A method for fabricating high-performance antireflection structures on infrared window surfaces based on femtosecond laser multi-focus multiplexing

Abstract

This invention discloses a method for fabricating high-performance anti-reflection structures on infrared window surfaces based on femtosecond laser multi-focus multiplexing, belonging to the field of laser processing technology. The method includes: sample fixation and dust removal system setup; sample stage leveling; and the fabrication of high aspect ratio, high transmittance subwavelength structures using multi-focus multiplexing combined with wet etching. This method utilizes a scanning galvanometer combined with a high-precision three-dimensional displacement platform to perform multiple scribing operations at the same horizontal position using a femtosecond laser. Specifically, during the first scribing operation, the laser focus is concentrated on the sample surface, forming a preliminary small-period subwavelength anti-reflection structure. Then, the laser focus is negatively defocused, and the laser beam polarization direction is changed, while increasing the laser power and scanning speed for a second scribing operation. This method solves problems such as rough surface morphology, low aspect ratio, and poor overall morphological consistency found in existing studies. Furthermore, it demonstrates that when fabricating anti-reflection structures with higher anti-reflection effects, shortcuts are not advisable; direct laser processing is necessary.

Description

Technical Field

[0001] This invention belongs to the field of laser processing technology, specifically relating to a method for preparing high-performance anti-reflection structures on the surface of infrared windows based on femtosecond laser multi-focus multiplexing. Background Technology

[0002] As a crucial component of infrared detection systems, infrared windows not only separate the environment from the imaging system to protect the detector but also enhance infrared radiation transmittance. Therefore, the selection of infrared window materials is paramount. Zinc sulfide, with its advantages of good thermal stability, high hardness, and high transmittance, is a common infrared window material widely used in radomes, infrared lenses, and infrared windows. However, the refractive index difference between air and the infrared window causes surface Fresnel reflection, leading to surface reflection loss and affecting the quality of infrared imaging. Traditional methods to reduce reflection loss involve depositing single or multiple antireflection coatings on the infrared window surface. However, this method suffers from drawbacks such as thermal expansion and lattice mismatch between the film and substrate materials, leading to easy film detachment, and limited antireflection effect on broad-spectrum infrared signals. Compared to depositing antireflection coatings on the infrared window surface, directly fabricating subwavelength high-performance antireflection structures on the sample surface using short-pulse lasers can reduce the refractive index difference between air and the interface, effectively reducing surface Fresnel reflection. Furthermore, single-pass processing using any parameters cannot meet the target requirements; therefore, a multi-focus multiplexing processing method is needed to obtain superior results. Summary of the Invention

[0003] To address the challenges of precisely controlling subwavelength micro / nano fabrication dimensions in existing technologies to improve antireflection, this invention provides a method for fabricating high-performance antireflection structures on infrared window surfaces using femtosecond laser multi-focus multiplexing. This method utilizes a scanning galvanometer combined with a high-precision three-dimensional displacement platform to perform multiple scribing operations at the same horizontal position using a femtosecond laser. In the first scribing operation, the laser focus is concentrated on the sample's upper surface, forming a preliminary small-period subwavelength antireflection structure. Then, the laser focus is negatively defocused (located below the surface), and the laser beam polarization direction is changed, while increasing the laser power and scanning speed for a second scribing operation. This method solves the problems of rough surface morphology, low aspect ratio, and poor overall morphological consistency found in existing studies. Furthermore, it demonstrates that when fabricating antireflection structures with higher antireflection effects, shortcuts are not advisable; direct laser processing is necessary.

[0004] This invention is achieved through the following technical solution:

[0005] The method for fabricating high-performance antireflection structures on infrared window surfaces based on femtosecond laser multi-focus multiplexing specifically includes the following steps:

[0006] (1) Sample fixation and dust removal system setup;

[0007] The specific steps are as follows: The sample is clamped to the edge using a special clamp and secured with screws and bolts; an air compressor provides the air source, consisting of pipes, a compatible needle-type air connector, and a pressure regulating valve; the needle-type air connector is fixed to the side of the displacement stage using a fixing rod, allowing it to directly act on the processing area, and then the gas is blown into the processing area through the nozzle of the needle-type air connector; the processing parameters are controlled by the pressure regulating valve to maintain a constant gas state before leveling the sample stage and during processing.

[0008] (2) Leveling the sample stage;

[0009] (3) Preparation of high aspect ratio and high transmittance subwavelength structures by combining multi-focus multiplexing with wet etching;

[0010] The specific steps are as follows: During the first scanning process, the laser is first focused on the upper surface of the sample, keeping the horizontal position unchanged. The laser power is set to W1 and the scanning speed is V1. A small-period subwavelength antireflection microstructure is prepared on the sample surface. Then, the femtosecond laser focus is negatively defocused, the laser power is increased to W2, and the scanning speed is increased to V2. At the same time, the beam polarization direction is rotated by 90°, and a second processing is performed at the same horizontal position. Finally, combined with wet etching, a high-quality, high aspect ratio subwavelength structure is prepared on the surface of the infrared window.

[0011] Furthermore, in step (2), the leveling of the sample stage specifically includes the following steps:

[0012] First, the femtosecond laser emitted from the laser passes through the first attenuator VDN1, then is incident on the first convex lens L1 and the second convex lens L2 for beam expansion. It then passes through the first half-wave plate HWP1, the reflector R1, and the first Glan prism GL1. After passing through the second half-wave plate HWP2, it is uniformly incident on the entrance pupil of the scanning galvanometer GS. After exiting the scanning galvanometer GS, it passes through the third convex lens L3 and the fourth convex lens L4, then is reflected by the reflector R2 and focused by the objective lens OL before being incident on the infrared window sample surface on the hollow sample stage. The illumination source LED is fixed above the hollow sample stage, and the white light emitted by the LED passes through the fifth convex lens L5 and the sixth convex lens L6 before being incident on the objective lens OL. The illumination light passes through the reflector R2, is reflected by the emitter R3, and is focused onto the seventh convex lens L7 and imaged onto the CCD. Finally, the three-dimensional displacement stage and the CCD are connected to a computer terminal to monitor the leveling process of the sample stage in real time and to level the sample stage accordingly.

[0013] The first half-wave plate and the first Glan prism constitute an optical attenuation system, which must ensure that the light intensity is precisely controlled within a precise range during processing to obtain the ideal structural dimensions.

[0014] Furthermore, the specific steps for leveling the sample stage are as follows:

[0015] Using the two perpendicular sides of the sample as the X and Y axes, the three-axis stepper motor is moved between the points (0, 0) and (1.5cm, 0) via computer adjustment. Simultaneously, the X-axis leveling knob is adjusted until the laser can draw a uniform damage line 1.5cm long and 5μm wide. At this point, X-axis leveling is complete. Similarly, the laser is used to draw a line between the points (0, 0) and (0, 1.5cm), and the Y-axis leveling knob is used to level the Y-axis. At this point, the laser can draw uniformly wide damage lines in both the X and Y directions, indicating that the sample stage is perpendicular to the focused laser beam. The leveling of the sample stage is complete. The position of the sample stage is then fixed.

[0016] Furthermore, in step (3), the laser scanning method used is point-by-point scanning, with the distance between each exposure point being... in, λ is the focused spot diameter, λ is the laser wavelength, and NA is the numerical aperture of the objective lens (OL).

[0017] Further, in step (3), the laser power W1 used in the first processing is twice the infrared window surface damage threshold, the laser power W2 used in the second processing is 2-3 times the infrared window surface damage threshold, the scanning speed V1 used in the first processing is 10-20μm / s, and the scanning speed V2 used in the second processing is 20-30μm / s.

[0018] Furthermore, in step (3), the period of the small-period subwavelength antireflection microstructure prepared in the first processing is less than 4 μm.

[0019] Furthermore, in step (3), rotating the beam polarization direction by 90° is specifically achieved by controlling the second half-wave plate to change the beam polarization direction to vertical polarization processing, with the scanning direction being two completely opposite directions perpendicular to the laser polarization.

[0020] Further, in step (3), the wet etching process involves heating the processed sample in a concentrated sulfuric acid solution water bath for a period of time, followed by etching with a sodium hydroxide solution with a mass fraction of 5%-10%.

[0021] The principle of the method for directly fabricating high-performance antireflection structures on the surface of an infrared window based on femtosecond laser multi-focus multiplexing is as follows:

[0022] The mechanism of subwavelength structure formation: Pulsed laser irradiation disrupts the material structure, thereby altering the material's surface and interior. For femtosecond short-pulse lasers, the material absorbs laser energy through photoionization and avalanche ionization, becoming excited in an extremely short time. Electron density and temperature rise rapidly, ultimately forming a high-density, high-temperature electron gas, which destroys the material through plasma explosion.

[0023] Subwavelength diffraction theory: When the incident light is perpendicular to the antireflection subwavelength structure, the period of the antireflection structure is constrained by the following conditions:

[0024]

[0025] Where, n ZnS and n air Here, λ represents the refractive index of the substrate and air, respectively, and λ is the incident wavelength in the antireflection band. Therefore, the optimal period for the antireflection microstructure in the long-wave infrared band (8-12 μm) is approximately 3 μm. Furthermore, according to the effective medium theory, the antireflection microstructure layer can be considered equivalent to a thin film. According to thin film interference theory, the thickness corresponding to the array antireflection height h should be:

[0026]

[0027] Where, n eff and k are the effective refractive index and constant of the antireflection structure, respectively (k = 0, 1, ..., k).

[0028] 2, 3...). Furthermore, the transmittance T of the antireflective microstructure is:

[0029]

[0030] To obtain the maximum T, the optimal n eff It should be 1.48. Therefore, for the long-wave infrared band, a structural height h of 1.8 μm or higher is more suitable.

[0031] The principle of multi-focus multiplexing control: Since a single processing operation, regardless of the parameters used, cannot meet the requirement of reducing Fresnel reflection loss, multiple scans are necessary to obtain better results. First, a laser beam with power exceeding the threshold is used to perform an initial scan of the material surface, forming a preliminary small-period subwavelength anti-reflection structure. Then, based on the first scan, the laser focus is negatively defocused, and the laser beam polarization direction is changed. The laser energy and scanning speed are then increased to perform a second or subsequent scan of the material surface. This step aims to further refine the micro / nano structure to the subwavelength level and optimize the shape and size of the structure. In summary, the multi-focus multiplexing method can progressively correct and optimize the shape, size, and positional accuracy of the subwavelength structure. By adjusting these parameters, the equivalent refractive index of the subwavelength structure can be precisely controlled to match the refractive index of the adjacent medium, thereby reducing reflection. It can also strengthen the internal connections and support structures of the subwavelength structure, improving its mechanical stability and durability.

[0032] Compared with the prior art, the advantages of the present invention are as follows:

[0033] (1) Compared with traditional femtosecond laser-induced surface subwavelength structures, the method of the present invention can realize subwavelength antireflection microstructures with a period of less than 4μm and a depth-to-diameter ratio of up to 18.

[0034] (2) By adjusting the beam polarization direction during the processing, the flatness of the structural edge and the uniformity of the microstructure can be flexibly improved.

[0035] (3) By using a dust removal system, the measurement error and poor processing quality caused by debris generated on the sample surface during processing can be improved. Attached Figure Description

[0036] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0037] Figure 1 This is a schematic diagram of the optical path of a femtosecond laser three-dimensional galvanometer processing system combined with multi-focus multiplexing to directly fabricate antireflection structures on the surface of an infrared window according to the present invention:

[0038] In the diagram, VND1 is the first attenuator, L1 is the first convex lens, L2 is the second convex lens, HWP1 is the first electrically controlled half-wave plate, R1 is the first reflector, GL1 is the first Glan prism, HWP2 is the second electrically controlled half-wave plate, GS is the scanning galvanometer, L3 is the third convex lens, L4 is the fourth convex lens, R2 is the second reflector, OL is the objective lens, the three-dimensional displacement stage (including the Z-axis piezoelectric positioning platform, the three-axis stepper motor, and the dual-axis adjustment platform), LED is the illumination source, R3 is the third reflector, L5 is the fifth convex lens, L6 is the sixth convex lens, L7 is the seventh convex lens, and CCD is the high-definition camera.

[0039] Figure 2 This is a schematic diagram of the reflective objective lens processing apparatus used in this invention;

[0040] Among them, (a) is the structural state after the first processing on the sample surface with the focus focused; (b) is the structural state after the second processing on the sample with the focus moved by the negative defocusing amount at the same horizontal position.

[0041] Figure 3 This diagram illustrates the simulation of the transmittance of subwavelength structures with different periods and depths, as well as the simulation of the optical field during laser processing.

[0042] Figure 4The graph shows the relationship between the structure depth, width, aspect ratio and Z-axis defocus position in a single scan at different scan speeds when preparing subwavelength structures with a power of 234 μW.

[0043] (a) represents the relationship between the structural depth and the Z-axis defocus position in a single scan at the same scanning speed; (b) represents the relationship between the structural width and the Z-axis defocus position in a single scan at the same scanning speed; and (c) represents the relationship between the depth-to-diameter ratio and the Z-axis defocus position in a single scan at the same scanning speed. Here, Z represents the defocus amount, and the scale bar is 1 μm.

[0044] Figure 5 The transmittance of a single scribing process for fabricating a subwavelength structure;

[0045] Wherein, (a) is the transmittance of a single scribing grating structure with different periods, and (b) is the transmittance of a variable focus scribing grating structure with different periods.

[0046] Figure 6 The diagram shows the relationship between the structure depth and the Z-axis defocus position when the laser was changed and different laser powers were used for the second scribing process at the same horizontal position to prepare the subwavelength structure. The first processing power was 234 μw.

[0047] Wherein, (a) represents the power of the second scribing process, which is 234 μW; (b) represents the power of the second scribing process, which is 468 μW; (c) represents the power of the second scribing process, which is 690 μW; and (d) represents the power of the second scribing process, which is 836 μW.

[0048] Figure 7 These are 3D laser microscope images of samples taken at different defocusing parameters during the fabrication of subwavelength structures.

[0049] Among them, (a) is a 3D measurement laser microscope image with the Z-axis defocus position inside the sample and 5 μm above the surface; (b) is a 3D measurement laser microscope image with the Z-axis defocus position inside the sample and 3 μm below the surface.

[0050] Figure 8 Transmittance and physical image of the photograting structure for preparing subwavelength structures;

[0051] Among them, (a) is the transmittance of the 3μm grating structure with variable focus processing period, and (b) is a physical image of the large-area subwavelength anti-reflection structure. Detailed Implementation

[0052] To clearly and completely describe the technical solution and its specific working process of the present invention, the specific embodiments of the present invention are as follows, in conjunction with the accompanying drawings:

[0053] Example 1

[0054] The femtosecond laser used in this embodiment has a wavelength of 800nm, a pulse width of 190fs, a repetition rate of 1kHz, and a beam diameter of approximately 3mm. The first electrically controlled half-wave plate HWP1 and the second electrically controlled half-wave plate HWP2 are driven by stepper motors. The focal lengths of the first convex lens L1, second convex lens L2, third convex lens L3, fourth convex lens L4, and fifth convex lens are 10cm, 10cm, and 20cm respectively. The first convex lens L1 and the second convex lens L2 form a beam amplification system, increasing the original femtosecond laser beam diameter by a factor of 2 to 6mm, matching the entrance pupil diameter of the scanning galvanometer GS. The distance from the exit of the scanning galvanometer GS to the entrance pupil of the objective lens OL is 100cm. The third convex lens L3 is placed 20cm from the exit of the scanning galvanometer GS, and the fourth convex lens L4 is placed 20cm from the entrance pupil of the objective lens OL. Thus, the third convex lens L3 and the fourth convex lens L4 form a 4f system, and the beam diameter reaching the entrance pupil of the objective lens OL is 6mm. The objective lens OL used is a Schwarzschild reflective objective lens with a working distance of 7.8mm, a numerical aperture NA = 0.55, and a magnification of 50×. Its structure consists of a circular convex reflector and an annular concave reflector, wherein the circular convex reflector is fixed by three radial blades. The three-dimensional displacement stage is assembled from four parts: a Z-axis piezoelectric positioning platform, a three-axis stepper motor, a dual-axis adjustment platform, and a hollow sample stage. It ensures the illumination source LED below is lit while enabling electronically controlled sample positioning and precise adjustment of the sample's Z-axis position during processing.

[0055] This embodiment provides a method for directly fabricating antireflection structures on the surface of an infrared window using a femtosecond laser three-dimensional galvanometer processing system combined with multi-focus multiplexing, specifically including the following steps:

[0056] (1) Sample fixation and dust removal system setup:

[0057] First, a specially designed clamp, tailored to the sample's shape, size, and material, is used to hold the sample at its edge. Then, screws and bolts are used to secure the clamp, ensuring a firm grip on the sample while preventing damage. An air compressor provides the air supply, with a route consisting of well-sealed and highly corrosion-resistant pipes, a compatible needle-type air nozzle, and a pressure regulating valve. A fixing rod secures the air nozzle to the side of the displacement stage, allowing it to directly act on the processing area. Gas is then blown directly into the processing area through a nozzle. The pressure regulating valve controls parameters such as gas flow rate, pressure, and blowing time.

[0058] (2) Leveling the sample stage:

[0059] like Figure 1 As shown, the femtosecond laser emitted from the laser is expanded by a first convex lens L1 and a second convex lens L2, doubling the beam size. It then passes sequentially through a first reflecting mirror R1, a scanning galvanometer GS, a third convex lens L3, and a fourth convex lens L4. The laser then enters the objective lens OL through the second reflecting mirror R2. The optical axis of the first electrically controlled half-wave plate HWP1 at the laser exit is rotated to ensure the laser pulse energy is 10 μJ behind the second reflecting mirror R2 and before the entrance pupil of the objective lens OL. The objective lens OL then focuses the laser onto the surface of the infrared window sample (diameter d = 12.7 mm, thickness 2 mm, from YUAN Guo Jing Hui). The illumination light emitted by the LED above the hollow sample stage passes through the fifth convex lens L5 and the sixth convex lens L6, the third reflecting mirror R3, the second reflecting mirror R2, and is then reflected by the objective lens OL and the fourth convex lens L7 before being focused onto the high-definition camera CCD. This allows for real-time monitoring of the sample stage leveling process on the control computer. Using the two perpendicular sides of the sample as the X and Y axes, the three-axis stepper motor is moved between the points (0, 0) and (1.5cm, 0) via computer adjustment. Simultaneously, the X-axis leveling knob is adjusted until the laser can draw a uniform damage line 1.5cm long and 5μm wide. At this point, X-axis leveling is complete. Similarly, the laser is used to draw a line between the points (0, 0) and (0, 1.5cm), and the Y-axis leveling knob is used to level the Y-axis. At this point, the laser can draw uniformly wide damage lines in both the X and Y directions, indicating that the sample stage is perpendicular to the focused laser beam. The leveling of the sample stage is complete. The position of the sample stage is then fixed.

[0060] (3) Multiplexing of multiple focus paths:

[0061] First, a reflective objective lens (OL) is used in conjunction with a CCD imaging system to focus the laser beam onto the surface of the infrared window sample. Then, using a scanning galvanometer and a high-precision three-dimensional displacement platform, a first scribing process is performed with a processing energy of 234 μJ. Subsequently, the laser spot focus position is changed, and a second scribing process is performed at the same horizontal position with appropriate laser power and scanning speed. Under laser pulse irradiation, an initial grating structure is generated at the original position. The power of the first half-wave plate (HWP1) is maintained or changed by controlling it, and the polarization direction is controlled by controlling the second half-wave plate (HWP2). The scanning speed is controlled by the galvanometer and the three-dimensional displacement platform. Then, the laser focus is positively defocused (above the surface) or negatively defocused (below the surface) for a second or multiple scribing processes. By using a multi-focus multiplexing method, subwavelength structures of special sizes can be fabricated, which can reduce the refractive index difference between air and the interface, effectively reduce surface Fresnel reflection, and thus improve transmittance.

[0062] In this embodiment, the laser power W1 used in the first processing is twice the infrared window surface damage threshold, the laser power W2 used in the second processing is two to three times the infrared window surface damage threshold, the scanning speed V1 used in the first processing is 10-20 μm / s, and the scanning speed V2 used in the second processing is 20-30 μm / s.

[0063] Scan results as follows Figure 4 As shown, the processing depth generally changes with the Z-axis defocus position, exhibiting a depth peak at both positive and negative laser defocus. Furthermore, the Z-axis defocus positions where these depth peaks occur are very similar across different scanning speeds. In most cases, the depth of the formed structure increases as the scanning speed decreases. At a scanning speed of 500 μm / s, the change in ablation structure depth is not significant, possibly due to the reduced effective pulse count on the infrared window surface. When the scanning speed decreases to 20 μm / s, the deepest structure is formed at a negative defocus position of 3 μm, with an average depth of approximately 3.5 μm, and the depth is relatively uniform. However, the reduced power density at positive defocus is detrimental to structure formation, while the increased power density caused by negative defocus allows for greater depth, contributing to a larger aspect ratio.

[0064] like Figure 5 As shown in (a), when the grating structure period is 2.5 μm, the transmittance increases significantly in the wavelength range of 4–14 μm, exceeds 80% in the wavelength range of 5.5–10.8 μm, and reaches its highest level of 85% at 6.5 μm, which is about 10% higher than the transmittance of the untreated infrared window. When the grating structure period is 3 μm, the transmittance exceeds 80% in the wavelength range of 6.5–10.5 μm, increases significantly in the wavelength range of 6–14 μm, and reaches its highest level of 85% at 7.7 μm, which is about 9% higher than the transmittance of the untreated infrared window. When the grating structure period is 3.5 μm, the transmittance increases significantly in the wavelength range of 7–14 μm, exceeds 80% in the wavelength range of 7.3–10.5 μm, and reaches its highest level of 83.5% at 8.5 μm, which is 7.5% higher than the transmittance of the untreated infrared window. The fabricated single-pass, single-sided infrared window grating structure exhibits the most significant increase in transmittance in the long-wave infrared band (8-14 μm), with a structural period of 2.5-3.5 μm. For example... Figure 5As shown in (b), when the grating structure period is 2.5 μm, the transmittance increases significantly in the range of 4–14 μm, exceeds 80% in the range of 5.2–10.2 μm, and reaches its highest level of 86% at 6.2 μm, which is about 11% higher than the transmittance of the untreated infrared window. When the grating structure period is 3 μm, the transmittance exceeds 80% in the range of 6.5–12 μm, reaches its highest level of 88% at 8.2 μm, which is about 12% higher than the transmittance of the untreated infrared window. When the grating structure period is 3.5 μm, the transmittance increases significantly in the range of 7–14 μm, exceeds 80% in the range of 7.7–12.2 μm, and reaches its highest level of 85% at 10 μm, which is 9% higher than the transmittance of the untreated infrared window.

[0065] The results show that when processing subwavelength structures on the surface of infrared windows, the use of multiple-focus multiplexing, i.e., the grating structure processed by moving focus twice, has significantly improved transmittance compared to the grating structure processed in a single step, and has a wider band coverage. The grating structure with a period within 4 μm has the best anti-transmittance effect in the long-wave infrared range.

[0066] This embodiment utilizes a method for fabricating an anti-reflection structure on the surface of an infrared window using femtosecond laser multi-focus multiplexing. An anti-reflection grating structure on the infrared window surface is obtained through a secondary processing method involving changing the focus position. For example... Figure 8 As shown, under optimal parameters, the antireflective structure exhibits a transmittance exceeding 80% in the 6.8-12.8 μm range, exceeding 90% in the 7.3-12.1 μm range, and reaching a maximum transmittance of 96% at 10 μm. This represents an increase of approximately 20% in transmittance compared to the untreated infrared window, demonstrating excellent transmission performance in the long-wave infrared band.

[0067] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0068] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0069] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A method for fabricating high-performance antireflection structures on the surface of an infrared window based on femtosecond laser multi-focus multiplexing, characterized in that, Specifically, the steps include the following: (1) Sample fixation and dust removal system setup; The specific steps are as follows: The sample is clamped to the edge using a special clamp and secured with screws and bolts; an air compressor provides the air source, consisting of pipes, a compatible needle-type air connector, and a pressure regulating valve; the needle-type air connector is fixed to the side of the displacement stage using a fixing rod, allowing it to directly act on the processing area, and then the gas is blown into the processing area through the nozzle of the needle-type air connector; the processing parameters are controlled by the pressure regulating valve to maintain a constant gas state before leveling the sample stage and during processing. (2) Leveling the sample stage; (3) Preparation of high aspect ratio and high transmittance subwavelength structures by combining multi-focus multiplexing with wet etching; The specific steps are as follows: During the first scanning process, the laser is first focused on the upper surface of the sample, the horizontal position remains unchanged, the laser power is set to W1, and the scanning speed is V1. A small-period subwavelength antireflection microstructure is prepared on the sample surface. Then, the femtosecond laser focus is negatively defocused, the laser power is increased to W2, the scanning speed is increased to V2, and the beam polarization direction is rotated by 90°. The second processing is performed at the same horizontal position. Finally, combined with wet etching, a high-quality, high aspect ratio subwavelength structure is prepared on the surface of the infrared window. In step (3), the laser power W1 used in the first processing is twice the infrared window surface damage threshold, the laser power W2 used in the second processing is 2-3 times the infrared window surface damage threshold, the scanning speed V1 used in the first processing is 10-20μm / s, and the scanning speed V2 used in the second processing is 20-30μm / s. In step (3), the period of the small-period subwavelength antireflection microstructure prepared in the first processing is less than 4 μm; In step (3), rotating the beam polarization direction by 90° is specifically achieved by controlling the second electronically controlled half-wave plate to change the beam polarization direction to vertical polarization processing, and the scanning direction is two directions that are completely opposite to the laser polarization. In step (3), the wet etching process involves heating the processed sample in a concentrated sulfuric acid solution water bath for a period of time, followed by etching with a sodium hydroxide solution with a mass fraction of 5%-10%.

2. The method for fabricating a high-performance antireflection structure on the surface of an infrared window based on femtosecond laser multi-focus multiplexing as described in claim 1, characterized in that, In step (2), the leveling of the sample stage specifically includes the following steps: First, the femtosecond laser emitted from the laser passes through the first attenuator (VND1), then is incident on the center of the first convex lens (L1) and the second convex lens (L2) for beam expansion. It then passes through the center of the first electrically controlled half-wave plate (HWP1), the first reflector (R1), and the first Glan prism (GL1). After passing through the second electrically controlled half-wave plate (HWP2), it is uniformly incident on the entrance pupil of the scanning galvanometer (GS). After exiting the scanning galvanometer (GS), it passes through the third convex lens (L3) and the fourth convex lens (L4), and then sequentially through the second reflector (R2). The light is reflected and focused by the objective lens (OL) and then incident on the infrared window sample surface on the hollow sample stage. The illumination source (LED) is fixed above the hollow sample stage, so that the white light emitted by the illumination source (LED) passes through the fifth convex lens (L5) and the sixth convex lens (L6) and then enters the objective lens (OL). The illumination light passes through the second reflecting mirror (R2) and is reflected by the third emitting mirror (R3) to the seventh convex lens (L7) and focused onto the CCD. Finally, the three-dimensional displacement stage and the CCD are connected to the computer terminal to monitor the leveling process of the sample stage in real time and level the sample stage.

3. The method for fabricating a high-performance antireflection structure on the surface of an infrared window based on femtosecond laser multi-focus multiplexing as described in claim 2, characterized in that, The specific steps for leveling the sample stage are as follows: Using the two perpendicular sides of the sample as the X and Y axes, the three-axis stepper motor is moved between the points (0, 0) and (1.5cm, 0) via computer adjustment. Simultaneously, the X-axis leveling knob is adjusted until the laser can draw a uniform damage line 1.5cm long and 5μm wide. At this point, X-axis leveling is complete. Similarly, the laser is used to draw a line between the points (0, 0) and (0, 1.5cm), and the Y-axis leveling knob is used to level the Y-axis. At this point, the laser can draw uniformly wide damage lines in both the X and Y directions, indicating that the sample stage is perpendicular to the focused laser beam. The leveling of the sample stage is complete. The position of the sample stage is then fixed.

Images