Thermal radiation micro-hole super surface for thermal camouflage and infrared encryption and preparation method thereof

Abstract

The application provides a thermal radiation micro-hole super surface for thermal camouflage and infrared encryption and a preparation method, and belongs to the technical field of micro-nano photonic devices.The thermal radiation micro-hole super surface is composed of deep hole structures arranged periodically on the surface of a metal material; the deep hole is a rotationally symmetric structure, and at least two different thermal emissivities exist, and the thermal emissivity is independently regulated by controlling the depth-width ratio of the deep hole cavity.The laser scanning processing of the thermal radiation micro-hole super surface can be realized by using a laser processing light path system based on a galvanometer scanning, and the prepared thermal radiation micro-hole super surface can be applied to thermal camouflage and infrared encryption technology, a pattern formed by the periodically arranged deep hole structures is processed on the surface of a metal material requiring thermal camouflage or infrared encryption, and the pattern is invisible under visible light and visible under infrared light.The application designs and prepares a high-resolution thermal radiation micro-hole super surface with adjustable local thermal radiation, and the thermal radiation micro-hole super surface can be widely applied in the fields of thermal camouflage and infrared encryption.

Description

Technical Field

[0001] This invention belongs to the field of micro-nano photonic device technology, specifically relating to a thermal radiation microporous metasurface for thermal camouflage and infrared encryption and its preparation method. Background Technology

[0002] To meet the urgent needs of critical equipment in aerospace, energy, and military sectors for high-performance thermal conduction, radiation, and conversion capabilities, and given the continuous advancements in infrared measurement equipment and technology in recent years, infrared thermal camouflage technology has attracted increasing attention. According to Planck's blackbody law, thermal radiation is the heat radiated by any object when its temperature is above absolute zero. Furthermore, the intensity of thermal radiation is proportional to the object's surface emissivity and the fourth power of its surface temperature. High-temperature objects emit stronger infrared signals than ordinary objects, making infrared stealth more difficult. Therefore, the fundamental problems of thermal camouflage technology mainly involve controlling temperature and surface emissivity. Firstly, temperature control is the most direct but difficult method because it requires additional cooling or heating equipment, especially for high-speed rotating mechanical power sources (such as aircraft engines), where the high-temperature heat cannot be fundamentally reduced. In contrast, controlling surface emissivity is a simpler and more effective approach; simply covering the object with a low-emissivity material (coating) or processing surface microstructures can effectively suppress thermal radiation signals.

[0003] Thermal camouflage methods fall into two categories: thermal stealth and thermal illusion. The former conceals the target within the environment, reducing the probability of detection and identification, such as an invisibility cloak. The latter artificially constructs false targets to confuse and deceive, increasing the survival probability of the real target. Traditional thermal stealth materials are metallic materials with ultra-wideband low emissivity or metallic particle coatings. However, mirror-polished metallic surfaces are easily detected by various multi-band detection technologies (such as lidar and vision). Another common approach is based on subwavelength structures such as thin films / metasurfaces, utilizing the electromagnetic resonance between infrared waves and micro / nano structures to modulate emissivity. A key drawback of metallic coatings and subwavelength structures is the inability to locally control the thermal emissivity of small areas, along with difficulties in large-area fabrication, high costs, and low mechanical wear resistance. On the other hand, the essence of thermal phantoms is to design infrared thermal imaging patterns that are hidden or undetectable in other wavelengths (such as the visible light band). However, the fabrication methods for similar thermal phantom surface structures, such as machining, multilayer film overlay, and subwavelength photonic crystal structures, suffer from problems such as complex processing, low processing precision, and low resolution of thermal imaging patterns. In particular, the adhesion of thin films or coatings on the metal surfaces of high-temperature equipment is low, making them prone to wear and subsequent functional failure. Infrared encryption, like thermal phantoms, uses artificial thermal infrared patterns, but its application is changed to the field of data encryption.

[0004] In summary, existing thermal camouflage mainly employs thin films or subwavelength structures, but their shortcomings include uniform and singular thermal radiation characteristics, angle dependence, and the inability to be directly fabricated on bulk materials (metallic materials). Summary of the Invention

[0005] To address the challenges of complex fabrication processes, low spatial resolution, and poor compatibility with metallic materials in existing thermal camouflage structures, directly fabricating efficient and stable thermal radiation functional interface micro / nano structures on the surface of metallic materials in high-temperature equipment has become a key technical issue. Therefore, this invention proposes a thermal radiation microporous metasurface for thermal camouflage and infrared encryption, along with its fabrication method. Based on a microcavity localized thermal radiation metasurface design, and utilizing a laser ablation subtractive processing method, each metasurface unit is independently controlled to fabricate a high-resolution thermal illusion or infrared encryption surface with tunable localized thermal radiation.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A thermal radiation microporous metasurface for thermal camouflage and infrared encryption is composed of deep hole structures periodically arranged on the surface of a metal material; the deep holes have a rotationally symmetric structure and at least two different thermal emissivitys, which are independently controlled by adjusting the aspect ratio of the deep hole cavity.

[0008] Furthermore, the deep hole is a blind hole structure, and adopts one or more of the following shapes: cylindrical, conical, and frustum.

[0009] Furthermore, the periodic arrangement adopts a grid or honeycomb pattern.

[0010] Furthermore, the surface of the metal material is a plane or a free-form surface.

[0011] Furthermore, when the thermal emissivity of a deep hole is independently controlled by adjusting the aspect ratio of the deep hole cavity, the upper opening diameter of the deep hole is fixed and only the depth or shape of the deep hole is changed. That is, the thermal emissivity can be controlled by combining various hole shapes (such as cylindrical, conical, and frustum shapes) and different depths.

[0012] Furthermore, the larger the aspect ratio of the deep cavity, the higher the thermal emissivity. The range of thermal emissivity adjustment is [ε0,1], where ε0 represents the thermal emissivity of the inner surface of the deep cavity.

[0013] Furthermore, combinations of deep holes with at least two different thermal emissivity on a thermally radiating microporous metasurface yield thermal phantoms or infrared encrypted patterns.

[0014] The above-mentioned method for preparing a thermally radiative microporous metasurface includes the following steps:

[0015] Step 1: Fix the metal sample on a triaxial displacement stage, set the laser polarization state and focus;

[0016] Step 2: Based on the pre-designed dimensions of the deep hole structure periodically arranged on the surface of the metal material, adjust the laser scanning processing parameters, including energy density, scanning speed, and number of processing layers;

[0017] Step 3: Align the laser with the area to be processed on the surface of the metal sample, open the high-pressure air valve, and align the high-pressure air nozzle with the area to be processed.

[0018] Step 4: After scanning one layer with the laser, the laser focus moves along the optical axis, causing the focus to move downwards. The feed amount of the downward movement of the focus is equal to the processing depth of each layer. This process is repeated until the preset number of processing layers is completed.

[0019] Step 5: Close the high-pressure air valve and remove the metal material sample;

[0020] Step 6: Perform ultrasonic cleaning on the metal material sample to obtain a thermally radiative microporous metasurface.

[0021] Furthermore, the laser scanning processing is implemented using a laser processing optical path system based on galvanometer scanning. The laser processing optical path system includes a polarizer, a half-wave plate, a quarter-wave plate, a galvanometer, a focusing lens, a triaxial displacement stage, and a high-pressure air nozzle. The polarizer, half-wave plate, quarter-wave plate, galvanometer, and focusing lens are arranged sequentially along the laser optical path. The high-pressure air nozzle is installed on one side of the triaxial displacement stage. The laser is focused onto the processing area on the surface of the metal material sample fixed on the triaxial displacement stage by the movement of the galvanometer.

[0022] The aforementioned application of thermally camouflaged microporous metasurfaces in thermal camouflage and infrared encryption technologies involves fabricating patterns of periodically arranged deep holes onto the surface of metallic materials requiring thermal camouflage or infrared encryption. These patterns are invisible under visible light but visible under infrared light. This utilizes the principle that visible light images cannot distinguish holes of different depths. By combining periodically arranged deep holes with varying thermal emissivity, an infrared pattern that is invisible under visible light and exhibits multi-level infrared radiation temperatures is created, achieving localized thermal emissivity modulation and high-resolution infrared imaging. Preferably, the metallic material is suitable for high-temperature environments.

[0023] The beneficial effects of this invention are:

[0024] 1. This invention designs a high spatial resolution and locally tunable thermal radiation microporous metasurface. Its highest spatial resolution depends on the period size of the metasurface unit and the laser processing spot size, while the tunable range of thermal emissivity depends on the geometry of the laser-processed holes. Furthermore, each metasurface unit consists of rotationally symmetric circular holes, enabling wide-bandwidth thermal radiation characteristics.

[0025] 2. This invention proposes a laser processing method to prepare this thermal radiation microporous metasurface, which features non-contact, maskless, single-step, and repeatable characteristics. Compared with traditional thermal radiation metasurface preparation methods, this invention does not require complex semiconductor processes, can be directly processed on the surface of metal materials, and is easy to process on the surface of high-temperature heating equipment.

[0026] 3. The locally tunable thermal radiation microporous metasurface designed in this invention can be applied to thermal stealth and infrared encryption. Compared with the prior art, the biggest feature of this method is that it can be prepared on the surface of metal materials. The process is simple and it is expected to be prepared directly on the surface of aerospace and high-temperature heat-generating equipment to achieve environmentally friendly and energy-saving infrared encryption or thermal stealth. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of a metasurface with adjustable thermal radiation micropores, which schematically shows the thermal illusion of the letter "A" and the infrared encryption pattern. The thermal emissivity of the white circular pore is ε1, and the thermal emissivity of the black circular pore is ε2.

[0028] Figure 2 Examples of several optional circular hole shapes;

[0029] Figure 3 Examples of several optional periodic arrangement methods;

[0030] Figure 4 This is a schematic diagram of a laser processing optical path system based on galvanometer scanning;

[0031] Figure 5 The results of the theoretical analysis of thermal radiation metasurfaces are shown, respectively, illustrating the relationship between the equivalent thermal emissivity of cylindrical (solid line) and conical (dashed line) holes and the aspect ratio, and compared with the original thermal emissivity ε0 (dotted line) of the material surface.

[0032] Figure 6 A multi-level temperature thermal illusion and infrared encryption design scheme for visible light stealth, taking the flame pattern as an example, ε1, ε2, ε3 represent three regions with different thermal emissivity;

[0033] Figure 7 Examples of thermal phantom and infrared encryption are shown, with the left column showing the fabrication drawing, the simulated visible light pattern, and the simulated infrared pattern, respectively.

[0034] The diagram shows: 1. A metasurface illustrating the thermal illusion and infrared encryption pattern of the letter "A"; 2. Metal sample material; 3. Cylindrical hole cross-section; 4. Frustum-shaped hole cross-section; 5. Conical hole cross-section; 6. Rectangular (grid-like) periodic distribution; 7. Rectangular (grid-like) periodic distribution rotated at a certain angle; 8. Regular hexagonal (honeycomb-like) periodic distribution; 9. Regular hexagonal (honeycomb-like) periodic distribution rotated at a certain angle; 10. Polarizer; 11. Half-wave plate; 12. Quarter-wave plate; 13. Laser optical path; 14. Galvanometer U-axis and its reflector; 15. Focusing lens; 16. Metal sample material; 17. Three-axis displacement stage; 18. High-pressure air nozzle; 19. Galvanometer V-axis and its reflector; 20. Design drawing of a QR code; 21. Visible light cloaking simulation of a QR code; 22. Infrared pattern simulation of a QR code; 23. Design drawing of a flame; 24. Visible light cloaking simulation of a flame; 25. Infrared pattern simulation of a flame. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0036] This invention proposes a design method for a metasurface with tunable thermal radiation micropores. The tunable thermal radiation micropore metasurface of this invention is composed of a series of periodic deep-hole cavities fabricated on the surface of a metallic material, with each period defined as a thermal radiation metasurface unit. The geometry of each deep-hole cavity can be independently controlled, thereby arranging it into a two-dimensional thermal illusion or infrared encryption pattern. For example, as... Figure 1 As shown, infrared patterns resembling the letter "A" are arranged according to the different thermal emissivity of the holes. Since visible light imaging is not easily able to discern processing depth, the letter "A" is invisible under visible light. Furthermore, to achieve angle-independent radiation characteristics, the deep hole shape of the metasurface unit is a rotationally symmetric structure. The deep hole shape can be, but is not limited to, rotationally symmetric structures such as cylindrical or conical holes, etc. Figure 2 As shown. Furthermore, the periodic arrangement of the thermal radiation metasurface units can take many forms, including, but not limited to, rectangular (grid-like) or regular hexagonal (honeycomb) periodic arrangements, such as... Figure 3 As shown.

[0037] By using high-degree-of-freedom laser subtractive processing, the geometric dimensions of each metasurface unit (including the taper angle, diameter, and depth of the hole) can be independently controlled, thereby achieving independent control of the thermal emissivity of each thermally radiating metasurface unit.

[0038] The principle behind enhanced local thermal emissivity: Based on the classic design concept of a spherical metallic blackbody cavity, light is reflected back and forth on the inner surface of the blackbody cavity, eventually leading to almost complete absorption, resulting in a thermal emissivity of approximately 1. Similarly, when the depth of a deep cavity reaches a certain level, light entering the cavity is completely absorbed, thus the thermal emissivity can also approach 1, making the deep cavity equivalent to a blackbody cavity. Based on classical thermal radiation theory, assuming no heat exchange between the inside of the deep cavity and the outside environment, the equivalent thermal emissivity of the deep adiabatic cavity can be expressed by the formula... The formula represents the equivalent thermal emissivity ε of the adiabatic cavity. a , the angular coefficient F of the inner surface of the deep cavity to the outside world, and the thermal emissivity of the inner surface of the deep cavity ε0.

[0039] Since the angle factor F is determined by the geometry of the structure, by controlling the geometry (diameter and depth) of the deep cavity, the angle factor F can be reduced, thereby achieving localized control of the equivalent thermal emissivity. Specific theoretical analysis results are as follows... Figure 5 As shown, by controlling the aspect ratio z / D of the deep cavity, the equivalent thermal emissivity ε can be achieved. a The control is defined within the range [ε0, 1], where ε0 represents the thermal emissivity of the inner surface of the deep cavity. In other words, the surface structure of the deep cavity enhances infrared radiation and light absorption.

[0040] Adopting such Figure 4 The laser processing optical path system based on galvanometer scanning shown is used to process deep hole cavities. After beam expansion and collimation, the laser beam passes through a polarizer 10, a half-wave plate 11, and a quarter-wave plate 12, allowing adjustment of the laser polarization state. In this embodiment, the laser polarization state is set to circular polarization to eliminate the anisotropy of the laser ablation material. Through the movement of the galvanometer, the laser optical path 13 achieves rapid and efficient laser scanning on the surface of the metal sample material 16 via the focusing lens 15. Based on experimental design and empirical methods, a set of laser scanning processing parameters (mainly including energy density, scanning speed, and fill volume) is selected, and the depth of single-layer processing is measured as z. L To achieve accurate depth control, the relationship between machining depth and the number of machining layers was experimentally calibrated, which can be expressed by the linear fitting expression z = kN. L +b represents the formula, where z is the total machining depth, the slope k is the machining depth per layer, and the intercept b is the depth offset.

[0041] To achieve a deep-hole cavity structure with a high aspect ratio, a high-speed laser scanning processing method based on a galvanometer is adopted. The specific process is as follows:

[0042] Step 1: The sample is a metallic material. Fix the sample on the triaxial displacement stage 17, set the laser polarization state, and perform laser focusing.

[0043] Step 2: Based on the morphology of different deep holes, adjust and select the appropriate laser scanning processing parameter set (energy density, scanning speed, filling amount), set the number of layers to be processed and the scanning method, and write the processing program.

[0044] Step 3: Position the laser processing area, check the settings, open the high-pressure air valve, and align the high-pressure air nozzle 18 with the laser processing area.

[0045] Step 4: Begin cyclic processing. After scanning one layer, the laser focus moves a certain distance along the optical axis, causing the focus to shift downwards. The feed rate is equal to the processing depth of each layer to ensure the laser focus is always on the material surface.

[0046] Step 5: After processing is complete, close the high-pressure air valve and remove the sample.

[0047] Step 6: Clean the sample with anhydrous ethanol (alcohol) using ultrasound to complete the preparation process.

[0048] Based on the aforementioned principle of enhanced local thermal emissivity and deep-hole cavity fabrication, infrared pattern design with visible light cloaking and multi-level infrared radiation temperatures can be achieved. The principle of visible light cloaking utilizes the depth-of-field limitation of a camera, where holes of different depths cannot be distinguished in a visible light image. Therefore, by fixing the hole diameter to D and changing the hole depth z, infrared patterns with visible light cloaking and multi-level infrared radiation temperatures can be achieved. When using a frustum-shaped deep hole, a fixed taper is preferred. Since the hole depth significantly enhances thermal emissivity, it enables infrared imaging.

[0049] In one specific embodiment of the present invention, a two-level temperature infrared pattern (infrared encrypted QR code pattern) is taken as an example, such as... Figure 7 Figures 20-22 show the infrared pattern design drawing, simulated visible light photograph, and infrared pattern, respectively. The sample material is titanium alloy. The specific preparation steps of this metasurface are as follows:

[0050] Step 1: Design the QR code drawing. In this embodiment, a periodic arrangement of regular hexagons is chosen. The diameter of the holes is set to D, but they have different thermal emissivity. The period is P = 2D. The thermal emissivity of the white holes is ε1, and the thermal emissivity of the black holes is ε2, satisfying ε1 < ε2. According to... Figure 5 Based on the theoretical analysis of the equivalent thermal emissivity of the deep-hole cavity shown, the preferred size of the cylindrical or conical hole is determined.

[0051] Step 2: For the selected titanium alloy material, calibrate the laser processing depth, set the laser processing parameter group (energy density, scanning speed, fill volume), and adjust the number of processing layers to achieve the target size.

[0052] Step 3: Based on the shape of the selected circular hole, write the laser processing program. To achieve better visible light cloaking and infrared pattern display effects, the laser processing parameters will be finely adjusted appropriately, with the goal of making the visible light appearance of the shallow white circular hole the same as that of the deep black circular hole.

[0053] Step 4: Fix the metal sample on the triaxial displacement stage, focus with the laser, check the parameter settings, and open the high-pressure air valve.

[0054] Step 5: Start the laser processing process. After scanning one layer, the laser focus moves a certain distance along the optical axis. The processing is repeated until the specified number of layers is reached, and then the processing ends.

[0055] Step 5: After processing is complete, close the high-pressure air valve and remove the sample.

[0056] Step 6: Clean the sample with anhydrous ethanol (alcohol) in an ultrasonic bath for about 5 to 10 minutes to complete the preparation process.

[0057] Step Seven: Visible Light Invisibility Test. Capture its visible light pattern using a camera, such as... Figure 7 21 shown.

[0058] Step 8: Infrared Pattern Stealth Test. Place the sample on a heating plate and heat it to 100°C. Capture its infrared pattern using an infrared camera. Figure 7 22 shown.

[0059] In one specific embodiment of the present invention, a multi-level temperature infrared pattern (a flame pattern for thermal camouflage) is taken as an example, such as... Figure 7 Figures 23-25 ​​show the infrared pattern design drawing, simulated visible light photograph, and infrared pattern, respectively. The sample material chosen is titanium alloy. The specific preparation steps of this metasurface are as follows:

[0060] Step 1: Design the flame diagram. In this example, a periodic hexagonal arrangement is chosen. All small holes have the same diameter D but different thermal emissivity, with a period of P = 2D. The thermal emissivity of the white holes is ε1, the black holes is ε2, and the gray holes is ε3, satisfying ε1 < ε2 < ε3. Based on... Figure 5 Based on the theoretical analysis of the equivalent thermal emissivity of the deep-hole cavity shown, the preferred size of the cylindrical or conical hole is determined.

[0061] Step 2: For the selected titanium alloy material, calibrate the laser processing depth, set the laser processing parameter group (energy density, scanning speed, fill amount), and preferably use cross scanning as the laser scanning method. Adjust the number of processing layers to achieve the target size.

[0062] Step 3: Based on the shape of the selected circular hole, write the laser processing program. To achieve better visible light stealth and infrared pattern display effects, the laser processing parameter group will be finely adjusted appropriately, with the goal of making the visible light appearance of white, black, and gray circular holes with different thermal emissivity the same.

[0063] Step 4: Fix the metal sample on the triaxial displacement stage, focus with the laser, check the parameter settings, and open the high-pressure air valve.

[0064] Step 5: Start the laser processing process. After scanning one layer, the laser focus moves a certain distance along the optical axis. The processing is repeated until the specified number of layers is reached, and then the processing ends.

[0065] Step 5: After processing is complete, close the high-pressure air valve and remove the sample.

[0066] Step 6: Clean the sample with anhydrous ethanol (alcohol) in an ultrasonic bath for about 5 to 10 minutes to complete the preparation process.

[0067] Step Seven: Visible Light Invisibility Test. Capture its visible light pattern using a camera, such as... Figure 7 24 shown.

[0068] Step 8: Infrared Pattern Stealth Test. Place the sample on a heating plate and heat it to 100°C. Capture its infrared pattern using an infrared camera. Figure 7 25 shown.

[0069] The above examples are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments and many variations are possible. All variations that can be directly derived or conceived by those skilled in the art from the disclosure of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a thermal radiation microporous metasurface for thermal camouflage and infrared encryption, characterized in that, Preparation methods include: Step 1: Fix the metal sample on a triaxial displacement stage, set the laser polarization state and focus; the surface of the metal sample can be a plane or a freeform surface. Step 2: Pre-design the dimensions of a periodically arranged deep hole structure on the surface of the metal material, ensuring at least two different thermal emissivity. The deep holes are rotationally symmetric, and the higher the depth-to-width ratio of the deep hole cavity, the higher the thermal emissivity. The thermal emissivity of the deep holes is independently controlled by adjusting the depth-to-width ratio of the deep hole cavity. During adjustment, the upper opening diameter of the deep holes is kept consistent, and only the depth or shape of the deep holes is changed. The deep holes are blind hole structures, employing one or more of cylindrical, conical, and frustum shapes. The periodic arrangement can be either grid-like or honeycomb-like. Adjust the laser scanning processing parameters, including energy density, scanning speed, and number of processing layers; Step 3: Align the laser with the area to be processed on the surface of the metal sample, open the high-pressure air valve, and align the high-pressure air nozzle with the area to be processed. Step 4: After scanning one layer with the laser, the laser focus moves along the optical axis, causing the focus to move downwards. The feed amount of the downward movement of the focus is equal to the processing depth of each layer. This process is repeated until the preset number of processing layers is completed. Step 5: Close the high-pressure air valve and remove the metal material sample; Step 6: Perform ultrasonic cleaning on the metal material sample to obtain a thermal radiation microporous metasurface, which is composed of deep hole structures periodically arranged on the surface of the metal material; the combination of at least two deep holes with different thermal emissivity on the thermal radiation microporous metasurface can obtain thermal phantom or infrared encryption pattern. The laser scanning processing is achieved using a laser processing optical path system based on galvanometer scanning. The laser processing optical path system includes a polarizer, a half-wave plate, a quarter-wave plate, a galvanometer, a focusing lens, a triaxial displacement stage, and a high-pressure air nozzle. The polarizer, half-wave plate, quarter-wave plate, galvanometer, and focusing lens are arranged sequentially along the laser optical path. The high-pressure air nozzle is installed on one side of the triaxial displacement stage. The laser is focused onto the processing area on the surface of the metal material sample fixed on the triaxial displacement stage by the movement of the galvanometer.

2. The application of the thermal radiation microporous metasurface prepared according to claim 1 in thermal camouflage and infrared encryption technology, characterized in that, Patterns formed by periodically arranged deep holes are processed onto the surface of metal materials that require thermal camouflage or infrared encryption. The patterns are invisible under visible light but visible under infrared light.

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