Tunable terahertz metamaterial sensor based on triple plasma induced transparency

Abstract

The invention relates to the technical field of terahertz metamaterial sensors, and provides a tunable terahertz metamaterial sensor based on triple plasma induced transparency in order to solve the problems that an existing terahertz wave band sensor is insufficient in sensitivity and tunability and the like. The designed single-layer graphene structure can be divided into three parts, wherein the middle part is composed of a cross-shaped ellipse and four elliptical rings, and the periphery is obtained by rotating a quarter of a circle. Wherein graphene is manufactured on the surface of a silicon dioxide substrate, and a layer of ionic gel with the dielectric constant of 1.82 is coated on a graphene pattern. Due to the central symmetry of the metamaterial, when incident light is terahertz waves, the transmission spectrum of the metamaterial can generate the same triple plasma induced transparency (PIT) phenomenon in two polarization directions. The terahertz metamaterial sensor formed by the invention has high sensitivity as high as 1.84 Thz / RIU, high quality factor and insensitivity to polarization and incident angle change. In addition, the device also has tunability, can control the frequency shift and switch-on and switch-off of the triple PIT window by adjusting the external voltage, and is suitable for a complex experimental environment.

Description

Technical Field

[0001] The invention relates to the technical field of terahertz metamaterial sensors, and in particular to an invention of a tunable terahertz metamaterial sensor based on triple plasma induced transparency. Background Art

[0002] Terahertz (THz) waves have fingerprint spectrum characteristics. Many biological molecules can resonate under the action of THz waves. Therefore, in the actual detection of THz band, biological molecules can be directly identified and their physical and chemical characteristics can be extracted. This information is as unique as fingerprints, which provides a new direction for label-free detection of biological molecules. Terahertz waves will not cause high-intensity radiation damage to the substances being tested, so non-destructive detection of the substances to be tested can be achieved, making THz technology highly valued by researchers in the field of sensing.

[0003] Metamaterials, that is, artificial electromagnetic media constructed on a subwavelength scale, are composite materials with physical properties such as negative refractive index effect. Metamaterials exhibit significant electromagnetic response in the terahertz band, making them ideal materials for modulating terahertz waves and widely used in fields such as perfect absorption, stealth technology, biosensing and communication antennas. Since traditional metal metamaterials cannot control their dielectric constants, the implementation of PIT faces the limitations of fixed spectral response and operating frequency. Therefore, the special electrical and optical properties of graphene give it better application prospects in metasurface structures.

[0004] Electromagnetic induced transparency (EIT) is a process in which the coherence effect of coupled light beams in atomic systems causes a specific transmission band to appear in an opaque medium. This transmission band also produces physical properties such as dispersion, which enables EIT to be applied in sensing, modulation, slow light and other fields. In order to overcome the experimental limitations required by EIT, many researchers have turned their attention to metamaterials with unique optical properties. Plasmon induced transparency (PIT) is an effect similar to EIT, which has attracted widespread attention and research due to its strong structural adaptability and simple implementation.

[0005] At present, terahertz sensors generally have the problem of low detection sensitivity in spectral detection technology. Most sensors using the PIT phenomenon can only form transparent windows in a single polarization direction and the number of transparent windows is small. In addition, the application frequency band of terahertz sensors designed using traditional metal materials is small and fixed. Therefore, it is particularly important to design a terahertz metamaterial sensor with a wide range of applications and high sensitivity. Summary of the invention

[0006] In view of the above problems, the present invention provides a highly sensitive, polarization-insensitive and tunable multi-PIT terahertz metamaterial sensor.

[0007] To achieve the above object, the present invention is implemented through the following technical solutions:

[0008] A tunable terahertz metamaterial sensor based on triple plasma induced transparency includes a silicon dioxide substrate layer, a single-layer graphene metamaterial layer attached to the substrate, an ion gel layer and a metal electrode, wherein the graphene metamaterial layer includes N×N square resonant units periodically arranged on the substrate plane, where N is a natural number greater than 0. The resonant unit consists of three parts: the middle part consists of a cross-shaped ellipse and four elliptical rings, and the surrounding parts are obtained by rotating a quarter circle.

[0009] In the above technical solution, the major axis radius of the intermediate graphene cross ellipse is preferably 4.9-5.2 μm, and the minor axis radius is 0.3-0.7 μm; the major axis radius of the four graphene elliptical rings is preferably 3.0-3.4 μm, and the minor axis radius is 1.6-2.0 μm; the inner diameter of the four graphene quarter rings is preferably 2.1-2.5 μm, and the outer diameter of the ring is 3.0-3.4 μm. The corresponding preferred unit structure has a period of 18-24 μm in the x-axis and y-axis directions.

[0010] In the above technical solution, it is preferred that each unit structure is in a cube shape, and the value of N is preferably ≥3.

[0011] As a preferred embodiment of the present application, the period of the structural unit in the x-axis and y-axis directions is preferably 20 μm; the major axis radius of the intermediate graphene cross ellipse is preferably 5.1 μm, and the minor axis radius is 0.5 μm; the major axis radius of the four graphene elliptical rings is preferably 3.2 μm, and the minor axis radius is 1.8 μm; the inner diameter of the four graphene quarter rings is preferably 2.3 μm, and the outer diameter of the ring is 3.2 μm.

[0012] In the above technical solution, the graphene layer is a single-layer graphene, and 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 one of silicon dioxide, polyimide, cycloolefin polymer and polytetrafluoroethylene (PTFE), etc. The present invention is preferably silicon dioxide, and the thickness of the dielectric layer is preferably 0.1 to 0.5 μm, more preferably 0.1 μm. The thickness of the ion gel layer is 0.1 to 0.5 μm, more preferably 0.1 μm.

[0013] Compared with the prior art, the present invention is characterized in that:

[0014] 1. The present invention uses graphene metamaterial as the basic model, and can achieve triple PIT effect in dual polarization directions through the coupling of a bright mode and three quasi-dark modes of the resonator. At the same time, the obtained sensor can be regulated in the frequency range of 2.0 to 5.0 THz, with a wide regulation range.

[0015] 2. The sensor of the present invention realizes the characteristic of polarization insensitivity. The transmission spectrum changes slightly when the incident angle exceeds 60°, but still has a high transmittance, showing excellent incident angle dependence. Therefore, the structure can be used in complex environments, greatly improving its applicability.

[0016] 3. The sensor of the present invention can change the Fermi level of graphene by adjusting the external voltage, thereby achieving regulation of the PIT window position of the sensor.

[0017] 4. The maximum sensitivity of the sensor of the invention can reach 1.84THz / RIU, which effectively solves the problem of low sensitivity of existing terahertz metamaterial sensors. Compared with existing peptide synthetic metamaterial sensors, the sensor proposed by the present invention can reduce detection costs, simplify experimental operations, and reduce experimental difficulty.

[0018] 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

[0019] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below according to specific embodiments of the present invention in conjunction with the accompanying drawings, wherein

[0020] Figure 1 The figure is a top view of a unit structure of the sensor described in the present invention.

[0021] Figure 2 for Figure 1 The schematic diagram of the three-dimensional structure of the sensor unit structure is shown in the plane periodic arrangement.

[0022] Figure 3 for Figure 2 Side view of the sensor structure shown.

[0023] In the figure, Au is the copper metal electrode, Ion gel is the ion gel layer, Graphene is the graphene layer, SiO2 is the silicon dioxide substrate layer, and Vg is the external voltage.

[0024] Figure 4 It is a transmission spectrum diagram of the sensor sample of the present invention in the x-polarization direction and transmission curves corresponding to different resonance units.

[0025] Figure 5 It is a transmission spectrum diagram of the sensor sample of the present invention in the y-polarization direction and transmission curves corresponding to different resonance units.

[0026] Figure 6 It is a simulation diagram of the sensor described in the present invention at different polarization angles.

[0027] Figure 7 It is a simulation diagram of the sensor described in the present invention at different incident angles.

[0028] Figure 8 This is a transmission spectrum diagram of the sensor described in the present invention as the Fermi level changes.

[0029] Fig. 9 The transmission spectrum of the sensor according to the present invention changes with the thickness of the analyte.

[0030] Fig.10 It is the sensitivity and FOM value of three PIT windows Peak1 to Peak3 in the transmission spectrum diagram of the sensor described in the present invention. DETAILED DESCRIPTION

[0031] The tunable terahertz metamaterial sensor based on triple plasma induced transparency described in the present invention is a periodic structure, including N×N structural units arranged periodically on the xy plane, N is a natural number greater than 0, and its value is preferably ≥3, each structure is rectangular, more preferably square, and the N×N structural units are continuously spliced ​​on the plane; the longitudinal structure of the sensor is "metal electrode-ion gel layer-graphene layer-dielectric layer-metal electrode".

[0032] The material of the metal electrode can be any one of stable metal elements such as copper, gold, silver, etc., and is more preferably metallic copper.

[0033] The material of the dielectric layer can be any one of silicon dioxide, polyimide, quartz crystal, cycloolefin polymer and polytetrafluoroethylene (PTFE), more preferably silicon dioxide; the thickness of the dielectric layer is usually 0.1-0.5 μm, more preferably 0.1 μm.

[0034] The graphene layer is a single-layer graphene, and the thickness is not calculated. Each unit structure of the graphene layer can be divided into three parts: the middle part is composed of a cross ellipse and four elliptical rings, and the surrounding parts are obtained by rotating a quarter circle.

[0035] In each unit structure, the radius of the major axis of the middle graphene cross ellipse is 4.9-5.2 μm, and the radius of the minor axis is 0.3-0.7 μm; preferably, the radius of the major axis of the four graphene elliptical rings is 3.0-3.4 μm, and the radius of the minor axis is 1.6-2.0 μm; preferably, the inner diameter of the four graphene quarter rings is 2.1-2.5 μm, and the outer diameter of the ring is 3.0-3.4 μm. The corresponding preferred unit structure has a period of 18-24 μm in the x-axis and y-axis directions.

[0036] The structure and preparation of the sensor of the present invention are described below in conjunction with preferred embodiments and drawings.

[0037] Example 1

[0038] by Figure 1 , Figure 2 , Figure 3 The sensor structure diagram shown illustrates the structure and preparation of the sensor described in the present invention.

[0039] See also Figure 2 and Figure 3 , the terahertz wave is incident vertically on the sensor sample to obtain the corresponding projection spectrum. The longitudinal structure of the sensor described in the present invention is "metal electrode-ion gel layer-graphene layer-dielectric layer-metal electrode", and the period of the structural unit in the x-axis and y-axis directions is 20μm;

[0040] The radius of the long axis of the middle graphene cross ellipse is 5.1 μm, and the radius of the short axis is 0.5 μm; preferably, the radius of the long axis of the four graphene elliptical rings is 3.2 μm, and the radius of the short axis is 1.8 μm; preferably, the inner diameter of the four graphene quarter rings is 2.3 μm, and the outer diameter of the ring is 3.2 μm. The dielectric layer material is silicon dioxide, with a relative dielectric constant of 3.9 and a thickness of 0.1 μm; the thickness of the graphene layer is 1 nm. Using the Drude model, the surface conductivity of graphene is simply obtained as:

[0041]

[0042] The carrier relaxation time is:

[0043] τ=μE F / eV F 2 (2)

[0044] The carrier mobility is about 4m 2 / V F , Fermi velocity V F 6m 2 / s, is Planck's constant, e is the electron charge, τ is the relaxation time, ω is the angular frequency, E F is the Fermi level.

[0045] A layer of ion gel with a dielectric constant of 1.82 is coated on the graphene pattern. The Fermi level of graphene can be modulated by applying a bias voltage to the upper and lower electrodes. Since the ion gel layer is thin and optically transparent, the absorption of terahertz waves is weak and can be ignored in the simulation.

[0046] The sensor designed in the present invention needs to be tested in a terahertz time-domain spectroscopy system (THz-TDS). Since the beam diameter on the focusing plane in the terahertz time-domain spectroscopy system is about 5 mm, the effective size of the sensor should be greater than 5 mm×5 mm, specifically, 15 mm×15 mm in this embodiment.

[0047] The preparation of the sensor includes the following steps:

[0048] 1) A 100 nm thick silicon dioxide wafer 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).

[0049] 2) The copper foil substrate is pretreated with ferric nitrate, and then methane is used as a carbon source, oxygen 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 helium layer is obtained on the copper foil for standby use.

[0050] 3) Use mechanical spin coating to spin-coat the polymethyl methacrylate solution onto the graphene surface on the copper pin, pre-spin at 600 r / min for 20 seconds, then spin at 3000 r / min for 100 seconds, and then spin at 600 r / min for 20 seconds. After spin coating, place it on a drying table and dry it at 120°C for 20 minutes to obtain a uranium pin with a polymethyl methacrylate film on the graphene surface.

[0051] 4) placing the copper box obtained in step 3) into an oxygen plasma etcher for etching at a power of 90%, a pressure of 0.2 Pa, and a time of 30 seconds; then soaking the treated copper in a saturated solution of iron oxide for 4 hours to completely etch away the copper, and when only graphene and polymethyl methacrylate are left floating in the solution, replacing them with deionized water for 5 times to completely replace the iron oxide, and finally using a 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.

[0052] 5) Using mechanical spin coating (spin coating at 600 rpm for 20 seconds, then at 4000 rpm for 60 seconds, and then at 600 rpm for 20 seconds) to coat a photoresist with a thickness of 1 on the graphene surface, and bake at 100°C for 60 seconds. After cooling, exposure, development and fixing processes are performed to obtain a photoresist mask for the graphene layer: oxygen plasma etching is then performed to etch away the graphene that is not masked by the photoresist, and the silicon wafer is then immersed in an acetone bath for 24 hours to remove the photoresist mask on the silicon wafer. After washing with deionized water three times, it is blown dry to obtain an imaged graphene structure, that is, an imaged graphene layer on the dielectric layer is obtained.

[0053] 6) Spin-coating photoresist on the graphene layer by mechanical spin coating with a coating thickness of 1 μm, and baking at 100°C for 60 seconds. After cooling, exposing, developing and fixing are performed to obtain a photoresist mask for preparing a metal gate electrode. Then, metal is deposited on the photoresist mask of the metal gate by a material growth process, and then the metal is immersed in an acetone solution for 24 hours to strip off the metal and remove the photoresist. After washing with deionized water three times, the layer is blown dry to obtain a metal electrode with a thickness of 1 μm.

[0054] 7) A 1 μm thick ion gel layer was spin-coated on the graphene layer by mechanical spin coating, and the terahertz effect graphene metamaterial device was prepared after baking at 100°C for 60 seconds.

[0055] Embodiment 2: Figure 1 The structure shown is a sensor sample with a unit structure, and the transmission characteristics, resonance mechanism, polarization characteristics, etc. of the obtained sensor sample are analyzed.

[0056] 1. Transmission characteristics

[0057] In order to explore the transmission characteristics of the sensor, the present invention performs numerical calculations using the finite element method based on CST Microwave studio software.

[0058] Figure 4 and Figure 5 : are the transmission spectra of the terahertz metamaterial sensor and each resonance unit. From the transmission spectra, it can be known that the present invention can form the same triple PIT transparent window in two polarization directions.

[0059] 2. Resonance mechanism analysis

[0060] The resonance mechanism of the terahertz metamaterial sensor is analyzed by transmission spectroscopy. For the convenience of description, we call the horizontal elliptical ring HER and the vertical elliptical ring VER.

[0061] Depend on Figure 4As shown in the figure, in the x-polarization direction, when CSE, QR, VER and HER exist alone, they can interact with the incident electromagnetic wave to form single transmission valleys at 3.06THZ, 3.465THz, 3.82THz and 4.155THz respectively; Figure 5 In the figure, under y-polarization, when CSE, QR, HER and VER exist alone, single transmission valleys are formed at 3.06THZ, 3.465THz, 3.82THz and 4.155THz, respectively. Finally, when CSE, QR and ER are combined, a triple PIT effect occurs. Among them, the resonant frequencies corresponding to the four transmission valleys are at 2.97THZ, 3.47THz, 3.87THz and 4.265THz, respectively; the resonant frequencies corresponding to the three transmission peaks are at 3.2THz, 3.65THz and 4.05THz, respectively.

[0062] 3. Analysis of polarization insensitive characteristics

[0063] In order to test the sensitivity of the sensor to changes in polarization angle, the present invention studies the transmission characteristics of terahertz waves at different polarization angles (0°, 20°, 40°, 60°, 80°). Figure 6 As shown in the figure, the transmission spectrum of the sensor hardly changes with the change of polarization angle. It can be seen that the sensor has the characteristic of polarization insensitivity.

[0064] 4. Incident Angle Characteristics Analysis

[0065] 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°, 80°). Figure 7 As shown, with the change of the incident angle, the transmission spectrum changes slightly when the incident angle exceeds 60°, but still has a high transmittance, showing excellent incident angle dependence.

[0066] 5. Tunability

[0067] 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 8 It can also be observed that when the Fermi level is between 0.7 and 1.0 eV, the transmission spectrum shows a very obvious blue shift, which shows that the resonant frequency of the sensor increases linearly with the increase of the Fermi level. 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.

[0068] 6. Sensing performance

[0069] When traditional terahertz metamaterial sensors detect the 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.

[0070] Sensor sensitivity is an important indicator to measure the sensor performance, indicating the degree of spectrum shift under the unit refractive index change of the object to be measured. The calculation formula of sensor sensitivity S is:

[0071] S=Δf / Δn (3)

[0072] Where Δf is the offset of the spectrum, and Δn is the change in the refractive index of the material being measured.

[0073] The FOM value is also an important indicator for measuring the sensing performance of the sensor, which is mainly used for comparison between sensors of the same type. The calculation formula of the FOM value is:

[0074] FOM=S / FWHM (4)

[0075] Where S is the sensor sensitivity and FWHM is the full width at half maximum of the PIT window.

[0076] Fig. 9 is the corresponding transmission spectrum of the sensor when it is placed with objects of different refractive indexes. Fig. 9 It can be seen that as the refractive index of the object to be measured increases, the transmission spectrum of the sensor red shifts. The sensitivity at the three transparent windows is calculated to be S1 = 1.84Thz / RIU, S2 = 1.60Thz / RIU and S3 = 1.30Thz / RIU, and the corresponding FOM values ​​are 7.36 / RIU -1 ,5.38 / RIU -1 and 4.06 / RIU -1 ,like Fig.10 This data proves that the terahertz metamaterial sensor provided by the present invention has excellent sensing performance and broad application prospects, and can be widely used in the field of terahertz sensing and detection.

[0077] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to specific implementation methods, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A tunable terahertz metamaterial sensor based on triple plasmon induced transparency, characterized in that: It includes a silicon dioxide substrate layer, a single-layer graphene metamaterial layer attached to the substrate, an ion gel layer and a metal electrode. The graphene metamaterial layer includes N×N square resonance units periodically arranged on the substrate plane, where N is a natural number greater than 0. The resonant unit consists of three parts: the middle part consists of a cross-shaped ellipse and four elliptical rings, and the surrounding parts are obtained by rotating a quarter circle.

2. The triple plasma induced transparent tunable terahertz metamaterial sensor according to claim 1, characterized in that: The silicon dioxide substrate layer has a thickness of 0.1 μm, and the unit structure is a square silicon dioxide layer with Px=Py=20 μm.

3. The triple plasma induced transparent tunable terahertz metamaterial sensor according to claim 1, characterized in that: The major axis radius R of the graphene cross ellipse 11 =5.1μm, minor axis radius R1 =0.5μm.

4. The triple plasma induced transparent tunable terahertz metamaterial sensor according to claim 1, characterized in that: The major axis radius R of the four graphene elliptical rings 22 =3.2μm, minor axis radius R2 =1.8μm.

5. The triple plasma induced transparent tunable terahertz metamaterial sensor according to claim 1, characterized in that: The inner diameter of the four graphene quarter rings Rin=2.3 μm, and the outer diameter of the rings Rout=3.2 μm.

6. The triple plasma induced transparent tunable terahertz metamaterial sensor according to claim 1, characterized in that The ion gel layer is added by a mechanical spin coating method, which can protect the graphene layer and connect the electrodes, making it easy for carriers to be enriched on the surface of the ion gel.

7. The triple plasma induced transparent tunable terahertz metamaterial sensor according to claim 1, characterized in that: The metal electrode material is copper metal, and the metal electrode is deposited on the top edge of the ion gel layer and the bottom edge of the silicon dioxide substrate by using a material growth process, so as to connect the sensor and the external power supply.

8. According to claims 1 to 7, the triple plasma induced transparent tunable terahertz metamaterial sensor generates four resonance peaks at 2.97 THz, 3.47 THz, 3.87 THz and 4.265 THz, and three transparent windows at 3.2 THz, 3.65 THz and 4.05 THz.

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