Electrically driven active VO2 / MXene metasurface terahertz modulator

Abstract

This invention relates to a method for fabricating an electrically driven active VO2 / MXene metasurface terahertz modulator, utilizing a vacuum filtration method to prepare an MXene thin film. The MXene metasurface is fabricated using a laser direct writing method. A vanadium dioxide thin film is fabricated using magnetron sputtering. The vanadium dioxide thin film on a quartz substrate is completely immersed in a buffered oxide etchant (BOE). Using the prepared MXene metasurface, a self-supporting vanadium dioxide thin film is retrieved from water, suspending the vanadium dioxide film on the MXene metasurface. The VO2 / MXene composite metasurface is then left to stand for 1 hour to allow them to bond and dry, resulting in the VO2 / MXene composite metasurface. The MXene metasurface offers advantages such as high flexibility and ease of fabrication. The VO2 / MXene terahertz modulator features actively controllable terahertz waves, high modulation depth, and low trigger power.

Description

Technical Field

[0001] This invention belongs to the field of metasurface terahertz modulators. Background Technology

[0002] Terahertz waves, with frequencies ranging from 0.1 to 10 THz, are a crucial electromagnetic band bridging electronics and photonics. The low energy, wide bandwidth, and high penetrability of terahertz photons make them essential for applications in aerospace communications, biological imaging, and security detection. However, the lack of materials for actively modulating terahertz waves presents a significant challenge in constructing high-performance and highly integrated active terahertz modulation devices. Metasurfaces are subwavelength-scale electromagnetic response unit arrays. By designing periodic structures, arrangements, and distributions, metasurfaces can respond to the wavelength, amplitude, phase, and polarization of terahertz waves. Researchers have extensively studied metallic and semiconductor metasurfaces. Although metasurface devices based on metallic and semiconductor materials can effectively manipulate terahertz waves, the inherent rigidity of traditional materials prevents the construction of flexible terahertz devices.

[0003] Carbon nanotube films possess excellent flexibility, making them suitable for constructing highly flexible terahertz devices. Most metasurfaces based on carbon nanotube films are constructed using films containing metallic carbon nanotubes. These metallic carbon nanotube films have lower electrical conductivity than metals, and their charge carrier concentration cannot be effectively tuned like that of semiconductors. Therefore, constructing active terahertz metasurfaces using metallic carbon nanotube-based films is challenging.

[0004] MXene is an emerging two-dimensional nanomaterial with ultra-high flexibility, easy large-area fabrication, and higher electrical conductivity than typical low-dimensional nanomaterials such as graphene and carbon nanotube films. MXene has an electrical conductivity of 2 × 10⁻⁶. 6 With a conductivity of S / m, MXene films have twice the conductivity of graphene films, an order of magnitude higher than that of oriented carbon nanotube films, and two orders of magnitude higher than that of randomly arranged carbon nanotube films. Therefore, MXene films are a novel candidate material for fabricating flexible metasurfaces. However, due to the lack of methods for actively controlling the conductivity of MXene films, active terahertz metasurface modulators based on MXene have not yet been realized.

[0005] Vanadium dioxide (VO2) has demonstrated excellent active terahertz modulation material due to its metal-to-insulator transition (MIT) near room temperature (68°C), offering advantages such as high modulation depth, wide bandwidth, fast modulation speed, and low insertion loss.

[0006] Therefore, by finding a way to combine MXene and VO2 films, we can construct a flexible, active terahertz metasurface with good performance.

[0007] Traditional metals and semiconductors, due to their rigidity and inflexibility, are unsuitable for many applications. Current flexible terahertz metasurfaces are mostly based on graphene and carbon nanotube films, both of which have low electrical conductivity, limiting further manipulation of terahertz waves. The current challenge is to find metasurface devices that combine high flexibility, high conductivity, ease of large-area fabrication, and the ability to actively control terahertz waves. Summary of the Invention

[0008] This invention achieves an active terahertz metasurface modulator with high modulation depth, low excitation power, and low response time by compositing a self-supporting vanadium dioxide thin film with a highly flexible and highly conductive MXene thin film.

[0009] The fabrication process of the VO2 / MXene thin film terahertz metasurface modulator is as follows: Figure 1 As shown:

[0010] 1. Preparation of MXene films using vacuum filtration. First, an MXene solution was prepared using a gentle etching method. Then, the MXene solution was vacuum filtered for 1 hour using a vacuum filtration pump. The sample was then placed on a heating plate and dried at 80°C for 2 hours to obtain a self-supporting MXene film with a diameter of 3.5 cm and a thickness of 2.5 μm.

[0011] 2. Fabrication of MXene metasurfaces using laser direct writing. The prepared self-supporting MXene film was transferred onto a quartz frame, and the sample was placed under a Nikon microscope. An 800nm ​​laser was focused onto the sample using a 50x objective lens. The metasurface unit size was designed to be 320×65μm. 2 A rectangular aperture was formed by an array of 15×15 holes with a period of 380μm in both the x and y directions to obtain an MXene metasurface.

[0012] 3. Vanadium dioxide thin films were prepared using magnetron sputtering. A quartz substrate was clamped on the stage of a magnetron sputtering instrument, and sputtering was performed using a pure vanadium target. The substrate was sputtered at room temperature, the cavity pressure was stabilized at 0.6 Pa, the target temperature was 30 °C, and the sputtering power was 55 W. After sputtering, the sample was transferred to a tube furnace and a 3.5 × 10⁻⁶ ppm vapor pressure was introduced. -2 With an oxygen concentration of mbar and a furnace pressure stabilized at 4.5 Pa, the tubular furnace was heated to 450°C and held for 10 minutes to complete the annealing process.

[0013] 4. Preparation of self-supporting vanadium dioxide thin films using nanopore infiltration etching. The vanadium dioxide thin film prepared above on a quartz substrate was completely immersed in a buffered oxide etching solution (BOE), which consisted of 40% NH4F and 49% HF in a volume ratio of 5:1. During the magnetron sputtering and annealing process, natural nanopores are formed in the vanadium dioxide thin film, allowing the BOE solution to penetrate into these pores from top to bottom, effectively removing the intermediate sacrificial layer. After the film separated from the quartz substrate, it was immediately transferred to deionized water for immersion, resulting in a clean, self-supporting vanadium dioxide thin film.

[0014] 5. A self-supporting vanadium dioxide thin film was transferred to an MXene metasurface to fabricate a VO2 / MXene active metasurface terahertz modulator. Using the MXene metasurface prepared above, the self-supporting vanadium dioxide thin film was retrieved from water, suspending it on the MXene metasurface. The VO2 / MXene composite metasurface was then left to stand for 1 hour to allow them to adhere and dry, resulting in the VO2 / MXene composite metasurface. This composite metasurface was then attached to a perforated ceramic sheet. Two electrodes were led out from both ends of the composite metasurface using conductive silver paste and attached to the electrodes at both ends of the ceramic sheet, thus fabricating the VO2 / MXene metasurface terahertz modulator.

[0015] like Figure 2 (a) shows a schematic diagram of a VO2 / MXene composite thin-film metasurface terahertz modulator. The bottom substrate is a quartz frame, the middle layer is 2.5 μm thick, and the dimension is 1 cm. 2 MXene metasurfaces, such as Figure 2 (b) The metasurface consists of unit cells with a size of 320 × 65 μm 2The composite metasurface consists of a rectangular aperture array with a period of 380 μm in both the x and y directions, topped by a 150 nm thick self-supporting vanadium dioxide film. Two electrodes are formed at both ends of the composite metasurface by bonding gold wires with conductive silver paste, resulting in a VO2 / MXene composite thin-film metasurface terahertz modulator. This modulator exhibits a resonant response to a specific wavelength (typically 0.43 THz) of incident terahertz waves and a polarization response. When the incident terahertz wave polarization state is in transverse magnetic polarization (TM) mode, the device's transmittance at 0.43 THz is 0.91 (when VO2 is the insulator phase). When the incident terahertz wave polarization state is in transverse polarization (TE) mode, the device's transmittance at 0.43 THz is 0.03 (when VO2 is the insulator phase). When the composite metasurface modulator is subjected to a voltage of 1.7V, vanadium dioxide undergoes a complete phase transition, thereby modulating the incident TM mode terahertz wave. This reduces the terahertz transmittance to 0.13 at 0.43THz, achieving the effect of modulating terahertz transmittance, with a modulation depth of 85.7% (at 0.43THz). Attached Figure Description

[0016] Figure 1 Schematic diagram of the fabrication of .VO2 / MXene active terahertz metasurface.

[0017] Figure 2 (a) Schematic diagram of the electrical excitation of the VO2 / MXene active terahertz metasurface modulator. (b) Schematic diagram of the MXene metasurface planar surface.

[0018] Figure 3 (a) Optical image of MXene thin film. (b) Transmission spectra of MXene metasurface incident with TM-polarized terahertz waves and TE-polarized terahertz waves (metasurface unit size 320 × 65 μm). 2 (c) Transmission spectra of VO2 / MXene active terahertz metasurface under electrical excitation (metasurface unit size is 320×65μm) 2 (d) Transient response of VO2 / MXene active terahertz metasurface modulator. Detailed Implementation

[0019] VO2 / MXene Active Metasurface Modulator Examples

[0020] 1. Preparation of MXene films using vacuum filtration. Firstly, Ti3C2T films were prepared using a mild etching method. xMXene solution. The method involves dissolving 2 g of LiF in 40 mL of HCl, stirring for 10 minutes, slowly adding 1 g of Ti3AlC2, and stirring at 50 °C for 24 hours. The mixture is centrifuged 6 times, and after the solvent pH returns to 6, the bottom precipitate is collected. The bottom precipitate is then dissolved in 100 mL of water and sonicated at 500 W in a 15 °C water bath under argon protection for 2 hours. Finally, the supernatant is collected after centrifugation at 3500 rpm for 1 hour. The MXene solution is then vacuum filtered using a vacuum filtration pump for 1 hour, and the sample is dried on a hot plate at 80 °C for 2 hours to obtain a self-supporting MXene film with a diameter of 3.5 cm.

[0021] 2. Fabrication of MXene metasurfaces using laser direct writing. A self-supporting MXene film was transferred onto a quartz frame. An 800nm ​​output from a femtosecond amplifier system (Legend, Coherent Company) with a pulse duration of 130 fs and a repetition rate of 1 kHz was used as the laser source for laser direct writing of the MXene metasurface. The 800nm ​​laser was focused onto the sample using a 50x objective lens, and the MXene metasurface was obtained through laser micromachining.

[0022] 3. Vanadium dioxide thin films were prepared using magnetron sputtering. Vanadium dioxide thin films were deposited on a quartz substrate using magnetron sputtering. Typical conditions for vanadium dioxide thin film preparation were: argon to argon-oxygen mixture (oxygen 4% by volume) at a ratio of 1:1, sputtering pressure of 0.60 Pa, sputtering power of 55 W, and sputtering time of 50 minutes. After sputtering, the samples were annealed in a furnace under typical low-pressure oxygen annealing conditions (oxygen pressure 3.5 × 10⁻⁶). -2 (mbar), temperature 450℃, annealing time 10min.

[0023] 4. Preparation of self-supporting vanadium dioxide thin films using nanopore infiltration etching. A self-supporting vanadium dioxide thin film was prepared using nanopore infiltration etching. The vanadium dioxide thin film on a quartz substrate was immersed in a buffered oxide etching solution (BOE) of 40% NH4F and 49% HF (volume ratio 5:1) for 30 minutes. During the magnetron sputtering and annealing process, natural nanopores are formed in the vanadium dioxide thin film, allowing the BOE solution to penetrate into these pores from top to bottom, effectively removing the intermediate sacrificial layer. The self-supporting vanadium dioxide thin film was obtained through nanopore infiltration etching and then transferred to deionized water to obtain a clean self-supporting vanadium dioxide thin film. Finally, the self-supporting vanadium dioxide thin film was transferred onto an MXene metasurface to obtain a VO2 / MXene composite metasurface.

[0024] 5. A VO2 / MXene active metasurface terahertz modulator was fabricated by transferring a self-supporting vanadium dioxide thin film onto an MXene metasurface. A typical modulator geometry is 1 cm² in area. 2 The VO2 / MXene composite metasurface. A typical MXene metasurface is an array of rectangular holes: the rectangular hole unit size is 320 × 65 μm. 2 The rectangular aperture has a period of 380 μm in both the x and y directions. The resistance of the VO2 / MXene active terahertz modulator is 2.5 Ω.

[0025] MXene metasurfaces have high flexibility (Figure 1) Figure 3 (a) The VO2 / MXene terahertz modulator has the advantages of being easy to fabricate and having the ability to actively control terahertz waves and high modulation depth (compared to...). Figure 3 (b), 3(c)), Low trigger power Figure 3 (d)) Advantages.

[0026] Table 1. Comparison of trigger power of different types of THz modulators

[0027] modulator Trigger power <![CDATA[Si3N4 / VO2]]> <![CDATA[145mW / mm 2 ]]> <![CDATA[Au / VO2]]> <![CDATA[21.7mW / mm 2 ]]> <![CDATA[MXene / VO2]]> <![CDATA[11.6mW / mm 2 (This experiment)

Claims

1. A method for fabricating an electrically driven active VO2 / MXene metasurface terahertz modulator, characterized in that, Includes the following steps: 1) Preparation of MXene films using vacuum filtration: First, an MXene solution was prepared, then the MXene solution was vacuum filtered for 1 hour using a vacuum filtration pump. The sample was then placed on a heating plate at 80°C. o The self-supporting MXene film was obtained by drying at C for 2 hours. 2) Prepare MXene metasurfaces using laser direct writing method; transfer the prepared self-supporting MXene film onto a quartz frame, then place the sample under a Nikon microscope, and focus an 800 nm laser onto the sample using a 50x objective lens to obtain the MXene metasurface; 3) Vanadium dioxide thin films were prepared using magnetron sputtering. A quartz substrate was clamped on the substrate stage of a magnetron sputtering instrument, and sputtering was performed using a pure vanadium target. The substrate was kept at room temperature, the cavity pressure was stabilized at 0.6 Pa, and the target temperature was maintained at 30°C. o C, magnetron sputtering was performed at a sputtering power of 55 W; after sputtering, the sample was transferred to a tube furnace and 3.5 × 10⁻⁶ W was introduced. -2 With an oxygen concentration of mbar and a furnace pressure stabilized at 4.5 Pa, the tubular furnace was heated to a temperature of 450°C. o Annealing is completed by holding at temperature C for 10 minutes; 4) A self-supporting vanadium dioxide film was prepared by nanopore permeation etching. The vanadium dioxide film on the quartz substrate prepared above was completely immersed in buffer oxide etching solution (BOE) to obtain a clean self-supporting vanadium dioxide film. 5) Using the MXene metasurface prepared above, retrieve the self-supporting vanadium dioxide film from the water, suspending the vanadium dioxide film on the MXene metasurface. Then, let the VO2 / MXene composite metasurface stand for 1 hour to allow the two to bond and dry, thus obtaining the VO2 / MXene composite metasurface. Attach the composite metasurface to a perforated ceramic sheet, and use conductive silver paste to attach gold wires to both ends of the composite metasurface to lead out two electrodes. Connect the two gold wires to the electrodes at both ends of the ceramic plate to prepare a VO2 / MXene metasurface terahertz modulator.

2. The method for fabricating the electrically driven active VO2 / MXene metasurface terahertz modulator according to claim 1, characterized in that, The metasurface unit size is designed to be 320 × 65 mm. 2 A rectangular aperture, in a 15×15 array with a period of 380 mm.

3. The method for fabricating the electrically driven active VO2 / MXene metasurface terahertz modulator according to claim 1, characterized in that, The bottom layer of the fabricated terahertz modulator is a quartz frame, with a thickness of 2.5 mm in the middle and a dimension of 1 cm. 2 The MXene metasurface has a 150 nm thick self-supporting vanadium dioxide film on top.

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