A transmissive-absorptive switching terahertz sensor based on vanadium dioxide-graphene super surface

Abstract

The application belongs to the field of terahertz sensor design, and specifically provides a transmission-absorption switching terahertz sensor based on a vanadium dioxide-graphene super surface, to solve the problems of complex structure, limited working frequency points, insufficient active regulation capacity and the like of existing terahertz super surface sensors. The terahertz sensor in the application is composed of a plurality of array-arranged sensing units, the sensing unit includes a vanadium dioxide layer 3, a dielectric layer 2 and a graphene super surface layer 1 which are sequentially stacked from bottom to top, and the graphene super surface layer is composed of an outer ring and an eight-equal-part concentric ring structure; the application creatively designs a compact vanadium dioxide-graphene composite super surface unit, so that it can change electromagnetic boundary conditions at different temperatures and realize transmission / absorption mode switching, and continuously adjust the multi-frequency resonance response by using the adjustable characteristics of the Fermi level of graphene, and finally realize multi-band sensing of environmental refractive index through resonance frequency shift.

Description

Technical Field

[0001] This invention belongs to the field of terahertz sensor design, specifically providing a transmissivity-absorption switching terahertz sensor based on a vanadium dioxide-graphene metasurface. Background Technology

[0002] Terahertz waves (0.1 ~ 10 THz) typically lie between microwaves and infrared, possessing strong spectral recognition capabilities, low photon energy, and the ability to penetrate many non-metallic materials. They hold significant promise for applications in biological detection, chemical identification, security imaging, communication modulation, and on-chip spectral sensing. As terahertz systems evolve towards miniaturization, integration, and multifunctionality, traditional fixed-frequency metasurface devices composed of metal resonant units are increasingly insufficient to meet the demands of multi-frequency detection, dynamic control, and mode switching in complex applications. Particularly in sensing applications, different analytes often correspond to different refractive indices, absorption characteristics, or molecular vibrational responses. Devices operating at a single frequency or with a single transmission / absorption mode struggle to balance sensitivity, selectivity, and applicability.

[0003] Metasurfaces, through their subwavelength periodic structures, can modulate the phase, amplitude, polarization, and local field distribution of electromagnetic waves, forming sharp resonance peaks or strong absorption peaks in the terahertz band, thereby significantly enhancing the interaction between the analyte and the electromagnetic field. In recent years, tunable materials such as graphene and vanadium dioxide have been introduced into terahertz metasurfaces for active modulation. The Fermi level of graphene can be adjusted by an applied bias voltage, thereby altering its surface conductivity and plasmon response. Vanadium dioxide undergoes a reversible insulator-metal phase transition around 68 °C, leading to significant changes in conductivity and optical response. Therefore, combining the phase-change switching characteristics of vanadium dioxide with the electrical tuning characteristics of graphene, and achieving dynamic switching between multi-frequency transmission and absorption modes through reasonable metasurface structure design, could potentially construct a novel, reconfigurable, multi-parameter-identifiable, and dynamically switchable multi-frequency transmission and absorption terahertz sensor suitable for complex terahertz detection scenarios.

[0004] Existing terahertz metasurface sensors mostly employ metal open-ended resonant rings, cross-shaped resonators, circular ring resonators, patch arrays, or multilayer metal-dielectric-metal structures. They detect the refractive index, concentration, thickness, or dielectric properties of the analyte by shifting the resonant peak frequency, changing the absorption peak intensity, or altering the transmission window. These devices typically rely on localized surface plasmon resonance, Fano resonance, electromagnetically induced transparency, or perfect absorption mechanisms to enhance the local electric field. Placing the analyte in a strong field region causes a shift in the resonant frequency, thereby achieving the sensing response. To improve the detection dimension, some existing schemes employ multi-resonant unit coupling or multilayer stacked structures to obtain dual-frequency, triple-frequency, or multi-frequency sensing peaks. However, their operating frequencies are mostly determined by fixed geometric dimensions, making them difficult to actively adjust after fabrication.

[0005] To address the insufficient controllability of fixed-frequency devices, graphene is widely used in tunable terahertz metasurfaces. Graphene possesses two-dimensional electron gas properties, and its surface conductivity can be altered with changes in the Fermi level. Through gate voltage, electrochemical modulation, or carrier concentration adjustment, resonant frequency shifts, absorption intensity adjustments, or transmission amplitude variations can be achieved. Existing graphene terahertz sensors can achieve multi-frequency refractive index detection and utilize bias voltage to adjust the position of each resonant peak or absorption intensity, improving the device's adaptability to different detection targets. For example, Chinese patent document CN119534385A discloses a terahertz sensor based on graphene Fano resonance, which generates two narrow-band absorption peaks in the 3–9 THz frequency range, achieving terahertz wave sensing with a maximum sensitivity of 1.548 THz / RIU. Furthermore, vanadium dioxide, as a typical temperature-controlled phase change material, can achieve reversible transitions between insulating and metallic states. Therefore, introducing graphene and vanadium dioxide into the same terahertz metasurface structure can fully leverage their complementary advantages: on the one hand, by utilizing the tunable Fermi level of graphene, continuous control of sensing frequency, transmission amplitude, and absorption intensity can be achieved; on the other hand, by utilizing the conductivity abrupt change caused by the VO2 insulating-metal phase transition, rapid switching of device operating modes and electromagnetic response reconstruction can be achieved; through the synergistic effect of the two, the metasurface can exhibit different resonance paths, local field distributions, and impedance matching states under different excitation conditions, thereby realizing multiple functions such as multi-band sensing, dynamic transmission / absorption switching, on / off modulation, multi-frequency tunability, and strong local field enhancement. Based on this, the present invention provides a transmissivity-absorption switching terahertz sensor based on a vanadium dioxide-graphene metasurface. Summary of the Invention

[0006] The purpose of this invention is to provide a terahertz sensor based on a vanadium dioxide-graphene metasurface, addressing the problems of complex structure, limited operating frequency, and insufficient active control capability in existing terahertz metasurface sensors. This invention constructs a compact vanadium dioxide-graphene composite metasurface unit, enabling it to change electromagnetic boundary conditions at different temperatures and achieve transmission / absorption mode switching. Furthermore, it utilizes the tunable Fermi level of graphene to achieve continuous adjustment of the multi-frequency resonant response, ultimately realizing multi-band sensing of the ambient refractive index through resonant frequency shift.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A transmissivity-absorption switching terahertz sensor based on a vanadium dioxide-graphene metasurface comprises several arrayed sensing units. The sensing unit comprises, from bottom to top, a vanadium dioxide layer 3, a dielectric layer 2, and a graphene metasurface layer 1. The graphene metasurface layer 1 is composed of an outer square ring and an eight-part concentric ring structure. The outer square ring is positioned along the edge of the upper surface of the dielectric layer, and the eight-part concentric ring structure is positioned along the center of the upper surface of the dielectric layer. The eight-part concentric ring structure consists of a central circle, an outer ring, and eight equal branches. The central circle and outer ring are concentrically positioned, and their centers coincide with the center of the upper surface of the dielectric layer. The eight equal branches are connected between the central circle and the outer ring, dividing the area between the central circle and the outer ring into eight equal parts.

[0009] Furthermore, the eight equal branches are located on the midline and diagonal of the upper surface of the medium layer.

[0010] Furthermore, the structural period (P) of the sensing unit x =P y The thickness is 25~35 μm.

[0011] Furthermore, the thickness of the graphene layer is 0.34 nm; the thickness (H1) of the vanadium dioxide layer is 0.2~1.0 μm; and the thickness (H2) of the dielectric layer is 2~4 μm.

[0012] Furthermore, in the graphene metasurface layer, the outer side length of the outer square ring is equal to the structural period of the sensing unit, and the inner side length is (L2) 20~30 μm; the outer circular ring is tangent to the outer square ring, the inner radius (R2) of the outer circular ring is 8~12 μm, the radius (R3) of the central circle is 2~4 μm, and the width (W) of the eight equally divided branches is 1~2 μm.

[0013] Furthermore, the transmissive-absorption switching terahertz sensor has a transmission mode and an absorption mode, with the graphene metasurface layer as the incident surface: when the operating temperature is lower than the phase transition temperature of vanadium dioxide, the transmissive-absorption switching terahertz sensor operates in the transmission mode; when the operating temperature is higher than the phase transition temperature of vanadium dioxide, the transmissive-absorption switching terahertz sensor operates in the absorption mode.

[0014] Furthermore, when the transmissive-absorption switching terahertz sensor operates in transmission mode, it has a dual-frequency transmission channel; when the transmissive-absorption switching terahertz sensor operates in absorption mode, it has a four-frequency absorption channel.

[0015] Based on the above technical solution, the beneficial effect of the present invention is that it provides a transmissivity-absorption switching terahertz sensor based on a vanadium dioxide-graphene metasurface, which has the following advantages:

[0016] (1) The present invention designs a compact vanadium dioxide-graphene composite metasurface sensing unit. Utilizing the phase transition characteristics of vanadium dioxide, the device can switch between multi-frequency transmission sensing mode and multi-frequency absorption sensing mode at temperatures of 310 K and 350 K, thereby improving the applicability of the same sensor in different detection scenarios.

[0017] (2) The sensing unit provided by the present invention has the advantages of compact structure and easy integration. Furthermore, multi-frequency response can be obtained through a single sensing unit, which reduces the dependence of traditional multi-frequency sensors on large-area unit combination or multi-layer stacked structure, which is beneficial to subsequent arraying, miniaturization and on-chip integration applications.

[0018] (3) The terahertz sensor proposed in this invention has excellent dynamic modulation characteristics in the working frequency band, which greatly increases the application range of the terahertz switchable sensor.

[0019] (4) The terahertz sensor proposed in this invention has excellent sensing performance. As the refractive index of the environment increases, the operating frequency band gradually shifts to lower frequencies, and the efficiency remains almost unchanged. The highest sensitivity S can reach 1283.3 GHz / RIU. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the unit structure of the transmembrane-absorbing switching terahertz sensor based on vanadium dioxide-graphene metasurface in this invention.

[0021] Figure 2 This is a schematic diagram of the graphene metasurface structure in this invention.

[0022] Figure 3 This is a schematic diagram of the top grid configuration of the transmembrane-absorption switching terahertz sensor based on a vanadium dioxide-graphene metasurface in this invention.

[0023] Figure 4 This is a schematic diagram of the operating mode of the vanadium dioxide-graphene metasurface-based transilluminated terahertz sensor in this invention at operating temperatures of 310 K and 350 K.

[0024] Figure 5 This is a graph showing the transmission efficiency of the vanadium dioxide-graphene metasurface-based transmissivity-absorption switching terahertz sensor in the transmission mode as a function of the graphene Fermi level.

[0025] Figure 6 This is a graph showing the transmission efficiency of the vanadium dioxide-graphene metasurface-based transmissivity-absorption switching terahertz sensor in transmission mode as a function of graphene relaxation time.

[0026] Figure 7This is a graph showing the transmission efficiency of the vanadium dioxide-graphene metasurface-based transmissivity-absorption switching terahertz sensor in transmission mode as a function of the ambient refractive index.

[0027] Figure 8 This is a graph showing the change in transmission frequency of the vanadium dioxide-graphene metasurface-based transmissivity switching terahertz sensor in transmission mode as a function of the ambient refractive index.

[0028] Figure 9 This is a graph showing the absorption efficiency of the vanadium dioxide-graphene metasurface-based transilluminated terahertz sensor in the absorption mode as a function of the graphene Fermi level.

[0029] Figure 10 This is a graph showing the change in absorption efficiency of the vanadium dioxide-graphene metasurface-based transilluminated terahertz sensor in absorption mode as a function of graphene relaxation time.

[0030] Figure 11 This is a graph showing the change in absorption efficiency of the vanadium dioxide-graphene metasurface-based transducer-absorption switching terahertz sensor in absorption mode as a function of the ambient refractive index.

[0031] Figure 12 This is a graph showing the change in absorption frequency of the vanadium dioxide-graphene metasurface-based transmissivity-absorption switching terahertz sensor in absorption mode as a function of the ambient refractive index. Detailed Implementation

[0032] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0033] This embodiment proposes a terahertz sensor based on a vanadium dioxide-graphene metasurface with switchable dynamic transmission and multi-frequency absorption. It achieves dynamic transmission and multi-frequency absorption switching sensing and detection in the terahertz frequency band and has the characteristics of simple structure, multi-frequency absorption and high sensitivity.

[0034] Specifically, the terahertz sensor based on the vanadium dioxide-graphene metasurface consists of several arrayed sensing units, which have a square structure from a top-view perspective, such as... Figure 1 As shown, it specifically includes a vanadium dioxide layer 3, a dielectric layer 2, and a graphene metasurface layer 1 stacked sequentially from bottom to top; wherein, the graphene metasurface layer 1 is composed of an outer square ring and an eight-part concentric ring structure, as shown in the figure. Figure 2As shown, the outer ring is arranged along the edge of the upper surface of the dielectric layer, and the eight-part concentric ring structure is arranged along the center of the upper surface of the dielectric layer and located inside the outer ring. The eight-part concentric ring structure consists of a central circle, an outer ring, and eight equal branches. The central circle and the outer ring are concentric, and their centers coincide with the center of the upper surface of the dielectric layer. The eight equal branches are connected between the central circle and the outer ring, dividing the area between the central circle and the outer ring into eight equal parts. Furthermore, the eight equal branches are located on the centerline and diagonal of the upper surface of the dielectric layer.

[0035] The structural period (P) of the sensing unit x =P y The thickness of the graphene layer is 0.34 nm; the thickness of the vanadium dioxide layer (H1) is 0.2~1.0 μm; the dielectric layer is made of FR-4 material with a dielectric constant of 3.8~4.6 and a thickness (H2) of 2~4 μm.

[0036] In the graphene metasurface layer, the outer side length of the outer ring (L1=P) x =P y The outer radius (R1) is 25~35 μm, the inner side length (L2) is 20~30 μm; in the eight-part concentric ring structure, the outer radius (R1=L2 / 2) of the outer ring is 10~15 μm, the inner radius (R2) is 8~12 μm, the radius (R3) of the central circle is 2~4 μm, and the width (W) of the eight-part branches is 1~2 μm; the pattern structure of the graphene metasurface layer can be realized by picosecond laser processing or photolithography.

[0037] In terms of working principle:

[0038] The optical properties of the vanadium dioxide layer can be described by the Drude model, with the following expression:

[0039] , ,

[0040] in, This represents the dielectric constant of vanadium dioxide. Indicates the frequency of electromagnetic waves. This indicates the electrical conductivity of vanadium dioxide. This represents the dielectric constant of vanadium dioxide in the high-frequency limit (value is 12). This indicates the plasma resonance frequency of vanadium dioxide. represents an imaginary number, This indicates the initial conductivity of vanadium dioxide. The collision frequency of vanadium dioxide is represented by a value of 5.75 × 10⁻⁶. 13 s -1 );

[0041] In addition, vanadium dioxide has high electrical conductivity. The conductivity of vanadium dioxide at different temperatures, which is related to ambient temperature, can be expressed as follows: , Represents the electric field modulation coefficient. Indicates the time-varying electric field strength. Indicates the modulation factor. Represents the Boltzmann constant. The ambient temperature (K) is represented. It can be seen that the conductivity of the vanadium dioxide layer increases with increasing ambient temperature, which also leads to changes in its response to electromagnetic waves. Regarding the various conductivity characteristics of vanadium dioxide phase change materials, it can be simulated as an insulating to a metallic state in the temperature range of 310 K to 350 K, with conductivity of 0.02 S / m and 2 × 10⁻⁶ S / m, respectively. 5 S / m.

[0042] Graphene layers are composed of carbon atoms arranged in sp... 2 Two-dimensional materials with a hexagonal honeycomb lattice composed of hybrid orbitals possess excellent optoelectronic properties such as high carrier mobility, strong conductivity, and strong bioadsorption. In this invention, the total conductivity of the graphene metamaterial is... It can be represented as:

[0043] ,

[0044] in, and These represent the intraband and interband conductivity of graphene metamaterials, respectively, and are specifically expressed as follows:

[0045] ,

[0046] ,

[0047] in, The charge of an electron. Represents the Boltzmann constant. Denotes the reduced Planck constant. and These refer to the Fermi level and relaxation time of graphene metamaterials, respectively.

[0048] Furthermore, by increasing the total conductivity Converted to bulk conductivity : , The thickness of the graphene metamaterial is represented by the dielectric constant of the graphene metamaterial. From bulk conductivity The calculation yielded the following:

[0049] ,in, Represents the vacuum permittivity;

[0050] As can be seen from the above, by controlling the Fermi level of graphene metamaterials to dynamically adjust the dielectric constant, a dielectric constant model of graphene metamaterials can be established to simulate and analyze its dynamic modulation properties in terahertz sensors. Based on this, in this invention, the Fermi level E of the patterned graphene metamaterial layer is set. F =0.6~1.0 eV, relaxation time =0.6~1.0Ps. This invention uses MATLAB software to calculate graphene metamaterials with different Fermi levels and relaxation times, and imports these results into the three-dimensional electromagnetic field simulation software CST for terahertz transmission-absorption switchable sensor structure modeling and simulation.

[0051] The beneficial effects of the present invention will be explained in detail below with reference to simulation tests.

[0052] During the simulation test, the frequency range of the incident electromagnetic wave was set to 2.0~7.0 THz, the transmission direction of the incident light wave was perpendicular to the XY direction and downward, periodic unit boundary conditions were selected in the X and Y directions, and free space boundary conditions were selected in the Z axis direction, and the simulation accuracy was automatically generated by the software.

[0053] like Figure 3 As shown, in this embodiment, the terahertz multi-frequency transducer-absorption switching sensor unit structure based on vanadium dioxide-graphene metasurface is arranged in a periodic array, with no spacing between any two adjacent sensing units; furthermore, the graphene Fermi level can be tuned through an ion gel top gate structure. Specifically, a source, drain, and gate are set on the metasurface array, and an ion gel is covered on the patterned graphene metasurface. The gate voltage V is then applied... g Adjusting the carrier concentration and surface conductivity of graphene through source-drain bias voltage V d and leakage current I d By monitoring the conductivity state, this gated structure allows for continuous adjustment of the terahertz resonant frequency and response intensity without altering the device geometry. Simultaneously, VO2 provides temperature-responsive switching freedom. At 310 K, VO2 is in a low-conductivity insulating state, and the sensor primarily exhibits a multi-frequency transmission response. When the temperature rises to 350 K, VO2 enters a high-conductivity metallic state, altering the equivalent current path and impedance matching conditions of the metasurface, and the sensor transitions from transmission to multi-frequency absorption operation. Thus, graphene provides continuously tunable electrical tuning capability, while VO2 provides discrete and reversible phase transition switching capability; the combination of these two achieves composite control of electrical tuning and thermal switching.

[0054] When terahertz electromagnetic waves are incident perpendicularly onto the surface of the terahertz multi-frequency transmission-absorption dynamically switchable sensor in this embodiment, if the total energy of the incident electromagnetic waves is normalized to 1, the device's reflection efficiency is... Transmission efficiency and absorption efficiency The law of conservation of energy is satisfied: Based on this relationship, this invention analyzes the modulating effect of vanadium dioxide phase transition on the transmission and absorption modes of terahertz sensors, such as... Figure 4 As shown, at an operating temperature of 310 K, vanadium dioxide is in a low-conductivity insulating state and exhibits transmission of incident terahertz waves. It can be seen that in the 2.0–7.0 THz range, the terahertz sensor shows two distinct transmission peaks at 3.67 THz and 5.61 THz, with transmission efficiencies of 80.1% and 65.2%, respectively. This indicates that at low temperatures, the structure can form a dual-frequency selective transmission channel. This dual-frequency transmission response can be used for refractive index detection of the analyte at different characteristic frequency bands, thereby increasing the dimension of sensing information and improving detection accuracy. When the operating temperature increases to 350 K, vanadium dioxide undergoes a phase transition from an insulating state to a metallic state, and its conductivity significantly increases to 2 × 10⁻⁶. 5 With the original transmission channel suppressed (S / m), the terahertz sensor transitions from transmission mode to a four-band absorption mode, forming four strong absorption peaks at 3.59 THz, 4.38 THz, 5.03 THz, and 5.89 THz, with absorption efficiencies of 99.98%, 96.56%, 99.44%, and 96.89%, respectively. This exhibits typical multi-band strong absorption characteristics, demonstrating that the VO2 phase transition can be driven by adjusting the operating temperature, enabling the same sensor to dynamically switch between low-temperature dual-frequency transmission mode and high-temperature multi-frequency absorption mode. Therefore, the terahertz sensor proposed in this invention not only achieves reversible conversion between transmission and absorption modes at different temperatures but also generates significant resonant responses at multiple terahertz frequencies. The dual-frequency transmission peak at room temperature is suitable for multi-frequency transmission sensing, while the multi-frequency absorption peak at high temperature is suitable for multi-frequency absorption sensing. This gives the device multi-band detection, transmission / absorption mode switching, and dynamically reconfigurable sensing capabilities, providing an effective technical solution for complex sample identification and multi-parameter terahertz sensing.

[0055] Furthermore, at a temperature of T = 310 K, this invention analyzes the influence of changes in the Fermi level and relaxation time of graphene on the transmission performance of the sensor. For terahertz sensors with fixed structural parameters, dynamic tunability has more significant application value; such as... Figure 5 As shown, when the graphene Fermi level E F As the voltage increases from 0.6 eV to 1.0 eV, the transmission spectrum of the sensor undergoes a significant frequency band change; with the increase of the graphene Fermi level E... FWith the increase of [missing information], the frequencies of both transmission peaks shifted to lower frequencies. The frequency modulation ranges of the transmission peaks were 4.01–3.29 THz and 5.97–4.86 THz, respectively, with frequency modulation depths of 0.72 THz and 1.11 THz, respectively. This indicates that by modulating the Fermi level E[missing information] of graphene... F This feature allows for adjustable shifting of the transmission resonant frequency, enabling terahertz sensors to actively adjust their operating frequency band based on the characteristic frequency or refractive index response of different analytes, thus improving the device's adaptability in complex detection scenarios.

[0056] At the same time, the electronic relaxation time τ of graphene can also be tuned, such as... Figure 6 As shown, under T = 310 K, the transmission efficiency of the device changes as the relaxation time τ of graphene increases from 0.6 Ps to 1.0 Ps. It can be seen that with the increase of relaxation time, the sensor still maintains obvious dual-frequency transmission characteristics, and the positions of the main transmission peaks remain basically stable, located at 3.67 THz and 5.61 THz, respectively. This indicates that the change in relaxation time does not disrupt the basic transmission resonance mode of the device. Meanwhile, the intensity of the dual-frequency transmission peaks decreases with the increase of relaxation time, with modulation ranges of 80.1%~73.4% and 65.2%~51.6%, respectively, and modulation depths of 6.7% and 13.6%, respectively. This demonstrates that the present invention can not only achieve dynamic switching between transmission and absorption modes by relying on the vanadium dioxide phase transition, but also further modify the multi-frequency sensing performance under low-temperature transmission mode by utilizing the electrically tunable properties of graphene.

[0057] Furthermore, to verify the sensing performance of the terahertz sensor in low-temperature transmission mode, this invention investigated the effect of environmental refractive index changes on the device's transmission spectrum. Since the analyte covering the sensor surface alters the equivalent dielectric environment around the metasurface, thus affecting the local electromagnetic field distribution and resonance conditions, refractive index sensing can be achieved by monitoring the shift in transmission peak frequency. Figure 7 As shown, under T = 310 K, when the ambient refractive index increases from 1.00 to 1.08, the sensor still maintains obvious dual-frequency transmission characteristics, corresponding to low-frequency transmission peak A and high-frequency transmission peak B, respectively. As the ambient refractive index gradually increases, both transmission peaks exhibit a regular redshift, i.e., the resonant frequency shifts towards lower frequencies, while the transmission efficiency remains basically unchanged. Specifically, the frequency variation range of transmission peak A is 3.67~3.57 THz, and the frequency variation range of transmission peak B is 5.61~5.41 THz. This result indicates that changes in the external refractive index can effectively control the transmission resonant frequency of the device, and the device possesses good refractive index response capability.

[0058] like Figure 8As shown, further extraction of the resonant frequencies of the dual-frequency transmission peaks under different refractive indices reveals that both transmission peak A and transmission peak B exhibit an approximately linear decreasing trend with increasing refractive index, indicating that the sensor possesses a stable and predictable sensing response. Here, the sensitivity S( , The change in resonant frequency. The change in ambient refractive index (RIU) characterizes the terahertz sensor's response to changes in the ambient refractive index. Calculations show that the sensitivity of transmission peak A is 975 GHz / RIU, and the sensitivity of transmission peak B is 1283.3 GHz / RIU, where RIU represents the refractive index unit. Combined with... Figure 7 and Figure 8 As can be seen, at a low temperature of 310K, the terahertz sensor in this invention can achieve dual-frequency transmission-type refractive index sensing, and both transmission peaks exhibit good frequency shift regularity and high sensitivity. The dual-frequency sensing response can simultaneously acquire information about the analyte in two different terahertz frequency bands, providing higher information content and recognition reliability compared to a single-frequency sensor. Therefore, the terahertz sensor proposed in this invention not only achieves multi-frequency response in low-temperature transmission mode but also utilizes dual transmission peaks for highly sensitive detection of changes in environmental refractive index, providing an effective technical solution for multi-band terahertz sensing.

[0059] Furthermore, when the temperature rises to T = 350 K, vanadium dioxide undergoes a phase transition from an insulating state to a metallic state, resulting in a significant increase in its conductivity. This alters the equivalent electromagnetic boundary conditions of the metasurface structure, causing the device to shift from a low-temperature transmission mode to a multi-frequency absorption mode. For example... Figure 9 As shown, at T = 350 K, the absorption spectrum of the sensor changes significantly when the graphene Fermi level increases from 0.6 eV to 1.0 eV. With the increase of the graphene Fermi level, the four resonant frequencies of the terahertz sensor all shift to higher frequencies, with ranges of 3.42–3.78 THz, 4.17–4.64 THz, 4.76–5.28 THz, and 5.57–6.18 THz, and modulation depths of 0.36 THz, 0.47 THz, 0.52 THz, and 0.61 THz, respectively. This result indicates that the graphene Fermi level can effectively modulate the absorption peak to dynamically shift between multiple terahertz frequencies, increasing the practical application efficiency of terahertz metamaterial sensors.

[0060] like Figure 10As shown, under the condition of T = 350 K, the change in sensor absorption efficiency was further analyzed as the graphene relaxation time τ increased from 0.6 Ps to 1.0 Ps. It can be seen that the device maintains obvious multi-frequency absorption characteristics at different relaxation times, indicating that changes in the graphene relaxation time do not alter the device's basic multi-frequency absorption mode. Simultaneously, as τ increases from 0.6 Ps to 1.0 Ps, the intensities of the four absorption peaks can be tuned, with tuning ranges of 72.12%–99.98%, 78.02%–96.56%, 67.33%–99.89%, and 57.88%–96.89%, respectively. Combined with… Figure 9 and Figure 10 It is known that, at a high temperature of 350 K, the terahertz sensor in this invention can exhibit a stable multi-frequency strong absorption response. Among them, the graphene Fermi level is mainly used to adjust the center frequency of the absorption peak, which can realize active tuning of the absorption frequency band. The graphene relaxation time mainly affects the absorption peak intensity and can be used to optimize the absorption efficiency and resonance quality. These results further prove that this invention can not only achieve dynamic switching between low-temperature transmission mode and high-temperature absorption mode by relying on vanadium dioxide phase transition, but also use the tunable electrical properties of graphene to finely control the absorption mode.

[0061] Furthermore, to verify the sensing performance of the terahertz sensor in high-temperature absorption mode, this invention analyzes the influence of environmental refractive index changes on the multi-frequency absorption response of the device. When the temperature is T = 350 K, vanadium dioxide is in a metallic state, and the metasurface structure can effectively confine and dissipate the incident terahertz wave energy, thereby forming a strong multi-frequency absorption response. After the analyte covers the sensor surface, it changes the equivalent refractive index environment around the metasurface, thus affecting the resonant frequencies of each resonant mode. Therefore, multi-frequency refractive index sensing can be achieved by monitoring the absorption peak frequency shift. Figure 11 As shown, under T = 350 K, when the ambient refractive index increases from 1.00 to 1.08, the sensor maintains a distinct four-frequency strong absorption characteristic in the terahertz band, corresponding to absorption peaks C, D, E, and F. All four main absorption peaks exhibit high absorption efficiency, with peak absorptivity exceeding 95%, indicating that the structure can achieve stable, near-perfect absorption across multiple frequency bands. With the gradual increase in ambient refractive index, each absorption peak exhibits a regular redshift towards lower frequencies, indicating that changes in the external medium environment can effectively modulate the absorption resonant frequency of the device. This phenomenon mainly stems from the increased equivalent dielectric constant around the metasurface after the refractive index increases, leading to an increase in the equivalent optical path corresponding to the resonant mode, thereby causing a decrease in the resonant frequency.

[0062] like Figure 12As shown, further extraction of the resonant frequencies of the four absorption peaks under different refractive indices reveals that absorption peaks C, D, E, and F all exhibit an approximately linear decreasing trend with increasing refractive index, indicating that the sensor possesses good linear sensing response in high-temperature absorption mode. Based on the sensitivity formula, the sensitivities corresponding to absorption peaks C, D, E, and F are calculated to be 625 GHz / RIU, 1025 GHz / RIU, 1062.5 GHz / RIU, and 875 GHz / RIU, respectively. Combined with... Figure 11 and Figure 12 It can be seen that at a high temperature of 350 K, the sensor can not only achieve a strong absorption response at four frequencies, but also each absorption peak shows a significant and stable frequency shift in response to changes in the refractive index of the environment. Compared with a single-frequency absorption sensor, the four-frequency absorption sensor can acquire information about the analyte at multiple frequency points at the same time, which helps to improve detection accuracy, anti-interference ability and identification reliability.

[0063] In summary, this invention proposes a transmissive-absorption switching terahertz sensor based on a vanadium dioxide-graphene metasurface. It possesses excellent multi-frequency refractive index sensing capabilities in both low-temperature dual-frequency transmission sensing mode and high-temperature absorption mode, enabling the realization of dual-mode, multi-frequency, dynamically switchable terahertz sensing functions.

[0064] The above description is merely a specific embodiment of the present invention. Any feature disclosed in this specification may be replaced by other equivalent or similar features unless otherwise specified. All disclosed features, or steps in all methods or processes, may be combined in any way except for mutually exclusive features and / or steps.

Claims

1. A transmissivity-absorption switching terahertz sensor based on a vanadium dioxide-graphene metasurface, comprising several arrayed sensing units, characterized in that, The sensing unit includes: a vanadium dioxide layer, a dielectric layer, and a graphene metasurface layer stacked sequentially from bottom to top; wherein, the graphene metasurface layer 1 is composed of an outer square ring and an eight-part concentric ring structure, the outer square ring is disposed along the edge of the upper surface of the dielectric layer, and the eight-part concentric ring structure is disposed along the center of the upper surface of the dielectric layer; the eight-part concentric ring structure is composed of a central circle, an outer ring, and eight equal branches, the central circle and the outer ring are concentrically disposed, and the center of the central circle coincides with the center of the upper surface of the dielectric layer, and the eight equal branches are connected between the central circle and the outer ring, dividing the area between the central circle and the outer ring into eight equal parts.

2. The vanadium dioxide-graphene metasurface-based transilluminated terahertz sensor according to claim 1, characterized in that, The eight equal branches are located on the midline and diagonal of the upper surface of the medium layer.

3. The vanadium dioxide-graphene metasurface-based transilluminated terahertz sensor according to claim 1, characterized in that, The structural period (P) of the sensing unit x =P y The thickness is 25~35 μm.

4. The vanadium dioxide-graphene metasurface-based transilluminated terahertz sensor according to claim 1, characterized in that, The thickness of the graphene layer is 0.34 nm; the thickness (H1) of the vanadium dioxide layer is 0.2~1.0 μm; and the thickness (H2) of the dielectric layer is 2~4 μm.

5. The vanadium dioxide-graphene metasurface-based transilluminated terahertz sensor according to claim 1, characterized in that, In the graphene metasurface layer, the outer side length of the outer square ring is equal to the structural period of the sensing unit, and the inner side length is (L2) 20~30 μm; the outer circular ring is tangent to the outer square ring, the inner radius (R2) of the outer circular ring is 8~12 μm, the radius (R3) of the central circle is 2~4 μm, and the width (W) of the eight equally divided branches is 1~2 μm.

6. The vanadium dioxide-graphene metasurface-based transilluminated terahertz sensor according to claim 1, characterized in that, The transmissive-absorption switching terahertz sensor has a transmission mode and an absorption mode, with the graphene metasurface layer as the incident surface: when the operating temperature is lower than the phase transition temperature of vanadium dioxide, the transmissive-absorption switching terahertz sensor operates in the transmission mode; when the operating temperature is higher than the phase transition temperature of vanadium dioxide, the transmissive-absorption switching terahertz sensor operates in the absorption mode.

7. The vanadium dioxide-graphene metasurface-based transilluminated terahertz sensor according to claim 6, characterized in that, When the transmissivity-absorption switching terahertz sensor operates in transmission mode, it has a dual-frequency transmission channel; when the transmissivity-absorption switching terahertz sensor operates in absorption mode, it has a four-frequency absorption channel.

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