Manufacturing method of frequency selective surface based on femtosecond laser induced graphene

Abstract

A frequency selection surface manufacturing method based on femtosecond laser induced graphene belongs to the technical field of frequency selection surface manufacturing and comprises the following steps: (1) graphene patterning: forming a graphene periodic micro-nano structure on the surface of a flexible substrate through induction by femtosecond laser; (2) a frequency selection unit structure design step: a nested resonance structure is obtained by layering and stacking graphene resonance units with different geometrical shapes, and selective transmission or reflection of multi-band electromagnetic waves is realized; the graphene resonance unit is the graphene periodic micro-nano structure; and (3) a large-area manufacturing step: processing a frequency selection surface with the area larger than 10cm < 2 > by adopting femtosecond laser dynamic scanning and energy regulation and control technologies. According to the invention, the local structure modification of the graphene is induced by femtosecond laser, and the metasurface design is combined, so that the large-area and multi-band frequency selection characteristic is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of frequency selective surface manufacturing, and particularly relates to a manufacturing method of a frequency selective surface based on femtosecond laser induced graphene. Background Art

[0002] Frequency selective surface (FSS) is an artificial structure that can selectively regulate electromagnetic waves (such as reflecting or transmitting specific frequency bands), and is widely used in fields such as radar stealth, communication antennas, and electromagnetic shielding. With the growth of the demand for multi-band, reconfigurability, and lightweight in modern electronic systems, the development of efficient, low-cost, and multi-band FSS manufacturing technologies has become a research hotspot.

[0003] Traditional FSS manufacturing methods (such as photolithography, electron beam lithography, nanoimprinting) have significant limitations. They require multiple steps (such as coating, masking, etching, etc.), are costly and time-consuming. And they rely on metal or semiconductor materials, making it difficult to achieve flexible or transparent devices. Multi-band FSS requires multi-layer stacking or complex unit designs, and traditional processes are difficult to ensure accuracy and consistency. Chemical etching is involved in the preparation process, resulting in pollution.

[0004] Graphene has become an ideal alternative material due to its high conductivity, mechanical flexibility, and tunability. However, its traditional preparation methods (such as chemical vapor deposition CVD) require high-temperature and high-pressure environments, and are prone to breakage when transferred to flexible substrates. Femtosecond laser induced graphene (FLIG) technology directly carbonizes on the polymer surface through an ultrafast laser to generate a porous graphene structure, which has the following advantages:

[0005] Maskless: Directly program the laser path to achieve rapid writing of complex patterns.

[0006] Environmentally friendly: It is carried out at room temperature and normal pressure, and is compatible with flexible substrates (such as polyimide).

[0007] High precision and controllability: The extremely short pulse of the femtosecond laser ( seconds) reduces thermal damage and can regulate the conductivity of graphene.

[0008] However, when the existing FLIG technology is applied to FSS, the following challenges are still faced:

[0009] Multi-band design: Complex micro-nano structures are required, and traditional laser processing is difficult to achieve efficiently.

[0010] Large-area uniformity: The laser scanning strategy and parameter control affect the structural consistency.

[0011] Performance optimization: The coupling mechanism between the conductivity of graphene, the pattern geometry, and the electromagnetic response needs to be studied in depth.

[0012] Currently, in the manufacturing method of large-area multi-band frequency selective surfaces, the following solutions also exist:

[0013] (1) Laser-induced graphene (LIG) for FSS: Graphene is generated on the polyimide surface using a CO2 laser or a nanosecond laser to form a conductive pattern. For example, a certain study prepared a single-frequency FSS by adjusting the laser power and scanning speed. However, limited by the laser type, the processing accuracy is low, and multiple patterning is required for multiple bands, making it difficult to cover multiple bands.

[0014] (2) Multilayer stacked metal FSS: Metal layers with different frequency bands are prepared separately by lithography and then stacked layer by layer to achieve multi-band response. For example, a certain patent uses three layers of metal patches corresponding to the Ku, K, and Ka bands respectively.

[0015] (3) Tunable FSS based on variable materials: Utilize liquid crystal, phase change materials, or voltage to regulate the Fermi level of graphene to achieve dynamic adjustment of the FSS frequency. For example, a certain scheme integrates a drive circuit on the graphene pattern and changes the resonance frequency through a bias voltage.

[0016] However, the following defects still exist in the above methods:

[0017] (1) Laser-induced graphene (LIG) for FSS: The thermal effect of the nanosecond laser is significant, and the edge accuracy is insufficient; multi-frequency design depends on the complexity of the unit structure, making it difficult to process uniformly over a large area.

[0018] (2) Multilayer stacked metal FSS: The process is complex, and the layer alignment error affects the performance; the metal structure cannot be dynamically tuned, and the weight and thickness increase.

[0019] (3) Tunable FSS based on variable materials: It depends on an external control system, and the structure is complex; it is difficult to ensure large-area uniformity, and it is difficult to regulate multiple independent frequency bands. Summary of the Invention

[0020] The present invention provides a manufacturing method for a frequency selective surface based on femtosecond laser-induced graphene to overcome the defects in the prior art.

[0021] To solve the above technical problems, the present invention provides the following technical solutions: A manufacturing method for a frequency selective surface based on femtosecond laser-induced graphene, including:

[0022] (1) Graphene patterning step: Use femtosecond laser to induce the formation of periodic graphene micro-nano structures on the surface of a flexible substrate;

[0023] (2) Frequency selective unit structure design step: By hierarchically stacking graphene resonance units with different geometric shapes, a nested resonance structure is obtained to achieve selective transmission or reflection of multi-band electromagnetic waves; the graphene resonance unit is the above-mentioned periodic graphene micro-nano structure;

[0024] (3) Large-area manufacturing step: Using femtosecond laser dynamic scanning and energy regulation technology to achieve the processing of frequency selective surfaces with an area greater than 10 cm 2 of frequency selective surfaces.

[0025] Further, the graphene periodic micro-nano structure is a grid-like pattern, and the grid unit is square, cross-shaped, double circular ring or fractal branch.

[0026] Further, the square resonant unit is used to reflect low-frequency electromagnetic waves, the cross-shaped resonant unit is used to transmit medium-frequency electromagnetic waves, the double circular ring resonant unit is used to reflect high-frequency electromagnetic waves of different frequencies, and the fractal branch resonant unit is used to transmit millimeter-wave electromagnetic waves.

[0027] Further, the nested resonant structure adopts a hierarchical design, and each layer adopts a structure of a dielectric substrate and a metal layer. The dielectric substrate is the flexible substrate described above, and the metal layer refers to the graphene periodic micro-nano structure described above.

[0028] Further, the FSS unit structure obtained in step (3) includes a multi-layer structure from bottom to top: 1) flexible substrate; 2) graphene resonant unit layer, composed of graphene periodic micro-nano structures induced by laser; 3) dielectric layer, composed of zinc oxide nanowire arrays covering the flexible substrate on which graphene periodic micro-nano structures have been formed; 4) protective layer.

[0029] Further, before the graphene patterning process, the flexible substrate needs to be pretreated. The flexible substrate is ultrasonically cleaned with ethanol and acetone, and then the flexible substrate is carbonized by femtosecond laser direct writing using a femtosecond laser galvanometer processing platform to generate graphene periodic micro-nano structures.

[0030] Further, when performing laser direct writing on the femtosecond laser galvanometer processing platform, multi-band electromagnetic responses are carried out, including conductivity regulation and geometric parameter regulation.

[0031] Further, the conductivity regulation process: 1) Set the laser power gradient to perform local annealing or etching on the graphene surface; 2) Quantitatively characterize the defect density through Raman spectroscopy; 3) Measure the Hall effect or use the four-probe method to obtain the conductivity σ, and establish the σ-P laser relationship curve; 4) Process the FSS unit and test the S parameters to verify the variation law of f r with σ.

[0032] Further, the geometric parameter regulation process: 1) Based on the target frequency band, calculate the initial size D D through the formula λ = c / f r where λ is the resonant wavelength corresponding to the target frequency band, c is the speed of light in vacuum, and f ris the target resonance frequency; 2) Optimize the shape and size of the FSS unit using full-wave simulation to ensure no conflict in multi-band resonance; 3) Manufacture FSS units of different sizes through laser direct writing or photolithography processes; 4) Measure the frequency response curve and iteratively adjust the size DD until the target frequency band is covered.

[0033] The beneficial effects achieved by the present invention are as follows: Through the local structure modification of graphene induced by femtosecond laser and combined with the metasurface design, the present invention realizes the frequency selection characteristics of large area and multi-bands (such as microwave, terahertz, infrared). The present invention has the following advantages:

[0034] ① Multi-band compatibility: Multiple frequency bands can be independently regulated on a single panel, reducing the complexity of traditional multi-layer structures.

[0035] ② High-precision processing: Femtosecond laser realizes sub-micron scale structure etching, avoiding the pollution problems of mechanical or chemical etching.

[0036] ③ Flexibility: Based on the flexible substrate of graphene, it is suitable for curved conformal design (such as smart skin, 5G antenna radome).

[0037] ④ Large area and high efficiency: The scanning speed > 100 mm / s, and the processing area reaches 30×30 cm².

[0038] ⑤ Environmental protection and low cost: No mask and chemical etching are required. Description of the Drawings

[0039] The drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation to the present invention. In the drawings:

[0040] Figure 1 is a schematic diagram of processing a large-area array pattern on a flexible substrate using a femtosecond laser galvanometer processing platform with a square fractal branch resonant unit as an example in an embodiment of the present invention.

[0041] Figure 2 is a schematic diagram of processing a large-area array pattern on a flexible substrate using a femtosecond laser galvanometer processing platform with a square resonant unit as an example in an embodiment of the present invention.

[0042] Figure 3 is the Raman spectrum described in the embodiment.

[0043] Figure 4 is the frequency response curve described in the embodiment. Detailed Embodiments

[0044] The following describes the preferred embodiments of the present invention with reference to the drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention and are not used to limit the present invention.

[0045] The manufacturing method of the frequency selective surface based on femtosecond laser-induced graphene of the present invention includes the following steps:

[0046] The first step, graphene patterning: Utilize the high-precision processing ability of femtosecond laser to induce periodic micro-nano structures of graphene on the surface of a flexible substrate (such as polyimide);

[0047] The periodic micro-nano structures of graphene are grid-like patterns, and the grid units are such as square, cross, double circular ring, and fractal branch. The periodic micro-nano structures of graphene constitute resonant units. By designing their geometric shapes, sizes, and periods, resonant responses (reflection or transmission) to electromagnetic waves of specific frequencies can be generated, thereby realizing the frequency selection function.

[0048] The geometric parameters of the graphene resonant unit and the corresponding frequency bands are shown in Table 1 below, and the unit period of the graphene resonant unit is 10 - 11 mm, and the line width is 0.05 - 0.1 mm.

[0049] Table 1 Corresponding table of geometric parameters of graphene resonant unit and corresponding frequency bands

[0050]

[0051] The second step, structural design of the frequency selection unit: By hierarchically stacking graphene resonant units of different geometric shapes, a nested resonant structure is obtained to achieve selective transmission or reflection of multi-band electromagnetic waves.

[0052] The nested resonant structure adopts a hierarchical design. Each layer adopts a structure of dielectric substrate + metal layer. The dielectric substrate is the above-mentioned flexible substrate, and the metal layer refers to the above-mentioned periodic micro-nano structures of graphene. The "metal layer" is a functional description of the periodic micro-nano structures of graphene. The distance between adjacent dielectric substrates is d = 0.5 mm, which is realized through a support structure or an air layer. Each layer is in turn from bottom to top:

[0053] Layer 1 (bottom layer): Dielectric substrate + square ring metal layer (reflect low-frequency 2.4 GHz);

[0054] Layer 2: Dielectric substrate + cross patch metal layer (transmit medium-frequency 5.8 GHz);

[0055] Layer 3: Dielectric substrate + double circular ring metal layer (reflect high-frequency 10 GHz and 18 GHz);

[0056] Layer 4 (top layer): Dielectric substrate + fractal branch metal layer (transmit millimeter wave 15 GHz).

[0057] Step 3, large-area manufacturing: Utilize the dynamic scanning and energy regulation technology of femtosecond lasers to ensure the consistency and efficiency of large-area (greater than 10 cm²) processing.

[0058] Through the above preparation process, the FSS unit structure prepared in this embodiment includes a multi-layer structure from bottom to top:

[0059] 1. Substrate: Flexible polyimide film, with a thickness of 10 - 50 μm;

[0060] 2. Graphene resonant unit layer: Composed of periodically arranged graphene micro-nano structures induced by lasers;

[0061] 3. Dielectric layer: Zinc oxide nanowire array, used to enhance dielectric regulation;

[0062] 4. Protective layer: Polydimethylsiloxane (PDMS) encapsulation layer, with a thickness < 5 μm, which plays a protective role to prevent the prepared FSS unit structure from being affected by external environmental factors (such as moisture, mechanical damage, etc.) and maintain its stable performance.

[0063] Among them, the dielectric layer covers the flexible substrate with the formed graphene periodic micro-nano structures, and the three form a stacked structure. Common methods for covering the dielectric layer on the micro-nano structures include physical deposition methods (such as electron beam evaporation, magnetron sputtering), chemical deposition methods (chemical vapor deposition, solution spin coating method), and self-assembly methods. In the self-assembly method, by utilizing the interactions between nano-materials (such as electrostatic force, van der Waals force, etc.), zinc oxide nanowires are spontaneously assembled and arranged on the surface of the graphene micro-nano structures in a solution environment to form an ordered dielectric layer.

[0064] In the above preparation process, before the graphene patterning treatment in the first step, the substrate needs to be pretreated. The PI film is ultrasonically cleaned with ethanol and acetone to remove impurities such as oil stains and dust on the surface of the PI film, improve the surface cleanliness and adhesion, and then use a femtosecond laser galvanometer processing platform to directly write with femtosecond lasers to carbonize the PI film to generate graphene periodic micro-nano structures.

[0065] The femtosecond laser galvanometer processing platform is a high-precision processing equipment based on ultrafast laser technology and a high-speed galvanometer scanning system. Its core components include:

[0066] 1. Femtosecond laser light source, parameters: Pulse width < 1 ps (usually 100 - 500 fs), wavelength 1030 nm or 515 nm (frequency-doubled), repetition frequency 10 kHz - 1 MHz. Function: Achieve "cold processing" through an ultrashort pulse width, reduce the heat-affected zone (HAZ), and is suitable for brittle materials and high-precision micro-nano processing.

[0067] 2. Galvo Scanner, composed of XY high-speed galvanometer, F-theta focusing lens, dynamic focusing module. Performance: scanning speed> 2 m / s, positioning accuracy ±1μm, field of view range 100×100 mm²~300×300 mm².

[0068] 3. Motion control platform, multi-axis linkage: Z-axis lifting platform (accuracy ±0.5μm) + rotation axis (optional), used for three-dimensional structure processing. Synchronous control: galvanometer and motion platform coordinated control to achieve large-area splicing and complex surface processing.

[0069] 4. Optical path and cooling system, optical path module: beam expander, energy attenuator, beam shaper (such as DOE). Cooling system: water cooling / air cooling, to ensure stable operation of the laser, temperature control accuracy ±0.1°C.

[0070] 5. Monitoring and feedback system, real-time monitoring: confocal sensor or CCD camera for processing position calibration and defect detection. Closed-loop control: adjust laser parameters (power, focus depth) and scanning path through feedback.

[0071] The present invention utilizes a laser galvanometer platform to achieve dynamic focusing, improve the aspect ratio processing capability (the side wall steepness of deep holes / grooves is increased by >30%), and support vertical stacking processing of multi-layer FSS structures; through multi-axis collaboration, complex three-dimensional structures (such as fractal branches and double ring nesting) can be formed in one go to reduce splicing errors; during the processing, the laser energy (10~500μJ) and pulse train mode (Burst Mode) are adjusted in real time based on the material reflectivity to improve the processing efficiency of the copper / dielectric substrate and avoid excessive ablation of the metal layer or carbonization of the dielectric; through image recognition technology, processing defects (such as edge burrs and microcracks) are automatically detected to improve the yield rate.

[0072] Specifically in this embodiment, during the laser-induced graphene patterning process, the laser parameters are: wavelength 1030nm, pulse width 250~300fs, repetition frequency 500kHz~1MHz; dynamic control: power (10~20W), scanning speed (100~800mm / s), defocus (±50μm); scanning path: spiral progressive filling (to avoid heat accumulation); post-processing: printing ZnO nanowires (temperature 400℃, time 1h); spin coating PDMS encapsulation (rotation speed 3000rpm, curing temperature 80℃).

[0073] like Figure 1 (a) and (b) show the real-time process of processing large-area (30×30 cm) array patterns using a femtosecond laser galvanometer processing platform. Figure 1 (c) and (d) are actual pictures of the processed samples, and it can be seen that the processing consistency is very good.

[0074] Figure 2 In (a) and (b), the physical diagrams of the processed resonant units are shown, Figure 2 As can be seen from the partial enlarged view of (c) in it, the unit structure has a very high consistency.

[0075] When performing laser direct writing on a femtosecond laser galvanometer processing platform, multi-band electromagnetic responses need to be carried out, including conductivity regulation and geometric parameter regulation.

[0076] 1. For conductivity regulation, an increase in laser power leads to a decrease in the defect density of graphene. At this time, the increase in conductivity causes the resonant frequency to shift towards higher frequencies. Therefore, by adjusting the femtosecond laser power, the defect density and conductivity of graphene are changed, and finally the resonant frequency of the frequency selective surface (FSS) is controlled.

[0077] The principle of conductivity regulation is:

[0078] (1) The relationship between laser power and defects: When the laser power is high (such as >5 W), the thermal effect of the femtosecond laser is sufficient to trigger local annealing of graphene, repair lattice defects (such as vacancies and edge disorder), and reduce the defect density; when the laser power is low (such as <2 W), the laser energy is not sufficient to repair the defects, but may increase the defects through ablation or oxidation (such as forming carboxyl or epoxy groups).

[0079] According to the Drude model, the conductivity of graphene is related to the carrier mobility and carrier concentration: a decrease in defect density → a decrease in scattering centers → an increase in carrier mobility and carrier concentration → an increase in conductivity. Therefore, it can be known that when the laser power is high, the conductivity increases, and when the laser power is low, the conductivity decreases.

[0080] (2) The correlation between conductivity and resonant frequency: According to the Drude model, an increase in conductivity reduces the surface impedance of graphene, resulting in a shift of the resonant frequency of the FSS towards higher frequencies (conversely, towards lower frequencies).

[0081] Conductivity regulation process: Step 1: Set the laser power gradient (such as 2~10 W), and perform local annealing or etching on the graphene surface; Step 2: Quantitatively characterize the defect density through Raman spectroscopy (the intensity ratio ID / IG of the D peak and G peak); Step 3: Measure the Hall effect or the four-probe method to obtain the conductivity σ. The relationship between its sweep rate, laser power, and conductivity is shown in Table 2, and establish the σ-P laser of σ-P laser relationship curve; Step 4: Process the FSS unit and test the S parameters to verify the variation law of the target resonant frequency f r with σ.

[0082] Table 2 Relationship between Scanning Speed, Laser Power and Conductivity

[0083] Figure 3 The Raman spectrum is given. In the figure, the core characteristic peaks and their physical meanings are as follows: ‌(1) D peak (~1350 cm -1 )‌, ‌Source‌: Related to carbon ring defects (such as five - membered / seven - membered rings), vacancies, and edge structures, belonging to the double - resonance scattering process (defects are required to break momentum conservation).

[0084] ‌(2) G peak (~1580 cm -1 )‌, ‌Source‌: In - plane stretching vibration of sp² carbon atoms (E2g symmetry), a common peak of all graphene.

[0085] ‌(3) 2D peak (‌G' peak, ~2700 cm -1 )‌, ‌Source‌: Double - phonon resonance scattering peak, a characteristic peak of graphene.

[0086] 2. For the regulation of geometric parameters, the regulation goal: By adjusting the structural size D of the resonant unit, directly control the resonant wavelength λ to cover multi - band requirements. Relationship between resonant wavelength and size: Achieve single - band coverage by changing D (e.g., D1 = 28mm, D1 = 28mm → 2.4 GHz, D2 = 7.2mm, D2 = 7.2mm → 10 GHz). Composite structure design: Nest sub - structures of different sizes (such as a square ring + double - ring) in the same FSS unit to excite multi - frequency resonance (2.4 / 10 / 18 GHz).

[0087] Geometric parameter regulation process: Step 1: Based on the target frequency band (such as 2~18 GHz), calculate the initial size DD of the resonant ring through the formula λ = c / fr, where λ is the resonant wavelength corresponding to the target frequency band, c is the speed of light in vacuum, and f r is the target resonant frequency; Step 2: Use full - wave simulation (CST / HFSS) to optimize the shape and size of the FSS unit to ensure no conflict in multi - band resonance; Step 3: Fabricate FSS units of different sizes through laser direct writing; Step 4: Measure the frequency response curve and iteratively adjust the size DD until the target frequency band is covered.

[0088] Among them, when the target frequency band is 2~18 GHz, the range of the resonant wavelength corresponding to this frequency band (in air) is λ = c / f， Then: f min = 2 GHz, then λmax = c / 2×10 9 ≈150 mm; f max = 18 GHz, then λ min = c / 18×10 9 ≈16.67 mm.

[0089] The target resonant frequency ( f r ) and the initial dimension DD: Here f r refers to the center frequency at which resonance is expected to occur in the desired structure, preset for the preliminary design. DD refers to the characteristic dimension of the key sub-structure that has the most direct relationship with the resonant frequency.

[0090] A more practical initial value setting: Considering that the unit period P needs to be much smaller than the wavelength to avoid grating lobes (usually P < λ / 2, for 2.4 GHz, λ / 2 ≈ 62.5 mm), and the simulation optimization space, the initial value of the side length of the outer square ring can be set within the range of 28 - 32 mm for trial.

[0091] 3. Co - optimization strategy: Material - structure joint design: At a fixed frequency band (such as 5.8 GHz), fine - tune f r (±5%) through conductivity regulation, and then coarsely adjust the geometric parameters to cover adjacent frequency bands.

[0092] The essence of geometric coarse - tuning: It refers to achieving a significant shift in the resonant frequency (usually > 10%) by significantly modifying the key geometric dimensions of the FSS unit (such as more than ±5%), and quickly covering adjacent frequency bands near the target frequency band (such as 5.8 GHz → 5.0 GHz or 6.5 GHz). When performing geometric coarse - tuning, select the dimensions that are linearly sensitive to the resonant frequency and are easy to process for coarse - tuning. For example: For the geometric parameters in Table 1, the arm length L2 of the cross - patch resonant unit can be coarsely adjusted, with an adjustment range of ±(8 - 15)%, and the frequency change range is 5.8 GHz → 5.0 GHz - 6.5 GHz, covering the adjacent frequency band change of 5.8 GHz → 5.0 GHz; Another example is that the side length L1 of the square ring can be coarsely adjusted, with an adjustment range of ±10%, and the frequency change range is 2.4 GHz → 2.2 GHz - 2.6 GHz, covering the adjacent frequency band change of 2.4 GHz → 2.6 GHz.

[0093] The present invention has the following advantages compared with the prior art:

[0094] (1)Single-step large-area manufacturing: Through femtosecond laser direct writing, multi-scale graphene structures are generated on flexible substrates at one time without masks or etching, significantly reducing costs.

[0095] (2)Multi-band integrated design: By adjusting laser parameters to control the microscopic morphology of graphene (such as porosity, number of layers), units with different resonance characteristics are designed in the same plane.

[0096] The physical mechanism of using laser parameters to control the microscopic morphology of graphene is shown in Table 3.

[0097] Table 3 Relationship between sweep rate, laser power and conductivity

[0098]

[0099] In the above table, 1) When the laser is at low power (≤2 W): The photon energy is not enough to completely break the carbon-carbon bonds, and only surface adsorbates are removed or slightly oxidized through local thermal effects, resulting in low porosity.

[0100] 2) When the laser is at medium power (2 - 5 W): The energy breaks through the graphene destruction threshold, and the upper layer in multi-layer graphene is selectively removed by ablation to form controllable holes.

[0101] 3) When the laser power is high (>5 W): The excessive energy density causes carbon atoms to vaporize (sublimation temperature ~4500 °C), triggering intense ablation and forming a disordered porous structure.

[0102] (3)Dynamic tunability potential: The conductivity of graphene can be flexibly adjusted by laser processing parameters or subsequent doping, laying a foundation for realizing tunable FSS in the future.

[0103] It should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention. The terms used in the description of this application are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to this application.

Claims

1. A method for manufacturing a frequency selective surface based on femtosecond laser induced graphene, characterized in that, Including: (1) Graphene patterning step: Using femtosecond laser to induce the formation of periodic graphene micro-nano structures on the surface of a flexible substrate; (2) Frequency selective unit structure design step: By hierarchically stacking graphene resonant units with different geometric shapes, a nested resonant structure is obtained to achieve selective transmission or reflection of multi-band electromagnetic waves; The graphene resonant unit is the aforementioned periodic graphene micro-nano structure; (3) Large-area manufacturing step: Using femtosecond laser dynamic scanning and energy regulation technology, achieve the processing of frequency selective surfaces with an area greater than 10 cm 2 2.

2. The method for manufacturing a frequency selective surface based on femtosecond laser induced graphene according to claim 1, wherein The periodic graphene micro-nano structure is a grid-like pattern, and the grid unit is square, cross-shaped, double circular ring or fractal branch.

3. The method for manufacturing a frequency selective surface based on femtosecond laser induced graphene according to claim 2, characterized in that, The square resonant unit is used to reflect low-frequency electromagnetic waves, the cross-shaped resonant unit is used to transmit medium-frequency electromagnetic waves, the double circular ring resonant unit is used to reflect high-frequency electromagnetic waves of different frequencies, and the fractal branch resonant unit is used to transmit millimeter-wave electromagnetic waves.

4. The method for manufacturing a frequency selective surface based on femtosecond laser induced graphene according to claim 1, characterized in that, The nested resonant structure adopts a hierarchical design, and each layer adopts a structure of a dielectric substrate and a metal layer. The dielectric substrate is the aforementioned flexible substrate, and the metal layer refers to the aforementioned periodic graphene micro-nano structure.

5. The method for manufacturing a frequency selective surface based on femtosecond laser induced graphene according to claim 1, characterized in that, The FSS unit structure obtained in step (3) includes a multi-layer structure from bottom to top: 1) Flexible substrate; 2) Graphene resonant unit layer, composed of periodic graphene micro-nano structures induced by laser; 3) Dielectric layer, formed by covering the flexible substrate with the formed periodic graphene micro-nano structure with an array of zinc oxide nanowires; 4) Protective layer.

6. The method for manufacturing a frequency selective surface based on femtosecond laser induced graphene according to claim 1, wherein Before the graphene patterning process, the flexible substrate needs to be pretreated. The flexible substrate is ultrasonically cleaned with ethanol and acetone, and then through femtosecond laser direct writing using a femtosecond laser galvanometer processing platform, the flexible substrate is carbonized to generate periodic graphene micro-nano structures.

7. The method for manufacturing a frequency selective surface based on femtosecond laser induced graphene according to claim 6, wherein, When performing laser direct writing on the femtosecond laser galvanometer processing platform, multi-band electromagnetic responses are carried out, including conductivity regulation and geometric parameter regulation.

8. The method for manufacturing a frequency selective surface based on femtosecond laser induced graphene according to claim 7, wherein Conductivity regulation process: 1) Set the laser power gradient and perform local annealing or etching on the graphene surface; 2) Quantitatively characterize the defect density through Raman spectroscopy; 3) Measure the Hall effect or use the four-probe method to obtain the conductivity σ , establish σ -P laser relationship curve; 4) Process the FSS unit and test the S parameters to verify the target resonance frequency f r with the conductivity σ variation law.

9. The method for manufacturing a frequency selective surface based on femtosecond laser induced graphene according to claim 7, wherein Geometric parameter regulation process: 1) Based on the target frequency band, calculate the initial size λ = c / f r through the formula DD , where λ is the resonant wavelength corresponding to the target frequency band, c is the speed of light in vacuum, f r is the target resonant frequency; 2) Use full-wave simulation to optimize the shape and size of the FSS unit to ensure no conflict in multi-band resonance; 3) Manufacture FSS units of different sizes through laser direct writing or lithography process; 4) Measure the frequency response curve and iteratively adjust the size DD until the target frequency band is covered.

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