A broadband terahertz perfect absorber based on MXene nanofilms and tunable fabry-perot cavity

Abstract

This invention relates to the field of terahertz electromagnetic wave manipulation technology, and discloses a broadband terahertz complete absorber based on an MXene nanofilm and a tunable Fabry-Perot cavity. The absorber comprises a two-dimensional MXene nanofilm layer, a Fabry-Perot resonant cavity layer, a metal reflective layer, and a displacement adjustment device. The relative position between the metal reflective layer and the two-dimensional MXene nanofilm layer is adjusted via the displacement adjustment device, enabling continuous adjustment of the cavity length. The two-dimensional MXene nanofilm layer exhibits low conductivity-dispersion characteristics in the terahertz frequency band, and its equivalent surface conductance can be controlled by the film thickness, thereby achieving near-free-space impedance matching over a wide terahertz frequency band. Through the synergistic effect of resonant cavity manipulation and impedance matching, efficient absorption of terahertz waves can be achieved at different resonant frequencies. This invention features a simple structure and flexible control methods, and can be used to achieve broadband, tunable terahertz complete absorption.

Description

Technical Field

[0001] This invention relates to the field of terahertz electromagnetic wave modulation and absorption technology, and in particular to a broadband terahertz complete absorber based on MXene nanofilm material and a tunable Fabry-Perot cavity. Background Technology

[0002] The terahertz band, situated between microwaves and infrared, holds significant value in non-destructive testing, imaging, communication, and security monitoring. However, achieving high-efficiency, tunable terahertz absorption remains a challenge due to the long wavelength and limited material response of terahertz waves. Existing terahertz absorbers are mostly based on multilayer dielectric structures of fixed thickness or subwavelength metasurfaces, with their operating frequencies typically determined by geometric dimensions or material parameters. This results in narrow absorption bandwidths and limited tuning flexibility. In particular, traditional quarter-wavelength absorption structures (such as the Salisbury Screen) have fixed absorption frequencies, making it difficult to achieve wideband tunable absorption while maintaining structural simplicity.

[0003] In recent years, two-dimensional conductive materials have been introduced into terahertz absorber design due to their excellent electromagnetic response characteristics. For example, two-dimensional materials such as graphene exhibit significant conductivity dispersion characteristics in the terahertz band, and their equivalent surface conductance varies significantly with frequency, typically only matching the free-space impedance at a few specific frequency points. When these materials are combined with tunable Fabry-Perot cavity structures, the resonant frequencies change under different cavity lengths, and the material impedance is difficult to simultaneously meet the impedance matching condition at different resonant frequencies, thus limiting their application in broadband, tunable terahertz complete absorbers.

[0004] Therefore, under the current technological conditions, how to achieve continuous tunability of the terahertz absorption frequency while maintaining a relatively simple structure, and how to maintain effective impedance matching over a wide frequency range, still requires further research. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a broadband terahertz complete absorber based on MXene nanofilms and a tunable Fabry-Perot cavity. The two-dimensional MXene nanofilm layer exhibits low conductivity-dispersion characteristics in the terahertz frequency band, and its equivalent surface conductance can be tuned by the film thickness, thereby achieving near-free-space impedance matching over a wide terahertz frequency range. By introducing a resonant cavity structure with an adjustable cavity length between the two-dimensional MXene nanofilm layer and the metal reflective layer, the absorption frequency can be continuously adjusted with variations in cavity length, thus achieving efficient and tunable complete absorption in the terahertz band.

[0006] The specific technical solution of the present invention is as follows:

[0007] A broadband terahertz complete absorber based on MXene nanofilms and a tunable Fabry-Perot cavity comprises a two-dimensional MXene nanofilm layer, a Fabry-Perot resonant cavity layer, and a metal reflective layer arranged sequentially from the incident direction. A tunable Fabry-Perot cavity is formed between the two-dimensional MXene nanofilm layer and the metal reflective layer. The height of the metal reflective layer along the terahertz wave incident direction is adjusted using a displacement adjustment device. Complete absorption within the terahertz frequency band is achieved by controlling the surface conductivity parameters of the two-dimensional MXene nanofilm layer and the cavity length of the Fabry-Perot cavity.

[0008] Furthermore, the surface of the metal reflective layer is a metal coating, made of gold or silver. The thickness of the metal reflective layer is greater than its skin depth in the terahertz frequency band.

[0009] Furthermore, the thickness of the two-dimensional MXene nanofilm layer is much smaller than the operating wavelength of terahertz waves in free space.

[0010] Compared with the prior art, the present invention has at least the following beneficial effects:

[0011] 1) By introducing a tunable Fabry-Perot cavity structure, the terahertz absorption frequency can be continuously adjusted;

[0012] 2) Utilizing the low conductivity and dispersion characteristics of the two-dimensional MXene nanofilm layer in the terahertz band, effective impedance matching is maintained over a wide frequency range;

[0013] 3) The complete absorption mechanism is based on multiple reflection interference and impedance matching effect. The structural design has a clear physical basis, and the design parameters can be guided by theoretical analysis.

[0014] 4) The structure is relatively simple, the control method is flexible, it is suitable for engineering implementation, and it is easy to process and integrate. Attached Figure Description

[0015] The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and form part of this application, do not constitute a limitation thereof. In the drawings:

[0016] Figure 1 This is a schematic diagram of the overall structure of the terahertz complete absorber according to an embodiment of the present invention;

[0017] Figure 2 This is a schematic diagram showing the variation of the real part of the impedance of two-dimensional MXene nanofilms of different thicknesses with frequency under different parameters.

[0018] Figure 3 A schematic diagram showing the absorption rate of a terahertz absorber composed of MXene thin films under different parameters with a fixed cavity length.

[0019] Figure 4 To fix the conductivity parameters of the two-dimensional MXene nanofilm layer, the absorption spectrum under different cavity length conditions was calculated by scanning the cavity length of the resonant cavity with multiple sets of different values.

[0020] The attached diagram shows the following components: 1-Two-dimensional MXene nanofilm layer, 2-Metal reflective layer, 3-Displacement adjustment device. Detailed Implementation

[0021] To better understand the purpose, structure, and function of this invention, the following detailed description of a broadband terahertz complete absorber based on MXene nanofilms and a tunable Fabry-Perot cavity is provided in conjunction with the accompanying drawings.

[0022] This invention provides a broadband terahertz complete absorber based on an MXene nanofilm and a tunable Fabry-Perot cavity, comprising a two-dimensional MXene nanofilm layer, a Fabry-Perot resonant cavity layer, and a metal reflective layer. A tunable Fabry-Perot cavity is formed between the two-dimensional MXene nanofilm layer and the metal reflective layer, and the cavity length is adjusted by a displacement adjustment device. In a preferred embodiment of this invention, the metal reflective layer is disposed on the displacement adjustment device, and the surface of the metal reflective layer is a metal coating, the material of which can be a high-conductivity metal such as gold or silver; the displacement adjustment device is used to adjust the height of the metal reflective layer along the incident direction of the terahertz wave, such as... Figure 1 The diagram shows the overall structure of the terahertz complete absorber based on a two-dimensional MXene nanofilm layer 1 and a tunable Fabry-Perot cavity according to the present invention. A metal reflective layer 2 is disposed on a displacement adjustment device 3. By adjusting the displacement adjustment device, the relative height between the metal reflective layer and the two-dimensional MXene nanofilm layer can be changed, thereby adjusting the cavity length of the Fabry-Perot cavity. Specifically, the displacement adjustment device may include a knob, a threaded drive structure, or other equivalent mechanical adjustment structure. By rotating the knob, the metal reflective layer is axially displaced relative to the substrate, thereby changing the relative distance between the metal reflective layer and the two-dimensional MXene nanofilm layer. In this way, continuous or discrete adjustment of the Fabry-Perot cavity length can be achieved to meet the resonance conditions at different terahertz frequencies.

[0023] By adjusting the surface conductivity parameters of the two-dimensional MXene nanofilm layer, it is made to have an equivalent surface conductivity close to free space impedance in the terahertz band, thereby enhancing the energy coupling of terahertz waves into the film. At the same time, by adjusting the cavity length of the Fabry-Perot cavity, the reflected waves after multiple reflections in the cavity undergo phase destructive phase at the film interface, thereby significantly suppressing reflection and achieving complete absorption.

[0024] The two-dimensional MXene nanofilm layer described in this invention can be prepared using a spraying process based on MXene solution. Specifically, the MXene solution is deposited onto the substrate surface by spraying to form a continuous film layer. The substrate is selected from flexible or rigid thin substrate materials with a thickness much smaller than the terahertz operating wavelength. Its influence on the phase accumulation and energy dissipation of electromagnetic wave propagation in the terahertz frequency band is negligible. Therefore, in subsequent structural design and absorption mechanism analysis, the substrate can be approximated as an electromagnetically transparent support layer.

[0025] The spraying process can employ pneumatic spraying, ultrasonic spraying, or other equivalent spraying methods. By adjusting the spraying time, number of spray passes, and spraying parameters, the thickness of the two-dimensional MXene nanofilm layer can be controlled.

[0026] The thickness of the two-dimensional MXene nanofilm layer prepared by the above-described spraying process can be adjusted within a wide range, thereby changing the equivalent surface conductivity and impedance characteristics of the film. By selecting appropriate process parameters, the equivalent impedance of the two-dimensional MXene nanofilm layer in the terahertz band can be made close to the free space impedance, thereby reducing the reflection loss of terahertz waves at the film interface and providing a material basis for achieving complete broadband terahertz absorption.

[0027] The physical mechanism of the terahertz complete absorber proposed in this invention originates from:

[0028] 1) Two-dimensional MXene nanofilms exhibit low conductivity and dispersion characteristics in the terahertz band. Their equivalent surface conductivity can be tuned by the film thickness, thereby achieving near-free space impedance matching in a wide terahertz band.

[0029] 2) Synergistic effect of multiple reflection phase modulation effect of Fabry-Perot cavity.

[0030] Next, from the perspective of electromagnetic energy coupling and interference, the conditions for achieving complete absorption by this structure are theoretically analyzed to illustrate the rationality and feasibility of the structural design of this invention. The following derivations are intended to clarify the design principles and technical effects of this invention, and are intended to guide those skilled in the art in structural design and parameter selection, and should not be construed as limiting the scope of protection of the claims of this invention.

[0031] Equivalent electromagnetic description of two-dimensional MXene nanofilms. In the terahertz band, because the physical thickness of two-dimensional MXene nanofilms is much smaller than the terahertz operating wavelength, they can be approximately equivalent to a zero-thickness conductive interface in electromagnetic analysis. Their electromagnetic response is described by introducing frequency-dependent surface conductivity parameters and applying them to electromagnetic boundary conditions.

[0032] For ease of analysis, the equivalent surface conductance σ_MXene of the MXene nanofilm is introduced, and a normalized admittance parameter y is further introduced to characterize the impedance relationship between the MXene nanofilm and free space, which is defined as:

[0033] y=Z0·σ_MXene

[0034] Where Z0 is the free-space wave impedance. This normalized surface admittance characterizes the impedance properties of the MXene nanofilm to terahertz waves, and its value can be effectively controlled by adjusting the thickness parameter of the MXene nanofilm.

[0035] The absorption properties of standalone MXene nanofilms are limited. When a two-dimensional MXene nanofilm layer is placed alone in free space, a terahertz plane wave is incident orthogonally on the interface of the suspended MXene nanofilm. According to the electromagnetic boundary conditions determined by Maxwell's equations, an induced current will be generated on the film surface, resulting in partial reflection, transmission, and absorption.

[0036] Theoretical analysis shows that, even without introducing a reflective layer, the maximum absorption rate of the MXene nanofilm is still limited to 50%, even if the impedance of the MXene nanofilm perfectly matches the free-space impedance. The maximum absorption rate of the MXene nanofilm alone for incident terahertz waves has a physical upper limit, making complete absorption of the incident energy impossible. Therefore, relying solely on a single MXene nanofilm is insufficient to meet the requirement of complete absorption.

[0037] This conclusion indicates that complete absorption of terahertz waves cannot be achieved by relying solely on MXene nanofilms; additional structural design techniques must be introduced to suppress transmission or further reduce reflection.

[0038] Therefore, to overcome the theoretical limitations of the absorption rate of a single MXene nanofilm, this invention introduces a metal reflective layer beneath the two-dimensional MXene nanofilm layer. The thickness of this metal reflective layer in the terahertz frequency band is greater than the corresponding skin depth, and can be approximated as an ideal reflection boundary, making the system's transmission component negligible. Thus, the system's absorption performance is primarily determined by the reflection suppression effect.

[0039] A Fabry-Perot cavity is formed between a two-dimensional MXene nanofilm layer and a metal reflective layer. Incident terahertz waves are partially reflected and partially transmitted at the MXene nanofilm. The transmitted waves enter the cavity, are reflected at the metal reflective layer, and return to the film interface. This structure allows the incident terahertz waves to propagate multiple times within the cavity, thus providing degrees of freedom for phase modulation between the reflected waves.

[0040] The destructive reflection mechanism of complete absorption: In a Fabry-Perot cavity structure, the overall reflection of the system is determined by both the direct reflected wave at the MXene nanofilm interface and the reflected wave returning after multiple reflections within the cavity. When the two waves have similar amplitudes and opposite phases, the overall reflection of the system will be significantly suppressed.

[0041] Theoretical analysis shows that for a system's reflection to approach zero, the following two conditions must be met simultaneously:

[0042] On the one hand, the equivalent impedance of the two-dimensional MXene nanofilm layer should be close to the free space wave impedance in order to enhance the energy coupling of the incident terahertz wave into the system and reduce the initial reflection;

[0043] On the other hand, the length of the Fabry-Perot cavity should satisfy a specific phase condition so that the reflected wave after multiple reflections within the cavity cancels out the phase of the reflected wave at the MXene nanofilm interface when it returns to the interface.

[0044] Provided that the metal reflective layer suppresses transmission, when the above conditions are met simultaneously, the energy of the incident terahertz wave will be mainly dissipated in the MXene nanofilm, thus achieving complete absorption.

[0045] The following is an explanation of the mechanism by which cavity length adjustment achieves complete absorption of broadband terahertz waves.

[0046] Two-dimensional MXene nanofilms exhibit broadband low-conductivity dispersion characteristics in the terahertz band, and their equivalent impedance can approach free-space impedance over a wide frequency range, providing a material basis for broadband absorption.

[0047] Meanwhile, the phase condition of the Fabry-Perot cavity is directly related to its length parameter. By continuously adjusting the cavity length, the reflection cancellation condition can be satisfied sequentially at different terahertz frequencies, thereby achieving a continuous shift of the complete absorption peak within the terahertz band.

[0048] Therefore, this invention achieves broadband, tunable terahertz complete absorption through the synergistic effect of the impedance-tunable properties of MXene nanofilms and the tunable Fabry-Perot cavity structure.

[0049] The above theoretical analysis, starting from the broadband low-conductivity dispersion characteristics of the two-dimensional MXene nanofilm layer in the terahertz band, and the fact that its equivalent impedance can approach the free-space impedance characteristics over a wide frequency range, combined with the multiple reflection interference effect of the Fabry-Perot cavity, systematically analyzes the physical mechanism of the structure proposed in this invention achieving complete terahertz absorption. The theoretical analysis shows that by reasonably controlling the thickness parameter of the MXene nanofilm and the cavity length of the resonant cavity, efficient and tunable complete absorption can be achieved in the terahertz band, providing a theoretical basis for subsequent structural design and experimental verification. It should be noted that the treatment of the two-dimensional MXene nanofilm layer in the above theoretical analysis aims to elucidate its electromagnetic modulation mechanism in the terahertz band. The modeling, which involves introducing surface conductivity parameters to modify the electromagnetic boundary conditions, should not be construed as limiting the scope of protection of this invention.

[0050] The following embodiments are used to further illustrate the technical solution of the present invention and its feasibility, but do not constitute a limitation on the scope of protection of the present invention.

[0051] Example 1: Simulation example of the effect of thin film parameters on complete terahertz absorption with fixed cavity length.

[0052] This embodiment aims to verify the influence of the equivalent surface conductivity of the two-dimensional MXene nanofilm layer on the terahertz complete absorption performance, and to determine the optimal process parameters for achieving system impedance matching.

[0053] The terahertz complete absorber structure used in this embodiment is as follows: Figure 1 As shown, starting from the incident direction, the structure sequentially includes: an air layer, a two-dimensional MXene nanofilm layer, a fixed-length Fabry-Perot resonant cavity layer, and a bottom metal reflective layer. In this embodiment, the two-dimensional MXene nanofilm layer is prepared on the surface of a polyimide (PI) flexible substrate using a spraying process. The thickness of the PI flexible substrate is much smaller than the operating wavelength of terahertz waves in free space, and its influence on the phase accumulation and energy dissipation of electromagnetic waves in the terahertz band is negligible. As an example, the operating wavelength of terahertz waves in free space is approximately 15 μm–3 mm, corresponding to the terahertz operating frequency band of 0.1–20 THz. With the flexible substrate thickness set to 3 μm, its impact on the absorption and resonant frequency of the structure is minimal. Therefore, in simulation and theoretical analysis, the PI flexible substrate can be approximated as an electromagnetically transparent support layer. Its presence does not affect the effective cavity length of the Fabry-Perot cavity or the resonant frequency position of the system, nor does it substantially affect the complete absorption characteristics of terahertz waves.

[0054] The Fabry-Perot resonator layer uses air as the medium, and the fixed cavity length d is set to 110 μm.

[0055] The Drude model is used to describe the electromagnetic response of gold (Au) in the terahertz band, where the plasma frequency ωp = 1.37 × 10^16 rad / s and the collision frequency is 1 × 10^14 rad / s. The thickness of the metal reflective layer is set to be greater than its skin depth in the terahertz band to ensure that the system transmittance is zero.

[0056] This embodiment utilizes the Drude-Smith model to characterize the surface conductivity of the two-dimensional MXene nanofilm. To simulate the effect of different coating thicknesses on the film performance during actual fabrication, six representative experimental parameters were selected for simulation comparison. The following parameters are for illustrative purposes only and do not constitute limitations. The parameters for each group are shown in Table 1.

[0057] Table 1: Physical parameters of MXene thin films under different process parameters.

[0058] Group 1 5.66 9.625 -0.82 Group 2 12.26 10.61 -0.79 Group 3 15.06 10.73 -0.76 Group 4 16.14 10.98 -0.72 Group 5 22.71 12.6 -0.68 Group 6 23.95 12.59 -0.66

[0059] To further demonstrate the material advantages of two-dimensional MXene nanofilms in the terahertz band, the frequency response characteristics of their equivalent surface conductance in the terahertz band were analyzed, such as... Figure 2 As shown, the red dashed line in the middle represents the free-space impedance (377Ω). Six color curves are used to represent the six sets of parameters mentioned above. It can be seen that the complex conductivity of the two-dimensional MXene nanofilm layer in the terahertz band exhibits a continuous and smooth trend with frequency variation, without obvious sharp resonance peaks or drastic fluctuations, demonstrating low conductivity dispersion characteristics. Compared with traditional two-dimensional conductive materials whose conductivity varies drastically with frequency in the terahertz band, this characteristic allows the two-dimensional MXene nanofilm layer to maintain a relatively stable equivalent impedance over a wider frequency range. This is beneficial for continuously meeting impedance matching requirements under different resonant cavity lengths, thereby achieving broadband, tunable, and complete terahertz absorption.

[0060] Simulation results and analysis: The above six sets of parameters were scanned and calculated within the frequency band from 0.1 THz to 20 THz to obtain the terahertz absorption rate under different parameters, such as... Figure 3 As shown, six color curves were used to represent the six parameter groups mentioned above, and the results are analyzed below:

[0061] Frequency response characteristics: Since the physical length d of the resonant cavity is fixed at 110 μm, the center frequencies of the absorption peaks corresponding to all parameter groups basically coincide. This indicates that the resonant frequency of the absorber is mainly determined by the geometry of the Fabry-Perot cavity, verifying the frequency control principle of the structural design.

[0062] Impedance matching analysis: Simulation results show that when using the first set of parameters, the absorption rate of the system reaches over 99% at multiple resonant frequencies, achieving complete absorption. This indicates that within this conductivity range, the equivalent admittance of the MXene film achieves optimal matching with the free-space impedance, effectively suppressing the reflection of incident waves at the interface.

[0063] In conclusion, this embodiment demonstrates that the absorption performance of a system can be precisely controlled under a fixed physical structure by adjusting the surface conductivity parameters (DC surface conductivity, scattering time, and backscattering parameters) of the two-dimensional MXene nanofilm layer. These simulation results provide key process parameter guidance for the structural optimization of broadband terahertz absorbers; that is, by selecting the first set of corresponding surface conductivity parameters, the optimal complete absorption effect can be obtained.

[0064] Example 2: Simulation example of achieving broadband coverage by adjusting cavity length.

[0065] In another embodiment, the film thickness corresponding to the first set of parameters in Example 1 was selected, while keeping the conductivity parameters of the two-dimensional MXene nanofilm layer essentially unchanged. The absorption spectrum under different cavity length conditions was calculated by scanning the cavity length of the resonant cavity with multiple sets of different values. The simulation results are as follows: Figure 4 As shown, 11 color curves are used to represent the cavity length d. air Data for 50μm, 65μm, 80μm, 95μm, 110μm, 125μm, 140μm, 155μm, 170μm, 185μm, and 200μm were used. The results show that when the cavity length d... air During continuous scanning within the range of 50μm to 200μm, for each given cavity length, the system satisfies the Fabry-Perot resonance condition at several specific frequency points, significantly reducing reflectivity and achieving an absorption rate approaching 100%. Different cavity lengths d air For different complete absorption frequency positions, the absorption peaks can be smoothly transitioned by continuously adjusting the cavity length, thus forming a broadband tunable terahertz absorption response as a whole.

[0066] It is understood that the present invention has been described through some embodiments, and those skilled in the art will recognize that various changes or equivalent substitutions can be made to these features and embodiments without departing from the spirit and scope of the invention. Furthermore, under the teachings of the present invention, these features and embodiments can be modified to adapt to specific situations and materials without departing from the spirit and scope of the invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the protection scope of the present invention.

Claims

1. A broadband terahertz complete absorber based on MXene nanofilms and a tunable Fabry-Perot cavity, characterized in that, It includes a two-dimensional MXene nanofilm layer, a Fabry-Perot resonant cavity layer and a metal reflective layer arranged sequentially from the incident direction. A tunable Fabry-Perot cavity is formed between the two-dimensional MXene nanofilm layer and the metal reflective layer. The height of the metal reflective layer along the terahertz wave incident direction can be adjusted by a displacement adjustment device.

2. The broadband terahertz complete absorber based on MXene nanofilm and a tunable Fabry-Perot cavity according to claim 1, characterized in that, Complete absorption in the terahertz frequency band was achieved by adjusting the surface conductivity parameters of the two-dimensional MXene nanofilm and the cavity length of the Fabry-Perot cavity.

3. A broadband terahertz complete absorber based on MXene nanofilm and a tunable Fabry-Perot cavity according to claim 2, characterized in that, The surface of the metal reflective layer is a metal coating, and the material is gold or silver.

4. A broadband terahertz complete absorber based on MXene nanofilm and a tunable Fabry-Perot cavity according to claim 3, characterized in that, The thickness of the two-dimensional MXene nanofilm layer is much smaller than the operating wavelength of terahertz waves in free space.

5. A broadband terahertz complete absorber based on MXene nanofilm and a tunable Fabry-Perot cavity according to claim 4, characterized in that, The thickness of the metal reflective layer is greater than its skin depth in the terahertz band.

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