invention provides a yttrium barium fluoride optical film, a preparation method thereof, and a mid-to-far infrared optical structure. The preparation method of the yttrium barium fluoride optical film comprises the following steps: providing an optical substrate and a yttrium barium fluoride composite, wherein the yttrium barium fluoride composite is a composite material having a metastable crystalline phase structure obtained by a high-temperature solid-phase reaction of yttrium fluoride and barium fluoride followed by quenching, and the yttrium barium fluoride composite has a submicron grain size and a metastable crystalline phase structure of a cubic phase with an 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; and using an electron gun evaporation method with the yttrium barium fluoride composite as an evaporation source to deposit the yttrium barium fluoride optical film on the optical substrate. The barium yttrium fluoride optical film of the present invention can still maintain a dense, continuous, and microcrack-free structural morphology when the single-layer film thickness reaches 2μm-6μm, and has excellent optical transparency, thermal stability and mechanical integrity.

Description

Technical Field

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

[0002] In mid- and far-infrared optical systems, achieving high transmittance output for high-power lasers or high signal-to-noise ratio imaging for thermal imaging systems typically requires depositing low-refractive-index optical films on the surfaces of optical components to create single- or multi-layer antireflection coating structures. According to optical film design principles, the thickness of a single-layer antireflection coating must ensure an optical path length of one-quarter the operating wavelength. Therefore, when operating in the 10.6μm band, the required film thickness typically reaches 2μm–4μm.

[0003] However, for traditional fluoride optical materials, due to their low mechanical strength and large difference in thermal expansion coefficient with commonly used optical substrates such as ZnSe, Ge and Si, when depositing a single-layer thin film with a thickness of more than 1μm, it is very easy to generate stress accumulation during the film growth process, which in turn causes structural damage such as cracks, peeling or warping. It is difficult to pass high and low temperature cycling or thermal shock environment reliability tests, which limits its practical application in thick film application scenarios.

[0004] In addition, in mid- and far-infrared laser systems with higher laser power densities, such as CO2 lasers, some existing technologies attempt to improve transmittance by using a multilayer interference film structure composed of alternating deposition of high-refractive-index materials such as ZnS and low-refractive-index fluoride materials. However, because the multilayer film system has more interface absorption sites and the energy accumulation effect between the films is significant, local temperature rise and micro-area thermal damage often occur, ultimately leading to optical failure of the film or shortened service life. If a low-absorption fluoride material such as YbF3 is used as a single-layer anti-reflection film, although higher transmittance can be achieved in theory, its inherent thermodynamic stability and thick film forming ability are still insufficient, making it difficult to achieve stable deposition greater than 2μm without sacrificing film integrity and environmental resistance. Summary of the Invention

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

[0006] According to one aspect of the present invention, a method for preparing a barium yttrium fluoride optical film is provided, comprising the following steps:

[0007] An optical substrate and a yttrium-barium fluoride composite are provided. 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 followed by a quenching treatment. The yttrium-barium fluoride composite has a submicron grain size, and the metastable crystal phase structure is a cubic phase having an Fm-3m space group and / or a tetragonal phase having a lattice-distorted P4 / nmm space group.

[0008] heating the optical substrate to 150° C.-200° C.;

[0009] 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.

[0010] Optionally, the preparation method of the yttrium barium fluoride complex comprises the following steps:

[0011] 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;

[0012] 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;

[0013] The barium yttrium fluoride frit is crushed, cleaned and dried to obtain the barium yttrium fluoride composite.

[0014] Optionally, the particles formed after the barium yttrium fluoride frit is crushed have an equiaxed or blocky morphology, and the particle size of the particles is 0.5 mm-2 mm.

[0015] Optionally, the method of depositing a yttrium barium fluoride optical thin film on the optical substrate using an electron gun evaporation method with the yttrium barium fluoride complex as an evaporation source comprises the following steps:

[0016] Evaporating the barium yttrium fluoride complex 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;

[0017] 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.

[0018] Optionally, before heating the optical substrate to 150° C.-200° C., the following steps are further included:

[0019] A barium yttrium fluoride seed film with an amorphous structure is deposited on the optical substrate, wherein the barium yttrium fluoride seed film has a thickness of 5 nm to 50 nm.

[0020] Optionally, depositing an amorphous barium yttrium fluoride seed film on the optical substrate comprises the following steps:

[0021] 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.

[0022] According to another aspect of the present invention, a yttrium barium fluoride optical film is provided. The yttrium barium fluoride optical film is prepared using the aforementioned preparation method. The yttrium barium fluoride optical film is a single-layer structure with a thickness of 2 μm-6 μm.

[0023] According to another aspect of the present invention, a mid- and far-infrared optical structure is provided, comprising an optical substrate and the aforementioned barium yttrium fluoride optical film.

[0024] Optionally, the optical substrate is made of ZnSe, Ge, CaF2 or a silicon-based material.

[0025] According to the solution of the present application, a yttrium barium fluoride optical film is deposited on the surface of an optical substrate. The film is formed by depositing a yttrium barium fluoride composite with a metastable crystalline phase structure formed by a 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 submicron level, effectively balancing the evaporation rate, thermal stability and particle density. In the subsequent electron beam evaporation process, it has excellent composition consistency and balanced thermal runaway behavior, avoiding problems such as splashing, 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, such as stacking faults, dislocations and non-ideal stacking 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 penetrating microcracks. During electron beam evaporation, the metastable crystalline phase of the yttrium-barium fluoride complex exhibits a moderate driving force for evaporation, a stable evaporation rate, and similar release rates for the components (barium fluoride and yttrium fluoride). This helps maintain consistent refractive index and optical uniformity in the film, avoiding localized devitrification, enhanced scattering, or refractive index fluctuations caused by phase separation or grain growth. Furthermore, existing fluoride materials (such as YF3, LaF3, and MgF2) are prone to large component vapor pressure differences, poor thermal stability, and uneven evaporation during evaporation, often accompanied by splashing, phase decomposition, or partial dissociation. To compensate for these structural and compositional instabilities during evaporation, existing technologies typically require heating the substrate to above 200°C to promote film adsorption and structural densification. However, this high-temperature process exacerbates thermal stress accumulation in the film substrate and poses limitations for certain heat-sensitive substrate materials. The yttrium barium fluoride composite material used in the embodiments of the present invention exhibits a metastable crystal phase structure and submicron particle size distribution. It can achieve stable and uniform evaporation deposition at relatively low temperatures (150°C-200°C), combining compositional consistency with structural density. This effectively reduces reliance on high-temperature substrates and enables a gentler and more compatible film-forming process. The resulting yttrium barium fluoride optical film maintains a dense, continuous, and microcrack-free structure with a single-layer thickness of 2μm-6μm, exhibiting excellent optical transparency, thermal stability, and mechanical integrity. This yttrium barium fluoride optical film is particularly suitable for use as a low-refractive-index optical thin film material in mid- and far-infrared laser systems or infrared imaging windows. It can be used to construct antireflection film structures, both as a single-layer antireflection coating and as a low-refractive-index sublayer in a multilayer interference film system. It effectively reduces optical interface reflection losses, improves system transmittance, and maintains a high laser damage threshold and film stability under high-power laser irradiation conditions.

[0026] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the following is a detailed description of the preferred embodiments of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 A schematic flow chart showing a method for preparing a barium yttrium fluoride optical film according to one embodiment of the present invention is shown;

[0028] Figure 2 A schematic flow chart of a method for preparing a yttrium-barium fluoride complex according to one embodiment of the present invention is shown;

[0029] Figure 3 shows a differential scanning calorimetry (DSC) test curve of a sub-molten composite reactant according to one embodiment of the present invention;

[0030] Figure 4 The X-ray diffraction (XRD) patterns of the samples obtained in the examples of the present invention and the comparative examples are shown;

[0031] Figure 5 FIG1 shows an appearance image of a barium yttrium fluoride frit at a macroscopic scale according to an embodiment of the present invention;

[0032] Figure 6 A scanning electron microscope image of a yttrium barium fluoride composite having an equiaxed particle morphology according to one embodiment of the present invention is shown;

[0033] Figure 7 shows a cubic phase structure diagram of the Fm-3m space group according to one embodiment of the present invention;

[0034] Figure 8 shows a tetragonal phase structure diagram of a P4 / nmm space group with lattice distortion according to one embodiment of the present invention;

[0035] Figure 9 shows an X-ray diffraction (XRD) pattern of a dual-phase coexistence structured yttrium barium fluoride composite according to one embodiment of the present invention;

[0036] Figure 10 A schematic flow chart of a method for depositing a barium yttrium fluoride optical thin film on an optical substrate using an electron gun evaporation method according to one embodiment of the present invention is shown;

[0037] Figure 11 A scanning electron microscope image of a cross section of a barium yttrium fluoride optical film according to one embodiment of the present invention is shown. DETAILED DESCRIPTION

[0038] The technical solution of the present invention is further described below in conjunction with specific embodiments.

[0039] Figure 1 FIG1 shows a schematic flow chart of a method for preparing a barium yttrium fluoride optical film according to an embodiment of the present invention. Figure 1 As shown, the preparation method comprises:

[0040] Step S100, providing an optical substrate and a yttrium barium fluoride composite. The yttrium barium fluoride composite is a composite material having a metastable crystal structure obtained by a high-temperature solid-phase reaction of yttrium fluoride and barium fluoride followed by a quenching treatment. The yttrium barium fluoride composite has a submicron grain size and a metastable crystal structure of a cubic phase with an Fm-3m space group and / or a tetragonal phase with a lattice-distorted P4 / nmm space group.

[0041] Step S200, heating the optical substrate to 150°C-200°C;

[0042] In step S300 , an electron gun evaporation method is used to deposit a barium yttrium fluoride optical thin film on an optical substrate using a barium yttrium fluoride complex as an evaporation source.

[0043] According to the solution of the embodiment of the present invention, a yttrium barium fluoride optical film is deposited on the surface of an optical substrate. The film is formed by depositing a yttrium barium fluoride composite with a metastable crystal phase structure formed by a 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 submicron level, effectively balancing the evaporation rate, thermal stability and particle density. In the subsequent electron beam evaporation process, it has excellent composition consistency and balanced thermal runaway behavior, avoiding problems such as splashing, 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, such as stacking faults, dislocations and non-ideal stacking 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 penetrating microcracks. During electron beam evaporation, the metastable crystalline phase of the yttrium-barium fluoride complex exhibits a moderate driving force for evaporation, a stable evaporation rate, and similar release rates for the components (barium fluoride and yttrium fluoride). This helps maintain consistent refractive index and optical uniformity in the film, avoiding localized devitrification, enhanced scattering, or refractive index fluctuations caused by phase separation or grain growth. Furthermore, existing fluoride materials (such as YF3, LaF3, and MgF2) are prone to large component vapor pressure differences, poor thermal stability, and uneven evaporation during evaporation, often accompanied by splashing, phase decomposition, or partial dissociation. To compensate for these structural and compositional instabilities during evaporation, existing technologies typically require heating the substrate to above 200°C to promote film adsorption and structural densification. However, this high-temperature process exacerbates thermal stress accumulation in the film substrate and poses limitations for certain heat-sensitive substrate materials. The yttrium barium fluoride composite material used in the embodiments of the present invention exhibits a metastable crystal phase structure and submicron particle size distribution. It can achieve stable and uniform evaporation deposition at relatively low temperatures (150°C-200°C), combining compositional consistency with structural density. This effectively reduces reliance on high-temperature substrates and enables a gentler and more compatible film-forming process. The resulting yttrium barium fluoride optical film maintains a dense, continuous, and microcrack-free structure with a single-layer thickness of 2μm-6μm, exhibiting excellent optical transparency, thermal stability, and mechanical integrity. This yttrium barium fluoride optical film is particularly suitable for use as a low-refractive-index optical thin film material in mid- and far-infrared laser systems or infrared imaging windows. It can be used to construct antireflection film structures, both as a single-layer antireflection coating and as a low-refractive-index sublayer in a multilayer interference film system. It effectively reduces optical interface reflection losses, improves system transmittance, and maintains a high laser damage threshold and film stability under high-power laser irradiation conditions.

[0044] In step S100 , the material of the optical substrate may be, for example, ZnSe, Ge, CaF 2 or silicon-based materials, which are optical substrates suitable for mid- and far-infrared bands.

[0045] Figure 2 FIG1 shows a schematic flow chart of a method for preparing a yttrium barium fluoride complex according to an embodiment of the present invention. Figure 2 As shown, the preparation method comprises:

[0046] Step S101: Yttrium fluoride and barium fluoride are mixed in a mass ratio of 6:4 to 4:6, and a high-temperature solid-phase reaction is carried out at 700° C. to 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;

[0047] Step S102, using a rapid quenching process to solidify the sub-molten composite reaction body into a barium yttrium fluoride frit at a cooling rate of 200°C / s-600°C / s;

[0048] Step S103: crushing, cleaning and drying the barium yttrium fluoride frit to obtain a barium yttrium fluoride composite.

[0049] In step S101, the mass ratio of yttrium fluoride to barium fluoride can be, for example, 3:2, 1:1, 5:4 or 2:3, or any other value between 6:4 and 4:6. This ratio range not only meets the stoichiometric balance required for the formation of yttrium barium fluoride crystals, but also helps to regulate the structural stability and evaporation behavior of the composite phase. Within this ratio range, by appropriately "barium-rich" or "yttrium-rich", the occupancy ratio and lattice constraint degree of different ions in the crystal can be adjusted, thereby affecting the ability of the material to form a metastable crystal phase structure during the high-temperature solid-phase reaction cooling process, improving the composition uniformity and grain boundary integrity, and suppressing the phase separation trend of barium fluoride or yttrium fluoride precipitation alone, while improving the grain size and grain boundary distribution, which is beneficial to the density and integrity of the subsequent film layer. This mass ratio control can also effectively alleviate the uneven component escape caused by the difference in vapor pressure of barium fluoride and yttrium fluoride during evaporation, making the escape rate of barium and yttrium elements more consistent, thereby improving the uniformity of the evaporated components and film formation stability.

[0050] During the high-temperature solid-phase treatment, a fluoride-rich atmosphere, supplemented by an inert gas shield, effectively inhibits the decomposition and volatilization loss of fluorides under high-temperature conditions, preventing the escape of fluorine, which can lead to deviations in the stoichiometric ratio and an incomplete crystal structure. Specifically, the fluoride atmosphere helps to establish a fluorination equilibrium environment on the material surface, reducing the driving force for the thermal decomposition of yttrium fluoride and barium fluoride in the reaction system and minimizing the risk of their conversion to oxides such as yttrium oxide and barium oxide at high temperatures. The fluoride-rich atmosphere is preferably a mixture of HF and NH₃ released by the thermal decomposition of ammonium fluoride (NH₄F). Alternatively, a low-concentration hydrogen fluoride mixture such as HF / Ar or HF / N₂, with a volume fraction of HF of 0.5% to 5%, can be used. Furthermore, the inert gas, acting as a non-reactive carrier gas, can isolate external oxygen molecules from entering the high-temperature region, preventing oxidation reactions from interfering with the formation of the composite crystal phase. For example, the inert gas can be high-purity argon or high-purity nitrogen. The above-mentioned atmosphere setting is conducive to forming a stable barium yttrium fluoride crystal structure and improving the uniformity of the material composition and structural integrity during the high-temperature solid-phase reaction process.

[0051] The temperature of the high-temperature solid-phase reaction can be, for example, 700°C, 750°C, 800°C, or 900°C, or any other value between 700°C and 900°C. Although the temperature range of 700°C to 900°C is lower than the melting points of yttrium fluoride and barium fluoride, 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, causing them to form local sub-molten regions at grain boundaries or particle contact areas, thereby promoting the formation of a sub-molten composite reaction body with uniform composition and stable structure. This process relies on the non-oxidizing, low-emission fluorine reaction environment constructed by the combined action of a fluorine-rich atmosphere and an inert gas, which can further reduce the reaction barrier and stabilize the reaction path, avoiding adverse phenomena such as fluorine loss, oxidation, or phase decomposition of the material. By controlling this temperature range, the synergistic effect of solid-solid reaction and local liquid-phase auxiliary mechanism can be achieved, thereby obtaining a sub-molten composite reaction body with reactive and structural controllability, providing a basis for subsequent quenching to form a yttrium-barium fluoride complex with a metastable crystalline phase structure. Here, "sub-molten composite reactant" refers to an intermediate composite material structure that has not yet completely melted but has undergone significant physical and chemical reactions. Its state is between the solid phase and the complete liquid phase. It does not have the overall liquid phase characteristics, but interfacial ion diffusion, preliminary lattice reconstruction and defect enrichment, interaction-induced short-range ordered structure generation and local liquid phase-assisted behavior occur at the grain boundaries, particle contact areas or local micro-regions.

[0052] Figure 3The following is a differential scanning calorimetry (DSC) test curve of a sub-molten composite reaction according to an embodiment of the present invention. In this embodiment, the mass ratio of yttrium fluoride to barium fluoride is 6:4, the fluoride ion-rich atmosphere is a 3% volume fraction of HF and Ar hydrogen fluoride mixture, and the inert gas is high-purity argon. A high-temperature solid-phase reaction is carried out at 900°C for 2 hours. The product formed after the reaction is ground and used for DSC testing. Figure 3 It can be seen that a broad endothermic peak appears at about 850°C (corresponding to the gray dotted line in the figure). This endothermic behavior does not show the typical characteristics of a melting phase transition, and the peak shape is relatively gentle, indicating that the material has not undergone an overall liquid phase transition in this temperature range. This endothermic peak reflects that the material mainly undergoes ion diffusion, preliminary reaction, and local structural reorganization at the particle contact interface during the heating process, which is a typical sub-melting reaction process. Therefore, this endothermic behavior can be reasonably inferred that the embodiment of the present invention formed in step S100 is a sub-melting composite reaction body in a metastable thermodynamic state. This structure has not undergone an overall liquid phase transition, but ion diffusion and local reaction processes have occurred in the particle interface area.

[0053] In step S102, the rapid quenching process refers to the instantaneous cooling operation of the sub-molten composite reactant at a cooling rate of 200°C / s-600°C / s, so that it solidifies rapidly in a very short time and forms a structurally uniform yttrium barium fluoride frit. 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 critical role. On the one hand, it can inhibit the long-range ion rearrangement and the formation of thermodynamically stable crystal phases during the solidification process of the material, thereby inducing the formation of a metastable crystal phase structure with slight lattice distortion, defect enrichment and high free energy state characteristics. On the other hand, rapid cooling can limit grain growth and promote the control of the grain size inside the frit at the submicron level, which is beneficial to the thermal uniformity and film formation consistency in the subsequent evaporation process. In addition, the quenching process can also avoid fluorine volatilization, component segregation and phase decomposition caused by excessively long high-temperature residence time, ensuring that the final yttrium barium fluoride frit has good structural stability and compositional uniformity.

[0054] It is important to explain that although the submolten composite reactant has not yet reached a fully liquefied state, ion diffusion and preliminary reactions have occurred locally at the particle contact interface or grain boundary region, forming a metastable intermediate with uniform composition and structural activity. This intermediate is thermodynamically unstable or metastable. Slow cooling can easily drive the system into a stable crystalline phase, leading to problems such as particle size growth, phase separation, or crystal coarsening, which in turn impairs the microstructural uniformity and optical consistency of subsequent film layers. The introduction of a rapid quenching process dynamically freezes and structurally captures this intermediate phase. Its high cooling rate effectively "locks" the non-equilibrium structural characteristics of the submolten composite reactant, inhibiting long-range diffusion and the formation of a stable phase. This results in the final solidified barium yttrium fluoride frit possessing metastable crystalline phase characteristics and submicron grain size. The two-step process synergistically controls the heat treatment and cooling paths, ensuring that the material maintains reactivity and structural stability while achieving compositional homogeneity, grain refinement, and structural compatibility during the subsequent evaporation process. This provides a key foundation for achieving optical uniformity, density, and low-stress properties in thick film deposition.

[0055] In order to verify the importance of the rapid quenching process, the inventors designed and implemented 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 fluoride ion-rich atmosphere was a hydrogen fluoride mixture 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 hours. Figure 4 The figure shows the X-ray diffraction (XRD) pattern comparison of the samples obtained in the embodiment of the present invention and the comparative example, and the sample is the obtained barium yttrium fluoride frit. Figure 4 It can be seen that the main peak positions in the XRD diffraction peaks of the samples in the embodiment of the present invention are located 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 the intensity is relatively low, indicating that its grain size is small and there is lattice distortion and defect enrichment, with metastable crystal phase characteristics. The diffraction peaks of the control sample are obviously sharp and have high intensity, indicating that its grain growth is sufficient, the crystallinity is high, a thermodynamically stable crystal phase is formed, the crystals are coarse, 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 submicron grain size.

[0056] The formed barium yttrium fluoride frit is a disc-shaped or flat block structure, and its physical dimensions are 10mm-40mm in diameter and 5mm-20mm in thickness. Figure 5 FIG1 shows the appearance image of the barium yttrium fluoride frit according to one embodiment of the present invention at a macroscopic scale. Figure 5In the embodiment, the mass ratio of yttrium fluoride to barium fluoride is 6:4, the fluoride ion-rich atmosphere is a mixture of HF and Ar with a volume fraction of 3%, the inert gas is high-purity argon, and the high-temperature solid-phase reaction is carried out at 900°C for 2h, with a cooling rate of 400°C / s. Figure 5 The barium yttrium fluoride frit exhibits a generally disc-shaped shape, with a relatively flat surface, sharp edges, and a regular geometric morphology. Furthermore, the barium yttrium fluoride frit exhibits no significant cracking, warping, or porosity, demonstrating that the high-temperature solid-phase-quenching control scheme employed in this embodiment of the present invention achieves excellent crystallization continuity and bulk uniformity.

[0057] In step S103, the crushing process can be performed using conventional mechanical methods known in the art, such as a ceramic ball mill, a vibrating crusher, or an impact mill, to form particles with an equiaxed or blocky morphology. Preferably, the frit is crushed to a particle size within the range of 0.5 mm to 2 mm to maintain uniform heat transfer and evaporation surface integrity during subsequent electron beam evaporation. This particle size range ensures dense accumulation of the evaporation source while avoiding uneven evaporation rates caused by excessive gaps between particles.

[0058] 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 ultrasonic cleaning for 5-10 minutes is followed by static sedimentation until the supernatant is clear, so as 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.

[0059] The aforementioned "equiaxed" particles, which have approximately equal length, width, and height, approaching a spherical or polyhedral shape, exhibit advantages such as good thermal uniformity, reduced stress concentration risk, and consistent thermal expansion before evaporation during electron beam evaporation under high vacuum conditions. Compared to flake- or needle-shaped particles, equiaxed particles are less susceptible to localized overheating or cracking, effectively preventing unstable behaviors such as material splashing and cracking during evaporation. Figure 6 A scanning electron microscope image of a yttrium barium fluoride composite with equiaxed particle morphology according to one embodiment of the present invention is shown. Figure 6In the embodiment, the mass ratio of yttrium fluoride to barium fluoride is 6:4, the fluoride ion-rich atmosphere 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 2 hours, 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 8 hours. Figure 6 The particles of the yttrium-barium fluoride composite are equiaxed, with a similar aspect ratio and a smooth, uniform surface, free of visible cracks or defects. This indicates that the crushed and processed yttrium-barium fluoride composite maintains good physical stability at both macro and micro scales. Furthermore, measurements show that the particle size range is controlled between 0.5 mm and 2 mm, meeting the requirements of the subsequent electron beam evaporation process.

[0060] The above-mentioned "blocky" particles refer to a type of particles with an irregular polyhedral morphology, a relatively thick overall outline, and a certain balanced volume and mass distribution. This morphology has many process compatibility advantages in a high-vacuum electron beam evaporation environment. First, due to their high mass concentration and large heat capacity, blocky particles can achieve a smoother heat conduction path during electron beam irradiation, thereby forming an evaporation interface with stronger thermal stability on the surface of the evaporation source, reducing the risk of localized melting unevenness or material splashing. Secondly, blocky particles have good filling and stacking characteristics. In the evaporation boat or crucible, a relatively dense and structurally continuous filling layer can be constructed, avoiding the high porosity and poor thermal contact problems that are prone to occur when fine powders are accumulated, thereby improving the overall heating uniformity of the material and the integrity of the evaporation surface. Compared with flaky or fragmented particles, blocky particles are not prone to warping or peeling due to local heating of the thin layer in the early stage of evaporation. They have higher morphological stability during electron beam energy loading, which is conducive to the controllability of the evaporation rate and the continuity of the film formation process.

[0061] In step S100 , the metastable crystal phase structure of the obtained barium yttrium fluoride complex is a cubic phase with an Fm-3m space group and / or a tetragonal phase with a P4 / nmm space group having a lattice distortion. Figure 7 FIG. 1 shows a cubic phase structure diagram of the Fm-3m space group according to an embodiment of the present invention. Figure 7 In the illustrated embodiment, yttrium fluoride and barium fluoride were mixed in a mass ratio of 6:4. The resulting mixture was subjected to a high-temperature solid-phase reaction at 900°C for 2 hours in a mixed atmosphere of 3% by volume HF and Ar, under high-purity argon protection. After the reaction, rapid quenching was performed at a cooling rate of 400°C / s. The resulting solid frit was crushed in a ceramic ball mill and ultrasonically cleaned with anhydrous ethanol. The cleaned sample was placed in a vacuum oven and dried at 60°C for 8 hours to obtain a yttrium barium fluoride complex with an Fm-3m space group structure. Figure 7 In the middle, the blue ball represents Ba 2+ , the green ball represents Y 3+ , the red ball represents F - .Depend on Figure 7 It can be seen that Ba 2+ Occupies the 8 corner points of the unit cell, forming a stable skeleton in the crystal, Y 3+ Located in the center of the unit cell, it is the core cation of the structure and dominates F - The coordination direction of F - Mainly distributed in the face center of the unit cell or the approximate coordination position, with the center Y 3+ Composition YF5 2- or YF6 3- Octahedral or tetragonal bipyramidal coordination structure.

[0062] Figure 8 FIG. 1 shows a tetragonal phase structure diagram of a P4 / nmm space group with lattice distortion according to an embodiment of the present invention. Figure 8 In the illustrated embodiment, yttrium fluoride and barium fluoride were mixed in a 5:5 mass ratio. This mixture was subjected to a high-temperature solid-phase reaction at 750°C for one hour in a mixed atmosphere of 1% by volume HF and N2, with high-purity nitrogen as an inert shielding gas. After the reaction, the mixture was rapidly quenched at a cooling rate of 600°C / s. The resulting solid frit was crushed in a ceramic ball mill, and the particles were ultrasonically cleaned with anhydrous ethanol. The cleaned sample was then dried in a vacuum oven at 60°C for 8 hours to obtain a yttrium barium fluoride complex with a P4 / nmm space group structure. Figure 8 In the figure, the blue sphere represents Ba 2+ ions, occupying the corners of the unit cell. The light green spheres are Y 3+ ions, occupying the center of the unit cell. The red and dark green spheres are F - ions, respectively, are distributed near the center and near the cell boundary. - Ions not only participate in the center Y 3+ Local coordination (such as YF6 3- or distorted YF5 2- polyhedron), and also periodically appear at the corners and boundaries of the unit cell. These ions form bridge bonds with multiple metal ions in the structure. This structure reflects the F - The multiple coordination characteristics and spatial periodic distribution features of ions in the crystal reflect that the crystal structure is a metastable crystal phase structure in a non-thermodynamically stable state.

[0063] Preferably, the metastable crystalline structure of the resulting yttrium-barium fluoride complex is a dual-phase coexistence structure of a cubic phase with an Fm-3m space group and a tetragonal phase with a lattice-distorted P4 / nmm space group. In this preferred embodiment, the mass ratio of yttrium fluoride to barium fluoride is controlled to be 1:1±5%, the high-temperature solid-phase reaction temperature is 780°C-850°C, the fluoride-rich atmosphere is selected to be HF or NH4F, and 95% or higher purity Ar or N2 is added, and the holding time is 20 min-45 min. The cooling rate is 300°C / s-450°C / s, the frit size is 10 mm-20 mm, and the crushed grain size is 300 nm-600 nm.

[0064] Figure 9 The figure shows an X-ray diffraction (XRD) pattern of a dual-phase coexistence structured yttrium barium fluoride composite according to one embodiment of the present invention. Figure 9 In the embodiment shown, yttrium fluoride and barium fluoride are uniformly mixed in a mass ratio of 1:1 (the allowable deviation does not exceed ±5%) and placed in a quartz crucible. A high-temperature solid-phase reaction is carried out in a fluorine-rich atmosphere consisting of a mixed gas of HF and N2 with a volume fraction of 2% (wherein the purity of N2 is not less than 99.999%). The reaction temperature is set to 820°C and the insulation reaction time is 1h. 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 frit with a size of about 15 mm. The frit is placed in a ceramic ball mill for mechanical crushing, and the obtained particles are ultrasonically cleaned in anhydrous ethanol medium for 15 minutes and dried at 60°C under vacuum conditions for 8 hours. Finally, a yttrium barium fluoride composite particle sample with a grain size in the range of 300nm-600nm is obtained. By Figure 9 Multiple diffraction peak splitting or shoulder characteristics are observed, for example, between 28°-29°, 47°-48°, and 56°-57.5°. This indicates that the sample contains more than a single crystalline phase, but rather the 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 and weak peaks reflects fine grains (submicron scale), lattice distortion, or enriched structural defects, consistent with a metastable dual-phase coexistence. This demonstrates that the metastable crystalline structure of the yttrium barium fluoride complex is a dual-phase coexistence of a cubic phase with the Fm-3m space group and a tetragonal phase with a lattice distortion with the P4 / nmm space group.

[0065] In this preferred embodiment, the cubic phase of the Fm-3m space group has a high degree of crystal symmetry and low internal stress characteristics, which can provide a structurally stable low-stress area for the film layer, while the distorted tetragonal phase of the P4 / nmm space group has slight lattice distortion and defect enrichment characteristics, and has a certain structural flexibility and stress relief ability. The two are synergistically nested at the microscale to construct a multi-scale stress relief path, effectively dispersing the internal stress generated in the film layer under deposition and heat load conditions, reducing the probability of the formation of through-going microcracks, thereby helping to achieve dense film formation and crack resistance stability of optical thin film structures with a film thickness greater than 2μm without ion-assisted deposition conditions. At the same time, this dual-phase coexistence structure can significantly improve the consistency of component volatilization during the evaporation process, reduce the refractive index fluctuation caused by vapor pressure differences, and improve the composition distribution and optical uniformity of the film layer. In addition, under high-power laser irradiation conditions, the distorted tetragonal phase grains can effectively inhibit the expansion of heat-induced microdefects through their lattice elasticity and dislocation energy absorption, thereby improving the laser damage threshold and environmental adaptability of the optical film. Furthermore, the dual-phase coexistence structure can provide heterogeneous sites with different lattice constants and crystallization rates during film nucleation and early growth, optimizing the film microstructure and promoting surface smoothness and thickness control. Therefore, this dual-phase coexistence structure has significant technical advantages in stress control, thick film deposition, evaporation stability, and high-energy laser compatibility.

[0066] In 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, thereby achieving effective construction of high adhesion and film-substrate interface density. The present invention improves the vapor stability and particle energy distribution uniformity of the evaporation source material under low temperature conditions by introducing a yttrium barium fluoride complex with a metastable crystal phase structure and submicron particle size, thereby enabling 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, and can also be any other value between 150°C and 200°C. Compared with the substrate temperature of more than 200°C commonly used in the prior art, this temperature range significantly reduces the risk of accumulation of thermal expansion mismatch stress in the film substrate, and is particularly suitable for mid- and far-infrared optical systems with heat-sensitive materials such as infrared-grade silicon, Ge, CaF2, etc. as substrates. Furthermore, the lower deposition temperature facilitates the formation of a quasi-amorphous or small-grained structure in the early stages of the film, inhibiting the formation of grain boundary dislocations and pores, and improving the overall structural integrity and stability of the film. The resulting barium yttrium fluoride optical film maintains excellent optical uniformity and density even at thicknesses of 2μm-6μm. The film structure is free of microcracks, exhibits stable transmission performance, and exhibits excellent laser damage threshold and thermal cycling reliability.

[0067] Figure 10 FIG1 shows a schematic flow chart of a method for depositing a barium yttrium fluoride optical thin film on an optical substrate using an electron gun evaporation method according to an embodiment of the present invention. Figure 10 As shown, step S300 includes:

[0068] Step S301 , evaporating the barium yttrium fluoride complex 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;

[0069] In step S302 , the yttrium barium fluoride complex is further evaporated 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 thin film on the optical substrate.

[0070] In step S300 of the embodiment of the present invention, the electron gun evaporation process of the yttrium barium fluoride complex is divided into two stages: an initial deposition step S301 performed at an evaporation rate of 0.1nm / s–0.3nm / s in the temperature range of 1150°C–1250°C, and a main deposition step S302 performed at an evaporation rate of 0.5nm / s–1.5nm / s in the temperature range of 1260°C–1350°C. This allows for the coordinated optimization of nucleation quality, film uniformity, and deposition efficiency throughout the deposition process. In the initial deposition stage, low-temperature, low-speed deposition significantly improves the directional adsorption and diffusion of active particles on the substrate surface, promoting the formation of a uniform and continuous nucleation layer, thereby enhancing film-substrate adhesion and inhibiting coarse grain aggregation. In the main deposition stage, a temperature-increasing and speed-increasing method is used to achieve stable evaporation output and efficient film formation of the yttrium barium fluoride complex, thereby quickly obtaining a dense film layer with a thickness of 2μm–6μm while ensuring film quality. This step-by-step evaporation process can effectively avoid the risks of grain coarsening, stress concentration, refractive index inhomogeneity, and devitrification that are common in traditional single-stage deposition, ensuring that the film layer maintains structural integrity and optical uniformity under high-power laser irradiation and multiple hot and cold shock environments.

[0071] 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 between 1150°C and 1250°C, and the evaporation rate can be, for example, 0.1nm / s, 0.2nm / s or 0.3nm / s, or any other value between 0.1nm / s and 0.3nm / s. This temperature range is in the critical region for effective evaporation of the material, which can ensure that the evaporation source components escape stably without violent decomposition or component segregation, and form physical adsorption and local reconstruction processes on the substrate surface. At the same time, this evaporation rate range can significantly prolong the residence and diffusion time of the active particles on the substrate surface, which is conducive to the formation of a continuous and uniform nucleation layer, improve the nucleation density and interface density, effectively inhibit 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 setting 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.

[0072] 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 between 1260°C and 1350°C, and the evaporation rate can be, for example, 0.5 nm / s, 1 nm / s or 1.5 nm / s, or any other value between 0.5 nm / s and 1.5 nm / s. This temperature range can significantly improve the vapor output capacity of the composite, ensuring the synchronous release of each component during the evaporation process, thereby reducing the refractive index fluctuation and component separation caused by the difference in vapor pressure. At the same time, it avoids the risk of structural instability caused by high-temperature decomposition or excessive fluorine evolution. The evaporation rate is controlled at 0.5 nm / s-1.5 nm / s, which can achieve rapid film formation while ensuring a continuous and dense film layer, meet the deposition requirements of 2 μm-6 μm thick low-refractive index films in mid- and far-infrared optical systems, and suppress problems such as porosity, thermal stress accumulation or non-uniform grain growth caused by excessive stacking rate. This main deposition parameter setting helps to achieve structural uniformity, optical stability and batch-to-batch repeatability of barium yttrium fluoride optical films while maintaining high evaporation efficiency.

[0073] Figure 11 FIG1 shows a cross-sectional scanning electron microscope image of a barium yttrium fluoride optical film according to an embodiment of the present invention. Figure 11 In the embodiment shown, an infrared-grade silicon substrate with a size of 25.4 mm × 1 mm was selected as the optical substrate. It was heated to 180°C and kept at this temperature for 10 minutes before evaporation to promote the surface adsorption and directional diffusion of active particles in the subsequent deposition process. The evaporation source of the yttrium barium fluoride complex used was Figure 9The particles of the metastable dual-phase coexistence structure shown have a grain size of about 400 nm. The evaporation process is carried out using an electron gun evaporation method with the following specific parameters: in step S301, the evaporation temperature is set to 1230°C, the evaporation rate is 0.2 nm / s, and the electron beam uses a spiral scanning mode to uniformly heat the evaporation source surface. The deposition time is about 20 minutes; in step S302, the evaporation temperature is increased to 1300°C, the evaporation rate is set to 1.0 nm / s, and the deposition is continued for 90 minutes. During the entire deposition process, the chamber base pressure is maintained at 3×10 -5 Pa below, without ion assist, the substrate is kept at a constant temperature of 180 ° C. After the film deposition is completed, the substrate is naturally cooled to room temperature and then taken out. The thickness of the deposited film is about 4 μm. Figure 11 The deposited barium yttrium fluoride optical thin film layer, located on the surface of the underlying optical substrate, exhibits good interfacial continuity and overall structural density, with no apparent defects such as porosity, delamination, or cracking. The film thickness indicated in the figure is approximately 4 μm. Furthermore, the interface between the film and the underlying substrate is smooth, with no apparent interfacial voids or desorption.

[0074] In a preferred embodiment, to further improve the nucleation uniformity and structural density of the yttrium barium fluoride optical film, the following step is further included before step S200: depositing an amorphous yttrium barium fluoride seed film on the optical substrate, wherein the thickness of the yttrium barium fluoride seed film is 5nm-50nm. The yttrium barium fluoride seed film is deposited by using an electron gun evaporation method, using a yttrium barium fluoride complex as an evaporation source, evaporating the yttrium barium fluoride complex at an evaporation rate of 0.05nm / s-0.2nm / s in a full-surface electron beam scanning manner at a temperature range of room temperature to 100°C, to obtain the yttrium barium fluoride seed film.

[0075] According to the solution of the embodiment of the present invention, the introduction of an amorphous barium yttrium 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 properties can shield the surface defects of the substrate and the interference of lattice orientation, 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 in the main film layer. At the same time, the seed film exhibits a good intermediate transition effect in terms of structure and thermal expansion characteristics, which can effectively alleviate the interfacial stress concentration caused by the mismatch of thermal expansion of the film substrate in the subsequent step 300 deposition stage, improve the adhesion stability of the main film layer, and reduce the risk of cracks, voids and interface delamination. Furthermore, the seed film thickness is controlled in the range of 5nm-50nm, and the overall structure is extremely thin, which has almost no adverse effect on the transmittance, phase delay or interference performance of the final optical film layer. Therefore, the introduction of the amorphous seed film significantly improves the optical uniformity, thick film integrity and stability of the deposition process of the main film layer.

[0076] According to another aspect of the present invention, a yttrium barium fluoride optical film is provided. The yttrium barium fluoride optical film is prepared using the aforementioned preparation method. The yttrium barium fluoride optical film has a single-layer structure and a thickness of 2 μm-6 μm.

[0077] According to the solution of the embodiment of the present invention, the obtained yttrium barium fluoride optical film maintains a highly dense, continuous, and microcrack-free morphology within a thickness range of 2μm to 6μm. Furthermore, the yttrium barium fluoride optical film exhibits mechanical properties of hardness, durability, and excellent thermal stability. In practical applications, it can withstand long-term heat treatment environments up to 300°C without experiencing destructive behaviors such as cracking, peeling, or shedding, fully meeting the stringent environmental adaptability and thermal stability requirements of mid- and far-infrared high-energy systems.

[0078] In addition, the film exhibits extremely low absorption and scattering losses in the 0.4μm to 12μm band and excellent optical transparency. 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 achieving low-reflectivity anti-reflection design. It is particularly suitable for anti-reflection film design in mid- and far-infrared windows, infrared detector packaging, 10.6μm laser systems, and infrared imaging optical components. Overall, the barium yttrium fluoride optical film prepared in the embodiment of the present invention shows comprehensive advantages over traditional fluoride coating materials in terms of optical performance, mechanical strength, thermal stability, and thick film reliability.

[0079] According to another aspect of the present invention, a mid- and far-infrared optical structure is provided, comprising an optical substrate and the aforementioned barium yttrium fluoride optical film. In one embodiment, the optical substrate is made of ZnSe, Ge, CaF2, or a silicon-based material.

[0080] The following specific examples and comparative examples are used to illustrate the criticality and importance of the process steps of this application.

[0081] Example 1:

[0082] The first embodiment of the present invention provides a method for preparing a yttrium barium fluoride optical film, and applies the yttrium barium fluoride optical film to a mid- and far-infrared optical structure as an antireflection film. The method for preparing the yttrium barium fluoride optical film includes:

[0083] Step 1: Yttrium fluoride and barium fluoride are mixed in a mass ratio of 1:1.05, and a high-temperature solid-phase reaction is carried out at 850° C. for 30 minutes under the protection of an inert gas of NH 4 F and 95% or more purity Ar to obtain a sub-molten composite reaction body;

[0084] Step 2: using a rapid quenching process to solidify the sub-molten composite reaction body into a yttrium barium fluoride frit at a cooling rate of 400°C / s;

[0085] Step 3, crushing, cleaning and drying the yttrium barium fluoride frit to obtain a yttrium barium fluoride complex;

[0086] Step 4: heating the optical substrate to 180° C., wherein the optical substrate is made of ZnSe material;

[0087] Step 5, evaporating the yttrium barium fluoride complex at a temperature of 1200° C. at an evaporation rate of 0.2 nm / s by electron beam full-surface scanning;

[0088] Step six: continue evaporating the yttrium barium fluoride complex 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.

[0089] Example 2:

[0090] The only difference between the second embodiment and the first embodiment of the present invention is that the second embodiment omits step five.

[0091] Comparative Example 1:

[0092] The only difference between this comparative example and Example 1 of the present invention is the temperature setting of the high-temperature solid-phase reaction in step 1. The temperature of the high-temperature solid-phase reaction in the comparative example 1 is 1400°C.

[0093] Comparative Example 2:

[0094] The only difference between this comparative example and Example 1 of the present invention is the cooling rate in step 2. In comparative example 2, the rapid quenching process in step 2 is replaced by cooling and solidifying the sub-molten composite reactant at a cooling rate of 80°C / s to obtain a barium yttrium fluoride frit.

[0095] Comparative Example 3:

[0096] The only difference between this comparative example and the first embodiment of the present invention is the heating temperature of the optical substrate in step 4. In the third comparative example, the heating temperature of the optical substrate is 220°C.

[0097] Table 1 below shows the key performance comparison results of the barium yttrium fluoride optical films prepared under different process parameters in the examples and the control examples.

[0098]

[0099] The film thickness in Table 1 above represents the deposited thickness of a single-layer film, and the refractive index is the value measured under working conditions in the 10.6μm band. During the test, the film thickness was measured using an ellipsometer with a test band range of 2-12μm. The ellipsometric data was fitted with the Cauchy model to evaluate the average thickness of the barium yttrium fluoride film in the deposition area, and the uniformity was confirmed by multi-point sampling. For samples with a film thickness greater than 3μm, a step profiler was used for cross-sectional verification. The refractive index was measured using variable-angle infrared spectroscopic ellipsometry (VASE) for reflectivity measurement, and the real refractive index at a wavelength of 10.6μm was obtained in combination with optical model fitting. The test environment was carried out in a dry nitrogen atmosphere to eliminate water vapor interference. The laser loss threshold was tested using the ISO 21254-1:2011 standard. A CO2 pulsed laser with a wavelength of 10.6 μm and a pulse width of 8 ns was used to apply a gradually increasing energy density to the sample surface. The energy density value at which the first observable damage occurred was recorded. At least 10 non-overlapping areas on the sample surface were selected for testing, and the average value was taken. The number of cracks in the film was determined by imaging the film surface at a magnification of 5000× using a scanning electron microscope. The number of cracks in the film was determined by imaging the film surface at a magnification of 100 μm or more in five randomly selected areas. 2 The number of cracks within the field of view was counted and the average value was taken. Transmittance improvement was measured using a Fourier transform infrared spectrometer covering the mid- and far-infrared ranges at a wavelength of 10.6 μm. The transmittance improvement was the difference between the transmittance of the sample after coating and the original transmittance of the substrate. Each sample group underwent baseline calibration before and after testing using the same light source and optical system.

[0100] The optical film obtained in Example 1 exhibited a higher film thickness (5.8μm), a lower refractive index (1.25 @ 10.6μm), a higher laser damage threshold (8.2J / cm²), and no cracks were detected within a 100μm² film area. Transmittance was improved by 5.4%. While the overall film structure remained dense, Example 2 achieved a film thickness approaching 5μm (4.9μm). Its refractive index was slightly increased (1.29), the laser damage threshold was reduced to 7.5J / cm², and a small number of cracks (1 / 100μm²) were observed. Transmittance improvement also decreased to 4.2%. This data clearly demonstrates that failing to divide the electron gun evaporation process of the yttrium barium fluoride complex into two stages—that is, performing only the main deposition step without the initial deposition step—can affect the nucleation density of the main film layer, the uniformity of the film interface transition zone, and the structural integrity of subsequent film layers.

[0101] In contrast, in Control Example 1, when the temperature of the high-temperature solid-phase reaction was set at 1400°C, although sufficient melting occurred, the thickness of the film formed was significantly reduced (1.7μm), and the refractive index increased to 1.38. The laser damage threshold was only 3.6J / cm², and there were as many as 10 cracks per 100μm² in the film, indicating that the high-temperature treatment caused abnormal grain growth, phase decomposition, or stress concentration, which seriously weakened the quality of the film. Although the film formed in Control Example 2 using a cooling rate of 80°C / s showed slight improvement, the film thickness was still insufficient, and the refractive index and the number of cracks did not show any fundamental improvement. In Control Example 3, when the substrate temperature was increased to 220°C, although the film adhesion was enhanced, the risk of thermal stress accumulation in the film increased, resulting in a decrease in the laser damage threshold.

[0102] In summary, by synergistically controlling the raw material ratio, high-temperature solid-phase reaction temperature, rapid quenching cooling rate, segmented evaporation process, and appropriate substrate heating temperature, an effective balance can be achieved between multiple performance dimensions of yttrium barium fluoride films, including structural density, low internal stress, thick film continuity, and optical uniformity. The obtained optical films exhibit excellent film integrity and high laser damage threshold, and are particularly suitable for the design of high-performance anti-reflection films in mid- and far-infrared optical structures.

[0103] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0104] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A method for preparing a barium yttrium fluoride optical film, characterized in that: The steps include: An optical substrate and a yttrium-barium fluoride composite are provided. 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 followed by a quenching treatment. The yttrium-barium fluoride composite has a submicron grain size, and the metastable crystal phase structure is a cubic phase having an Fm-3m space group and / or a tetragonal phase having a lattice-distorted P4 / nmm space group. heating the optical substrate to 150° C.-200° C.; Using an electron gun evaporation method, the yttrium barium fluoride complex is used as an evaporation source to deposit a yttrium barium fluoride optical thin film on the optical substrate; 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; crushing, cleaning and drying the barium yttrium fluoride frit to obtain the barium yttrium fluoride composite; The method of using an electron gun evaporation method to deposit a yttrium barium fluoride optical thin film on the optical substrate using the yttrium barium fluoride complex as an evaporation source comprises the following steps: Evaporating the barium yttrium fluoride complex 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.

2. The preparation method according to claim 1, 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 0.5 mm-2 mm.

3. The preparation method according to any one of claims 1 to 2, 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, wherein the barium yttrium fluoride seed film has a thickness of 5 nm to 50 nm.

4. The preparation method according to claim 3, characterized in that The method of depositing an amorphous barium yttrium fluoride seed film 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.

5. 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 4. The barium yttrium fluoride optical film is a single-layer structure with a thickness of 2 μm to 6 μm.

6. 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 5.

7. The mid- and far-infrared optical structure according to claim 6, characterized in that: The optical substrate is made of ZnSe, Ge, CaF2 or silicon-based materials.

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