A method for manufacturing a concave mirror

Abstract

The application provides a preparation method of a concave mirror, and belongs to the field of ultra-precision machining of optical elements. The preparation method comprises the following steps: forming a metal conductive layer on a fused quartz substrate; using a focused ion beam milling method, the fused quartz substrate with the metal conductive layer is milled by using a focused ion beam, so that the milled fused quartz substrate forms a concave mirror with a target three-dimensional curved surface morphology; wherein the radius of curvature of the concave mirror ranges from 5 to 100 microns, and the depth ranges from 100 nanometers to 5 microns; the ion energy of the focused ion beam ranges from 5 to 30 keV, and the beam current ranges from 1 pA to 10 nA; the metal conductive layer on the milled fused quartz substrate is removed by using an etching solution; after the metal conductive layer is removed, the milled fused quartz substrate is subjected to at least one annealing treatment in a vacuum or inert gas environment to reduce the surface roughness of the concave mirror, and the annealing temperature of each annealing treatment ranges from 980 to 1020 DEG C. The root mean square roughness of the concave mirror prepared in the embodiment of the application is less than or equal to 0.15 nm.

Description

Technical Field

[0001] At least one embodiment of the present invention relates to the field of ultra-precision machining of optical components, and particularly to a method for preparing a concave mirror. Background Technology

[0002] Miniaturized optical microcavities have become a key component in many scientific and technological applications, such as cavity quantum electrodynamics, quantum precision measurement, and quantum computing. High-performance miniaturized optical microcavities can be used to study novel mechanisms of light-matter interaction, paving crucial pathways for manipulating quantum states, developing new quantum devices, and exploring fundamental physical phenomena. They have typical applications in various physical systems, including quantum dots, NV color centers, two-dimensional materials, and atoms. Furthermore, cavity-enhanced spectroscopy can significantly improve the signal-to-noise ratio, enabling highly sensitive measurement of environmental parameters and becoming an important platform in the field of quantum sensing. Simultaneously, it holds great potential for integrated, miniaturized optical clocks, laying a solid foundation for the development of next-generation portable high-precision time and frequency systems.

[0003] One typical high-performance miniaturized optical microcavity is the open resonator, whose most important technical specifications are the quality factor Q and low mode volume. The quality factor Q is inversely proportional to the total integrated scattering (TIS) of the resonator, while the total scattering loss is directly proportional to the square of the surface roughness. Therefore, stringent requirements are placed on the surface roughness of the concave mirrors that make up the microcavity. On the other hand, the mode volume of the microcavity is closely related to the morphology of the concave mirrors. To achieve a small mode volume, it is necessary to fabricate concave mirrors with small radii of curvature, and to be able to independently control the radius of curvature (ROC), depth (h), and ellipticity of the concave mirrors. Parameters such as these.

[0004] In cutting-edge fields such as photon-based quantum technology applications, it is possible to simultaneously achieve a high quality factor Q and a low mode volume V. m One of the core technologies of open resonant cavities (i.e., highly miniaturized Fabry-Perot cavities) is the fabrication of concave spherical mirrors with extremely low roughness and small radius of curvature. Currently, carbon dioxide (CO2) laser ablation is mainly used to fabricate these concave spherical mirrors. The advantage of this technology is that it can achieve excellent surface quality, with surface roughness as low as 0.2 nm. However, its drawbacks include the inability to accurately control the microcavity surface morphology; it typically only has a near-spherical profile in the central region, and the inability to independently adjust parameters such as radius of curvature (ROC), cavity depth (h), and cross-sectional diameter (d), making it difficult to minimize the mode volume (V). m .

[0005] Currently, a method combining CO2 laser ablation with reactive ion etching has been proposed to improve the coupling constraints between parameters (such as cavity depth and cross-sectional diameter) of concave mirrors (i.e., cavity mirrors). However, this method still cannot achieve the accurate generation of arbitrary curved surface morphologies. Summary of the Invention

[0006] In view of this, the present invention provides a method for preparing a concave mirror, comprising:

[0007] A conductive metal layer is formed on a fused silica substrate;

[0008] A focused ion beam milling method is used to mill a fused silica substrate with a conductive metal layer using a focused ion beam, forming a concave mirror with a target three-dimensional curved surface morphology. The radius of curvature of the concave mirror ranges from 5 to 100 μm, and the depth ranges from 100 nm to 5 μm. The ion energy of the focused ion beam is 5 to 30 keV, the beam current is 1 pA to 10 nA, and the material of the conductive metal layer is selected from gold or aluminum, with a thickness of 5 to 20 nm.

[0009] The conductive metal layer on a milled fused silica substrate is removed using an etching solution;

[0010] After removing the conductive metal layer, the milled fused silica substrate is annealed at least once in a vacuum or inert gas environment to reduce the surface roughness of the concave mirror; the annealing temperature for any one annealing treatment is 980~1020℃. According to an embodiment of the present invention, performing at least one annealing treatment on the milled fused silica substrate includes repeating the following operations until the surface roughness of the concave mirror meets a preset condition:

[0011] If the surface roughness of the concave mirror after the (i-1)th annealing process does not meet the preset conditions, then the concave mirror after the (i-1)th annealing process will be subjected to the i-th annealing process.

[0012] According to an embodiment of the present invention, the annealing time for any annealing process is 2 to 24 hours.

[0013] According to an embodiment of the present invention, a concave mirror includes a mirror body and a recessed portion formed by inward indentation from the center of the mirror body;

[0014] The concave mirror that has undergone the (i-1)th annealing process is subjected to the i-th annealing process, including:

[0015] The concave mirror is placed in the annealing equipment with the opening of the concave part facing upwards, and the concave mirror that has undergone the (i-1)th annealing process is subjected to the i-th annealing process.

[0016] According to an embodiment of the present invention, the preset condition is that the root mean square roughness Rq of the concave mirror is less than or equal to 0.15 nm, and the root mean square roughness Rq characterizes the irregularity of the surface unevenness.

[0017] According to an embodiment of the present invention, the concave mirror is a spherical mirror or an aspherical mirror.

[0018] According to an embodiment of the present invention, a focused ion beam milling method is used to mill a fused silica substrate with a conductive metal layer using a focused ion beam, thereby forming a concave mirror with a target three-dimensional curved surface morphology on the milled fused silica substrate, including:

[0019] Identify the graphic file used to characterize the morphology of the concave mirror;

[0020] Based on the graphic file, a focused ion beam dwell time distribution map is obtained; in the dwell time distribution map, the pixels correspond one-to-one with the positions on the fused silica substrate; the pixel value of the pixel represents the relative dwell time of the focused ion beam at the position corresponding to the pixel;

[0021] The focused ion beam is controlled to scan and mill the position on the surface of the fused silica substrate according to the residence time distribution map, forming a concave mirror with the target three-dimensional curved surface morphology.

[0022] According to an embodiment of the present invention, the heating rate of any annealing process is 10°C / min, and the cooling rate is 5°C / min or furnace cooling.

[0023] According to an embodiment of the present invention, the annealing pressure is less than or equal to 0.01 MPa.

[0024] The present invention has the following technical effects: According to the embodiments of the present invention, concave mirrors are precisely processed by focused ion beam (FIB) milling, which can realize the free combination and independent control of parameters within the range of curvature radius of 5~100μm and depth of 100nm~5μm. The morphology of concave mirrors can be customized according to the actual application needs of quantum optics, quantum information and other fields, breaking through the limitations of traditional process parameter coupling, minimizing the volume of optical microcavity modes, and adapting to the high-performance design requirements of open resonant cavities.

[0025] Focused ion beam milling can achieve high-precision shaping of the three-dimensional curved surface of concave mirror targets, with a high degree of agreement between the surface shape and the theoretical design value. It can also accommodate complex morphology designs such as elliptical cavities and dual-mode coupled open cavities. Annealing in a vacuum or inert gas atmosphere can reduce the surface roughness of the concave mirror to the atomic level through a self-organizing process driven by the surface tension of the material, while maintaining the original surface shape after milling to the greatest extent. The concave mirror prepared in the embodiments of this invention combines the core advantages of both processes, overcoming the defects of high surface roughness of concave mirrors prepared by traditional FIB milling and low morphology control precision of CO2 laser ablation. It also reduces the optical scattering loss of subsequent growth of optical high-reflectivity thin films, thus making it applicable to cavity quantum electrodynamics and quantum information research in various systems such as quantum dots.

[0026] This invention combines FIB milling with high-temperature annealing to achieve the fabrication of concave mirrors with small radius of curvature, low surface shape error, and atomic-level surface flatness. Furthermore, the annealing process is stable, highly repeatable, maintains good shape, and allows for the processing of multiple samples at once. Attached Figure Description

[0027] The above and other objects, features and advantages of the present invention will become more apparent from the following description of embodiments of the invention with reference to the accompanying drawings, in which:

[0028] Figure 1 A flowchart of a method for preparing a concave mirror according to an embodiment of the present invention is shown.

[0029] Figure 2 A schematic diagram of a focused ion beam processing apparatus according to Embodiment 1 of the present invention is shown.

[0030] Figure 3 A schematic diagram of a tubular furnace according to an embodiment of the present invention is shown.

[0031] Figure 4 A three-dimensional topographic image of a concave mirror prepared according to Embodiment 1 of the present invention is shown.

[0032] Figure 5A It shows Figure 4 A schematic diagram of the profile fitting of the concave mirror in the central section perpendicular to the y-axis.

[0033] Figure 5B It shows Figure 4 A schematic diagram of the profile fitting of the concave mirror in the central section perpendicular to the x-axis.

[0034] Figure 6A It shows Figure 4 A schematic diagram of the profile fitting of a concave mirror in a section perpendicular to the y-axis.

[0035] Figure 6B It shows Figure 6A The difference between the measured value and the fitted curve.

[0036] Figure 6C The surface roughness of different cross-sections of the concave mirror prepared in Example 1 is shown.

[0037] Figure 7A A schematic diagram of the profile fitting of a concave mirror prepared according to another embodiment of the present invention before annealing along a section perpendicular to the y-axis is shown.

[0038] Figure 7B A schematic diagram of fitting a cross-sectional profile along a direction perpendicular to the x-axis before annealing of a concave mirror prepared according to another embodiment of the present invention is shown.

[0039] Figure 8A It shows Figure 7A A schematic diagram of the fitted radius of curvature of the central section along the direction perpendicular to the y-axis after annealing of the concave mirror.

[0040] Figure 8B It shows Figure 7B A schematic diagram of the fitted radius of curvature of the concave mirror after annealing along the central section perpendicular to the x-axis.

[0041] Figure 9A The surface morphology of the annealed fused silica substrate prepared in Comparative Example 1 is shown.

[0042] Figure 9B The surface morphology of the annealed fused silica substrate prepared in Comparative Example 2 is shown.

[0043] Figure 9C The surface morphology of the annealed fused silica substrate prepared in Comparative Example 3 is shown.

[0044] Figure 9D The surface morphology of the annealed fused silica substrate prepared in Comparative Example 4 is shown.

[0045] Figure 9E The surface morphology of the annealed fused silica substrate prepared in Comparative Example 5 is shown.

[0046] Figure 10A The surface morphology and roughness of the concave mirror sample 1 of Comparative Example 9 before annealing are shown.

[0047] Figure 10B The surface morphology and roughness of the concave mirror sample 1 of Comparative Example 9 after annealing are shown.

[0048] Figure 10C A top view and roughness of the surface morphology of the concave mirror sample 2 before annealing are shown in Comparative Example 9.

[0049] Figure 10D A top view and roughness of the surface morphology of the concave mirror sample 2 after annealing are shown in Comparative Example 9.

[0050] Figure 10E A top view and roughness of the surface morphology of the concave mirror sample 3 before annealing are shown in Comparative Example 9.

[0051] Figure 10F A top view and roughness of the surface morphology of the concave mirror sample 3 after annealing are shown in Comparative Example 9.

[0052] Figure 10G A top view and roughness of the surface morphology of the concave mirror sample 4 before annealing are shown in Comparative Example 9.

[0053] Figure 10H A top view and roughness of the surface morphology of the concave mirror sample 4 after annealing are shown in Comparative Example 9.

[0054] Figure 10I A top view and roughness of the surface morphology of the concave mirror sample 5 before annealing are shown in Comparative Example 9.

[0055] Figure 10J A top view and roughness of the surface morphology of the concave mirror sample 5 after annealing are shown in Comparative Example 9.

[0056] Figure 11A A top view of the surface morphology of the dual-cavity concave mirror sample of Comparative Example 10 before annealing is shown.

[0057] Figure 11B The cross-sectional profiles of the dual-cavity concave mirror sample of Comparative Example 10 before annealing are shown along the directions perpendicular to the x-axis and y-axis.

[0058] Figure 11C A 3D morphological profile of the recessed portion of the dual-cavity membrane sample of Comparative Example 10 after annealing with the opening facing downwards is shown. Detailed Implementation

[0059] In the process of realizing this invention, it was discovered that concave spherical mirrors with extremely low roughness and small radius of curvature can also be prepared using focused ion beam (FIB) milling. The advantage of this technique is that it allows for precise control of the surface morphology through point-by-point removal of the material surface, thereby enabling the fabrication of arbitrary curved surfaces. However, the drawback of this technique is that the surface quality after processing is relatively poor; the optimal roughness reported in the literature is approximately 0.7 nm. Alternatively, CO2 laser ablation technology can be used to post-process the concave mirror surface after FIB milling, reducing the surface roughness to 0.2 nm. However, this method has a narrow effective process window, and small changes in laser energy can easily lead to uneven surface reconstruction, thus affecting the stability and repeatability of the process.

[0060] Therefore, a new preparation method is needed to combine the morphology control advantages of focused ion beam milling with the low surface roughness advantage of carbon dioxide laser ablation, thereby preparing materials with both a high quality factor Q and a small mode volume V. m Optical microcavities.

[0061] This invention provides a novel method for fabricating concave mirrors with atomically smooth surfaces and small radii of curvature, aiming to overcome the aforementioned deficiencies of existing technologies. The method first employs focused ion beam milling to precisely machine the morphology of a concave mirror (or cavity mirror) onto the surface of a fused silica substrate. Subsequently, a independently developed high-temperature annealing technique is used to further reduce the surface roughness of the concave mirror. This method allows for free combination of parameters within the range of radii of curvature from 5 to 100 μm and depths from 100 nm to 5 μm, and the root mean square roughness of the concave mirror surface is as low as 0.15 nm, breaking through the limits of existing technologies.

[0062] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the invention. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of the invention for ease of explanation. However, it will be apparent that one or more embodiments may be practiced without these specific details. Furthermore, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0063] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0064] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein are to be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.

[0065] Figure 1 A flowchart of a method for preparing a concave mirror according to an embodiment of the present invention is shown.

[0066] like Figure 1 As shown, the method for preparing the concave mirror includes operations S1 to S4.

[0067] In operation S1, a metallic conductive layer is formed on a fused silica substrate.

[0068] In operation S2, a focused ion beam milling method is used, utilizing a focused ion beam (e.g., Ga). +A focused ion beam (FIF) is used to mill a fused silica substrate with a conductive metallic layer, forming a concave mirror with a target three-dimensional surface morphology (the morphology of the concave mirror is characterized using atomic force microscopy). The radius of curvature of the concave mirror ranges from 5 to 100 μm, and its depth ranges from 100 nm to 5 μm. The ion energy of the FIF beam is 5–30 keV, and the beam current is 1 pA–10 nA. The conductive metallic layer is made of gold or aluminum and has a thickness of 5–20 nm. The radius of curvature of the concave mirror can be, for example, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 50μm, 55μm, 60μm, 65μm, 70μm, 75μm, 80μm, 85μm, 90μm, 95μm, or 100μm. The depth range can be, for example, 100nm, 200nm, 300nm, 400nm, 500nm, 600nm, 700nm, 800nm, 900nm, 1μm, 2μm, 3μm, 4μm, or 5μm. Ion energies, for example, are 5keV, 6keV, 7keV, 8keV, 9keV, 10keV, 20keV, 25keV, and 30keV; beam currents, for example, are 1pA, 10pA, 50pA, 100pA, 500pA, 1nA, 2nA, 3nA, 4nA, 5nA, 6nA, 7nA, 8nA, 9nA, and 10nA; and the thickness of the conductive metal layer, for example, is 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, 13nm, 15nm, and 20nm. In operation S3, the conductive metal layer on the milled fused silica substrate is removed using an etching solution. When the conductive metal layer is made of gold, the etching solution is an iodine-potassium iodide solution; when the conductive metal layer is made of aluminum, the etching solution is a potassium hydroxide solution. The specific process involves placing a milled fused silica substrate with a conductive metal layer in an etching solution and using a wet etching process to etch the conductive metal layer off the milled fused silica substrate.

[0069] In operation S4, after removing the conductive metal layer, the milled fused silica substrate is annealed at least once in a vacuum or inert gas (e.g., nitrogen) environment to reduce the surface roughness of the concave mirror; the annealing temperature for any annealing process is 980~1020℃. According to embodiments of the present invention, concave mirrors are precisely machined using focused ion beam (FIB) milling, enabling free combination and independent control of parameters within the range of curvature radius 5~100μm and depth 100nm~5μm. This allows for customization of the concave mirror morphology according to practical application needs in fields such as quantum optics and quantum information, breaking through the limitations of traditional process parameter coupling and minimizing the optical microcavity mode volume V. m It meets the high-performance design requirements of open resonant cavities.

[0070] Focused ion beam milling (FIB) can achieve high-precision shaping of the three-dimensional curved surface of concave mirror targets, with a high degree of agreement between the surface shape and the theoretical design value. It can also accommodate complex morphology designs such as elliptical cavities and dual-mode coupled open cavities. Annealing can reduce the surface roughness of concave mirrors obtained by FIB milling to the atomic level through a self-organizing process driven by the surface tension of the material, while maintaining the original surface shape after milling to the greatest extent. The concave mirror prepared by the embodiments of this invention combines the core advantages of both processes, overcoming the defects of high surface roughness of concave mirrors prepared by traditional FIB milling and low morphology control precision of CO2 laser ablation. It also reduces the optical scattering loss of subsequent growth of optical high-reflectivity thin films, thus enabling its application in cavity quantum electrodynamics and quantum information research of various systems such as quantum dots.

[0071] This invention combines FIB milling with high-temperature annealing to achieve the fabrication of concave mirrors with small radius of curvature, low surface shape error, and atomic-level surface flatness. Furthermore, the annealing process is stable, highly repeatable, maintains good shape, and allows for the processing of multiple samples at once.

[0072] According to an embodiment of the present invention, the preset condition is that the root mean square roughness Rq of the concave mirror is less than or equal to 0.15 nm. The root mean square roughness Rq characterizes the irregularity of the surface unevenness. It should be noted that surface roughness includes surface arithmetic mean roughness, surface root mean square roughness, and surface maximum height roughness. In the embodiments of the present invention, surface roughness is characterized by surface root mean square roughness. Unless otherwise specified, roughness or surface roughness mentioned below refers to surface root mean square roughness.

[0073] By limiting the root mean square roughness Rq of the concave mirror to less than or equal to 0.15 nm, the surface flatness of the concave mirror can be quantitatively and accurately controlled, so that the micro-unevenness of the surface of the concave mirror can be controlled within a small range, thereby reducing optical scattering loss and meeting the requirements of the concave mirror as a high-performance optical microcavity.

[0074] According to embodiments of the present invention, the concave mirror prepared using the preparation method of the present invention can be a spherical mirror or an aspherical mirror. Therefore, the preparation method of the present invention has a wider range of applications and can meet the preparation of concave mirrors with different structures and optical design requirements, satisfying different practical application scenarios.

[0075] According to an embodiment of the present invention, in operation S1, the fused silica substrate used has high thermal stability, high optical transmittance, low expansion coefficient and good surface processing characteristics, which can be adapted to the subsequent high-temperature annealing process, and facilitates the formation of high-precision three-dimensional curved surface morphology by focused ion beam milling method, which is beneficial to obtaining concave mirror with low defects and high flatness, and meets the requirements for use of high-performance optical resonator.

[0076] According to an embodiment of the present invention, in operation S2, a focused ion beam milling method is used to mill a fused silica substrate with a metal conductive layer using a focused ion beam, so that the milled fused silica substrate forms a concave mirror with a target three-dimensional curved surface morphology, including operations S21 to S23.

[0077] In operation S21, a graphic file is determined to characterize the morphology of the concave mirror.

[0078] In this operation, the first step is to determine the graphic file used to characterize the three-dimensional curved surface morphology of the concave mirror target. The morphological features of the concave mirror to be processed are defined through this graphic file, providing a morphological basis for subsequent ion beam milling.

[0079] In operation S22, a focused ion beam dwell time distribution map is obtained based on the graphic file. The pixels in the dwell time distribution map correspond one-to-one with their positions on the fused silica substrate. The pixel value represents the relative dwell time of the focused ion beam at the position corresponding to the pixel. By controlling the total processing time, the depth of the concave spherical surface can be controlled. The relative dwell time distribution map is used to precisely control the relative residence time of the focused ion beam at different positions on the fused silica substrate, thereby achieving control over the milling depth at different positions and providing a control basis for the subsequent formation of the target three-dimensional curved surface morphology.

[0080] In operation S23, the focused ion beam is controlled to scan and mill the position on the surface of the fused silica substrate according to the residence time distribution map, forming a three-dimensional curved surface with a target depth distribution.

[0081] Based on the obtained focused ion beam dwell time distribution map, the focused ion beam is controlled to scan and mill various positions on the surface of the fused silica substrate. The focused ion beam mills at different positions on the fused silica substrate according to the corresponding dwell time, thereby forming a three-dimensional curved surface with a target depth distribution on the fused silica substrate, and finally obtaining a concave mirror with the target three-dimensional curved surface morphology.

[0082] According to an embodiment of the present invention, in operation S3, the milled fused silica substrate is subjected to at least one annealing process, including repeating the following operations until the surface roughness of the concave mirror meets a preset condition: if the surface roughness of the concave mirror after the (i-1)th annealing process does not meet the preset condition, then the concave mirror after the (i-1)th annealing process is subjected to an i-th annealing process. The detailed process is as follows.

[0083] First, the concave mirror formed by focused ion beam milling undergoes a first vacuum high-temperature annealing. After completion, its surface roughness is measured. If it meets the preset standard, the annealing process ends. If the roughness does not meet the standard after the first annealing, the concave mirror undergoes a second annealing (i.e., i=2, i-1=1); and so on. Roughness is measured after each annealing. If the previous (i-1) annealing fails to meet the preset conditions, the next (i) annealing is performed until the surface roughness of the concave mirror meets the design requirements. This method does not involve a fixed single annealing, but rather ensures that the roughness ultimately reaches the preset standard of atomic-level flatness through at least one annealing, avoiding the problem of substandard roughness caused by fluctuations in equipment process parameters or operational errors during a single annealing.

[0084] According to embodiments of the present invention, the annealing temperature for any annealing treatment is 980-1020℃, for example, 980℃, 990℃, 1000℃, 1010℃, or 1020℃. The annealing time for any annealing treatment is 2 to 24 hours, for example, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, or 10 hours. If the temperature is too low or the time is insufficient, there may be no smoothing effect or the surface may not achieve atomic-level flatness. If the temperature is too high or the time is too long, it may damage the concave mirror.

[0085] According to an embodiment of the present invention, a concave mirror includes a mirror body and a recessed portion formed by inward indentation from the center of the mirror body. Performing a first annealing process on the concave mirror that has undergone the (i-1)th annealing process includes: placing the concave mirror, which has undergone the (i-1)th annealing process, in an annealing apparatus with the opening of the recessed portion facing upwards, and then performing the first annealing process on the concave mirror that has undergone the (i-1)th annealing process.

[0086] According to embodiments of the present invention, the number of recesses can be one, two, or more. Performing the i-th annealing process on the concave mirror that has undergone the (i-1)-th annealing process includes: placing the concave mirror, with the opening of the recess facing upwards, on the annealing equipment, and then performing the i-th annealing process on the concave mirror that has undergone the (i-1)-th annealing process. When the opening of the recess faces downwards, under high-temperature softening conditions, gravity will compress the recess and the annealing equipment, causing the originally precision-milled small radius of curvature recess structure to collapse and its shape to distort, directly damaging the core parameters (radius of curvature, depth, and pattern volume) of the recess. Conversely, if the opening faces upwards, the recess is suspended and not under pressure; gravity and surface tension work together, and after annealing, both roughness can be reduced, and the original milled shape can be completely preserved, avoiding this type of failure.

[0087] According to an embodiment of the present invention, the heating rate of any annealing process is 10°C / min, and the cooling rate is 5°C / min or furnace cooling.

[0088] According to an embodiment of the present invention, the annealing pressure is less than or equal to 0.01 MPa to ensure that the concave mirror is not contaminated by the environment during the annealing process.

[0089] Example 1

[0090] The following describes in detail the method for preparing the concave mirror according to an embodiment of the present invention, taking into account the specific structure of the focused ion beam processing equipment and the tube furnace.

[0091] Step A: The first step of machining the concave mirror, or shaping, is performed on the fused silica substrate using focused ion beam milling. This step is mainly completed using focused ion beam processing equipment.

[0092] It should be noted that before processing, a 10nm gold conductive layer (i.e., a gold film) needs to be prepared on the surface of the fused silica sample using methods such as electron beam evaporation to make the fused silica sample conductive, and then it is diced into 12×12mm pieces. 2 A fused silica substrate of a certain size. In this embodiment of the invention, a commercially available 4-inch, 0.5 mm thick, double-sided polished fused silica sample was used, with an initial planar surface of 5 × 5 μm. 2 The root mean square roughness of the surface within the specified range is no higher than 0.5 nm. The objective of this invention is to fabricate and design a concave mirror with an atomically smooth surface and a small radius of curvature, having a radius of curvature of 20 μm and a depth of 1 μm.

[0093] Figure 2 A schematic diagram of a focused ion beam processing apparatus according to Embodiment 1 of the present invention is shown.

[0094] like Figure 2 As shown, the focused ion beam processing equipment includes an ion gun 201, an electron gun 202, a displacement stage 203, a first vacuum pump group 205, and a housing 206.

[0095] The focused ion beam processing equipment includes an ion gun 201 for providing a focused ion beam, and a housing 206 forming an internal containment space. A displacement stage 203, located within this space, supports and moves a fused silica substrate 204, enabling scanning and processing of different positions on the substrate 204 using the focused ion beam. This focused ion beam acts as a cutting tool, achieving precise milling of the substrate 204 through physical sputtering with the substrate material, forming a concave mirror with a three-dimensional curved surface. An electron microscope provides imaging and observation capabilities for real-time monitoring of the substrate 204's morphology and the processing procedure. A first vacuum pump unit 205 provides a high-vacuum environment for the entire processing chamber, preventing collisions between the focused ion beam and air molecules, thus ensuring the processing accuracy and stability of the focused ion beam.

[0096] Next, the fused silica substrate 204 is fixed to the sample holder and mounted on the displacement stage 203. The first vacuum pump group 205 is turned on and the pressure is evacuated to less than 5 × 10⁻⁶. -3 After Pa, turn on the electron gun 202 and ion gun 201, adjust the stage 203 to make the fused silica substrate 204 visible under the electron microscope, and adjust the focus to make the sample image clear. Specifically, first, perform a coarse adjustment: adjust the vertical distance between the fused silica substrate 204 and the electron gun 202 to about 4 mm, so that the fused silica substrate 204 is located at the coaxial point of the electron gun 202 and ion gun 201. Then, perform a fine adjustment: find a characteristic marker on the fused silica substrate 204, and further adjust the angle between the stage 203 and the horizontal plane to 7 degrees and 52 degrees respectively under low magnification (1000x) and high magnification (5000x) electron microscopes, and finely adjust the height of the stage at both angles until the position of the marker remains basically unchanged in the electron microscope image, completing the fine adjustment of the coaxial point position.

[0097] Subsequently, by using a pre-generated bitmap (BMP) file describing the concave mirror morphology generated by the recognition software, the dwell time of the focused ion beam at each point is adjusted to determine the amount of material removed from the surface of the fused silica substrate 204, thus achieving the processing of a concave mirror with a three-dimensional curved surface. The depth parameter h of the concave mirror can be controlled by adjusting the total milling time. Specifically: the angle between the displacement stage 203 and the horizontal plane is adjusted to 52 degrees, and the concave mirror morphology graphic file in 24-bit BMP format required for processing is imported. Based on the design graphic, the size of the processing area is determined, the voltage of the ion gun 201 is set to 30kV (i.e., the ion energy of the focused ion beam is 30keV), the current is set to 0.43nA, and the processing time is set. Finally, the ion gun 201 is turned on. Since the actual material removal amount is basically linearly related to the processing time, the linear relationship between processing depth and time can be fitted by the material removal results of a single processing of the fused silica substrate 204, thereby accurately determining the actual required processing time. Multiple processing parameters can be set in a single processing flow to meet the processing requirements of concave mirrors with different parameters, and a concave mirror array including multiple concave mirrors can be processed simultaneously.

[0098] Step B involves shaping the fused silica substrate 204 using FIB milling technology, followed by characterizing the surface morphology of the concave mirror using atomic force microscopy (AFM). The AFM characterization results are then imported into processing software for fitting the machining depth h, radius of curvature (ROC), and ellipticity. The parameters were compared to determine the degree of agreement between the actual morphology produced and the theoretical design values. If the machining depth did not meet the design requirements, the total milling time was adjusted. Subsequently, a gold etching solution (iodine-potassium iodide mixed solution) was used to remove the residual gold film in the unprocessed area of ​​the concave mirror.

[0099] Step C involves high-temperature annealing of the concave mirror. This step is primarily performed in a tube furnace.

[0100] Figure 3 A schematic diagram of a tubular furnace according to an embodiment of the present invention is shown.

[0101] like Figure 3 As shown, the tube furnace includes a furnace body 301, a furnace tube 302, a temperature control component 304, and a second vacuum pump assembly 305. The furnace body 301 is the main outer shell of the tube furnace, providing insulation and structural support; its interior is the heating chamber. The furnace tube 302 is a high-temperature resistant quartz / ceramic tube that penetrates the furnace body; it is the reaction chamber where the concave mirror is located. Protective gas can be introduced or a vacuum can be created to provide a controllable high-temperature environment for the concave mirror 303. The concave mirror 303 is placed in a crucible and pushed into the high-temperature zone of the furnace tube for annealing. The temperature control component 304 is used to precisely set, monitor, and control the furnace temperature, achieving programmed control of heating, holding, and cooling to ensure the temperature accuracy of the annealing process. The second vacuum pump assembly 305 is connected to the furnace tube and can extract air from the furnace tube to create a vacuum or low-oxygen environment. It can also be used in conjunction with the introduction of inert gases (such as argon or nitrogen) to create a protective atmosphere and prevent contamination of the concave mirror.

[0102] The concave mirror 303 was placed in a quartz crucible and annealed in a tube furnace under vacuum at 1000 degrees Celsius for 10 hours. This process allowed the concave mirror to undergo a self-organizing process driven by surface tension, thereby reducing its surface roughness. The specific steps are as follows.

[0103] The concave mirror 303 is placed in a quartz crucible in the central region of the furnace tube 302 for accurate thermocouple temperature measurement. The pressure inside the furnace tube 302 is evacuated to 0.01 MPa using a second vacuum pump assembly 305. Subsequently, a heating program is set via the temperature control assembly 304: heating to 1000°C at a rate of 10°C / min, holding at that temperature for 10 hours, and then cooling to room temperature at a rate of 5°C / min or with the furnace. This process performs high-temperature annealing and smoothing on the concave mirror, reducing its surface roughness while preserving its original surface morphology to the greatest extent possible.

[0104] Step D: The surface roughness of the concave mirror is characterized again using an atomic force microscope.

[0105] Step E: If the root mean square roughness of the concave mirror surface is higher than 0.15 nm, repeat steps C and D. If the root mean square roughness is lower than or equal to 0.15 nm, the concave mirror fabrication is complete. Specifically, the fabricated concave mirror is characterized using atomic force microscopy, scanning the area including the concave region. The characterization results are imported into software to fit the surface roughness. If the surface roughness does not meet the requirements, the parameters are calibrated and steps C and D are repeated.

[0106] Figure 4A three-dimensional topographic image of a concave mirror prepared according to Embodiment 1 of the present invention is shown.

[0107] like Figure 4 The image shown is a contour map of the three-dimensional morphology of the concave mirror prepared in Example 1, obtained by scanning with an atomic force microscope (AFM). The colors represent height differences (black for concavity and yellow for plane). In this example, the scanning range of AFM in the x-axis and y-axis directions is 20 μm × 20 μm.

[0108] exist Figure 4 In Example 1, the concave mirror prepared along the x-axis direction ( Figure 4 (represented by x in Chinese), y-axis direction ( Figure 4 The design value for the radius of curvature (represented by y) is 20 μm, and the height h along the z-axis is 0.99 μm. The central region is a downward-sloping depression. The color scale on the right indicates the height of different positions of the concave mirror, with yellow representing the highest point (~1.0 μm) and black representing the lowest point (0 μm). Figure 4 The depth of the depression is visually represented; the depth of the depression is approximately 1 μm (i.e., 1 × 10⁻⁶). -6 m).

[0109] Figure 5A It shows Figure 4 A schematic diagram of the profile fitting of the concave mirror in the central section perpendicular to the y-axis.

[0110] The central section of a concave mirror perpendicular to the y-axis can be called the first central section, such as... Figure 5A As shown, the horizontal axis represents the position coordinates of each test point on the first central profile along the x-axis, and the vertical axis represents the position coordinates of each test point on the first central profile along the z-axis (represented by z in the figure). Figure 5A The scatter plot represents the measured values ​​of each test point, and the solid line represents the corresponding fitted curve. In this embodiment, the test point refers to... Figure 4 The concave mirror in the image is selected from a series of specific locations along the first central section (i.e., a section perpendicular to the y-axis and parallel to the xz plane).

[0111] Figure 5B It shows Figure 4 A schematic diagram of the profile fitting of the concave mirror in the central section perpendicular to the x-axis.

[0112] A concave mirror's central section perpendicular to the x-axis can be called its second central section, such as... Figure 5B As shown, the horizontal axis represents the position coordinates of each test point on the second central profile along the y-axis, and the vertical axis represents the position coordinates of the test points on the central profile along the z-axis. Figure 5BThe scatter plots represent measured values, and the solid line represents the corresponding fitted curve. In this embodiment, the test points refer to... Figure 4 The concave mirror in the middle is selected from a series of specific locations along the second central section (i.e., the section perpendicular to the x-axis and parallel to the yz plane).

[0113] like Figure 5A and Figure 5B As shown, the fitted radius of curvature of the profile of the concave mirror prepared in Example 1 along the central section in the xz plane direction is 21.66 μm, and the fitted radius of curvature of the profile of the profile along the central section in the yz plane direction is 21.85 μm. The ellipticity of the concave part is calculated to be 0.84.

[0114] Figure 6A It shows Figure 4 A schematic diagram of the profile fitting of a concave mirror in a section perpendicular to the y-axis.

[0115] like Figure 6A As shown, the horizontal axis represents this cross-section (different from the one shown). Figure 5A The x-axis represents the position coordinates of each test point on the cross-section, and the y-axis represents the coordinates of each test point on the z-axis. Figure 6A The scatter plots represent measured values, and the solid line represents the corresponding fitted curve. In this embodiment, the test points refer to... Figure 4 The concave mirror in the image is selected from a series of specific locations on a cross-section perpendicular to the y-axis.

[0116] Figure 6B It shows Figure 6A The difference between the measured value and the fitted curve.

[0117] like Figure 6B As shown, the horizontal line is the zero line, which facilitates comparison of the difference between the measured value and the fitted curve.

[0118] Roughness is essentially the degree to which a measured surface deviates from an ideal surface (fitted curve). Therefore, this difference data is the direct basis for calculating roughness. Figure 6B As shown, the root mean square roughness (RMS) of this specific profile is calculated to be 0.069814 nm based on this difference, which is approximately 0.07 nm. This is an ultra-high smoothness at the atomic level, indicating that the surface at this location is extremely smooth.

[0119] Figure 6C The surface roughness of different cross-sections of the concave mirror prepared in Example 1 is shown.

[0120] like Figure 6CAs shown, the horizontal axis represents the fitted cross-section sequence, used to refer to different cross-sections (including multiple cross-sections perpendicular to the y-axis and multiple cross-sections perpendicular to the x-axis), and the vertical axis represents the surface roughness, which is the root mean square surface roughness. This figure reflects the morphological information of the entire concave mirror cavity (recessed part). Figure 6C The results show that the average root mean square roughness of the surface of the concave mirror prepared in Example 1 is 0.12 nm, which is less than 0.15 nm.

[0121] Figure 7A A schematic diagram of the profile fitting of a concave mirror prepared according to another embodiment of the present invention before annealing along a section perpendicular to the y-axis is shown.

[0122] like Figure 7A As shown, the horizontal axis represents the position coordinates of each test point on the x-axis along a cross-section perpendicular to the y-axis before annealing of the concave mirror, and the vertical axis represents the coordinates of each test point on the z-axis along the cross-section. Figure 7A The scatter plots represent the measured values ​​of each test point, and the solid line represents the corresponding fitted curve. Figure 7A It can be concluded that the radius of curvature of the concave mirror prepared in this embodiment before annealing along a certain cross-section perpendicular to the y-axis is 32.56 μm. Here, the test points in this embodiment refer to a series of specific locations selected along a certain cross-section perpendicular to the y-axis of the concave mirror prepared in this embodiment.

[0123] Figure 7B A schematic diagram of fitting a cross-sectional profile along a direction perpendicular to the x-axis before annealing of a concave mirror prepared according to another embodiment of the present invention is shown.

[0124] like Figure 7B As shown, the horizontal axis represents the position coordinates of each test point on the y-axis along a section of the concave mirror perpendicular to the x-axis. Figure 7B The scatter plots represent the measured values ​​of each test point, and the solid line represents the corresponding fitted curve. Figure 7B It can be concluded that the radius of curvature of the concave mirror prepared in this embodiment before annealing along a certain cross-section perpendicular to the x-axis is 32.13 μm. Here, the test points in this embodiment refer to a series of specific locations selected along a certain cross-section perpendicular to the x-axis of the concave mirror prepared in this embodiment.

[0125] Figure 8A It shows Figure 7A A schematic diagram of the fitted radius of curvature of the central section along the direction perpendicular to the y-axis after annealing of the concave mirror.

[0126] like Figure 8A As shown, the horizontal axis represents the position coordinates of each point on the central section perpendicular to the y-axis after the concave mirror has been annealed, and the vertical axis represents the coordinates of each point on the z-axis. Figure 8AThe scatter plots represent the measured values ​​of each test point, and the solid line represents the corresponding fitted curve.

[0127] Figure 8A It can be concluded that the radius of curvature of the central section of the concave mirror after annealing along the direction perpendicular to the y-axis is 32.79 μm.

[0128] Figure 8B It shows Figure 7B A schematic diagram of the fitted radius of curvature of the concave mirror after annealing along the central section perpendicular to the x-axis.

[0129] like Figure 8B As shown, the horizontal axis represents the position coordinates of each test point on the y-axis along the central section of the concave mirror perpendicular to the x-axis. Figure 8B The scatter points represent the measured values ​​of the test points, and the solid line represents the corresponding fitted curve. Figure 8B It can be concluded that the radius of curvature of the concave mirror prepared in this embodiment after annealing along a certain section perpendicular to the x-axis is 32.27 μm.

[0130] Combination Figure 7A , Figure 7B , Figure 8A and Figure 8B It can be seen that annealing has almost no effect on the radius of curvature of the concave mirror.

[0131] The following are several comparative examples. Using multiple fused silica substrates as research objects, comparative experiments were conducted in tube furnaces with different heat treatment processes (heating rate, holding temperature / time, cooling rate) to explore the influence of process parameters of the annealing process on the fused silica substrate during the preparation of concave mirrors. The annealing processes and results of each experimental group are shown in Comparative Examples 1 to 8.

[0132] Comparative Example 1

[0133] The fused silica substrate was heated to 1200℃ at a rate of 10℃ / min, held at that temperature for 2 hours, and then cooled in the furnace for annealing.

[0134] Figure 9A The surface morphology of the annealed fused silica substrate prepared in Comparative Example 1 is shown.

[0135] Figure 9A This indicates that the fused silica substrate turned white as a whole after annealing, and large flaky cracks were visible on the fused silica substrate under a light microscope.

[0136] Comparative Example 2

[0137] The fused silica substrate was heated to 1200℃ at a heating rate of 10℃ / min, held at that temperature for 10 min, and then cooled at a rate of 5℃ / min for annealing.

[0138] Figure 9B The surface morphology of the annealed fused silica substrate prepared in Comparative Example 2 is shown.

[0139] Figure 9B This indicates that small fish-scale-like cracks are visible on the fused silica substrate under a light microscope after annealing.

[0140] Comparative Example 3

[0141] The fused silica substrate was heated to 1200℃ at a rate of 5℃ / min, held at that temperature for 10 min, and then cooled at a rate of 2.5℃ / min.

[0142] Figure 9C The surface morphology of the annealed fused silica substrate prepared in Comparative Example 3 is shown.

[0143] like Figure 9C As shown, after annealing, irregular cracks can be seen on the surface of the fused silica substrate under a light microscope. The center of the fused silica substrate has high transparency, while the surrounding area turns white.

[0144] Comparative Example 4

[0145] The fused silica substrate was heated to 1100℃ at a heating rate of 10℃ / min, held at that temperature for 10min, and then annealed at a cooling rate of 5℃ / min.

[0146] Figure 9D The surface morphology of the annealed fused silica substrate prepared in Comparative Example 4 is shown.

[0147] like Figure 9D As shown, after annealing, a small area in the center of the fused silica substrate turns white under a light microscope, and cracks appear in this area. It can be inferred that the white cracked phase on the fused silica substrate is cristobalite.

[0148] Comparative Example 5

[0149] The fused silica substrate was annealed by heating to 1050℃ at a rate of 10℃ / min, holding at that temperature for 3 hours, and then cooling down at a rate of 5℃ / min.

[0150] Figure 9E The surface morphology of the annealed fused silica substrate prepared in Comparative Example 5 is shown.

[0151] like Figure 9E As shown, after annealing, the central region of the fused silica substrate turns white under an optical microscope, accompanied by the formation of cracks.

[0152] Comparative Example 6

[0153] The fused silica substrate was annealed by heating to 800°C at a rate of 10°C / min, holding at that temperature for 10 min, and then cooling at a rate of 5°C / min. After annealing, no obvious changes were observed in the fused silica substrate sample under an optical microscope.

[0154] Comparative Example 7

[0155] The fused silica substrate was heated to 1000℃ at a rate of 10℃ / min, held at that temperature for 10 min, and then cooled at a rate of 5℃ / min. After annealing, no obvious changes were observed in the fused silica substrate under an optical microscope.

[0156] Comparative Example 8

[0157] The fused silica substrate was heated to 1000℃ at 10℃ / min and held for 2 hours, then cooled at 5℃ / min. No obvious changes were observed under an optical microscope after annealing.

[0158] Comparative Examples 1 to 8 show that when the annealing temperature is too high (e.g., reaching 1050℃ or above), the fused silica substrate undergoes crystallization (whitening), accompanied by microcracks (fracture) caused by thermal stress. This indicates that simply increasing the temperature cannot achieve surface smoothing of the fused silica substrate; instead, it will damage the structural integrity of the material. This invention limits the temperature to around 1000℃, far below the softening point of fused silica (~1500℃), not only to smooth the surface but, more importantly, to avoid phase transformation and damage to the quartz material at extreme high temperatures. This demonstrates the precision of the process window selection in this approach. However, experiments that only target surfaces without damaging the material plane cannot provide insights for preparing concave mirrors with small radii of curvature. At high temperatures, stress release and viscous flow of the material can still damage the concave morphology of surfaces with extremely small radii of curvature.

[0159] The following comparative examples use concave mirror samples obtained by pre-shaping using CO2 laser ablation technology as the research object. A control experiment was conducted in a tube furnace to investigate the effect of CO2 laser ablation technology on the shaping of fused silica substrates, combined with different annealing processes, on the roughness of the concave mirrors. The results are shown in Comparative Examples 9 and 10.

[0160] Comparative Example 9

[0161] Multiple concave mirror samples were obtained by pre-shaping a fused silica substrate using CO2 laser ablation technology. After heating the multiple concave mirror samples to 1000℃ at 10℃ / min, holding for 2h, and then cooling at a rate of 5℃ / min, the roughness of the multiple concave mirror samples before and after annealing showed different changes.

[0162] Figure 10A The surface morphology and roughness of the concave mirror sample 1 of Comparative Example 9 before annealing are shown.

[0163] Figure 10B The surface morphology and roughness of the concave mirror sample 1 of Comparative Example 9 after annealing are shown.

[0164] Figure 10C A top view and roughness of the surface morphology of the concave mirror sample 2 before annealing are shown in Comparative Example 9.

[0165] Figure 10D A top view and roughness of the surface morphology of the concave mirror sample 2 after annealing are shown in Comparative Example 9.

[0166] Figure 10E A top view and roughness of the surface morphology of the concave mirror sample 3 before annealing are shown in Comparative Example 9.

[0167] Figure 10F A top view and roughness of the surface morphology of the concave mirror sample 3 after annealing are shown in Comparative Example 9.

[0168] Figure 10G A top view and roughness of the surface morphology of the concave mirror sample 4 before annealing are shown in Comparative Example 9.

[0169] Figure 10H A top view and roughness of the surface morphology of the concave mirror sample 4 after annealing are shown in Comparative Example 9.

[0170] Figure 10I A top view and roughness of the surface morphology of the concave mirror sample 5 before annealing are shown in Comparative Example 9.

[0171] Figure 10J A top view and roughness of the surface morphology of the concave mirror sample 5 after annealing are shown in Comparative Example 9.

[0172] Figures 10A to 10J Each part (a) represents a top view of the surface morphology of the corresponding concave mirror sample, and part (b) represents the root mean square roughness of the surface of the corresponding concave mirror sample. The roughness on the vertical axis is the root mean square roughness of the surface.

[0173] like Figures 10A to 10J As shown, the root mean square roughness of the surfaces of concave mirror sample 1 and concave mirror sample 2 both increased to 3 nm, while the surface roughness of concave mirror sample 3 to concave mirror sample 5 all decreased to about 0.2 nm.

[0174] Comparative Example 9 shows that annealing at 1000℃ on concave mirror samples prepared by CO2 laser ablation resulted in a decrease in the root mean square roughness (RMS RMS) of some samples (approximately 0.2 nm), but the process exhibited significant randomness and instability. The roughness of some concave mirror samples increased instead of decreasing, presumably due to non-uniform reconstruction of submicron-level melt residue or surface contamination generated during laser thermal processing. In contrast, the process scheme in this embodiment of the invention allows some sputtering contamination from the focused ion beam processing to deposit on the metal conductive layer and be removed along with it.

[0175] The following comparative examples are based on a double-cavity concave mirror sample (i.e., a concave mirror sample with two recesses) etched by focused ion beam milling. However, the opening of the recess is placed downwards, and the morphology of the concave mirror sample is found to be different from that of a normal concave mirror sample with the opening facing upwards. The results are shown in Comparative Example 10.

[0176] Comparative Example 10

[0177] For the double-cavity concave mirror sample obtained by pre-shaping using focused ion beam milling, the opening of the concave part is placed downwards during tube furnace annealing. The annealing parameters are: heating to 1000℃ at a rate of 10℃ / min, holding at 10h, and then cooling down at a rate of 5℃ / min.

[0178] Figure 11A A top view of the surface morphology of the dual-cavity concave mirror sample of Comparative Example 10 before annealing is shown.

[0179] Figure 11B The cross-sectional profiles of the dual-cavity concave mirror sample of Comparative Example 10 before annealing are shown along the directions perpendicular to the x-axis and y-axis.

[0180] Figure 11C The 3D morphological profile of the recessed portion of the dual-cavity membrane sample of Comparative Example 10 after annealing with the opening facing downwards is shown.

[0181] in, Figure 11B In The x-axis or y-axis Figure 11C It was obtained using atomic force microscopy. For example... Figures 11A-11C As shown, the double-cavity concave mirror sample before annealing has a good double-cavity concave cavity morphology. After annealing, the morphology of the central region of the double-cavity concave mirror sample collapses and no longer has the morphology after opening.

[0182] Comparative Example 10 shows that when the concave part is annealed by placing it upside down with the opening facing down, the gravity direction of the microstructure and the surface tension direction are adversely superimposed in the high-temperature softening state, resulting in insufficient support in the small radius of curvature region and structural collapse. In contrast, the double-cavity concave mirror sample with the opening facing up maintains its morphology and has reduced roughness.

[0183] The opening of this invention faces upwards, placing the recessed portion in a suspended state under stress. In this state, surface tension is cleverly used to drive atomic smoothing, thus avoiding the crushing and damage to the concave contour caused by gravity. This operational detail is crucial for maintaining a high-precision surface shape with a small radius of curvature.

[0184] This invention combines two micro / nano fabrication techniques: focused ion beam milling of fused silica substrates and tube furnace thermal annealing. It is the first to develop a high-temperature annealing technique for concave mirrors on fused silica substrates. Through the softening and self-organization process of the silica surface, the root mean square roughness of the concave mirror surface can be further reduced to 0.15 nm while preserving the original surface shape to the greatest extent possible. The focused ion beam milling method offers excellent surface shape control, high consistency with design values, and compatibility with various upper cavity mirror designs, such as elliptical cavities and dual-mode coupled open cavities. The thermal annealing temperature is set at 1000℃, lower than the conventional softening point of fused silica (1500℃), ensuring the stability of the concave surface morphology with a small radius of curvature. The tube furnace equipment used is easy to operate, highly robust, and highly repeatable. The combination of these two techniques facilitates the mass production of high-quality, low-roughness concave mirrors.

[0185] The embodiments of the present invention have been described above. However, these embodiments are merely illustrative and not intended to limit the scope of the invention. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. Those skilled in the art can make various substitutions and modifications, all of which should fall within the scope of the present invention.

Claims

1. A method for preparing a concave mirror, characterized in that, include: A metallic conductive layer is formed on a fused silica substrate; A focused ion beam milling method is used to mill the fused silica substrate with a conductive metal layer using a focused ion beam, forming a concave mirror with a target three-dimensional curved surface morphology. The concave mirror has a radius of curvature ranging from 5 to 100 μm and a depth ranging from 100 nm to 5 μm. The focused ion beam has an ion energy of 5 to 30 keV and a beam current of 1 pA to 10 nA. The conductive metal layer is made of gold or aluminum and has a thickness of 5 to 20 nm. The conductive metal layer on a milled fused silica substrate is removed using an etching solution; After removing the conductive metal layer, the milled fused silica substrate is annealed at least once in a vacuum or inert gas environment to reduce the surface roughness of the concave mirror; the annealing temperature of any annealing treatment is 980~1020℃; the annealing time of any annealing treatment is 2~24 hours. The step of performing at least one annealing treatment on the milled fused silica substrate includes repeating the following operations until the surface roughness of the concave mirror meets a preset condition: If the surface roughness of the concave mirror after the (i-1)th annealing treatment does not meet the preset conditions, then the concave mirror after the (i-1)th annealing treatment will be subjected to the i-th annealing treatment. The preset condition is that the root mean square roughness Rq of the concave mirror is less than or equal to 0.15 nm, and the root mean square roughness Rq characterizes the irregularity of the surface unevenness.

2. The preparation method according to claim 1, characterized in that, The concave mirror includes a mirror body and a recessed portion formed by the inward indentation of the middle part of the mirror body; The step of performing the i-th annealing process on the concave mirror that has undergone the (i-1)-th annealing process includes: The concave mirror is placed in an annealing apparatus with the opening of the concave portion facing upwards, and the concave mirror that has undergone the (i-1)th annealing process is subjected to the i-th annealing process.

3. The preparation method according to claim 1 or 2, characterized in that, The concave mirror can be a spherical mirror or an aspherical mirror.

4. The preparation method according to claim 1 or 2, characterized in that, The method employing focused ion beam milling utilizes a focused ion beam to mill the fused silica substrate with a conductive metal layer, thereby forming a concave mirror with a target three-dimensional curved surface morphology on the milled fused silica substrate. This includes: Identify the graphic file used to characterize the morphology of the concave mirror; Based on the graphic file, a focused ion beam dwell time distribution map is obtained; wherein, the pixels in the dwell time distribution map correspond one-to-one with the positions on the fused silica substrate; the pixel value of the pixel represents the relative dwell time of the focused ion beam at the position corresponding to the pixel; The focused ion beam is controlled to scan and mill the fused silica substrate according to the dwell time distribution map to form a concave mirror with the target three-dimensional curved surface morphology.

5. The preparation method according to claim 1 or 2, characterized in that, The heating rate for any annealing treatment is 10℃ / min, and the cooling rate is 5℃ / min or furnace cooling.

6. The preparation method according to claim 1 or 2, characterized in that, The annealing process is performed at a pressure less than or equal to 0.01 MPa.

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