A method for preparing an ultra-long focal length thin film concave mirror array

Abstract

The application belongs to the technical field of thin film materials, and particularly relates to a preparation method of an ultralong-focus thin film concave mirror array. Step (1) is to perform solvent wiping, nitrogen drying on the base material, and placing the base material into a vacuum chamber; step (2) is to apply a two-dimensional permanent magnetic array on the non-coating surface of the base material, and adjusting the relative positions of the two-dimensional permanent magnetic array, the base material and the target material; step (3) is to perform argon ion cleaning on the base material by vacuumizing the vacuum chamber to a vacuum degree better than 1*10-3 Pa, and the time is 5-20 min; and step (4) is to perform coating by adopting a pulse laser deposition technology, and the laser performs two-dimensional uniform scanning on the target material, and the coating time is 20-30 min. The prepared thin film concave mirror array has the characteristic of ultralong focus, and the focal length can be adjusted by changing the film thickness difference of the thin film. The directionality and error correction are stronger than those of a single reflecting mirror with the same size, and the thin film concave mirror array provides a better reflecting device for spatial light detection and optical communication, and has the advantages of high precision, low manufacturing cost, and can be used for scientific research and communication.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of thin film materials, and particularly relates to a preparation method of an ultra-long focal length thin film concave mirror array. BACKGROUND

[0002] The ultra-long focal length concave mirror array plays an important role in space light detection and optical communication. The longer the focal length of the concave mirror is, the greater the radius of curvature is, and the smaller the curvature (the reciprocal of the radius of curvature) is. When the focal length exceeds dozens to hundreds of meters, the curvature tends to zero, and it is difficult to realize the ultra-long focal length concave mirror array by using the traditional mechanical polishing method.

[0003] According to the search, the paper “Fabrication of Concave Microlens Array by Femtosecond Laser and Acid Etching” (Li Ming, Acta Photonica Sinica, 2009, Vol. 39, No. 3, 547-560) adopts the method of femtosecond laser lithography glass substrate and hydrogen fluoride (HF) etching to fabricate a concave lens array. The technology has the advantages of high precision and small unit size. However, the technology has high requirements for equipment (high-priced femtosecond laser, high-precision CCD and three-position moving equipment), the fabrication process is complex (laser beam homogenization and beam quality control are required), and HF acid with corrosive property is used.

[0004] Although the patents CN201710449031.9 “Method for pasting mirror array on concave structure, concave mirror array and lamp” and CN201120476751.2 “Array type unit focusing mirror” mention “array”, the “array” in the above patents refers to a small-size mirror “fragment” set assembled into a single large-size plane mirror or concave mirror, and the reflection characteristic precision is low, which is only suitable for engineering and life lighting and cannot be used for scientific research and communication. SUMMARY

[0005] In order to solve the above technical problems, the application provides a preparation method of an ultra-long focal length thin film concave mirror array, so that the prepared thin film concave mirror array has the characteristic of ultra-long focal length, and the focal length size can be adjusted by changing the film thickness difference of the thin film. The directionality and error correction are stronger than those of a single reflecting mirror of the same size, and a better reflector is provided for space light detection and optical communication.

[0006] The technical problems solved by the application are realized by the following technical solutions.

[0007] A preparation method of an ultra-long focal length thin film concave mirror array, and the specific steps are as follows:

[0008] Step (1) solvent wiping, nitrogen drying and placing in a vacuum chamber are performed on a substrate;

[0009] Step (two) is to apply a two-dimensional permanent magnet array on the non-coating surface of the substrate, and adjust the relative positions of the two-dimensional permanent magnet array, the substrate and the target material;

[0010] Step (three) is to perform argon ion cleaning on the substrate for 5-20 minutes after vacuumizing the vacuum chamber to a vacuum degree of 1*10 -3 Pa.

[0011] Step (four) is to perform coating by using the pulse laser deposition technology, and the laser performs two-dimensional uniform scanning on the target material, and the coating time is 20-30 minutes.

[0012] Preferably, the substrate in step (one) is any one of silicon, germanium or quartz glass.

[0013] Preferably, the substrate in step (one) has a length, a width and a thickness, wherein the length and the width are not greater than 120 mm respectively, and the thickness is not greater than 15 mm, and the surface roughness of the coating surface of the substrate is better than 60-40, and the aperture number is less than 1.

[0014] Preferably, the two-dimensional permanent magnet array in step (two) is composed of 9-100 same cuboid single permanent magnets, and the N and S poles of any two adjacent single permanent magnets are opposite.

[0015] Preferably, the single permanent magnet has a residual magnetism of 0.5-5 T, a length of 5-30 mm, a width of 5-30 mm and a thickness of 5-50 mm, and the length and the width of the two-dimensional permanent magnet array composed of the single permanent magnets are not less than 120% of the length and the width of the substrate respectively.

[0016] Preferably, the two-dimensional permanent magnet array in step (two) is fixed on the non-coating surface of the substrate, and the distance between the two-dimensional permanent magnet array and the substrate is not greater than 10 mm, and the two-dimensional permanent magnet array and the substrate are kept synchronous rotation or static.

[0017] Preferably, the target material in step (two) is any one of gold, silver and aluminum.

[0018] Preferably, the target material in step (two) has a length and a width, wherein the length and the width are not less than the length and the width of the substrate respectively, and the target material is opposite and parallel to the coating surface of the substrate with a distance of 50-200 mm.

[0019] Preferably, the pulse energy density of the laser on the surface of the target material in step (four) is 0.1-40 J / cm 2 , or the peak power density is 0.1-10*10 12 W / cm 2 .

[0020] Preferably, the scanning range of the two-dimensional uniform scanning of the laser on the target material in step (four) covers the substrate, and the two-dimensional uniform scanning is two-dimensional translation or two-dimensional galvanometer of the laser.

[0021] The present application has the following beneficial effects:

[0022] This invention employs a two-dimensional permanent magnet array applied to the uncoated surface of a substrate and uses pulsed laser deposition technology for coating. Compared with existing technologies, this results in a thin-film concave mirror array with an ultra-long focal length. The focal length can be adjusted by changing the thickness difference of the thin film. Its directionality and error correction are stronger than those of a single reflective mirror of the same size, providing a better reflective device for space light detection and optical communication. It features high precision, low manufacturing cost, and applicability to scientific research and communication. Attached Figure Description

[0023] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0024] Figure 1 A cross-sectional view of a single concave reflective film;

[0025] Figure 2 This is a schematic diagram of the preparation equipment in this invention;

[0026] Figure 3 The magnetic induction intensity distribution diagram is shown for a 7×7 two-dimensional permanent magnet array.

[0027] In the figure: 1. Two-dimensional permanent magnet array; 2. Substrate; 3. Target material; 4. Laser; 5. Plasma. Detailed Implementation

[0028] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0029] This invention employs an external magnetic field-controlled pulsed laser deposition method for film deposition, and utilizes the film thickness non-uniformity and its rotational symmetry to treat the reflective film as an equivalent concave mirror. For example, as... Figure 1 The above, Figure 1 The circular reflective film attached to substrate 2 has a thickness that is thinner in the middle and thicker at the edges. The film diameter (equal to or close to the diameter of substrate 2) is D, and the thicknesses at the edges and center are h1 and h2, respectively. Generally, the dimension D is from millimeters to centimeters, while (h2-h1) is on the thickness scale of tens of nanometers to several micrometers. Therefore, its radius of curvature r ≈ D. 2 If the radius of curvature is 8(h2-h1), then its radius of curvature can reach hundreds of meters or even thousands of meters. Furthermore, if this optical thin film is fabricated into a two-dimensional array, it can form an array of ultra-long focal length thin-film concave mirrors.

[0030] A method for fabricating an ultra-long focal length thin-film concave mirror array, the specific steps of which are as follows:

[0031] Step (1) The substrate 2 is wiped with solvent, dried with nitrogen, and placed in a vacuum chamber.

[0032] Specifically, the substrate 2 is any one of silicon, germanium or quartz glass, each of the length and width is not more than 120 mm, the thickness is not more than 15 mm, the surface finish of the film-coated surface of the substrate 2 is better than 60-40, and the aperture number is less than 1.

[0033] Based on the cost and polishing difficulty, the above-mentioned materials have low cost, and more importantly, moderate hardness and brittleness, and are easy to polish to high precision, and can easily reach the surface finish and aperture number requirements under lower conditions and cost. The length and width of the substrate 2 refer to the two orthogonal lengths on the film-coated surface, and the thickness refers to the length perpendicular to the film-coated surface.

[0034] In the embodiment, the substrate 2 is quartz glass, each of the length and width is 100 mm, the thickness is 10 mm, the surface finish is 60-40, and the aperture number is less than 1.

[0035] The purpose of solvent wiping and nitrogen drying of the substrate 2 is to remove the pollution on the surface of the substrate 2.

[0036] Step (two) is to apply the two-dimensional permanent magnetic array 1 to the non-film-coated surface of the substrate 2, and adjust the relative positions of the two-dimensional permanent magnetic array 1, the substrate 2 and the target material 3.

[0037] Specifically, the two-dimensional permanent magnetic array 1 is composed of 9-100 identical cuboid single permanent magnets, and the N and S poles of any two adjacent single permanent magnets are opposite. The magnetic induction intensity distribution of the MxK two-dimensional magnetic array presents (M-1)x(K-1) two-dimensional "magnetic vortex". The center of a single "magnetic vortex" corresponds to the intersection point of four single permanent magnets, and the magnetic induction intensity is the lowest. Figure 2 For example, a 7x7 array, the magnetic induction intensity distribution formed is as shown in Figure 3 The magnetic induction intensity distribution of the two-dimensional permanent magnetic array 1 presents a 6x6 two-dimensional "magnetic vortex" array. Since the magnetic induction intensity distribution of the MxK two-dimensional magnetic array presents (M-1)x(K-1) two-dimensional "magnetic vortex", therefore, the vertical and horizontal cell numbers M and K should not be less than 3, so as to form a two-dimensional array, and therefore the size of the two-dimensional permanent magnetic array 1 should not be less than 3x3, that is, the number of single permanent magnets should not be less than 9.

[0038] The residual magnetism of the single permanent magnet is 0.5-5T, each of the length and width is 5-30 mm, the thickness is 5-50 mm, and the length and width of the two-dimensional permanent magnetic array 1 composed of the single permanent magnets are not less than 120% of the length and width of the substrate 2. Further, the length and width of the single permanent magnet refer to the two orthogonal lengths perpendicular to the plane of the two poles, and the thickness refers to the length in the magnetization direction.

[0039] In the embodiment, the two-dimensional permanent magnet array 1 is composed of 10x10 identical cuboid single permanent magnets arranged in an array, and the N and S poles of any two adjacent single permanent magnets are opposite. The residual magnetism of the single permanent magnets is 0.6T, and the length and width of each single permanent magnet are both 12mm, and the thickness is 15mm.

[0040] The laser 4 in focus or a certain defocus ablates the target 3 to form a plasma 5 containing a large number of charged ions. During the expansion (flight) of the ions toward the substrate 2, the ions are subjected to the Lorentz force formed by the magnetic field, and the direction of the Lorentz force is always perpendicular to the direction of the ion flight, so the ions always fly in a spiral manner. At the same time, most of the ions are constrained in the "magnetic vortex" closest to the laser ablation point, and until they expand (fly) to the surface of the substrate 2 and condense into a film. Due to the rotational symmetry of the magnetic induction intensity of the "magnetic vortex", the distribution of the ions reaching the surface of the substrate 2 also presents rotational symmetry of non-uniformity, i.e. less in the center and more at the edges, so the film thickness presents the condition of thin in the center and thick at the edges.

[0041] Further, the distance between the two-dimensional permanent magnet array 1 and the substrate 2 is limited to no more than 10mm, mainly considering that a larger distance will cause the magnetic induction intensity near the film-coated surface of the substrate 2 to be weak, and the "magnetic vortex" effect is not obvious, which cannot realize the constraint of the plasma 5. The synchronous rotation or static state between the two is maintained, i.e. keeping the relative static state, otherwise it will cause the uniformization of the film thickness.

[0042] In this step, the target 3 is any one of gold, silver and aluminum, because the film layers prepared from these metals have high reflection coefficients. The requirement that the length and width of the target 3 are not less than the length and width of the substrate 2 is to enable all "magnetic vortex" regions on the surface of the substrate 2 to have plasma 5 constrained therein.

[0043] The distance between the substrate 2 and the target 3 is 50-200mm, which is set by experimental experience: less than the minimum limit of 50mm, the laser 4 may not be able to ablate some areas of the target 3 due to the shielding of the substrate 2; higher than the maximum limit of 200mm, the distance between the target 3 and the two-dimensional permanent magnet array 1 is too far, and the plasma 5 is very small or even not constrained by the "magnetic vortex" in the initial expansion stage, and presents a free diffusion in the case of no magnetic field, and after expanding to the region with high magnetic induction intensity near the substrate 2, it has lost the significance of constraint.

[0044] In the embodiment, the target 3 is made of aluminum, and the length and width of the target 3 are both 100mm, and the target 3 is opposite to and parallel with the film-coated surface of the substrate 2 with a distance of 120mm.

[0045] Step (three) vacuumize the vacuum chamber to a vacuum degree better than 1x10 -3 Pa, and clean the substrate 2 with argon ions for 5-20min.

[0046] The step (four) is implemented by using the pulse laser deposition technology. The laser 4 scans the target material 3 in two dimensions uniformly, and the film coating time is 20-30 min.

[0047] Specifically, the pulse energy density of the laser 4 on the surface of the target material 3 is 0.1-40 J / cm 2 , or the peak power density is 0.1-10×10 12 W / cm 2 .

[0048] The pulse energy density and the peak power density are respectively for nanosecond laser and femtosecond laser. The nanosecond laser generally has a large pulse energy, which can reach hundreds of mJ or even several J. Therefore, the pulse energy per unit area (i.e. pulse energy density) is measured. The pulse energy of the femtosecond laser is generally small, but the pulse width is extremely narrow, so that the pulse energy is fully applied to the metal atoms without thermal effect, which is directly related to the pulse width. Therefore, the pulse energy density per unit time and per unit area (i.e. peak power density) is measured.

[0049] The two-dimensional uniform scanning of the laser 4 is to continuously change the ablation points of the laser 4 on the surface of the target material 3 according to a certain rule, so as to achieve two main purposes. One is to realize the expansion (flight) of the plasma 5 in different single "magnetic vortexes", that is, to form films on different positions of the substrate 2 surface, that is, to form a two-dimensional thin film concave mirror array corresponding to the "magnetic vortex" of the two-dimensional permanent magnet array 1. The other is to realize uniform ablation of the target material 3, so that the surface of the target material 3 does not have obvious roughness, so that the distribution of the formed plasma 5 is basically unchanged, that is, it has strong stability. The two-dimensional uniform scanning of the laser 4 can be realized by two-dimensional galvanometer or two-dimensional translation of the laser.

[0050] Adjusting the thickness difference between the center and the edge of the film can change the curvature radius (i.e. focal length) of the concave reflecting film. The specific implementation methods mainly include the following three: first, adjusting the energy density of the laser 4 or the spot size of the laser 4 on the surface of the target material 3, so as to change the distribution of the plasma 5. Second, adjusting the magnetic induction intensity distribution, that is, changing the two-dimensional magnetic array parameters, such as the size of a single permanent magnet, the residual magnetism or the distance between the magnetic array and the substrate 2. Third, changing the preparation time or the total number of laser pulses. Under certain magnetic induction intensity distribution conditions, the ratio of the center thickness to the edge thickness of a single film is certain. Increasing or decreasing the preparation time (or the total number of laser pulses) means increasing or decreasing the thickness difference (h2-h1).

[0051] In this embodiment, the laser 4 used in this step is a nanosecond pulse laser, and the pulse energy density on the surface of the target material 3 is 1.2 J / cm 2The laser 4 ablates the target material 3 by two-dimensional galvanometer scanning, and the scanning range covers the length and width of the substrate 2. The coating time of the pulse laser deposition technology is 30 min.

[0052] After the coating implementation is completed, a two-dimensional concave mirror array with a 9×9 array and a unit size of 12 mm is obtained on the surface of the substrate 2 made of quartz glass. The focal length of the array is about 763.5 m after testing.

[0053] The above shows and describes the basic principles, main features and advantages of the present application. It should be understood by those skilled in the art that the present application is not limited to the above examples, and the above examples and descriptions in the specification are only the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A method of fabricating an ultra-long focal length thin film concave mirror array, the method comprising: The specific steps are as follows: ​ Step (1): solvent wiping, nitrogen drying and placing the substrate (2) in a vacuum chamber; Step (2): applying a two-dimensional permanent magnetic array (1) on the non-coated surface of the substrate (2), adjusting the relative positions of the two-dimensional permanent magnetic array (1), the substrate (2) and the target material (3); The two-dimensional permanent magnetic array (1) is composed of 9-100 identical cuboid single permanent magnets, and the N and S poles of any two adjacent single permanent magnets are opposite; the residual magnetism of the single permanent magnet is 0.5-5T, the length and width are each 5-30mm, the thickness is 5-50mm, the length and width of the two-dimensional permanent magnetic array (1) are not less than 120% of the length and width of the substrate (2); the two-dimensional permanent magnetic array (1) is fixed on the non-coated surface of the substrate (2), the distance between them is not more than 10mm, and they are kept synchronous rotation or static; the target material (3) is any one of gold, silver and aluminum; the length and width of the target material (3) are not less than the length and width of the substrate (2), the target material (3) is opposite and parallel to the coated surface of the substrate (2) with a distance of 50-200mm; Step (three) vacuum the chamber to better than 1 x 10 -3 Pa, argon ion clean the substrate (2) for 5-20 min. Step (4): coating by pulse laser deposition technology, the laser (4) performs two-dimensional uniform scanning on the target material (3), and the coating time is 20-30min.

2. The method of claim 1, wherein the method further comprises: The substrate (2) in step (1) is any one of silicon, germanium or quartz glass.

3. The method of claim 1, wherein the method further comprises: The length and width of the substrate (2) in step (1) are each not more than 120mm, the thickness is not more than 15mm, the surface finish of the coated surface of the substrate (2) is better than 60-40, and the aperture number is less than 1.

4. The method of claim 1, wherein the method further comprises: The pulse energy density of the laser (4) on the surface of the target material (3) in step (four) is 0.1-40 J / cm 2 or the peak power density is 0.1-10 x 10 12 W / cm 2 .

5. The method of claim 1, wherein the method further comprises: In step (4), the scanning range of the two-dimensional uniform scanning of the laser (4) on the target material (3) covers the substrate (2), and the two-dimensional uniform scanning is two-dimensional translation or two-dimensional galvanometer of laser.

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