Dual-frequency terahertz metamaterial absorber and method of manufacturing the same

Abstract

The application discloses a dual-frequency terahertz metamaterial absorber, which is arranged by a periodic array of M*N super surface units with a multilayer structure, wherein M and N are positive integers; each super surface unit comprises, from bottom to top, a metal substrate, a first dielectric layer, a first graphene resonance layer, a second dielectric layer and a second graphene resonance layer. The absorber can achieve nearly perfect absorption of terahertz waves in a specific frequency band, has a dynamic tunable function based on graphene Fermi level regulation, can independently adjust the center frequency and absorption intensity of two absorption peaks, and has a simple structure and is easy to prepare and integrate. The application further discloses a preparation method of the dual-frequency terahertz metamaterial absorber.

Description

Technical Field

[0001] This invention belongs to the field of terahertz wave technology, specifically relating to a dual-frequency terahertz metamaterial absorber, and also to a method for preparing a dual-frequency terahertz metamaterial absorber. Background Technology

[0002] Terahertz waves are electromagnetic waves that lie between microwaves and infrared waves, with frequencies ranging from 0.1 THz to 10 THz, corresponding to wavelengths of 30 μm to 3000 μm. Because this band lies at the boundary between microwaves and infrared light, terahertz waves possess the physical characteristics of adjacent frequency bands, exhibiting both the strong penetrating power of microwaves and the wave characteristics of near-infrared radiation. As a crucial research direction in cutting-edge interdisciplinary fields of the 21st century, terahertz technology offers significant advantages such as high security, broad spectral characteristics, strong penetrating power, and sub-millimeter spatial resolution, demonstrating groundbreaking application potential in areas such as precision spectral analysis, high-speed communication systems, biomedical imaging, and the development of novel sensors.

[0003] Metamaterials are artificially designed unit structures using existing materials, typically composed of subwavelength-scale artificial atoms, capable of performing specific electromagnetic functions. Thanks to their high designability, metamaterials provide an effective way to control the propagation characteristics of electromagnetic waves and have shown significant application potential in electromagnetic absorption, polarization modulation, imaging, and sensing. With the rapid development of terahertz technology, terahertz metamaterial absorbers based on metamaterials have gradually become a research hotspot due to their compact structure, ease of integration, and flexible controllable absorption performance.

[0004] As research on terahertz metamaterial absorbers continues to deepen, the following dilemma has emerged: Once the design and parameters of a terahertz metamaterial absorber made of traditional materials are determined, due to the limitations of its material properties, it can only maintain a high absorption rate at a certain frequency and cannot be dynamically tuned. Summary of the Invention

[0005] The first objective of this invention is to provide a dual-frequency terahertz metamaterial absorber that can achieve near-perfect absorption of terahertz waves in a specific frequency band, has a dynamically tunable function based on graphene Fermi level modulation, can independently adjust the center frequency and absorption intensity of the two absorption peaks, and has a simple structure that is easy to fabricate and integrate.

[0006] The second objective of this invention is to provide a method for fabricating a dual-frequency terahertz metamaterial absorber. By synergistically controlling the structural parameters and the chemical potential of graphene, the method achieves efficient absorption and dynamic tuning of terahertz waves while ensuring stable absorption performance over a wide azimuth range. This method can meet the application requirements of multi-frequency terahertz detection, communication, and other scenarios.

[0007] The first technical solution adopted in this invention is a dual-frequency terahertz metamaterial absorber, which is composed of M×N metasurface units with multilayer structures arranged in a periodic array, where M and N are both positive integers; a single metasurface unit includes a metal substrate, a first dielectric layer, a first graphene resonant layer, a second dielectric layer, and a second graphene resonant layer arranged sequentially from bottom to top.

[0008] The invention is further characterized in that: The metal substrate, the first dielectric layer, and the second dielectric layer all have square rectangular parallelepiped structures in the xy plane. The side lengths of the cross sections of the metal substrate, the first dielectric layer, and the second dielectric layer are all equal, ranging from 8 μm to 8.3 μm. The thickness t of the metal substrate is 0.2 μm to 0.5 μm. The thickness H1 of the first dielectric layer is 8.8 μm to 9.1 μm. The thickness H2 of the second dielectric layer is 3.0 to 3.5 μm.

[0009] The metal substrate is made of gold and has an electrical conductivity of 4.1 × 10⁻⁶. 7 S / m; Both the first and second dielectric layers are made of silicon dioxide, and both have a relative permittivity of 3.9.

[0010] The first graphene resonant layer is a square ring structure composed of a single layer of graphene. The outer frame is square with a side length a of 7.2 μm to 7.7 μm. The inner frame is square with a side length d of 6.2 μm to 6.7 μm. The width D of the square ring is 1.0 μm. The center of the first graphene resonant layer coincides with the center of the metasurface unit.

[0011] The second graphene resonant layer is a regular hexagonal structure composed of a single layer of graphene, with the circumcircle radius R of the hexagon ranging from 1.6 μm to 1.8 μm; the center of the second graphene resonant layer coincides with the center of the metasurface unit; the thickness of the second graphene resonant layer is 0.34 nm.

[0012] The second technical solution adopted in this invention is the preparation method of the above-mentioned dual-frequency terahertz metamaterial absorber, specifically as follows: Step 1: Preparation of the metal substrate; Step 2: Deposition of the first dielectric layer; Step 3: Preparation of the first graphene resonant layer; Step 4: Deposition of the second dielectric layer; Step 5: Preparation of the second graphene resonant layer.

[0013] The invention is further characterized in that: Step 1 is as follows: Step 1.1, Substrate cleaning: High-purity silicon was selected as the substrate and ultrasonically cleaned with acetone, ethanol and deionized water for 10 min-15 min in sequence to remove surface contaminants; after drying with nitrogen, it was dried in an oven at 110℃-120℃ for 30 min-40 min. Step 1.2, Metal Thin Film Deposition: Gold thin films were deposited using electron beam evaporation. Step 1.3, Annealing treatment: Annealing at 250℃-300℃ for 30min-40min yields a metal substrate; Step 2 is as follows: Step 2.1, Preparation of the dielectric layer thin film: Silicon dioxide was deposited on a metal substrate using plasma-enhanced chemical vapor deposition. Step 2.2, Curing treatment: The first dielectric layer is obtained by curing at 150℃-160℃ for 1-2 hours.

[0014] Step 3 specifically involves: Step 3.1, Graphene Growth and Transfer: A single layer of graphene was grown on copper foil using chemical vapor deposition and then transferred to the surface of the first dielectric layer using a wet process. Step 3.2, Graphene Patterning: Electron beam lithography is used to define a pattern that matches the square ring structure, and then oxygen plasma etching is used to remove excess areas to form a square ring graphene structure, thus obtaining the first graphene resonant layer. Step 4 specifically involves: Step 4.1, Preparation of silicon dioxide thin film: A silicon dioxide thin film was deposited on the first graphene resonant layer using chemical vapor deposition. Step 4.2, Curing treatment: The second dielectric layer is obtained by curing at 150℃-160℃ for 1-2 hours. Step 5 specifically involves: Step 5.1, Graphene Growth and Transfer: A single layer of graphene was grown on copper foil again using chemical vapor deposition and then transferred to the surface of the second dielectric layer by a wet process. Step 5.2, Graphene Patterning: Electron beam lithography is used to define a pattern that matches the hexagonal structure, and then oxygen plasma etching is used to remove excess areas to form a hexagonal graphene structure, thus obtaining the second graphene resonant layer.

[0015] The beneficial effects of this invention are: (1) The dual-frequency terahertz metamaterial absorber of the present invention achieves near-perfect absorption in both 1.24THz and 4.23THz bands through the synergistic effect of two patterned graphene resonant layers; by utilizing the tunable characteristics of graphene Fermi level, the center frequency and absorption intensity of the two absorption peaks can be independently controlled to realize the dynamic tuning function of dual bands; the structure adopts a planar periodic unit design, and the simulation results show that it still has stable absorption performance in a wide azimuth range, and the structure is simple, easy to prepare and integrate.

[0016] (2) The dual-frequency terahertz metamaterial absorber of the present invention has a simple structure and mature technology. It can be applied to terahertz detection, spectrum imaging, electromagnetic stealth and other scenarios, and has good engineering application value. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the periodic structure of Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the unit structure of Embodiment 1 of the present invention; Figure 3 This is a schematic diagram of the structure of the first graphene resonant layer in Embodiment 1 of the present invention; Figure 4 This is a schematic diagram of the structure of the second graphene resonant layer in Embodiment 1 of the present invention; Figure 5 This is the absorption spectrum of the terahertz metamaterial absorber in Embodiment 1 of the present invention; Figure 6 This is a diagram showing the relative impedance characteristics of the terahertz metamaterial absorber in Embodiment 1 of the present invention; Figure 7 The diagram shows the electric field distribution of the graphene resonant layer unit in the absorber of Embodiment 1 of the present invention at 1.24 THz. Figure 8 The diagram shows the electric field distribution of the graphene resonant layer unit in the absorber of Embodiment 1 of the present invention at 1.24 THz. Figure 9 The diagram shows the electric field distribution of the graphene resonant layer unit in the absorber of Embodiment 1 of the present invention at 4.23 THz. Figure 10 The diagram shows the electric field distribution of the graphene resonant layer unit in the absorber of Embodiment 1 of the present invention at 4.23 THz. Figure 11 The curves showing the effect of different first layer thicknesses H1 on the absorption rate in the absorbers of Embodiments 1-3 of the present invention are shown. Figure 12 The curves showing the effect of different thicknesses H2 of the second dielectric layer on the absorption rate in the absorbers of Embodiments 1-3 of the present invention are shown. Figure 13This is a graph showing the absorption rate of the absorber as a function of the square ring width D, under the same conditions as the absorber in Example 1. Figure 14 The absorption rate diagrams are for different side lengths a of the outer frame of the first graphene resonant layer in the absorbers of embodiments 1-3 of the present invention. Figure 15 The diagram shows the absorption rates of different second graphene resonant layer radii R in the absorbers of Examples 1-3 of this invention. Figure 16 The absorption rate diagram for the absorber of Embodiment 1 of the present invention is shown when the chemical potential of the first graphene resonant layer is fixed at 0.4 eV. Figure 17 The absorption rate diagram for the absorber of Embodiment 1 of the present invention is shown when the chemical potential of the first graphene resonant layer is fixed at 0.5 eV. Figure 18 The absorption rate diagram for the absorber of Embodiment 1 of the present invention is shown when the chemical potential of the first graphene resonant layer is fixed at 0.6 eV. Figure 19 The absorption rate diagram for the absorber of Embodiment 1 of the present invention is shown when the chemical potential of the first graphene resonant layer is fixed at 0.7 eV. Figure 20 The absorption rate diagram for the absorber of Embodiment 1 of the present invention is shown when the chemical potential of the second graphene resonant layer is fixed at 0.6 eV. Figure 21 The absorption rate diagram for the absorber of Embodiment 1 of the present invention is shown when the chemical potential of the second graphene resonant layer is fixed at 0.7 eV. Figure 22 The absorption rate diagram for the absorber of Embodiment 1 of the present invention is shown when the chemical potential of the second graphene resonant layer is fixed at 0.8 eV. Figure 23 The absorption rate diagram for the absorber of Embodiment 1 of the present invention is shown when the chemical potential of the second graphene resonant layer remains unchanged and the chemical potential of the graphene resonant layer is 0.9 eV. Figure 24 The electric field distribution of the absorber in Embodiment 1 of the present invention varies with the polarization angle; Figure 25 The electric field distribution of the absorber under TE polarization as a function of the incident angle in Embodiment 1 of the present invention; Figure 26 This is the electric field distribution of the absorber TM under polarization as a function of the incident angle in Embodiment 1 of the present invention.

[0018] In the figure, 1. metal substrate, 2. first dielectric layer, 3. first graphene resonant layer, 4. second dielectric layer, 5. second graphene resonant layer. Detailed Implementation

[0019] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0020] This invention provides a dual-frequency terahertz metamaterial absorber, such as... Figures 1-4 As shown, the absorber is composed of M×N metasurface units with multilayer structures arranged in a periodic array, where M and N are both positive integers; a single metasurface unit includes a metal substrate 1, a first dielectric layer 2, a first graphene resonant layer 3, a second dielectric layer 4, and a second graphene resonant layer 5 arranged sequentially from bottom to top.

[0021] The metal substrate 1, the first dielectric layer 2, and the second dielectric layer 4 are all rectangular parallelepiped structures with square cross-sections in the xy plane. The side lengths of the cross-sections of the metal substrate 1, the first dielectric layer 2, and the second dielectric layer 4 are all equal, ranging from 8 μm to 8.3 μm. The thickness t of the metal substrate 1 is 0.2 μm to 0.5 μm. The thickness H1 of the first dielectric layer 2 is 8.8 μm to 9.1 μm. The thickness H2 of the second dielectric layer 4 is 3.0 μm to 3.5 μm. The metal substrate 1 serves as a continuous metal reflective layer to suppress terahertz wave transmission.

[0022] The metal substrate 1 is made of gold and has an electrical conductivity of 4.1 × 10⁻⁶. 7 S / m; Both the first dielectric layer 2 and the second dielectric layer 4 are made of silicon dioxide, and both have a relative permittivity of 3.9. The first dielectric layer 2 and the second dielectric layer 4 of the metal substrate 1 are arranged periodically in the xy plane, and the unit period is Px and Py. The unit period Px and the unit period Py are equal and both have a value of 8μm ~ 8.3μm.

[0023] The first graphene resonant layer 3 is a square ring structure composed of a single layer of graphene. The outer frame is square with a side length a of 7.2 μm to 7.7 μm. The inner frame is square with a side length d of 6.2 μm to 6.7 μm. The width D of the square ring is 1.0 μm. The center of the first graphene resonant layer 3 coincides with the center of the metasurface unit. The thickness of the first graphene resonant layer 3 is 0.34 nm.

[0024] The second graphene resonant layer 5 is a regular hexagonal structure composed of a single layer of graphene. The radius R of the circumscribed circle of the hexagon is 1.6 μm to 1.8 μm, and the side length of the regular hexagon is 1.6 μm to 1.8 μm. The center of the second graphene resonant layer 5 coincides with the center of the metasurface unit. The thickness of the second graphene resonant layer 5 is 0.34 nm.

[0025] This invention also provides a method for preparing the above-mentioned dual-frequency terahertz metamaterial absorber, specifically as follows: Step 1: Preparation of metal substrate 1; Step 1 is as follows: Step 1.1, Substrate cleaning: High-purity silicon was selected as the substrate and ultrasonically cleaned with acetone, ethanol and deionized water for 10 min-15 min in sequence to remove surface contaminants; after drying with nitrogen, it was dried in an oven at 110℃-120℃ for 30 min-40 min. Step 1.2, Metal Thin Film Deposition: A gold thin film was deposited using electron beam evaporation as the bottom reflective layer; Step 1.3, Annealing treatment: Annealing at 250℃-300℃ for 30min-40min improves the crystallinity and conductivity of the gold film, resulting in metal substrate 1; Step 2, Deposition of the first dielectric layer 2; Step 2 is as follows: Step 2.1, Preparation of the dielectric layer thin film: Silicon dioxide (SiO2) was deposited on metal substrate 1 using plasma-enhanced chemical vapor deposition. Step 2.2, Curing treatment: The film is cured at 150℃-160℃ for 1-2 hours to ensure the film's density and dielectric properties are stable, thus obtaining the first dielectric layer 2.

[0026] Step 3: Preparation of the first graphene resonant layer 3; Step 3 specifically involves: Step 3.1, Graphene Growth and Transfer: A single layer of graphene was grown on copper foil using chemical vapor deposition and then transferred to the surface of the first dielectric layer 2 by a wet process. Step 3.2, Graphene Patterning: Electron beam lithography is used to define a pattern that matches the square ring structure, and then oxygen plasma etching is used to remove excess areas to form a square ring graphene structure, thus obtaining the first graphene resonant layer 3. Step 4: Deposition of the second dielectric layer 4; Step 4 specifically involves: Step 4.1, Preparation of silicon dioxide thin film: A silicon dioxide thin film was deposited on the first graphene resonant layer 3 using chemical vapor deposition; Step 4.2, Curing treatment: Curing at 150℃-160℃ for 1-2 hours yields the second dielectric layer 4. Step 5: Preparation of the second graphene resonant layer 5; Step 5 specifically involves: Step 5.1, Graphene Growth and Transfer: A single layer of graphene was grown on copper foil again using chemical vapor deposition and then transferred to the surface of the second dielectric layer 4 by a wet process. Step 5.2, Graphene Patterning: Electron beam lithography is used to define a pattern that matches the hexagonal structure, and then oxygen plasma etching is used to remove excess areas to form a hexagonal graphene structure, thus obtaining the second graphene resonant layer 5.

[0027] The first graphene resonant layer 3 and the second graphene resonant layer 5 can be independently tuned by an external bias voltage to achieve dynamic control of the absorption efficiency and absorption center frequency of the terahertz metamaterial absorber. The first graphene resonant layer 3 is mainly used to control the low-frequency absorption peak, while the second graphene resonant layer 5 is mainly used to control the high-frequency absorption peak. The two work together to achieve separate dynamic tuning of the absorption center frequency and absorption rate of the two bands.

[0028] Example 1 like Figures 1-4 As shown, the dual-frequency terahertz metamaterial absorber in this embodiment includes 3×3 metasurface units. The number of units does not affect the overall performance. Each metasurface unit includes a metal substrate 1, a first dielectric layer 2, a first graphene resonant layer 3, a second dielectric layer 4, and a second graphene resonant layer 5 arranged sequentially from bottom to top.

[0029] The cross-sections of the metal substrate 1, the first dielectric layer 2, and the second dielectric layer 4 in the xy plane are all square cuboid structures. The side lengths of the cross-sections of the metal substrate 1, the first dielectric layer 2, and the second dielectric layer 4 are all equal, with a side length of 8 μm. The thickness t of the metal substrate 1 is 0.2 μm. The thickness H1 of the first dielectric layer 2 is 8.8 μm. The thickness H2 of the second dielectric layer 4 is 3.0 μm. The metal substrate 1 serves as a continuous metal reflective layer to suppress terahertz wave transmission.

[0030] The metal substrate 1 is made of gold and has an electrical conductivity of 4.1 × 10⁻⁶. 7 S / m; Both the first dielectric layer 2 and the second dielectric layer 4 are made of silicon dioxide, and both have a relative permittivity of 3.9. The first dielectric layer 2 and the second dielectric layer 4 of the metal substrate 1 are arranged periodically in the xy plane, and the unit period is Px and Py. The unit period Px and the unit period Py are equal and both have a value of 8μm.

[0031] The first graphene resonant layer 3 is a square ring structure composed of a single layer of graphene. The outer frame is square with a side length a of 7.2 μm, the inner frame is square with a side length d of 6.2 μm, and the width D of the square ring is 1.0 μm. The center of the first graphene resonant layer 3 coincides with the center of the metasurface unit. The thickness of the first graphene resonant layer 3 is 0.34 nm.

[0032] The second graphene resonant layer 5 is a regular hexagonal structure composed of a single layer of graphene. The radius R of the circumscribed circle of the hexagon is 1.6 μm, and the side length of the regular hexagon is 1.6 μm. The center of the second graphene resonant layer 5 coincides with the center of the metasurface unit. The thickness of the second graphene resonant layer 5 is 0.34 nm.

[0033] The first graphene resonant layer 3 and the second graphene resonant layer 5 can be independently tuned by an external bias voltage to achieve dynamic control of the absorption efficiency and absorption center frequency of the terahertz metamaterial absorber. The first graphene resonant layer 3 is mainly used to control the low-frequency absorption peak, while the second graphene resonant layer 5 is mainly used to control the high-frequency absorption peak. The two work together to achieve separate dynamic tuning of the absorption center frequency and absorption rate of the two bands.

[0034] Figure 5 This is the absorption spectrum of the terahertz metamaterial absorber in Embodiment 1 of the present invention. In the figure, the absorption rate of the absorber reaches 99.9% at 1.24 THz and 4.23 THz, which is close to perfect absorption.

[0035] Figure 6 This is a relative impedance characteristic diagram of the terahertz metamaterial absorber in Embodiment 1 of the present invention. Near the two resonant frequencies of 1.24 THz and 4.23 THz, Re(Zr)≈1 and Im(Zr)≈0, indicating that it has good impedance matching characteristics.

[0036] To further investigate the physical mechanism of this terahertz metamaterial absorber, the electric field distribution of the metasurface unit at each peak frequency was measured, and the results are as follows: Figures 7-10 As shown, at a frequency of 1.24 THz, the electric field distribution of the second graphene resonant layer 5 is relatively dispersed and its intensity is low; while the first graphene resonant layer 3 exhibits a significant dipole resonance phenomenon, with the electric field mainly concentrated at its upper and lower ends, and its electric field intensity is much higher than that of the second graphene resonant layer 5. Therefore, the 1.24 THz absorption peak is mainly caused by the dipole resonance of the first graphene resonant layer 3.

[0037] At a frequency of 4.23 THz, the electric field response of the second graphene resonant layer 5 is very weak, and the electric field strength is at an extremely low level. The first graphene resonant layer 3 becomes the core region where the electric field gathers, and a significant hexapole resonance occurs here. The electric field is distributed in six discrete strong regions. This high-order resonance mode leads to strong electromagnetic energy localization. Therefore, the absorption peak at 4.23 THz is dominated by the hexapole resonance of the first graphene resonant layer 3.

[0038] The above electric field distribution analysis shows that both absorption peaks mainly originate from the different orders of plasmon resonances in the bottom graphene, while the top graphene plays an auxiliary role in the low-frequency range and hardly participates in the resonance in the high-frequency range.

[0039] Figure 11 The figures show the effect of different first layer thicknesses H1 on the absorption rate in the absorbers of Embodiments 1-3 of this invention. When the thickness H1 of the first dielectric layer 2 is 8.8 μm, 8.9 μm, and 9.0 μm, the center frequency of the low-frequency absorption peak remains basically unchanged, while the second absorption peak shows a slight blue shift. When H1 = 8.8 μm, the absorption rate of the first absorption peak is 99.7%, and when H1 = 8.9 μm, the absorption rate of the second absorption peak is 99.9%. When H1 = 9.0 μm, the second absorption peak shows a further blue shift, and the peak absorption rate is still close to 100%. This indicates that changing the thickness of the first dielectric layer mainly affects the resonant frequency of the high-frequency absorption peak, while having a smaller impact on the low-frequency absorption peak.

[0040] Figure 12 The figures show the effect of different thicknesses H2 of the second dielectric layer on the absorption rate in the absorbers of Examples 1-3 of this invention. When H2 increases from 3.0 μm to 3.4 μm, the low-frequency absorption peaks essentially overlap and show almost no change; however, the high-frequency absorption peaks exhibit a blue shift. When H2 = 3.0 μm, the absorption rates of the first and second absorption peaks are 99.8% and 99.2%, respectively. When H2 = 3.2 μm, the absorption rates of the first and second absorption peaks are 99.9% and 99.9%, respectively. When H2 = 3.4 μm, the absorption rates of the first and second absorption peaks are relatively low, at 99.8% and 99.9%, respectively.

[0041] Figure 13 This is an absorptivity graph of the absorber as a function of the square ring width D, under the same conditions as the absorber in Example 1. When D increases from 0.8 μm to 1.2 μm, the first and second absorption peaks exhibit a significant blue shift. When D = 0.8 μm, the absorptivity of the first, second, and third absorption peaks is 93.2% and 99.7%, respectively; when D = 1.0 μm, the absorptivity of the first, second, and third absorption peaks is 99.9% and 99.9%, respectively; when D = 1.2 μm, the absorptivity of the first and second absorption peaks is 97.4% and 98.5%, respectively; therefore, D = 1.0 μm is determined.

[0042] Figure 14 This diagram shows the absorptivity of the first graphene resonant layer 3 with different outer frame side lengths 'a' in the absorbers of Embodiments 1-3 of the present invention. When 'a' is 7.2 μm, 7.4 μm, and 7.6 μm, the center frequencies of the first and second absorption peaks exhibit a slight redshift. When 'a' = 7.2 μm, 7.4 μm, and 7.6 μm, the absorptivity of the first absorption peak is 99.7%, 99.9%, and 98.9%, respectively.

[0043] Figure 15 This diagram shows the absorptivity of the second graphene resonant layer 5 with different circumcircle radii R in the absorbers of Examples 1-3 of this invention. When R is 1.6 μm, 1.7 μm, and 1.8 μm, the low-frequency absorption peak remains essentially unchanged, while the high-frequency absorption peak shows a redshift. When R = 1.6 μm, 1.7 μm, and 1.8 μm, the absorptivity of the first absorption peak is 99.9%, 99.8%, and 99.6%, respectively.

[0044] Figures 16-19 The chemical potential of the first graphene resonant layer 3 is fixed in the absorber of Embodiment 1 of the present invention. The chemical potential of the second graphene resonant layer 5 remains unchanged. The changing absorbance graph; the chemical potential of the fixed first graphene resonant layer 3 The chemical potential of the second graphene resonant layer 5 remains unchanged. The low-frequency absorption peak remained relatively stable, while the high-frequency absorption peak showed a slight blue shift, mainly within the range of 0.4 eV to 0.7 eV. Figures 20-23 The chemical potential of the second graphene resonant layer 5 is fixed in the absorber of Embodiment 1 of the present invention. The chemical potential of the first graphene resonant layer 3 remains unchanged. Variation in absorbance; chemical potential of the fixed second graphene resonant layer 5 The chemical potential of the first graphene resonant layer 3 remains unchanged. Starting from 0.6 eV, The voltage was increased in increments of 0.1 eV to 0.9 eV. The center frequency of the first absorption peak remained relatively stable, while the second absorption peak showed a significant blue shift, indicating that... It mainly affects the second absorption peak.

[0045] from Figures 16-23 It can be seen that, in the chemical potential of graphene Within the range of 0.4–0.9 eV, the device maintains high absorption characteristics across both frequency bands. With... With the increase of [value], the center frequencies of both absorption peaks exhibit a significant blue shift: the first absorption peak gradually shifts from approximately 1.15 THz to 1.31 THz, and the second absorption peak shifts from approximately 3.825 THz to 4.64 THz. In terms of absorption performance, both absorption peaks maintain near-perfect absorption (approximately 99%) throughout the entire tuning range. The low-frequency absorption peak [values ​​missing in original text]. The percentage decreased slightly from about 99.9% to 96.9%, while the high-frequency absorption peak remained above 99%, with the overall change being relatively small.

[0046] The results show that by adjusting the chemical potential of graphene, the continuous frequency modulation of the dual-frequency absorption peaks can be achieved without losing absorption efficiency, demonstrating the good dynamic tunability of the structure.

[0047] Figure 24 This shows the electric field distribution of the absorber in Embodiment 1 of the present invention as a function of the polarization angle. Under normal incidence, the absorption spectrum remains essentially unchanged with the electromagnetic wave polarization angle ψ, exhibiting excellent polarization insensitivity. This characteristic is mainly due to the rotational symmetry of the graphene layer in the absorber.

[0048] Figure 25 This is the electric field distribution of the absorber under TE polarization as a function of the incident angle in Embodiment 1 of the present invention. For TE waves, when the incident angle is between 0° and 60°, the center frequencies and absorptivity of the two absorption peaks do not show significant fluctuations; when the angle θ > 60°, the absorption peak at the low frequency begins to change, and the absorptivity begins to decay.

[0049] Figure 26 This diagram shows the electric field distribution of the absorber in Embodiment 1 of the present invention under TM polarization as a function of the incident angle. For TM waves, when the incident angle is within the range of 0°-30°, the two absorption peaks remain essentially unchanged; when the incident angle is greater than 30°, the first absorption peak changes, and the absorptivity begins to gradually decrease, while the center frequency and absorptivity of the second absorption peak remain stable; when the incident angle is greater than 50°, the second absorption peak begins to exhibit a redshift. Therefore, it can be seen that this absorber not only exhibits polarization insensitivity but also possesses large-angle incident characteristics within the 0-30° range.

[0050] In summary, the dual-frequency terahertz metamaterial absorber in this embodiment achieves three core performance characteristics through independent voltage regulation of the bilayer graphene: near-perfect dual-frequency absorption, adjustable impedance matching, and robustness against angular polarization. The specific conclusions are as follows: (1) The device achieves near-perfect absorption at both resonant frequencies of 1.24 THz and 4.23 THz, with absorption rates exceeding 99%. The relative impedance at these two frequencies has a real part close to 1 and an imaginary part close to 0, exhibiting excellent impedance matching characteristics. The electric field distribution indicates that the low-frequency absorption at 1.24 THz is mainly dominated by the dipole resonance of the lower graphene layer, while the high-frequency absorption at 4.23 THz originates from the multipolar resonance of the upper graphene layer. The two graphene layers correspond to independent absorption channels.

[0051] (2) Tunable chemical potential of graphene: The two graphene layers can be independently biased to change the Fermi level. The chemical potential of the lower first graphene resonant layer 3 modulates the low-frequency resonant peak, and the chemical potential of the upper second graphene resonant layer 5 modulates the high-frequency resonant peak. Increasing the corresponding graphene chemical potential results in a blue shift of the resonant frequency and a high absorption rate. The dual-band absorption center frequency can be dynamically tuned independently and continuously without interference.

[0052] (3) Robustness to polarization and incident angle: Under vertical incident, the absorption spectra of the device for TE and TM polarized waves are completely coincident, and it has polarization insensitivity. Within the incident angle range of 0°~30°, the position of the double absorption peaks and the absorption rate are almost unaffected, and it still maintains high-efficiency absorption at large angles, and has excellent wide-angle working performance.

[0053] This device utilizes a dual-layer patterned graphene layered resonance and voltage-adjustable mechanism to achieve independent dynamic tuning of dual frequencies, ultra-high absorption, and stable polarization / angle response. It has a simple structure and is easy to fabricate, overcoming the shortcomings of traditional single-frequency absorbers, such as limited tuning and operating bandwidth.

[0054] Example 2 The dual-frequency terahertz metamaterial absorber in this embodiment 2 includes 3×4 metasurface units. The number of units does not affect the overall performance. Each metasurface unit includes, from bottom to top, a metal substrate 1, a first dielectric layer 2, a first graphene resonant layer 3, a second dielectric layer 4, and a second graphene resonant layer 5.

[0055] The cross-sections of the metal substrate 1, the first dielectric layer 2, and the second dielectric layer 4 in the xy plane are all square cuboid structures. The side lengths of the cross-sections of the metal substrate 1, the first dielectric layer 2, and the second dielectric layer 4 are all equal, with a side length of 8.1 μm. The thickness t of the metal substrate 1 is 0.3 μm. The thickness H1 of the first dielectric layer 2 is 8.9 μm. The thickness H2 of the second dielectric layer 4 is 3.2 μm. The metal substrate 1 serves as a continuous metal reflective layer to suppress terahertz wave transmission.

[0056] The metal substrate 1 is made of gold and has an electrical conductivity of 4.1 × 10⁻⁶. 7 S / m; Both the first dielectric layer 2 and the second dielectric layer 4 are made of silicon dioxide, and both have a relative permittivity of 3.9. The first dielectric layer 2 and the second dielectric layer 4 of the metal substrate 1 are arranged periodically in the xy plane, and the unit period is Px and Py. The unit period Px and the unit period Py are equal and both have a value of 8.1 μm.

[0057] The first graphene resonant layer 3 is a square ring structure composed of a single layer of graphene. The outer frame is square with a side length a of 7.4 μm, the inner frame is square with a side length d of 6.4 μm, and the width D of the square ring is 1.0 μm. The center of the first graphene resonant layer 3 coincides with the center of the metasurface unit. The thickness of the first graphene resonant layer 3 is 0.34 nm.

[0058] The second graphene resonant layer 5 is a regular hexagonal structure composed of a single layer of graphene. The radius R of the circumscribed circle of the hexagon is 1.7 μm, and the side length of the regular hexagon is 1.7 μm. The center of the second graphene resonant layer 5 coincides with the center of the metasurface unit. The thickness of the second graphene resonant layer 5 is 0.34 nm.

[0059] The first graphene resonant layer 3 and the second graphene resonant layer 5 can be independently tuned by an external bias voltage to achieve dynamic control of the absorption efficiency and absorption center frequency of the terahertz metamaterial absorber. The first graphene resonant layer 3 is mainly used to control the low-frequency absorption peak, while the second graphene resonant layer 5 is mainly used to control the high-frequency absorption peak. The two work together to achieve separate dynamic tuning of the absorption center frequency and absorption rate of the two bands.

[0060] Example 3 The dual-frequency terahertz metamaterial absorber in this embodiment 3 includes 4×4 metasurface units. The number of units does not affect the overall performance. Each metasurface unit includes, from bottom to top, a metal substrate 1, a first dielectric layer 2, a first graphene resonant layer 3, a second dielectric layer 4, and a second graphene resonant layer 5.

[0061] The cross-sections of the metal substrate 1, the first dielectric layer 2, and the second dielectric layer 4 in the xy plane are all square cuboid structures. The side lengths of the cross-sections of the metal substrate 1, the first dielectric layer 2, and the second dielectric layer 4 are all equal, with a side length of 8.2 μm. The thickness t of the metal substrate 1 is 0.4 μm. The thickness H1 of the first dielectric layer 2 is 9.0 μm. The thickness H2 of the second dielectric layer 4 is 3.4 μm. The metal substrate 1 serves as a continuous metal reflective layer to suppress terahertz wave transmission.

[0062] The metal substrate 1 is made of gold and has an electrical conductivity of 4.1 × 10⁻⁶. 7 S / m; Both the first dielectric layer 2 and the second dielectric layer 4 are made of silicon dioxide, and both have a relative permittivity of 3.9. The first dielectric layer 2 and the second dielectric layer 4 of the metal substrate 1 are arranged periodically in the xy plane, and the unit period is Px and Py. The unit period Px and the unit period Py are equal and both have a value of 8.2 μm.

[0063] The first graphene resonant layer 3 is a square ring structure composed of a single layer of graphene. The outer frame is square with a side length a of 7.6 μm, the inner frame is square with a side length d of 6.6 μm, and the width D of the square ring is 1.0 μm. The center of the first graphene resonant layer 3 coincides with the center of the metasurface unit. The thickness of the first graphene resonant layer 3 is 0.34 nm.

[0064] The second graphene resonant layer 5 is a regular hexagonal structure composed of a single layer of graphene. The radius R of the circumscribed circle of the hexagon is 1.8 μm, and the side length of the regular hexagon is 1.8 μm. The center of the second graphene resonant layer 5 coincides with the center of the metasurface unit. The thickness of the second graphene resonant layer 5 is 0.34 nm.

[0065] The first graphene resonant layer 3 and the second graphene resonant layer 5 can be independently tuned by an external bias voltage to achieve dynamic control of the absorption efficiency and absorption center frequency of the terahertz metamaterial absorber. The first graphene resonant layer 3 is mainly used to control the low-frequency absorption peak, while the second graphene resonant layer 5 is mainly used to control the high-frequency absorption peak. The two work together to achieve separate dynamic tuning of the absorption center frequency and absorption rate of the two bands.

[0066] Example 4 The dual-frequency terahertz metamaterial absorber in this embodiment 4 includes 5×5 metasurface units. The number of units does not affect the overall performance. Each metasurface unit includes, from bottom to top, a metal substrate 1, a first dielectric layer 2, a first graphene resonant layer 3, a second dielectric layer 4, and a second graphene resonant layer 5.

[0067] The cross-sections of the metal substrate 1, the first dielectric layer 2, and the second dielectric layer 4 in the xy plane are all square cuboid structures. The side lengths of the cross-sections of the metal substrate 1, the first dielectric layer 2, and the second dielectric layer 4 are all equal, with a side length of 8.3 μm. The thickness t of the metal substrate 1 is 0.5 μm. The thickness H1 of the first dielectric layer 2 is 9.0 μm. The thickness H2 of the second dielectric layer 4 is 3.5 μm. The metal substrate 1 serves as a continuous metal reflective layer to suppress terahertz wave transmission.

[0068] The metal substrate 1 is made of gold and has an electrical conductivity of 4.1 × 10⁻⁶. 7 S / m; Both the first dielectric layer 2 and the second dielectric layer 4 are made of silicon dioxide, and both have a relative permittivity of 3.9. The first dielectric layer 2 and the second dielectric layer 4 of the metal substrate 1 are arranged periodically in the xy plane, and the unit period is Px and Py. The unit period Px and the unit period Py are equal and both have a value of 8.3 μm.

[0069] The first graphene resonant layer 3 is a square ring structure composed of a single layer of graphene. The outer frame is square with a side length a of 7.7 μm, the inner frame is square with a side length d of 6.7 μm, and the width D of the square ring is 1.0 μm. The center of the first graphene resonant layer 3 coincides with the center of the metasurface unit. The thickness of the first graphene resonant layer 3 is 0.34 nm.

[0070] The second graphene resonant layer 5 is a regular hexagonal structure composed of a single layer of graphene. The radius R of the circumscribed circle of the hexagon is 1.6 μm, and the side length of the regular hexagon is 1.6 μm. The center of the second graphene resonant layer 5 coincides with the center of the metasurface unit. The thickness of the second graphene resonant layer 5 is 0.34 nm.

[0071] The first graphene resonant layer 3 and the second graphene resonant layer 5 can be independently tuned by an external bias voltage to achieve dynamic control of the absorption efficiency and absorption center frequency of the terahertz metamaterial absorber. The first graphene resonant layer 3 is mainly used to control the low-frequency absorption peak, while the second graphene resonant layer 5 is mainly used to control the high-frequency absorption peak. The two work together to achieve separate dynamic tuning of the absorption center frequency and absorption rate of the two bands.

[0072] Example 5 The fabrication method of the dual-frequency terahertz metamaterial absorber is as follows: Step 1: Preparation of metal substrate 1; Step 1 is as follows: Step 1.1, Substrate cleaning: High-purity silicon was selected as the substrate and ultrasonically cleaned with acetone, ethanol and deionized water for 10 min in sequence to remove surface contaminants; after drying with nitrogen, it was dried in an oven at 110℃ for 30 min. Step 1.2, Metal Thin Film Deposition: A gold thin film was deposited using electron beam evaporation as the bottom reflective layer; Step 1.3, Annealing treatment: Annealing at 250℃ for 30 min improves the crystallinity and conductivity of the gold film, resulting in metal substrate 1; Step 2, Deposition of the first dielectric layer 2; Step 2 is as follows: Step 2.1, Preparation of the dielectric layer thin film: Silicon dioxide was deposited on metal substrate 1 using plasma-enhanced chemical vapor deposition; Step 2.2, Curing treatment: The film was cured at 150°C for 1 hour to ensure its density and dielectric properties were stable, thus obtaining the first dielectric layer 2.

[0073] Step 3: Preparation of the first graphene resonant layer 3; Step 3 specifically involves: Step 3.1, Graphene Growth and Transfer: A single layer of graphene was grown on copper foil using chemical vapor deposition and then transferred to the surface of the first dielectric layer 2 by a wet process. Step 3.2, Graphene Patterning: Electron beam lithography is used to define a pattern that matches the square ring structure, and then oxygen plasma etching is used to remove excess areas to form a square ring graphene structure, thus obtaining the first graphene resonant layer 3. Step 4: Deposition of the second dielectric layer 4; Step 4 specifically involves: Step 4.1, Preparation of silicon dioxide thin film: A silicon dioxide thin film was deposited on the first graphene resonant layer 3 using chemical vapor deposition; Step 4.2, Curing treatment: The second dielectric layer 4 was obtained by curing at 150°C for 1 hour. Step 5: Preparation of the second graphene resonant layer 5; Step 5 specifically involves: Step 5.1, Graphene Growth and Transfer: A single layer of graphene was grown on copper foil again using chemical vapor deposition and then transferred to the surface of the second dielectric layer 4 by a wet process. Step 5.2, Graphene Patterning: Electron beam lithography is used to define a pattern that matches the hexagonal structure, and then oxygen plasma etching is used to remove excess areas to form a hexagonal graphene structure, thus obtaining the second graphene resonant layer 5.

[0074] Example 6 The fabrication method of the dual-frequency terahertz metamaterial absorber is as follows: Step 1: Preparation of metal substrate 1; Step 1 is as follows: Step 1.1, Substrate cleaning: High-purity silicon was selected as the substrate and ultrasonically cleaned with acetone, ethanol and deionized water for 12 minutes in sequence to remove surface contaminants; after drying with nitrogen, it was dried in an oven at 115℃ for 35 minutes. Step 1.2, Metal Thin Film Deposition: A gold thin film was deposited using electron beam evaporation as the bottom reflective layer; Step 1.3, Annealing treatment: Annealing at 260℃ for 35 min improves the crystallinity and conductivity of the gold film, resulting in metal substrate 1; Step 2, Deposition of the first dielectric layer 2; Step 2 is as follows: Step 2.1, Preparation of the dielectric layer thin film: Silicon dioxide was deposited on metal substrate 1 using plasma-enhanced chemical vapor deposition; Step 2.2, Curing treatment: The film was cured at 155°C for 1.5 hours to ensure its density and dielectric properties were stable, thus obtaining the first dielectric layer 2.

[0075] Step 3: Preparation of the first graphene resonant layer 3; Step 3 specifically involves: Step 3.1, Graphene Growth and Transfer: A single layer of graphene was grown on copper foil using chemical vapor deposition and then transferred to the surface of the first dielectric layer 2 by a wet process. Step 3.2, Graphene Patterning: Electron beam lithography is used to define a pattern that matches the square ring structure, and then oxygen plasma etching is used to remove excess areas to form a square ring graphene structure, thus obtaining the first graphene resonant layer 3. Step 4: Deposition of the second dielectric layer 4; Step 4 specifically involves: Step 4.1, Preparation of silicon dioxide thin film: A silicon dioxide thin film was deposited on the first graphene resonant layer 3 using chemical vapor deposition; Step 4.2, Curing treatment: Curing at 155℃ for 1-2 hours yields the second dielectric layer 4. Step 5: Preparation of the second graphene resonant layer 5; Step 5 specifically involves: Step 5.1, Graphene Growth and Transfer: A single layer of graphene was grown on copper foil again using chemical vapor deposition and then transferred to the surface of the second dielectric layer 4 by a wet process. Step 5.2, Graphene Patterning: Electron beam lithography is used to define a pattern that matches the hexagonal structure, and then oxygen plasma etching is used to remove excess areas to form a hexagonal graphene structure, thus obtaining the second graphene resonant layer 5.

[0076] Example 7 The fabrication method of the dual-frequency terahertz metamaterial absorber is as follows: Step 1: Preparation of metal substrate 1; Step 1 is as follows: Step 1.1, Substrate cleaning: High-purity silicon was selected as the substrate and ultrasonically cleaned with acetone, ethanol and deionized water for 15 minutes in sequence to remove surface contaminants; after being dried with nitrogen, it was dried in an oven at 120℃ for 40 minutes. Step 1.2, Metal Thin Film Deposition: A gold thin film was deposited using electron beam evaporation as the bottom reflective layer; Step 1.3, Annealing treatment: Annealing at 300℃ for 40 min improves the crystallinity and conductivity of the gold film, resulting in metal substrate 1; Step 2, Deposition of the first dielectric layer 2; Step 2 is as follows: Step 2.1, Preparation of the dielectric layer thin film: Silicon dioxide was deposited on metal substrate 1 using plasma-enhanced chemical vapor deposition; Step 2.2, Curing treatment: The film was cured at 160°C for 2 hours to ensure its density and dielectric properties were stable, thus obtaining the first dielectric layer 2.

[0077] Step 3: Preparation of the first graphene resonant layer 3; Step 3 specifically involves: Step 3.1, Graphene Growth and Transfer: A single layer of graphene was grown on copper foil using chemical vapor deposition and then transferred to the surface of the first dielectric layer 2 by a wet process. Step 3.2, Graphene Patterning: Electron beam lithography is used to define a pattern that matches the square ring structure, and then oxygen plasma etching is used to remove excess areas to form a square ring graphene structure, thus obtaining the first graphene resonant layer 3. Step 4: Deposition of the second dielectric layer 4; Step 4 specifically involves: Step 4.1, Preparation of silicon dioxide thin film: A silicon dioxide thin film was deposited on the first graphene resonant layer 3 using chemical vapor deposition; Step 4.2, Curing treatment: The second dielectric layer 4 was obtained by curing at 160℃ for 2 hours. Step 5: Preparation of the second graphene resonant layer 5; Step 5 specifically involves: Step 5.1, Graphene Growth and Transfer: A single layer of graphene was grown on copper foil again using chemical vapor deposition and then transferred to the surface of the second dielectric layer 4 by a wet process. Step 5.2, Graphene Patterning: Electron beam lithography is used to define a pattern that matches the hexagonal structure, and then oxygen plasma etching is used to remove excess areas to form a hexagonal graphene structure, thus obtaining the second graphene resonant layer 5.

Claims

1. A dual-frequency terahertz metamaterial absorber, characterized in that, The absorber is composed of M×N metasurface units with multilayer structures arranged in a periodic array, where M and N are both positive integers; a single metasurface unit includes a metal substrate (1), a first dielectric layer (2), a first graphene resonant layer (3), a second dielectric layer (4), and a second graphene resonant layer (5) arranged sequentially from bottom to top.

2. The dual-frequency terahertz metamaterial absorber according to claim 1, characterized in that, The metal substrate (1), the first dielectric layer (2) and the second dielectric layer (4) are all rectangular parallelepiped structures with square cross sections in the xy plane. The side lengths of the cross sections of the metal substrate (1), the first dielectric layer (2) and the second dielectric layer (4) are all equal, and the side lengths are all 8μm-8.3μm. The thickness t of the metal substrate (1) is 0.2μm~0.5μm. The thickness H1 of the first dielectric layer (2) is 8.8μm~9.1μm. The thickness H2 of the second dielectric layer (4) is 3.0~3.5μm.

3. The dual-frequency terahertz metamaterial absorber according to claim 2, characterized in that: The metal substrate (1) is made of gold and has an electrical conductivity of 4.1 × 10⁻⁶. 7 S / m; Both the first dielectric layer (2) and the second dielectric layer (4) are made of silicon dioxide and have a relative permittivity of 3.

9.

4. The dual-frequency terahertz metamaterial absorber according to claim 1, characterized in that: The first graphene resonant layer (3) is a square ring structure composed of a single layer of graphene. The outer frame is square with a side length a of 7.2μm~7.7μm. The inner frame is square with a side length d of 6.2μm~6.7μm. The width D of the square ring is 1.0μm. The center of the first graphene resonant layer (3) coincides with the center of the metasurface unit.

5. The dual-frequency terahertz metamaterial absorber according to claim 1, characterized in that: The second graphene resonant layer (5) is a regular hexagonal structure composed of a single layer of graphene, and the radius R of the circumscribed circle of the hexagon is 1.6μm ~ 1.8μm; the center of the second graphene resonant layer (5) coincides with the center of the metasurface unit.

6. The method for preparing a dual-frequency terahertz metamaterial absorber according to any one of claims 1-5, characterized in that, Specifically: Step 1, Preparation of the metal substrate (1); Step 2, Deposition of the first dielectric layer (2); Step 3: Preparation of the first graphene resonant layer (3); Step 4, Deposition of the second dielectric layer (4); Step 5: Preparation of the second graphene resonant layer (5).

7. The method for preparing a dual-frequency terahertz metamaterial absorber according to claim 6, characterized in that, Step 1 is as follows: Step 1.1, Substrate cleaning: High-purity silicon was selected as the substrate and ultrasonically cleaned with acetone, ethanol and deionized water for 10 min-15 min in sequence to remove surface contaminants; after drying with nitrogen, it was dried in an oven at 110℃-120℃ for 30 min-40 min. Step 1.2, Metal Thin Film Deposition: Gold thin films were deposited using electron beam evaporation. Step 1.3, Annealing treatment: Annealing at 250℃-300℃ for 30min-40min yields a metal substrate (1); Step 2 is as follows: Step 2.1, Preparation of the dielectric layer thin film: Silicon dioxide was deposited on a metal substrate (1) by plasma-enhanced chemical vapor deposition; Step 2.2, Curing treatment: The first dielectric layer (2) is obtained by curing at 150℃-160℃ for 1-2 hours.

8. The method for preparing a dual-frequency terahertz metamaterial absorber according to claim 6, characterized in that, Step 3 specifically involves: Step 3.1, Graphene Growth and Transfer: A single layer of graphene was grown on copper foil by chemical vapor deposition and then transferred to the surface of the first dielectric layer (2) by wet transfer. Step 3.2, Graphene Patterning: Electron beam lithography is used to define a pattern that matches the square ring structure, and then oxygen plasma etching is used to remove excess areas to form a square ring graphene structure, thus obtaining the first graphene resonant layer (3). Step 4 is as follows: Step 4.1, Preparation of silicon dioxide thin film: A silicon dioxide thin film was deposited on the first graphene resonant layer (3) by chemical vapor deposition; Step 4.2, Curing treatment: Curing at 150℃-160℃ for 1-2 hours yields the second dielectric layer (4). Step 5 specifically involves: Step 5.1, Graphene Growth and Transfer: A single layer of graphene was grown on copper foil again using chemical vapor deposition and then transferred to the surface of the second dielectric layer (4) by wet transfer. Step 5.2, Graphene Patterning: A pattern matching the hexagonal structure is defined by electron beam lithography, and then excess areas are removed by oxygen plasma etching to form a hexagonal graphene structure, thus obtaining the second graphene resonant layer (5).

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