A terahertz metamaterial sensor with tunable dual polarization direction and preparation method thereof

Abstract

The present invention discloses a terahertz metamaterial sensor with dual polarization direction tunability and a preparation method thereof. The terahertz metamaterial sensor with dual polarization direction tunability comprises N×N structural units arranged periodically on a plane, each structural unit is in the shape of a cuboid, each structural unit comprises a dielectric layer and a graphene layer arranged on the dielectric layer, the graphene layer is two concentrically nested graphene ring structures, wherein the inner ring is a complete graphene ring, and the outer ring is a split graphene ring; the top view of the outer ring is composed of a relative left split ring and a right split ring, wherein the upper and lower notches of the left split ring are on the same vertical vertical line, and the upper and lower notches of the right split ring are also on the same vertical vertical line, but they do not overlap with the upper and lower notches of the left split ring. The terahertz metamaterial sensor of the present invention has two transparent windows in both polarization directions, a wide control range, and high sensitivity.

Description

Technical Field

[0001] The present invention relates to the technical field of terahertz metamaterial sensors, and in particular to a terahertz metamaterial sensor with tunable dual polarization directions and a preparation method thereof. Background Art

[0002] Terahertz (THz) waves refer to electromagnetic waves with a frequency range of 0.1 to 10 THz. They are in the transition band from macroscopic electronics to microscopic optoelectronics. They have unique physical properties such as fingerprint spectrum, safety and penetration in biological molecules, cells and tissues, and have good non-destructive detection characteristics. Therefore, terahertz spectroscopy technology is particularly suitable for non-destructive detection of biological molecules and cells.

[0003] Metamaterial is a new type of artificial material with special properties and composed of sub-wavelength units. Its unique physical properties have attracted much attention from researchers, and graphene, which is composed of a single layer of carbon atoms, is a hot topic in metamaterial research. Graphene has nearly perfect optoelectronic properties such as ultra-low transmission loss and extremely high carrier mobility. In recent years, people have gradually proposed and studied various metamaterial devices based on graphene. Compared with traditional metal elements in nature, graphene can adjust its own Fermi level by changing the gate voltage, thereby changing the conductivity inside the graphene. It has more advantages and potential in tunability.

[0004] Electromagnetically induced transparency (EIT) is a quantum interference phenomenon, and the dispersion characteristics in its transparent window have good application prospects in sensing and slow light. Plasmon induced transparency (PIT) is a kind of EIT effect, which can produce obvious transparency peaks in the transmission spectrum, which has attracted widespread attention. Generally speaking, the excitation of PIT phenomenon is mainly concentrated in two ways: the mutual coupling of bright mode and dark mode and the mutual coupling of bright mode and bright mode.

[0005] With the continuous research on the structural characteristics of terahertz metamaterial sensors, terahertz metamaterial sensors in the shape of metal split ring, h-type, disk, cross, etc. have been discovered in recent years. However, most sensors can only form transparent windows in a single polarization direction, and the regulation range and modulation depth can only be within a very small range. The graphene-based terahertz metamaterial sensors that appeared later can adjust their own Fermi level by changing the gate voltage, thereby changing the conductivity inside the graphene. For example, the invention patent with publication number CN108390156A discloses a terahertz-adjustable polarization wave-insensitive electromagnetically induced transparent device based on metamaterials, including several basic units arranged periodically in the same direction; each basic unit includes an upper graphene layer and a lower dielectric layer, and the dielectric layer is fixed on the substrate device; by carving the graphene, a graphene square ring with adjacent edges opened is formed; when the graphene layer is carved once, the middle of the substrate device is hollowed out and finally coated with a layer of conductive glue. The invention has the same electromagnetically induced transparency phenomenon for linearly polarized light polarized in the x-direction and the y-direction. When the polarization mode of the incident wave changes, there is no need to replace the device. Moreover, the invention can achieve adjustable electromagnetically induced transparency by adjusting the applied voltage without changing the device structure. However, the regulation described in the invention is still in a single polarization direction.

[0006] It can be seen that the existing terahertz metamaterial sensors that can realize the PIT phenomenon can only form a transparent window in one polarization direction, and their control range is small and their sensing performance is generally low. How to design a terahertz metamaterial sensor with a wide control range, high sensitivity and easy processing is a problem worth thinking about and studying. Summary of the invention

[0007] The technical problem to be solved by the present invention is to provide a dual-polarization direction tunable terahertz metamaterial sensor having two transparent windows in both dual-polarization directions, a wide control range, high sensitivity, and a preparation method thereof.

[0008] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0009] A dual-polarization direction tunable terahertz metamaterial sensor comprises N×N structural units periodically arranged on a plane, N is a natural number greater than 0, each structural unit is in a rectangular parallelepiped shape, and the N×N structural units are continuously spliced ​​on a plane; each structural unit comprises a dielectric layer and a graphene layer arranged on the dielectric layer, the graphene layer is two concentrically nested graphene ring structures, wherein the inner ring is a complete graphene ring, and the outer ring is a split graphene ring; the top view of the outer ring consists of a relative left split ring and a right split ring, wherein the upper and lower notches of the left split ring are on the same vertical longitudinal line, and the upper and lower notches of the right split ring are also on the same vertical longitudinal line, but they do not overlap with the upper and lower notches of the left split ring.

[0010] In the above technical solution, it is preferred that the x component of the left split ring from the center of the circle is 1 to 3 μm, and the x component of the right split ring from the center of the circle is 3 to 5 μm. Correspondingly, it is preferred that the outer ring diameter of the inner ring in each structural unit is 10 to 20 μm, the outer ring diameter of the outer ring is 15 to 30 μm, and the ring widths of the inner and outer rings are both 1 to 3 μm; and it is preferred that the period of the structural unit in the x-axis direction is 15 to 45 μm, and the period in the y-axis direction is 15 to 45 μm. It is further preferred that the outer ring diameter of the inner ring in each structural unit is 10 to 15 μm, the outer ring diameter of the outer ring is 18 to 22 μm, and the ring widths of the inner and outer rings are both 1 to 3 μm; and the period of the structural unit in the x-axis direction is 25 to 35 μm, and the period in the y-axis direction is 25 to 35 μm.

[0011] In the above technical solution, preferably each structural unit is in the shape of a cube, and the value of N is preferably ≥3.

[0012] As a preferred embodiment of the present application, the preferred structural unit has a period of 30 μm in the x-axis direction and a period of 30 μm in the y-axis direction; the outer ring diameter of the inner ring is 12 μm, the outer ring diameter of the outer ring is 20 μm, and the ring widths of the inner and outer rings are both 2 μm; the x component of the left split ring from the center of the circle is 2 μm, and the x component of the right split ring from the center of the circle is 4 μm.

[0013] In the above technical solution, the thickness of the graphene layer is preferably 1 to 5 nm, more preferably 1 to 2 nm. The material and thickness of the dielectric layer are the same as those in the prior art. Specifically, the material of the dielectric layer can be any one of silicon dioxide, polyimide, quartz crystal, cycloolefin polymer and polytetrafluoroethylene (PTFE), etc., and silicon dioxide is preferred in this application. The thickness of the dielectric layer is preferably 0.1 to 0.5 μm, more preferably 0.1 to 0.2 μm.

[0014] The method for preparing the dual-polarization direction tunable terahertz metamaterial sensor of the present invention comprises the following steps:

[0015] 1) selecting a dielectric material as a dielectric layer and performing pretreatment to obtain a dielectric layer;

[0016] 2) Preparing graphene on a metal substrate;

[0017] 3) spin coating the polymethyl methacrylate solution on the graphene surface on the metal substrate to obtain a substrate with a polymethyl methacrylate film on the graphene surface;

[0018] 4) etching the substrate with the polymethyl methacrylate film on the graphene surface to remove the substrate, and then transferring the graphene and the polymethyl methacrylate film on the surface to the dielectric layer;

[0019] 5) Based on the dielectric layer obtained in step 4), coating, exposure, development and fixing are performed to obtain a photoresist mask of the graphene layer, and then etching and stripping processes are performed to obtain the result.

[0020] The operations not described in detail in the above preparation method are the same as those in the prior art. The above preparation method is used to obtain a dielectric layer and an imaged graphene layer located on the dielectric layer. In specific applications, a metal gate electrode and a conductive layer need to be prepared on the graphene layer, and the specific operations are the same as those in the prior art.

[0021] Compared with the prior art, the present invention is characterized in that:

[0022] 1. The present invention uses a metamaterial basic model. In the x-polarization direction, after the excitation of the terahertz wave, the inner ring is equivalent to the bright film in the three-harmonic oscillator model, and the two split rings constituting the outer ring act as two quasi-dark films; in the y-polarization direction, the right split ring in the outer ring acts as the bright film in the three-harmonic oscillator model, and the left split ring and the inner ring in the outer ring are equivalent to two quasi-dark films. Since the bright film and the two quasi-dark films in the two polarization directions are not of the same structure, the one bright film and the two quasi-dark films in different polarization directions interfere destructively with each other (couple), so that the sensor can form two different transparent windows in the dual polarization directions. At the same time, the obtained sensor can be regulated in the frequency range of 0.5 to 4.5 THz, with a wide regulation range.

[0023] 2. The sensor of the present invention realizes the characteristic of being insensitive to the angle of incident light, which is conducive to reducing detection errors and realizing rapid and highly sensitive detection of the object to be detected.

[0024] 3. The sensor described in the present invention can achieve dual regulation of changing the polarization direction of incident light and the Fermi level of graphene.

[0025] 4. The maximum sensing sensitivity of the sensor described in the present invention can reach 1.1THz / RIU, which effectively solves the problem of low sensitivity of existing terahertz metamaterial sensors. Compared with existing terahertz metamaterial sensors, the sensor proposed in the present invention can reduce detection costs, simplify experimental operations, and reduce experimental difficulty.

[0026] 5. The sensor of the present invention has a simple structure, superior performance, and an easy manufacturing process, which meets the demand for cost-effectiveness in the sensor design process. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a top view of a structural unit of the dual-polarization direction tunable terahertz metamaterial sensor of the present invention.

[0028] Figure 2 For Figure 1 The transmission spectrum diagram of the sensor sample in the dual polarization directions is obtained by arranging the sensor structural units of the structure shown in a periodic manner of 13×13 on the plane.

[0029] Figure 3 For Figure 1 The electric field distribution diagram of the sensor sample at positions F1 to F5 in the x-polarization direction is obtained by arranging the sensor structural units of the structure shown in a periodic manner of 13×13 on a plane.

[0030] Figure 4 For Figure 1 The electric field distribution diagram of the sensor sample at positions f1 to f5 in the y polarization direction is obtained by arranging the sensor structural units of the structure shown in a periodic manner of 13×13 on a plane.

[0031] Figure 5 For Figure 1 The transmission spectrum diagram of the sensor sample obtained by arranging the sensor structural units of the structure in a periodic manner of 13×13 on the plane at different incident angles in the x-polarization direction.

[0032] Figure 6 For Figure 1 The transmission spectrum diagram of the sensor sample obtained by arranging the sensor structural units of the structure shown in a periodic manner of 13×13 on the plane at different incident angles in the y-polarization direction.

[0033] Figure 7 For Figure 1 The transmission spectrum of the sensor sample obtained by arranging the sensor structural units of the structure shown in a 13×13 periodic manner on the plane changes with the Fermi level in the x-polarization direction.

[0034] Figure 8 For Figure 1The transmission spectrum of the sensor sample obtained by arranging the sensor structural units of the structure shown in a 13×13 periodic manner on a plane changes with the Fermi level in the y-polarization direction.

[0035] Fig. 9 For Figure 1 The frequency variation diagram of the sensor sample obtained by arranging the sensor structural units of the structure in a periodic manner of 13×13 on a plane when the Fermi level changes in each valley in the x-polarization direction.

[0036] Fig.10 For Figure 1 The frequency variation diagram of the sensor sample obtained by arranging the sensor structural units of the structure shown in a 13×13 periodic manner on a plane when the Fermi level changes in each valley in the y-polarization direction.

[0037] Fig.11 For Figure 1 The sensor structure units of the structure shown are arranged periodically in 13×13 on a plane, and the sensor sample corresponds to the transmission spectrum of the object to be tested with different refractive indices when it is in the x-polarization direction.

[0038] Fig.12 For Figure 1 The sensor structure units of the structure shown are arranged periodically in 13×13 on a plane, and the sensor sample corresponds to the transmission spectrum of the object to be tested with different refractive indices when it is in the y-polarization direction.

[0039] Fig.13 For Figure 1 The sensor sample with the structure shown is obtained by arranging the sensor structural units in a 13×13 periodic manner on a plane, and the frequency shift curve of the transmission window in the dual polarization directions when the refractive index of the object to be measured changes.

[0040] Fig.14 This is a three-dimensional structural diagram of a structural unit of the dual-polarization direction tunable terahertz metamaterial sensor of the invention.

[0041] Fig.15 This is a structural diagram of the terahertz graphene metamaterial device.

[0042] The numbers in the figure are:

[0043] 1 dielectric layer, 2 inner ring, 3 outer ring, 3-1 left split ring, 3-2 right split ring, 4 metal gate electrode, 5 ion gel layer, W ring width, x1 x component of the left split ring, x2 x component of the right split ring, Ф1 outer ring diameter of the inner ring, Ф2 outer ring diameter of the outer ring. DETAILED DESCRIPTION

[0044] The dual-polarization direction tunable terahertz metamaterial sensor described in the present invention is a periodic structure, including N×N structural units periodically arranged in the xy plane, N is a natural number greater than 0, and its value is preferably ≥3, each structural unit is in the shape of a cuboid, more preferably in the shape of a cube, and the N×N structural units are continuously spliced ​​in the plane; each structural unit includes a dielectric layer 1 and a graphene layer arranged on the dielectric layer 1.

[0045] The material of the dielectric layer 1 can be any one of silicon dioxide, polyimide, quartz crystal, cycloolefin polymer and polytetrafluoroethylene (PTFE), etc. The thickness of the dielectric layer 1 is usually 0.1-0.5 μm, and more preferably 0.1-0.2 μm.

[0046] The graphene layer is a structure of two concentrically nested graphene rings, specifically a concentrically nested inner ring 2 and outer ring 3, wherein the inner ring 2 is a complete graphene ring, and the outer ring 3 is a split graphene ring; the top view of the outer ring 3 is composed of a relative left split ring 3-1 and a right split ring 3-2, wherein the upper and lower notches of the left split ring 3-1 are on the same vertical vertical line, and the upper and lower notches of the right split ring 3-2 are also on the same vertical vertical line, but the upper and lower notches of the right split ring 3-2 do not overlap with the upper and lower notches of the left split ring 3-1. The thickness of the graphene layer is preferably 1 to 5 nm, and more preferably 1 to 2 nm.

[0047] In each structural unit, the x component x1 of the left split ring 3-1 constituting the outer ring 3 from the center of the circle and the x component x2 of the right split ring 3-2 from the center of the circle have an important influence on whether the sensor can have two transparent windows in both polarization directions. In this application, the x component x1 of the left split ring 3-1 from the center of the circle is 1 to 3 μm, and the x component x2 of the right split ring 3-2 from the center of the circle is 3 to 5 μm. Correspondingly, the period in the x-axis direction is 15 to 45 μm, and the period in the y-axis direction is 15 to 45 μm. In each structural unit, the outer ring diameter Φ1 of the inner ring 2 is 10 to 15 μm, the outer ring diameter Φ2 of the outer ring 3 is 18 to 22 μm, and the ring width W of the inner ring 2 and the outer ring 3 are both 1 to 3 μm. It is further preferred that in each structural unit, the outer ring diameter Φ1 of the inner ring 2 is 10 to 15 μm, the outer ring diameter Φ2 of the outer ring 3 is 18 to 22 μm, and the ring width W of the inner ring 2 and the outer ring 3 are both 1 to 3 μm; and the period of the structural unit in the x-axis direction is 25 to 35 μm, and the period in the y-axis direction is 25 to 35 μm.

[0048] The method for preparing the dual-polarization direction tunable terahertz metamaterial sensor comprises the following steps:

[0049] 1) selecting a dielectric material for dielectric layer 1 and performing pretreatment to obtain dielectric layer 1;

[0050] 2) preparing graphene on a metal substrate (which can be any one of gold, silver, copper, aluminum, etc.);

[0051] 3) spin coating the polymethyl methacrylate solution on the graphene surface on the metal substrate to obtain a substrate with a polymethyl methacrylate film on the graphene surface;

[0052] 4) etching the substrate with the polymethyl methacrylate film on the graphene surface to remove the substrate, and then transferring the graphene and the polymethyl methacrylate film on the surface to the dielectric layer 1;

[0053] 5) Based on the dielectric layer 1 obtained in step 4), coating, exposure, development and fixing are performed to obtain a photoresist mask of the graphene layer, and then etching and stripping processes are performed to obtain the result.

[0054] The operations not described in detail in the above preparation method are the same as those in the prior art. The above preparation method is used to obtain the dielectric layer 1 and the imaged graphene layer on the dielectric layer 1. In specific applications, it is also necessary to prepare a metal gate electrode 4 and a conductive layer (i.e., an ion gel layer 5) on the graphene layer, and the specific operations are the same as those in the prior art.

[0055] The present invention is further described in detail below in conjunction with preferred embodiments and drawings to provide a better understanding of the present invention.

[0056] Example 1

[0057] by Figure 1 and Fig.14 The structural diagram shown illustrates the structure and preparation of the sensor described in the present invention.

[0058] See also Figure 1 and Fig.14 , each structural unit of the sensor of the present invention includes a dielectric layer 1 and a graphene layer arranged on the dielectric layer 1, and the period of the structural unit in the x-axis direction and the period in the y-axis direction are both 30μm; the outer ring diameter Φ1 of the inner ring 2 constituting the graphene layer is 12μm, the outer ring diameter Φ2 of the outer ring 3 is 20μm, and the ring width W of the inner ring 2 and the outer ring 3 are both 2μm; the x component x1 of the left split ring 3-1 constituting the outer ring 3 is 2μm from the center of the circle, and the x component x2 of the right split ring 3-2 is 4μm from the center of the circle. The material of the dielectric layer 1 is silicon dioxide, whose dielectric constant is 3.9 and the thickness is 0.1μm; the thickness of the graphene layer is 1nm. The surface conductivity of graphene is simply obtained using the Drude model:

[0059]

[0060] The carrier relaxation time is:

[0061]

[0062] The carrier mobility μ is about 4m 2 / Vs, Fermi velocity V F For 10 6 m / s, is Planck's constant, e is the electron charge, τ is the relaxation time (s), ω (rad / s) is the angular frequency, E F (eV) is the Fermi level.

[0063] The structure adsorbed on graphene is a non-dispersive ion gel layer (dielectric constant is 1.82), and the ultrathin ion gel layer as an electrode medium has almost no effect on the terahertz spectrum, so it can be ignored in the simulation.

[0064] The sensor designed in this application needs to be tested in a THz time-domain spectroscopy system. Since the diameter of the THz beam on the focusing plane in the THz time-domain spectroscopy system is about 5 mm, the effective size of the sensor should be greater than 5 mm × 5 mm. Specifically, it is 15 mm × 15 mm in this embodiment.

[0065] The preparation method of the sensor comprises the following steps:

[0066] 1) A silicon dioxide wafer with a thickness of 100 nm was placed in an acetone solution for ultrasonic treatment for 15 minutes, then placed in anhydrous ethanol for ultrasonic treatment for 20 minutes, and finally placed on a heating plate and heated at 110° C. for 10 minutes to obtain a pretreated silicon dioxide wafer (hereinafter referred to as silicon wafer).

[0067] 2) The copper foil substrate is pretreated with ferric nitrate, and then methane is used as a carbon source, argon and hydrogen are used as a protective gas and a reducing gas, respectively. The pretreated copper foil substrate is heated to 1080°C, and then the temperature is reduced to 1020°C, and then further reduced to room temperature. After completion, a graphene layer is obtained on the copper foil for standby use.

[0068] 3) The polymethyl methacrylate solution was spin-coated onto the graphene surface on the copper foil by mechanical spin coating, pre-spinning at 600 r / min for 20 seconds, then spinning at 3000 r / min for 100 seconds, and then spinning at 600 r / min for 20 seconds. After spin coating, the copper foil was placed on a drying table and dried at 120° C. for 20 minutes to obtain a copper foil with a polymethyl methacrylate film on the graphene surface.

[0069] 4) placing the copper foil obtained in step 3) in an oxygen plasma etcher for etching at a power of 90%, a pressure of 0.2 Pa, and a time of 30 seconds; then placing the treated copper foil in a saturated solution of ferric chloride and soaking it for 4 hours to etch away all the copper, and when only graphene and polymethyl methacrylate are left floating in the solution, replacing it with deionized water for 5 times to completely replace the ferric chloride, and finally using the pretreated silicon wafer to fish out the graphene with polymethyl methacrylate from the solution, drying it, and soaking it in an acetone solution for 9 hours to remove the polymethyl methacrylate, and washing it with deionized water for 3 times to complete the transfer of the graphene to the pretreated silicon wafer.

[0070] 5) using a mechanical spin coating method (first spin coating at a speed of 600 rpm for 20 seconds, then spin coating at a speed of 4000 rpm for 60 seconds, and then spin coating at a speed of 600 rpm for 20 seconds) to coat a photoresist with a thickness of 1 μm on the graphene surface, and bake at 100° C. for 60 seconds, and perform exposure, development and fixing processes after cooling to obtain a photoresist mask for the graphene layer; then perform oxygen plasma etching to etch away the graphene not masked by the photoresist, and then soak the silicon wafer in an acetone solution for 24 hours to remove the photoresist mask on the silicon wafer, wash it with deionized water three times and blow dry it to obtain an imaged graphene structure, that is, obtain an imaged graphene layer on the dielectric layer 1, as shown in FIG. Fig.14 shown.

[0071] 6) Spin-coat a photoresist on the graphene layer using a mechanical spin coating method, with a coating thickness of 1 μm, and bake at 100° C. for 60 seconds. After cooling, expose, develop and fix to obtain a photoresist mask for preparing a metal gate electrode 4. Then, deposit metal on the photoresist mask for the metal gate electrode 4 using a material growth process, and then soak it in an acetone solution for 24 hours to strip off the metal and remove the photoresist. After washing with deionized water three times, blow dry to obtain a metal gate electrode 4 with a thickness of 1 μm.

[0072] 7) A 1 μm thick ion gel layer 5 is spin-coated on the graphene layer by mechanical spin coating, and the terahertz graphene metamaterial device is prepared after baking at 100° C. for 60 seconds. Fig.15 shown.

[0073] Embodiment 2: Figure 1 The sensor structural units of the structure shown are arranged periodically in a 13×13 pattern on a plane to obtain a sensor sample, and the transmission characteristics, resonance mechanism, incident angle insensitivity characteristics, etc. of the obtained sensor sample are analyzed.

[0074] 1. Transmission characteristics

[0075] In order to explore the transmission characteristics of the sensor, the present invention performs numerical calculations of the finite element method based on CST Microwave studio software.

[0076] Figure 2 Figure 1 is the transmission spectrum of the terahertz metamaterial sensor. It can be seen from the figure that the sensor forms double transparent windows in both polarization directions (i.e., the x-polarization direction and the y-polarization direction). The frequency range of the low-frequency double transparent window is 1 to 3.5 THz, and the frequency range of the high-frequency double transparent window is 2.5 to 4.5 THz. From the transmission spectrum, it can be seen that the present invention can achieve the effect of double transparent windows in dual polarization directions.

[0077] 2. Resonance mechanism analysis

[0078] In order to explore the resonance mechanism of the terahertz metamaterial sensor, the present invention displays the electric field diagrams in two polarization directions through CST software. Figure 3 and Figure 4 They are the electric field distributions of the terahertz metamaterial sensor at f1 (F1) to f5 (F5) in the dual polarization directions. Figure 3 It can be seen that in the x polarization direction, F1 = 2.804 THz, F3 = 3.276 THz, F5 = 3.744 THz, ( Figure 4 It shows that the electric field of f1=1.672THz, f3=1.968THz, f5=2.956THz in the y-polarization direction is mainly distributed on the edge or structure of the inner graphene ring, the outer left graphene ring and the outer right graphene ring. This is because these three parts are directly coupled with the incident light at the corresponding frequency, so the corresponding inclination angle is formed in the corresponding transmission spectrum. The electric field distribution of F2=2.996THz and F4=3.578THz in the x-polarization direction (the corresponding frequencies in the y-polarization direction are f2=1.810THz and f4=2.296THz) shows that it is not the resonance of a single local structure, but the result of overall resonant coupling, that is, the coupling interference of two quasi-dark films and one bright film.

[0079] 3. Analysis of incident angle insensitivity characteristics

[0080] In order to test the sensitivity of the sensor to changes in the incident angle, the present invention studies the transmission characteristics of terahertz waves under different incident angles (0°, 20°, 40°, 60° and 80°). Figure 5 and Figure 6 As shown in the figure, as the incident angle changes continuously, the resonant frequency in the transmission spectrum in the dual polarization direction is affected very little. Moreover, when the incident angle exceeds 60°, the transmittance of the dual transparent window is still over 80%. It can be seen that the terahertz metamaterial sensor is insensitive to changes in the incident angle.

[0081] 4. Tunability

[0082] Based on the above research, we know that the special advantage of graphene lies in its tunability (changing the resonant frequency of the plasma by changing the Fermi level of graphene). Figure 7 and Figure 8 It can also be observed that when the Fermi level is adjusted between 0.7 and 1.0 eV, the transmission spectrum in the dual polarization direction shows a very obvious blue shift. Fig. 9 and Fig.10 The frequency variation diagram of the three troughs in the dual polarization directions, namely F1(f1), F3(f3), and F5(f5), as the Fermi level changes. It can be seen from the figure that the resonant frequency of the sensor increases linearly with the increase of the Fermi level in both the x-polarization direction and the y-polarization direction. This is because the resonant frequency of each component of the sensor also increases linearly with the change of the Fermi level, resulting in the blue shift of the final coupled resonant frequency. Figure 7 and Figure 8 It can be seen that the change of Fermi level has no significant effect on the shape of the transmission spectrum, so the change of Fermi level mainly affects the change of resonance frequency. By utilizing the tunability of graphene, the metamaterial sensor can be finely modulated within the adjustable range.

[0083] 5. Sensing performance

[0084] When a traditional terahertz metamaterial sensor detects an object to be tested, the object to be tested is attached to the surface of the periodic structure layer. Therefore, the sensitivity characteristics of the sensor can be obtained by studying the response degree of the sensor to changes in the physical parameters of the surrounding medium.

[0085] Sensor sensitivity is an important indicator to measure the sensor performance, which indicates the degree of spectrum shift under the unit refractive index change of the object to be measured. The calculation formula of sensor sensitivity is:

[0086]

[0087] Where Δf is the spectrum shift and Δn is the change in the refractive index of the object to be measured.

[0088] Fig.11 and Fig.12 They are the transmission spectra corresponding to the objects with different refractive indices in the x-polarization direction and the y-polarization direction of the sensor. Fig.11 and Fig.12 It can be seen that with the increase of the refractive index of the object to be measured, the transmission spectrum of the sensor red-shifts. It is calculated that the sensitivities at the double transparent windows f2 and f4 in the y-polarization direction are S1=0.5THz / RIU and S2=0.7THz / RIU, respectively, and the sensitivities at the double transparent windows F2 and F4 in the x-polarization direction are S3=0.9THz / RIU and S4=1.1THz / RIU, respectively.

[0089] Fig.13The frequency variation of the dual transparent windows of the sensor in the dual polarization directions is given by changing the refractive index of the object to be measured. Fig.13 It can be seen that, no matter in the x-polarization direction or the y-polarization direction, the frequency shift of the two transparent windows relative to the refractive index of the object to be measured increases linearly, which also indirectly proves that the sensing performance of the sensor is very good.

Claims

1. A dual-polarization direction tunable terahertz metamaterial sensor, comprising N×N structural units periodically arranged on a plane, N being a natural number greater than 0, each structural unit being in a rectangular parallelepiped shape, and the N×N structural units being continuously spliced ​​on a plane; wherein: Each structural unit comprises a dielectric layer (1) and a graphene layer arranged on the dielectric layer (1), wherein the graphene layer is a structure of two concentrically nested graphene rings, wherein the inner ring (2) is a complete graphene ring, and the outer ring (3) is a split graphene ring; the outer ring (3) is formed of a left split ring (3-1) and a right split ring (3-2) in a top view, wherein the upper and lower notches of the left split ring (3-1) are on the same vertical longitudinal line, and the upper and lower notches of the right split ring (3-2) are also on the same vertical longitudinal line, but they do not overlap with the upper and lower notches of the left split ring (3-1).

2. The dual-polarization direction tunable terahertz metamaterial sensor according to claim 1, characterized in that: The x component (x1) of the left split ring (3-1) from the center of the circle is 1 to 3 μm, and the x component (x2) of the right split ring (3-2) from the center of the circle is 3 to 5 μm.

3. The dual-polarization direction tunable terahertz metamaterial sensor according to claim 1, characterized in that: The outer ring diameter (Φ1) of the inner ring (2) is 10 to 20 μm, the outer ring diameter (Φ2) of the outer ring (3) is 15 to 30 μm, and the ring widths (W) of the inner ring 2 and the outer ring (3) are both 1 to 3 μm.

4. The dual-polarization direction tunable terahertz metamaterial sensor according to claim 1, characterized in that: The period of the structural unit in the x-axis direction is 15 to 45 μm, and the period in the y-axis direction is 15 to 45 μm.

5. The dual-polarization direction tunable terahertz metamaterial sensor according to claim 1, characterized in that: Each structural unit is in the shape of a cube.

6. The dual-polarization direction tunable terahertz metamaterial sensor according to claim 1, characterized in that: The period of the structural unit in the x-axis direction is 30 μm, and the period in the y-axis direction is 30 μm; the outer ring diameter (Φ1) of the inner ring (2) is 12 μm, the outer ring diameter (Φ2) of the outer ring (3) is 20 μm, and the ring width (W) of the inner ring (2) and the outer ring (3) are both 2 μm; the x component (x1) of the left split ring (3-1) from the center of the circle is 2 μm, and the x component (x2) of the right split ring (3-2) from the center of the circle is 4 μm.

7. The method for preparing the dual-polarization direction tunable terahertz metamaterial sensor according to claim 1, comprising the following steps: 1) selecting a dielectric material to be used as the dielectric layer (1), and performing pretreatment to obtain the dielectric layer (1); 2) Preparing graphene on a metal substrate; 3) spin coating the polymethyl methacrylate solution on the graphene surface on the metal substrate to obtain a substrate with a polymethyl methacrylate film on the graphene surface; 4) etching the substrate with the polymethyl methacrylate film on the surface of the graphene to remove the substrate, and then transferring the graphene and the polymethyl methacrylate film on the surface to the dielectric layer (1); 5) Based on the dielectric layer (1) obtained in step 4), coating, exposure, development and fixing are performed to obtain a photoresist mask of the graphene layer, and then etching and stripping processes are performed to obtain the result.

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