A terahertz coding metamaterial with tunable transmission and reflection based on vanadium dioxide

Abstract

The present invention provides a terahertz coding metamaterial with tunable transmission and reflection based on vanadium dioxide. The material unit structure consists of a metal layer and a dielectric layer, and the metal layers are separated by dielectric layers. The metal layer includes two structures. The first structure is used to achieve coding, and the second structure is used to achieve the transition between transmission and reflection. In the present invention, four different unit structures are obtained by adjusting the different lengths of the metal and vanadium dioxide in the first structure. The electromagnetic response phases between the four unit structures differ by π / 2, achieving 2-bit coding. Utilizing the temperature-controlled phase change characteristic of vanadium dioxide, the second structure can realize the conversion of vertically incident electromagnetic waves between two optical transmission phenomena, namely transmission and reflection. When vanadium dioxide is in the dielectric state, extraordinary optical transmission can be achieved, and when vanadium dioxide is in the metallic state, extraordinary optical reflection can be achieved.

Description

Technical Field

[0001] The present invention belongs to the field of tunable coding metamaterials, and particularly relates to a terahertz coding metamaterial with tunable transmission and reflection based on vanadium dioxide, which is mainly used for the transmission and reflection regulation of terahertz band light beams and can realize functions such as superlenses and vortex beams. Background Art

[0002] Metamaterials are artificially designed materials whose unit structural size is much smaller than the wavelength. They are usually composed of periodic or quasi-periodic structures of sub-wavelength size. It is an artificial material with unique properties. Through reasonable design, some special physical phenomena that natural materials do not possess can be realized. Metamaterials are usually characterized by the equivalent medium theory and cannot achieve real-time and flexible adjustment of electromagnetic fields and electromagnetic waves. With the development of science and technology, in 2014, the research group of Professor Cui Tiejun at Southeast University proposed the concepts of "digital coding metasurface" and "programmable coding metasurface" (Cui T J et al, Light: Science & Applications, 2014), successfully connecting the traditional physical world and the digital world, greatly simplifying the design and optimization of metamaterial structures. By discretizing and binary coding the 2π phase response of metamaterials to electromagnetic waves, a digital coding metamaterial unit structure with corresponding bit positions can be generated. For example, for 1-bit coding, only need to discretize the 2π phase into two phases, corresponding to two unit structures, and the phase difference between them will be π, which can be encoded as 0 and 1. For example, if 2-bit digital coding metamaterials are needed, then the 2π phase needs to be discretized into four phases, the phase difference between them is π / 2, and correspondingly four unit structures will be formed, which can be encoded as 00, 01, 10, and 11. It can be found that the digital coding concept of metamaterials is very similar to the high and low levels in the digital circuit field, successfully introducing the concept of the digital circuit field into the metamaterials in the physical world, which also lays the foundation for the development of digital coding metamaterials in the field of digital information processing and is conducive to the development and application of metamaterials in real life. Spatiotemporal coding digital metamaterials have important applications in wireless communication, cognitive radar, multiple input multiple output systems, OAM beam generation, adaptive beamforming, and holographic imaging.

[0003] By adding tunable materials, the functions of metamaterials can be made more diverse, improving the capabilities of metamaterials in practical applications. The electromagnetic properties of tunable materials or components can be changed by external stimuli, resulting in changes in the effective electromagnetic response of metamaterials and the tuning of electromagnetic output. Heat is an available stimulus factor for tunable metamaterials. For effective thermally tunable metamaterials, the subatomic or sub-material substrate should exhibit strong sensitivity to temperature changes. Due to this sensitivity, the electromagnetic response of metamaterials changes significantly with temperature changes, thus obtaining the ability to dynamically adjust the response of metamaterials.

[0004] At room temperature, the crystal lattice structure of vanadium dioxide is a monoclinic rutile structure. When its temperature exceeds 68 °C, it can undergo a transition from an insulating state to a metallic state. As a commonly used tunable material, before and after the phase change, the reflectivity, absorptivity, transmittance, resistivity, magnetic susceptibility, etc. of the vanadium dioxide thin film will change greatly. Especially the resistivity changes by 3 to 5 orders of magnitude. Therefore, due to these diverse stimuli, low operating temperature and good performance, vanadium dioxide has become a popular choice for potential applications. Summary of the Invention

[0005] The purpose of the present invention is to provide a terahertz coding metamaterial with tunable transmission and reflection based on vanadium dioxide. The present invention utilizes the temperature-controlled phase change characteristics of vanadium dioxide to realize the regulation of terahertz beams, realizes 2-bit coding by changing the lengths of the gold and vanadium dioxide rectangular bars, and utilizes temperature regulation to realize the reversible transition of vanadium dioxide from the metallic state to the insulating state, realizing the dynamic regulation of terahertz beam transmission and reflection, and can realize functions such as superlenses and vortex beams.

[0006] The purpose of the present invention is achieved as follows:

[0007] A terahertz coding metamaterial with tunable transmission and reflection based on vanadium dioxide is composed of metamaterial unit structures. The metamaterial unit structure is composed of a metal layer and a dielectric layer, and the metal layers are separated by dielectric layers. There are two structures for the metal layer. The first structure is composed of a square metal and a vanadium dioxide rectangular bar, which is used for coding. The second structure is composed of a square metal embedded with a cross-shaped vanadium dioxide, which is used for realizing the transition between transmission and reflection; arranged in the order of the first structure - dielectric layer - first structure - dielectric layer - second structure - dielectric layer - first structure - dielectric layer - first structure, 2-bit coding is realized by changing the lengths of the four first-structure metals and vanadium dioxide, and the electromagnetic response phases between the four first structures differ by π / 2; utilizing the temperature-controlled phase change characteristics of vanadium dioxide, the second structure can realize the conversion of vertically incident electromagnetic waves between the two optical transmission phenomena of transmission and reflection: when vanadium dioxide is in the dielectric state, extraordinary transmission of light can be realized, and when vanadium dioxide is in the metallic state, extraordinary reflection of light can be realized. Combining multiple structural units can realize the functions of superlenses and vortex beams.

[0008] Furthermore, the dielectric layer is polyimide;

[0009] Furthermore, the metal is gold;

[0010] The present invention is composed of a phase change material, a metal, and a dielectric. The unit structure of the metamaterial is composed of a metal layer and a dielectric layer, and the metal layers are separated by dielectric layers. By changing the lengths of the gold and vanadium dioxide bars, 2-bit coding is achieved, and by using temperature regulation, a reversible transition of vanadium dioxide from an insulating state to a metallic state is realized, achieving the conversion of the transmission and reflection phenomena of electromagnetic waves by the metamaterial. Since this metamaterial is a sub-wavelength structure, the thickness of the structure can be ignored in practical applications. By using the change in temperature, vanadium dioxide can achieve a phase change on a time scale of several nanoseconds or even picoseconds, and the ultrafast dynamic switching effect can flexibly control the transmission and reflection switching of terahertz beams. When vanadium dioxide is in the dielectric state, light transmission can be achieved, and when vanadium dioxide is in the metallic state, light reflection can be achieved.

[0011] Compared with the prior art, the advantages of the present invention are as follows:

[0012] 1. By adjusting the temperature to make vanadium dioxide transition from the metallic state to the insulating state, the regulation of terahertz beams can be achieved;

[0013] 2. 2-bit coding can be simply achieved by changing the lengths of the metal and vanadium dioxide. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1a Schematic diagram of each layer of the geometric structure of the tunable coding metamaterial unit separated;

[0015] Figure 1b Schematic diagram of Structure 2 for realizing the transition between transmission and reflection;

[0016] Figure 1c Schematic diagram of Structure 1 for realizing coding;

[0017] Figure 2a Transmission phase diagram of four coding metamaterial unit structures when vanadium dioxide is in the dielectric state;

[0018] Figure 2b Transmission amplitude diagram of four coding metamaterial unit structures when vanadium dioxide is in the dielectric state;

[0019] Figure 2c Reflection phase diagram of four coding metamaterial unit structures when vanadium dioxide is in the metallic state;

[0020] Figure 2d Reflection amplitude diagram of four coding metamaterial unit structures when vanadium dioxide is in the metallic state;

[0021] Figure 3a Far-field diagram of the metamaterial with the coding units arranged periodically in the x direction when vanadium dioxide is in the insulating state;

[0022] Figure 3bMetamaterials with coding units arranged periodically in the x - direction, far - field pattern when vanadium dioxide is in the metallic state;

[0023] Figure 3c Metamaterials with coding units arranged periodically in the x - direction, two - dimensional far - field pattern when vanadium dioxide is in the insulating state;

[0024] Figure 3d Metamaterials with coding units arranged periodically in the x - direction, two - dimensional far - field pattern when vanadium dioxide is in the metallic state;

[0025] Figure 4a Phase distribution diagram of the metamaterial;

[0026] Figure 4b Distribution diagram of the coding units and the overall structure of the metamaterial;

[0027] Figure 4c Phase distribution diagram of the transmitted vortex beam when vanadium dioxide is in the insulating state;

[0028] Figure 4d Phase distribution diagram of the reflected vortex beam when vanadium dioxide is in the metallic state. Detailed implementation mode

[0029] The present invention will be described in more detail with reference to the accompanying drawings as follows:

[0030] Example 1:

[0031] The unit structure of the metamaterial of the present invention is as shown in Figure 1a . In the figure, 1 is the first structure of the metal layer, 2 is the second structure of the metal layer, and 3 is the dielectric layer; the unit structure of the metamaterial is composed of a dielectric layer, metal, and vanadium dioxide. As shown in Figure 1a , the period p of the unit structure is 100 μm, the dielectric layer is made of polyimide, with a thickness of h = 12.7 μm, a dielectric constant of ε = 2.4, and a tangent loss of tanδ = 0.005 / 2.4. The metal layers are all composed of a mixture of gold and vanadium dioxide, the overall structure is symmetric about the middle, and the metal layers are separated by a dielectric layer, and the dielectric layer is a cuboid. As shown in Figure 1b , it shows the metal layer used to achieve the transition between transmission and reflection, where the width and length of the cross - shaped vanadium dioxide are set to lc = 15 μm and wc = 75 μm respectively. The metal layers for realizing coding are all the same, Figure 1cShown is a schematic diagram of one of the metal layers implementing encoding, where the length of gold is set to L, the total length of gold and vanadium dioxide is set to Lv, and the total width is set to Wv. According to the principle of encoded metamaterials, in order to achieve beam steering using 2-bit encoded metamaterials, four unit structures with different electromagnetic response phases are required. The metal layer used to achieve the transition between transmission and reflection is a cross structure composed of a gold layer embedded with vanadium dioxide, which is also the key to the transmission and reflection switching of the metamaterial. That is, when vanadium dioxide is in the insulating state, it can be regarded as a dielectric at this time, and electromagnetic waves can pass through well. When vanadium dioxide is in the metallic state, it can be regarded as a metal at this time, and the entire intermediate layer is equivalent to a metal for electromagnetic waves and has a good reflection effect on electromagnetic waves.

[0032] According to the design principle of encoded metamaterials, the dimensions L (Lv) of the four encoded unit structures are respectively set to 2.5 μm (2.5 μm), 38.5 μm (48.5 μm), 58 μm (61 μm), and 69 μm (72.8 μm). Wv is fixed at 62.5 μm. As Figure 2a –b shows, when vanadium dioxide is in the dielectric state, the electromagnetic response intensity and phase of the four encoded metamaterial unit structures are respectively shown. At this time, the metamaterial can achieve the transmission of electromagnetic waves. It can be found from the figure that the transmission amplitudes of the four encoded unit structures are close, and the electromagnetic response phases between every two unit structures differ by π / 2, which conforms to the design principle of encoded metamaterials. Figure 2c –d shows the electromagnetic response intensity and phase of the four encoded metamaterial unit structures when vanadium dioxide is in the metallic state. At this time, the metamaterial can achieve the reflection of electromagnetic waves. Similarly, it can be found from the simulation result figure that the reflection amplitudes of the four encoded unit structures are close at this time, and the reflection phases between every two unit structures differ by π / 2, which conforms to the design principle of encoded metamaterials.

[0033] Example 2:

[0034] In order to observe the electromagnetic effects achieved by the metamaterial composed of these four unit structures and to reduce the coupling effects between different unit structures, four identical encoded unit structures of 2×2 are used to form a super subunit. In this way, there will be four super subunits, and then these four super subunits are arranged periodically along the x direction in sequence 0001 10 11 00 01 10 11…, and repeated along the y direction. Then, according to the formula θ = arcsin(λ / Г), the deflection angle of the light beam should be 19°. In order to verify this result, the metamaterial is simulated, and the electromagnetic wave is set to be incident along the -z direction. The three-dimensional far-field simulation result can be obtained as shown in Figure 3, where Figure 3a shown is the far-field diagram when vanadium dioxide is in the insulating state, Figure 3bIt shows the far-field pattern when vanadium dioxide is in the metallic state. It can be found that by switching the state of vanadium dioxide, the incident electromagnetic wave can be switched between extraordinary transmission and extraordinary reflection. According to the principle of the coded metamaterial mentioned above, the different electromagnetic responses of the coded metamaterial to electromagnetic waves depend on the spatial arrangement order of the coding units. Therefore, during the simulation process, the only factor that changes in this metamaterial is the different states of vanadium dioxide, and the arrangement order of the metamaterial unit structure does not change. So, the beam deflection effects of the two states of vanadium dioxide conversion on electromagnetic waves are similar, both realizing the extraordinary transmission of the light beam, and the deflection of the light beam is along the x direction, and the deflection angles are both 19°. The only difference is that they respectively realize the extraordinary transmission and reflection of the light beam. To illustrate that when the state of vanadium dioxide is converted, the beam deflection angle does not change, Figure 3c –d shows the two-dimensional far-field pattern, from which the magnitude of the deflection angle can be clearly observed. Figure 3c Shown is the two-dimensional far-field pattern simulated when vanadium dioxide is in the insulating state. It can be clearly seen from the figure that the beam deflection occurs at θ = 161°, that is, the deflection angle is 19°, Figure 3d Shown is the two-dimensional far-field pattern simulated when vanadium dioxide is in the metallic state. Similarly, it can be clearly found from the figure that the beam deflection occurs at θ = 19°. So the beam deflection angle is also equal to 19°. Obviously, in these two cases, the metamaterial realizes the switching of electromagnetic wave transmission and reflection, and has a deflection effect on the beam transmission, and the deflection angles are the same, both equal to 19°, which is in good agreement with the theoretical calculation results.

[0035] Example 3:

[0036] To further illustrate the function of the proposed coded metamaterial, a metamaterial capable of realizing a vortex beam is constructed using a 2-bit coding unit structure. A vortex beam with l = 1 is constructed using a 2-bit coding unit structure. When vanadium dioxide is in the insulating state, the constructed metamaterial will realize a transmissive vortex beam. When vanadium dioxide is in the metallic state, the metamaterial will switch to form a reflective vortex beam. Based on the design principle of the vortex beam, the phase distribution of the designed metamaterial is as Figure 4a shown. It can be clearly found that the entire metamaterial is divided into four regions, the phases within each region are equal, the phase difference between adjacent regions is π / 2, and the entire metamaterial realizes a 2π phase coverage. The front view of the coding unit and the overall structure distribution is shown in Fig. 4(b), and the light beam travels along the -z direction. The simulation results are as Figure 4c -d shown, where Figure 4c represents the phase distribution of the transmissive vortex beam when vanadium dioxide is in the insulating state, and the viewing angle is along the +z direction, Figure 4dShown is the phase distribution of the reflected vortex beam when vanadium dioxide is in the metallic state. The viewing angle is along the -z direction. It can be found from the simulation results that the transmitted beam realized by this metamaterial has obvious vortex beam characteristics. This fully demonstrates that the proposed 2-bit metasurface can achieve the switching of transmitted and reflected vortex beams, and also fully verifies the design of the 2-bit coding unit structure for achieving transmission and reflection switching. This also provides a basis and direction for future communication transmission research.

[0037] The present invention provides a terahertz coding metamaterial with tunable transmission and reflection based on vanadium dioxide. This tunable coding metamaterial is composed of a phase change material, a metal, and a dielectric. The multi-layer metamaterial unit structure is composed of a metal layer and a dielectric layer, and the metal layers are separated by dielectric layers. The metal layer includes two structures. The first structure is used for coding, and the second structure is used for achieving the transition between transmission and reflection. In the present invention, four different unit structures are obtained by adjusting the lengths of the metal and vanadium dioxide in the first structure, and the electromagnetic response phases between the four unit structures differ by π / 2, achieving 2-bit coding. Utilizing the temperature-controlled phase change characteristic of vanadium dioxide, the second structure can achieve the conversion of vertically incident electromagnetic waves between two optical transmission phenomena of transmission and reflection. When vanadium dioxide is in the dielectric state, extraordinary optical transmission can be achieved, and when vanadium dioxide is in the metallic state, extraordinary optical reflection can be achieved. By using the change in temperature, vanadium dioxide can achieve a phase change on a time scale of several nanoseconds or even picoseconds. The ultrafast dynamic switching effect can flexibly control the transmission and reflection switching of terahertz beams, thereby achieving dynamic beam control and rich device functions.

Claims

1. A terahertz coding metamaterial with tunable transmission and reflection based on vanadium dioxide, characterized in that: It is composed of metamaterial unit structures. The metamaterial unit structures are composed of a metal layer and a dielectric layer. The metal layers are separated by dielectric layers. There are two structures for the metal layer. The first structure is composed of a square metal in the middle and vanadium dioxide rectangular strips around it, which is used to achieve coding. The second structure is composed of a square metal embedded with a cross-shaped vanadium dioxide, which is used to achieve the transition between transmission and reflection. They are sorted in the order of the first structure - dielectric layer - the first structure - dielectric layer - the second structure - dielectric layer - the first structure - dielectric layer - the first structure. By changing the lengths of the four first-structure metals and vanadium dioxide, 2-bit coding can be achieved, and the electromagnetic response phases between the four first structures differ by π / 2. Utilizing the temperature-controlled phase change characteristics of vanadium dioxide, the second structure can achieve the conversion of vertically incident electromagnetic waves between two optical transmission phenomena, namely transmission and reflection: when vanadium dioxide is in the dielectric state, extraordinary transmission of light can be achieved, and when vanadium dioxide is in the metallic state, extraordinary reflection of light can be achieved. Combining multiple structural units can achieve the functions of a superlens and a vortex beam.

2. The terahertz coding metamaterial with tunable transmission and reflection based on vanadium dioxide according to claim 1, characterized in that : The dielectric layer is polyimide.

3. The terahertz coding metamaterial with tunable transmission and reflection based on vanadium dioxide according to claim 1, characterized in that : The metal is gold.

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