A switchable terahertz dual-function device based on vanadium dioxide metasurface

Abstract

The present invention relates to the field of electromagnetic metasurfaces, and specifically to a switchable terahertz dual-function device based on a vanadium dioxide metasurface, wherein the switchable metasurface is composed of a diagonally placed vanadium dioxide pattern layer, a polysilicon dielectric layer, a gold pattern layer composed of a gold split ring resonator and an L-shaped gold bar, a vanadium dioxide film layer, a polyimide dielectric layer, and a gold reflective layer arranged in sequence from the top layer to the bottom layer, and when the vanadium dioxide is in a metallic state, the switchable metasurface realizes a broadband absorption function; when the vanadium dioxide is in an insulating state, the switchable metasurface realizes a reflective linear polarization conversion function. The present invention utilizes the phase change characteristics of vanadium dioxide materials to integrate the characteristics of these two dual-function devices, a broadband absorber and a reflective linear polarization converter, into a metasurface structure, thereby solving the problem of single function in traditional metasurface structures and making their application more flexible and extensive.

Description

Technical Field

[0001] The present invention relates to the field of electromagnetic metasurfaces, and in particular to a switchable terahertz dual-function device based on a vanadium dioxide metasurface, which can realize broadband absorption and reflective linear polarization conversion functions. Background Art

[0002] Terahertz wave manipulation technology has attracted increasing attention due to its wide applications in wireless communications, biomedical imaging, and radar stealth. However, the key issue affecting the practical application of terahertz is the lack of functional devices with excellent performance. This is mainly due to the lack of materials that can directly interact with terahertz waves in nature. To solve this problem, metamaterials are considered to be an ideal platform for terahertz functional devices. Metamaterials are artificially designed electromagnetic composites that have a wide range of applications in superlenses, perfect absorbers, and polarization converters. Metasurfaces can be regarded as the two-dimensional counterpart of metamaterials. In recent years, many interesting applications have been proposed based on metasurfaces, such as lens antennas, absorbers, and polarization converters.

[0003] Terahertz absorbers are one of the most attractive research topics in the terahertz field. Due to their important application prospects in detection, imaging and modulation, they have also received more and more attention. With the introduction of the concept of metasurfaces, terahertz metasurface absorbers have been developed.

[0004] Polarization is an important property of light waves, and it has potential application value in many fields such as electronic countermeasures and satellite communications. With the rapid development of terahertz technology, many functional terahertz devices have been reported. However, due to the defects of natural materials, the control of terahertz polarization state remains a huge technical challenge. In recent years, metasurfaces, as an artificially designed two-dimensional structure, have continued to attract attention due to their exotic electromagnetic properties. Introducing metasurfaces into the designed structure can change the amplitude and phase distribution of the incident electromagnetic wave, thereby realizing the manipulation of the polarization state of the incident electromagnetic wave.

[0005] As we all know, most terahertz metasurface devices have only one function at the operating frequency, which is a significant disadvantage. Therefore, how to design terahertz metasurface multifunctional devices has become a hot topic for researchers in this field. Summary of the invention

[0006] In order to overcome the problem of single function at the operating frequency, the present invention proposes a broadband absorber and a reflective linear polarization converter based on a vanadium dioxide switchable metasurface.

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

[0008] A switchable terahertz dual-function device based on a vanadium dioxide metasurface, wherein the switchable metasurface is composed of a diagonally placed vanadium dioxide pattern layer, a polysilicon dielectric layer, a gold pattern layer composed of a gold split ring resonator and an L-shaped gold bar, a vanadium dioxide thin film layer, a polyimide dielectric layer and a gold reflective layer arranged in sequence from the top layer to the bottom layer. When the vanadium dioxide is in a metallic state, the switchable metasurface realizes a broadband absorption function; when the vanadium dioxide is in an insulating state, the switchable metasurface realizes a reflective linear polarization conversion function.

[0009] As a further design of this scheme, the conductivity of vanadium dioxide is changed by external excitation to realize the reversible phase transition process of vanadium dioxide from a metallic state to an insulating state. The external excitation can be light, temperature, voltage, etc.

[0010] As a further design of this scheme, when vanadium dioxide is in a metallic state, a three-layer structure of a diagonally placed vanadium dioxide pattern layer, a polysilicon dielectric layer and a vanadium dioxide thin film layer realizes a broadband absorption function; when vanadium dioxide is in an insulating state, a three-layer structure of a gold pattern layer composed of a gold open ring resonator and an L-shaped gold bar, a polyimide dielectric layer and a gold reflective layer realizes a reflective linear polarization conversion function.

[0011] As a further design of this scheme, in the terahertz frequency band, the relative dielectric constant of vanadium dioxide is described by the Drude model as follows:

[0012]

[0013] in, is the high frequency dielectric constant; , The plasma frequency at can be approximated by express, , ;

[0014] When the conductivity of vanadium dioxide When in the metallic state, the conductivity of vanadium dioxide When the conductivity of the vanadium dioxide material of the metasurface unit changes, the vanadium dioxide converts between the metallic state and the insulating state, and finally the working state of the designed metasurface device switches between a broadband absorber and a reflective linear polarization converter.

[0015] As a further design of this solution, the dielectric constant of the polysilicon dielectric material is 3.

[0016] As a further design of this solution, the conductivity of the gold material of the gold pattern layer and the gold reflective layer is .

[0017] As a further design of this solution, the composite dielectric constant of the polyimide dielectric material is .

[0018] As a further design of this solution, a gold pattern consisting of a gold split ring resonator and an L-shaped gold bar is arranged along Symmetrical.

[0019] The present invention has the following beneficial effects:

[0020] 1) The metasurface structure of the present invention is actively controllable, and can realize a reversible phase transition of vanadium dioxide from an insulating state to a metallic state through external excitation, thereby realizing the dual functions of switching between a broadband absorber and a reflective linear polarization converter.

[0021] 2) The broadband absorber of the present invention has a wide bandwidth (the bandwidth where the absorption rate is greater than 90% is 1.82 THz, and the relative bandwidth is 86%). and The absorption rates at and are respectively 0.99 and 0.98, which are close to perfect absorption. The absorber has polarization-independent characteristics and large incident angle absorption characteristics.

[0022] 3) The reflective linear polarization converter of the present invention has a high polarization conversion efficiency (greater than 92% in the frequency range of 0.8-1.86 THz), and its polarization conversion efficiency is close to 1 at the frequency points of 1.12 THz and 1.57 THz, achieving perfect linear polarization conversion. In addition, the polarizer also has a wide operating bandwidth (the bandwidth with polarization conversion efficiency greater than 92% is 1.06 THz).

[0023] 4) Since the gold pattern composed of the gold split ring resonator and the L-shaped gold strip in the designed reflective metasurface linear polarization converter is along Symmetric, so the reflection coefficient is symmetrical, that is .

[0024] 5) By utilizing the phase change characteristics of vanadium dioxide materials, two dual-functional devices, a broadband absorber and a reflective linear polarization converter, are integrated into a metasurface structure, thus solving the problem of single function in traditional metasurface structures and making its application more flexible and extensive. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0026] Figure 1 This is a schematic diagram of a unit structure of a switchable terahertz dual-function device based on a vanadium dioxide supersurface according to an embodiment of the present invention;

[0027] In the figure: 1-vanadium dioxide pattern layer; 2-polysilicon dielectric layer; 3-gold pattern layer; 4-vanadium dioxide thin film layer; 5-polyimide dielectric layer; 6-gold reflective layer.

[0028] Figure 2 It is a schematic diagram of the periodic structure of the present invention;

[0029] Figure 3 It is a unit structure parameter diagram of the present invention;

[0030] In the figure: (a) Top view of the diagonally placed vanadium dioxide pattern layer, (b) Top view of the gold pattern layer consisting of a gold split ring resonator and an L-shaped gold bar.

[0031] Figure 4 The electrical conductivity of vanadium dioxide Absorption spectrum of the metasurface absorber in (metallic state);

[0032] Figure 5 The electrical conductivity of vanadium dioxide Distribution of the real and imaginary parts of the relative impedance of the metasurface absorber in the metallic state;

[0033] Figure 6 The electrical conductivity of vanadium dioxide Absorption spectra of the metasurface absorber at different incident angles in the (metallic state);

[0034] Figure 7 The electrical conductivity of vanadium dioxide Reflection coefficient of the linear polarization converter in (insulating state);

[0035] Figure 8 The electrical conductivity of vanadium dioxide Polarization conversion efficiency of the linear polarization converter in (insulating state).

[0036] Fig. 9 The electrical conductivity of vanadium dioxide Absolute value of polarization azimuth of linear polarization converter in (insulated state) .

[0037] Fig.10 The electrical conductivity of vanadium dioxide Ellipticity of the linear polarization converter in (insulated state) . DETAILED DESCRIPTION

[0038] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several variations and improvements may be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0039] like Figure 1-2 As shown, a switchable terahertz dual-function device based on a vanadium dioxide metasurface according to an embodiment of the present invention, wherein the switchable metasurface is composed of a diagonally placed vanadium dioxide pattern layer 1, a polycrystalline silicon dielectric layer 2, a gold pattern layer 3 composed of a gold split ring resonator and an L-shaped gold bar, a vanadium dioxide thin film layer 4, a polyimide dielectric layer 5 and a gold reflective layer 6 arranged in sequence from the top layer to the bottom layer, wherein the diagonally placed vanadium dioxide pattern layer 1 is attached to the upper surface of the polycrystalline silicon dielectric layer 2; the gold pattern layer 3 composed of the gold split ring resonator and the L-shaped gold bar is located below the polycrystalline silicon dielectric layer 2, and the gold pattern composed of the gold split ring resonator and the L-shaped gold bar is arranged along the Symmetrically, the polysilicon dielectric layer 2 is attached to the upper surface of the gold pattern layer 3 composed of the gold split ring resonator and the L-shaped gold bar; the vanadium dioxide film layer 4 is located below the gold pattern layer 3 composed of the gold split ring resonator and the L-shaped gold bar, and the gold pattern layer 3 composed of the gold split ring resonator and the L-shaped gold bar is attached to the upper surface of the vanadium dioxide film layer 4; the polyimide dielectric layer 5 is located below the vanadium dioxide film layer 4, and the vanadium dioxide film layer 4 is attached to the upper surface of the polyimide dielectric layer 5; the gold reflective layer 6 is located below the polyimide dielectric layer 5, and the polyimide dielectric layer 5 is attached to the upper surface of the gold reflective layer 6. The thickness of the diagonally placed vanadium dioxide pattern layer 1 is , the thickness of polysilicon dielectric layer 2 is The thickness of the gold pattern layer 3 consisting of a gold split ring resonator and an L-shaped gold bar is The thickness of the vanadium dioxide film layer 4 is , the thickness of the polyimide dielectric layer 5 is , the thickness of the gold reflective layer 6 is . The reversible phase transition of vanadium dioxide from a metallic state to an insulating state is achieved by changing the conductivity of vanadium dioxide through external excitation, and the external excitation can be light, temperature, voltage, etc. When vanadium dioxide is in a metallic state, the three-layer structure of a diagonally placed vanadium dioxide pattern layer, a polysilicon dielectric layer, and a vanadium dioxide film layer realizes a broadband absorption function; when vanadium dioxide is in an insulating state, the three-layer structure of a gold pattern layer composed of a gold split ring resonator and an L-shaped gold bar, a polyimide dielectric layer, and a gold reflective layer realizes a reflective linear polarization conversion function.

[0040] like Figure 3 As shown, it is a unit structure parameter diagram of the present invention, in which, , , .

[0041] During the simulation process, the finite element method was used to perform full-wave electromagnetic simulation and simulate the designed metasurface structure. The frequency domain solver was used in the simulation process, and the meshing was done using tetrahedral adaptive meshing. Unit cell boundary conditions were used in the x and y directions, and open (add space) boundary conditions were used in the z direction.

[0042] When the conductivity of vanadium dioxide (Metallic state), the designed metasurface structure mainly consists of a three-layer structure of a diagonally placed vanadium dioxide pattern layer 1, a polysilicon dielectric layer 2, and a vanadium dioxide film layer 4 to achieve broadband absorption function. In addition, the absorption rate calculation formula of the metasurface absorber is as follows:

[0043]

[0044] Among them, the reflectivity , transmittance The absorption spectra of the metasurface absorber in TE and TM polarization states are shown in Figure 4 As shown in the figure, in the frequency range of 1.21-3.04 THz, the bandwidth with an absorption rate greater than 90% is 1.82 THz, and its relative bandwidth is 86%. In addition, the absorption spectra under TE and TM polarization modes are consistent, indicating that the designed metasurface broadband absorber has polarization-independent characteristics.

[0045] In order to better understand the absorption mechanism of the absorber, the impedance matching theory is introduced, and the relative impedance calculation formula of the metasurface absorber is:

[0046]

[0047] The real and imaginary parts of the relative impedance of the terahertz wave under normal incidence are as follows: Figure 5 As shown in Figure 2, in the frequency range of 1.21-3.04 THz, the real part gradually approaches 1 and the imaginary part gradually approaches 0, especially in and At the two frequency points, the real part is closest to 1 and the imaginary part is closest to 0, indicating that the designed absorber impedance is most matched with the free space impedance and is closest to perfect absorption at these two frequency points.

[0048] Finally, the dependence of the absorber performance on the incident angle is studied, e.g. Figure 6 shown. Figure 6 (a) shows the change of absorption spectrum when the incident angle changes under TE polarization. When the incident angle is greater than The absorption rate gradually decreases. Figure 6(b) is the absorption spectrum corresponding to different incident angles under TM polarization. When the incident angle is greater than When , the absorption bandwidth gradually narrows. In summary, the designed metasurface absorber has It can maintain good absorption performance within the range and has absorption characteristics at large incident angles.

[0049] When the conductivity of vanadium dioxide In the insulating state, the designed metasurface structure comprises a gold pattern layer 3 composed of a gold split ring resonator and an L-shaped gold bar, a polyimide dielectric layer 5 and a gold reflective layer 6, which realizes the reflective linear polarization conversion function.

[0050] In the simulation process, the reflective linear polarization conversion function under the incidence of y-polarized terahertz waves is mainly studied. is the cross-polarization (y-to-x polarization) reflection coefficient, is the co-polarization (y-to-y polarization) reflection coefficient. Figure 7 As shown in the figure, the gold pattern layer composed of the gold split ring resonator and the L-shaped gold strip in the designed reflective metasurface linear polarization converter is arranged along Symmetric, so the reflection coefficient is symmetrical, that is .

[0051] In order to characterize the performance of the linear polarization converter, the polarization conversion efficiency of the polarization converter was studied. PCR , and its calculation formula is:

[0052]

[0053] like Figure 8 As shown, in the frequency range of 0.8-1.86 THz PCR The value is always greater than 0.92, and the corresponding working bandwidth is 1.06 THz, which shows that the designed terahertz metasurface linear polarization conversion has a wide linear polarization conversion frequency band.

[0054] In addition, combined Figure 7 and Figure 8 , at 1.12 THz and 1.57 THz, the co-polarization reflection coefficient is close to zero, indicating that only cross-polarization reflection coefficients exist at these two frequency points. , so that PCR The value is close to 1, thus achieving perfect linear polarization conversion.

[0055] In addition to using PCR In addition to measuring the performance of linear polarization converters, the polarization azimuth can also be used and ellipticity They are used to describe the polarization direction and linear polarization purity of the reflected wave respectively. as well as , indicating that the linear polarization converter has a high polarization purity. The corresponding calculation formulas are:

[0056]

[0057]

[0058] Fig. 9 is the polarization azimuth of the y-polarized normally incident terahertz wave The absolute value of , in the frequency range of 0.93-1.75 THz , and at 0.99 THz, 1.51 THz and 1.84 THz Close to , indicating that at these frequency points, when the y-polarized positive incident wave is transmitted along the -z axis, the reflected wave rotates by about , so that the reflected wave is converted into an x-polarized wave.

[0059] Fig.10 is the ellipticity of the y-polarized normally incident terahertz wave , in the frequency range of 0.79-1.86 THz , especially at 0.83 THz, 1.12 THz, 1.57 THz and 1.80 THz It is almost 0, indicating that the designed metasurface linear polarization converter has excellent linear polarization purity at these frequency points.

[0060] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A terahertz broadband absorber and reflective linear polarization converter based on a vanadium dioxide switchable metasurface, characterized in that: The switchable metasurface is composed of a diagonally placed vanadium dioxide pattern layer, a polysilicon dielectric layer, a gold pattern layer composed of a gold split ring resonator and an L-shaped gold bar, a vanadium dioxide film layer, a polyimide dielectric layer, and a gold reflective layer arranged in sequence from the top layer to the bottom layer. When the vanadium dioxide is in a metallic state, the switchable metasurface realizes a broadband absorption function; when the vanadium dioxide is in an insulating state, the switchable metasurface realizes a reflective linear polarization conversion function. The reversible phase transition of vanadium dioxide from a metallic state to an insulating state is achieved by changing the conductivity of vanadium dioxide through external stimulation, and the external stimulation can be light, temperature, and voltage; When vanadium dioxide is in a metallic state, a three-layer structure of a diagonally placed vanadium dioxide pattern layer, a polysilicon dielectric layer, and a vanadium dioxide thin film layer realizes a broadband absorption function; when vanadium dioxide is in an insulating state, a three-layer structure of a gold pattern layer composed of a gold split ring resonator and an L-shaped gold bar, a polyimide dielectric layer, and a gold reflective layer realizes a reflective linear polarization conversion function; In the terahertz frequency band, the relative dielectric constant of vanadium dioxide is described by the Drude model: Among them, ε ∞ =12 is the high frequency dielectric constant; γ = 5.75×10 13 rad / s, the plasma frequency at σ can be approximated by It means that σ0=3×10 5 S / m,ω p (σ0)=1.4×10 15 S / m; When the conductivity of vanadium dioxide σ=2×10 5 S / m is in the metallic state, and the conductivity of vanadium dioxide is σ=2×10 2 S / m is in the insulating state; The gold pattern consisting of a gold split ring resonator and an L-shaped gold bar is symmetrical along 45°.

2. The terahertz broadband absorber and reflective linear polarization converter based on vanadium dioxide switchable metasurface according to claim 1, characterized in that: The dielectric constant of polysilicon dielectric material is 3.

3. The terahertz broadband absorber and reflective linear polarization converter based on vanadium dioxide switchable metasurface according to claim 1, characterized in that: The electrical conductivity of the gold material in the gold pattern layer and the gold reflective layer is 4.56×10 7 S / m.

4. The terahertz broadband absorber and reflective linear polarization converter based on vanadium dioxide switchable metasurface according to claim 1, characterized in that: The composite dielectric constant of polyimide dielectric material is 3.5+0.00945i.

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