Transparent and reflective switchable multifunctional polarization converter based on metasurface

Abstract

The invention relates to the technical field of electromagnetic wave polarization conversion, in particular to a transmission and reflection switchable multifunctional polarization converter based on a metasurface, and the polarization converter comprises four layers, namely a TOPAS (cycloolefin copolymer) dielectric layer, a crossed I-shaped metal resonance layer, a silicon dioxide dielectric layer and a metal-VO2 alternating grating layer from top to bottom. According to the design, conversion between a transmission function and a reflection function can be realized by changing the conductivity of vanadium dioxide: 1, in a transmission mode, incident polarized waves can be converted into cross polarized waves with broadband characteristics; and 2, in a reflection mode, incident linearly polarized waves can be converted into left-hand circularly polarized waves in a frequency band of 0.8 THz-1. 83 THz, and the incident linearly polarized waves can be converted into right-hand circularly polarized waves in a frequency band of 2.7 THz-3. 06 THz. According to the invention, the conversion functions are more, cross polarization conversion and linear-circular polarization conversion can be realized, and the engineering applicability is higher.

Description

Technical Field

[0001] The present invention relates to the technical field of electromagnetic wave polarization conversion, and in particular to a terahertz polarization converter based on a metasurface with switchable transflection and reflection, which can realize cross-polarization conversion and linear-circular polarization conversion functions. Background Art

[0002] Terahertz waves are electromagnetic waves with a frequency range of 0.1THz to 10THz, which is between microwaves and infrared light. The unique spectrum makes terahertz technology have broad application prospects in spectral imaging, telecommunications, non-destructive testing and sensitivity testing.

[0003] Metasurfaces are artificial layered materials with a thickness less than the wavelength. They have new electromagnetic properties that do not exist in nature, such as asymmetric transmission, conversion between circularly polarized waves and linearly polarized waves, negative refractive index, optical magnetism, and high-power microwave emission. In addition, metasurfaces have lighter structures and better performance and are widely used in micro-nano devices. Therefore, they have opened up a new technical path for controlling the amplitude, phase, polarization, and propagation mode of electromagnetic waves.

[0004] Polarization is one of the basic characteristics of electromagnetic waves, which refers to the vibration direction of the electric field of electromagnetic waves during propagation. Effectively controlling the polarization direction of electromagnetic waves is crucial for the utilization of electromagnetic waves. The polarization of electromagnetic waves can be divided into linear polarization, circular polarization and elliptical polarization. Different systems often have different requirements for the polarization of electromagnetic waves. For example, circularly polarized waves have the characteristics of resistance to multipath fading and polarization matching. Therefore, satellite communications usually use circularly polarized electromagnetic waves, and radar detection systems use two orthogonal linear polarizations or two orthogonal circularly polarized electromagnetic waves for detection scanning.

[0005] Polarization converters control the form of polarization according to actual needs and realize different polarization conversion functions to meet the needs of specific applications. For example, in the field of communications, the polarization direction of the signal is changed to avoid interference from other signals and improve communication quality. In terms of radar cross-section reduction, it can effectively reduce the reflection of radar waves, thereby helping stealth aircraft or other military equipment to evade detection by enemy radars. In optics and nanotechnology, it is used to control the propagation direction and intensity of optical signals to achieve efficient optical communications and optical sensing systems. Therefore, polarization converters provide important technical support for multiple fields such as communications, radar, imaging, and optics by controlling the polarization state of electromagnetic waves, and their versatility and tunability make them an indispensable part of modern science and technology.

[0006] From the perspective of the interaction mode with the incident wave, polarization converters can be divided into two types: reflective and transmissive. Reflective polarization converters have obvious advantages in energy utilization and wide incident angle adaptability; transmissive polarization converters have good broadband performance, low insertion loss, and can provide greater flexibility for polarization control in complex electromagnetic environments. Therefore, the present invention designs a transmissive and reflective switchable polarization converter to achieve multifunctional performance to meet more practical application needs. Summary of the invention

[0007] The present invention aims to solve the problem that the existing polarization converter is untunable and has a single function. It provides a multifunctional polarization converter based on metasurface transflective switchable. 2 The conductivity of VO 2 The shape and function of the polarization converter enable the polarization converter to achieve the goal of freely switching between the transmission and reflection functions, thereby expanding the bandwidth and improving the sensitivity and integration of the device. The present invention is achieved through the following technical solutions:

[0008] A multifunctional polarization converter based on metasurface transflective switchable, characterized in that it comprises four layers, from top to bottom, namely, a TOPAS dielectric layer, a cross-I-shaped metal resonance layer, a SiO 2 Dielectric layer and metal-VO 2 Alternating grating layer;

[0009] The metal-VO 2 The alternating grating layer is made of metal and VO 2 In the o-xyz space rectangular coordinate system, the supersurface microstructure is formed by periodically arranging along the x and y directions respectively, with o being the origin of the coordinate system;

[0010] The multifunctional polarization converter based on the switchable transflection and transmission of the metasurface has a structural size of 50 μm×50 μm×36 μm.

[0011] The TOPAS dielectric layer and SiO 2 The dielectric layers are square with the same side length p = 50 μm, but different thicknesses; the upper TOPAS dielectric layer has a dielectric constant of 2.35, a loss tangent of 0.00007, and a thickness of d2 = 15 μm; the lower SiO 2 The dielectric constant of the dielectric layer is 3.90, the loss tangent is 0.001, and the thickness d1 = 20 μm.

[0012] The dielectric layer causes the terahertz wave to generate phase accumulation inside the device, meeting the requirements of polarization conversion.

[0013] The cross-I-shaped metal resonance layer is symmetrical about the diagonal in the xoy plane, and is composed of two I-shaped metal structures of different sizes that are rotated and superimposed, that is, the first I-shaped is rotated 45° counterclockwise and the second I-shaped is rotated 45° clockwise in the xoy plane and then superimposed, and the centers of the first I-shaped and the second I-shaped are the same; wherein the length of the middle vertical axis of the first I-shaped metal structure is 17-23μm, the width is 5-9μm, the length of the horizontal axes on both sides is 5-9μm, and the width is 29-35μm, the length of the middle vertical axis of the second I-shaped metal structure is 30-45μm, the width is 1-3μm, the length of the horizontal axes on both sides is 0.5-3μm, and the width is 1-10μm.

[0014] The metal-VO 2 In the alternating grating layer, the width of each metal grating is w3 = p / (4+4 / g), VO 2 The width w4=w3 / g, g is 1-3, and p is the side length.

[0015] The cross-I-shaped metal resonant layer and the metal-VO 2 In the alternating grating layer, the metal material used is gold, and the conductivity is 4.56×10 7 S / m, thickness d=0.5μm.

[0016] The relative dielectric constant of the vanadium dioxide film It is expressed as follows:

[0017]

[0018] Where ε represents the relative dielectric constant, VO 2 represents vanadium dioxide, represents the relative dielectric constant of vanadium dioxide, δ is the volume fraction of vanadium dioxide in the metallic state, ε insulating represents the relative dielectric constant of the insulating state of vanadium dioxide, ε metallic Represents the relative dielectric constant of metallic vanadium dioxide.

[0019] The metal-VO 2 Alternating grating layers, changing VO 2 Conductivity can realize the switching between transmission and reflection, VO 2 The conductivity is 2×10 5 S / m, it behaves in a metallic state, the device behaves in a reflective state, and VO 2 When the conductivity is 10S / m, it behaves in an insulating state and the device behaves in a transmission type.

[0020] In the above scheme, the bottom metal-VO 2 The thickness of the alternating grating layer and the intermediate metal resonance layer must be greater than the skin depth of the electromagnetic wave. The materials used are all with a conductivity of 4.56×107 The bottom metal layer is used to reflect the terahertz wave that passes through the dielectric layer, so that the device forms a multiple interference cavity model to achieve a high polarization conversion rate; the metal resonance layer resonates with the incident wave and forms impedance matching with the incident wave, so that the terahertz wave can generate different types of induced electric fields in different directions.

[0021] The present invention has the following advantages and beneficial effects:

[0022] 1. The transflective multifunctional polarization converter proposed in the present invention does not require additional active devices, but only needs to control VO 2 The conductivity can realize dynamic switching between transmission mode and reflection mode in the terahertz band.

[0023] 2. The transmission type polarization converter has a relatively wide working bandwidth. When the bottom metal grating width is equal to VO 2 When the width ratio is 1:1, cross-polarization conversion can be achieved in the frequency range of 1THz-2.85THz; when the width ratio is 1:2, cross-polarization conversion can be achieved in the frequency range of 0.77THz-3.2THz; when the width ratio is 1:3, cross-polarization conversion can be achieved in the frequency range of 0.58THz-3.26THz.

[0024] 3. The reflective polarization converter can achieve different rotation directions in different bands, achieving left rotation in the 0.8THz-1.83THz band and right rotation in the 2.7THz-3.06THz band.

[0025] 4. The designed polarization converter is conducive to the miniaturization and integration of devices, can reduce losses and improve efficiency, and its manufacturing cost is low during actual processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A schematic diagram of the exploded structure of a metasurface-based transflective multifunctional polarization converter is provided for a preferred embodiment of the present invention.

[0027] Figure 2 Bottom view of a metasurface-based transflective multifunctional polarization converter structure.

[0028] Figure 3 Side view of a metasurface-based transflective multifunctional polarization converter structure designed.

[0029] Figure 4 (a) is a first I-shaped structure diagram of the designed cross-I-shaped metal resonance layer.

[0030] Figure 4 (b) is a second I-shaped structure diagram of the designed cross-I-shaped metal resonance layer.

[0031] Figure 4 (c) is the overall picture of the designed cross-I-shaped metal resonance layer.

[0032] Figure 5 The co-polarization and cross-polarization transmission coefficient diagrams are obtained by using the method of the present invention under y-polarization vertical incidence conditions for a designed metasurface-based transflective multifunctional polarization converter.

[0033] Figure 6 For VO 2 The linear polarization conversion rate change curve of the polarization converter when it is in the insulating state and the device behaves in the transmission mode.

[0034] Figure 7 The co-polarization and cross-polarization reflection coefficient diagrams are obtained by using the method of the present invention under y-polarization vertical incidence conditions for a designed metasurface-based transflective multifunctional polarization converter.

[0035] Figure 8 For VO 2 Phase difference curve of the polarization converter when in the metallic state and the device behaves in reflection mode.

[0036] Fig. 9 For VO 2 AR curve of the polarization converter when in the metallic state and the device behaves in reflection mode.

[0037] Fig.10 For VO 2 Normalized ellipticity curve of the polarization converter when in the metallic state and the device behaves in reflection mode.

[0038] In the figure: 1TOPAS dielectric layer; 2Cross-I-shaped metal resonant layer; 3SiO 2 Dielectric layer; 4 metal-VO 2 Alternating grating layer. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and do not limit the present invention. On the contrary, these embodiments are provided to make the content of the present invention more thorough and comprehensive. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0040] Combination Figures 1 to 4As shown, the present invention provides a transflective multifunctional polarization converter based on a metasurface. The structure consists of four layers from top to bottom, including two dielectric layers and two metal layers. The upper dielectric layer is TOPAS and the lower dielectric layer is SiO 2 The middle cross-shaped metal resonance layer 2 is formed by rotating the first I-shaped structure and the second I-shaped structure and then cross-overlaying them. The bottom is made of VO 2 The device is attached to a metal grating to switch between transmission and reflection. The device is periodically arranged in the x and y axis directions to form a metasurface microstructure.

[0041] The frequency domain solver in CST electromagnetic simulation software is used to simulate and analyze the model. The device structure is set as periodic boundary conditions along the x-axis and y-axis directions, and as open boundary conditions in the z-axis direction. The incident electromagnetic wave is a plane electromagnetic wave, incident along the -z axis direction.

[0042] Working principle: When electromagnetic waves irradiate anisotropic structures, they exhibit different equivalent electromagnetic parameters in two orthogonal directions. By adjusting the physical size of the polarization converter, the amplitude and phase of the outgoing electromagnetic wave can be changed, so that the states of the incident wave and the reflected wave or the transmitted wave are different, and the directions of the two are orthogonal, thereby producing the effect of converting linear polarization to cross polarization or circular polarization, and realizing polarization conversion.

[0043] This embodiment provides a VO-based 2 The transflective switchable multifunctional polarization converter, when VO 2 When the device is in an insulating state, it acts as a transmission-type polarization converter and can achieve perfect broadband cross-polarization conversion. 2 When in the metallic state, the device is a reflective polarization converter, with left-hand circular polarization waves in the range of 0.8-1.83THz and right-hand circular polarization waves in the range of 2.7-3.06THz. The device has the characteristics of high polarization efficiency and dynamically adjustable polarization mode.

[0044] The terahertz polarization converter based on the "I" shaped cross structure proposed in the present invention has a structure as follows: Figures 1 to 4 As shown in the figure, it is similar to a "sandwich" structure, which consists of TOPAS dielectric layer 1, cross-I-shaped metal resonant layer 2, SiO 2 Dielectric layer 3, metal-VO 2 Alternating grating layer 4.

[0045] like Figure 2 The bottom view of the polarization converter is shown, with the bottom metal-VO 2 The alternating grating layer 4 is formed by 2 The width of the metal grating is w3 = 5.55 μm, VO 2The width w4 = 5.55 μm, the length p = 50 μm, and the thickness d = 0.5 μm.

[0046] like Figure 3 As shown, the dielectric constant of the upper TOPAS dielectric layer 1 of the present invention is 2.35, the loss tangent is 0.00007, the thickness d2 is 15 μm, and the lower SiO 2 The dielectric constant of the dielectric layer 3 is 3.90, the loss tangent is 0.001, the thickness d1=20 μm, and the length and width p of the dielectric substrate are 50 μm.

[0047] like Figure 4 As shown, Figure 4 (a) is a first I-shaped metal structure, Figure 4 (b) is a second I-shaped metal structure, Figure 4 (c) is the middle crossed I-shaped metal resonance layer, which is formed by rotating the first I-shaped metal structure counterclockwise by 45° and the second I-shaped metal structure clockwise by 45° in the xoy plane and then superimposing them, wherein the length of the middle vertical axis of the first I-shaped metal structure is L1=20μm, the width is w1=7.5μm, the length of the horizontal axes on both sides is L2=8.5μm, the width is w2=32μm, the length of the middle vertical axis of the second I-shaped metal structure is b=40μm, the width is a=2μm, the length of the horizontal axes on both sides is e=1μm, and the width is c=5μm.

[0048] According to the design method of the tunable polarization converter of the present invention, the thickness of the bottom and top metal layers must be greater than the skin depth of the electromagnetic wave, and both use gold with a thickness of d = 0.5 μm and a conductivity of 4.56×10 7 S / m.

[0049] In summary, the bottom metal-VO in the polarization converter 2 The alternating grating layer and the I-shaped metal resonant layer in the middle form a Fabry-Perot interference structure, which further enhances the cross-polarization transmission and reflection efficiency, polarization conversion ratio and working bandwidth, and can realize the conversion of the linearly polarized incident wave in the y-direction into the reflected electromagnetic wave and the transmitted electromagnetic wave of the x-direction polarized wave or the circularly polarized wave.

[0050] The transmission-type polarization converter proposed in the present invention generates two independent polarization modes for the incident electromagnetic wave. The intensity of the cross-polarization conversion of the transmitted electromagnetic wave can be determined according to the amplitude difference between the two independent transmission coefficients. The polarization conversion ratio (PCR) is introduced to describe the conversion efficiency of the polarization converter, which can be defined as:

[0051]

[0052] Where, t xyIt is the cross-polarization transmission coefficient formed by the electromagnetic wave incident along the y-axis direction and transmitted along the x-axis direction after polarization rotation. yy It is the co-polarization transmission coefficient formed by the electromagnetic wave incident along the y-axis direction with almost no polarization rotation and directly transmitted from the y-axis direction. The methods for obtaining the two can be expressed as follows:

[0053]

[0054] Where E represents the electric field, the subscripts i and t represent the incident electromagnetic wave and the transmitted electromagnetic wave respectively, x and y represent the polarization direction of the electromagnetic wave respectively, and the x and y coordinate axes are rotated counterclockwise by 45° to obtain the u and v coordinate axes respectively. is the phase difference, m represents any working frequency point within the working bandwidth, It represents the phase difference of the co-polarized reflections along the u and v axes at the frequency m.

[0055] The reflective polarization converter of the present invention realizes dual-band linear-circular polarization conversion, and introduces the axial ratio AR and the normalized ellipticity χ to evaluate the mode of the reflected electromagnetic wave.

[0056] According to the calculation formula of the axis ratio AR:

[0057]

[0058] in and a represents an intermediate variable, represents the phase difference between the cross-polarization reflection coefficient and the co-polarization reflection coefficient, r xy It is the cross-polarization reflection coefficient formed by the electromagnetic wave incident along the y-axis direction and reflected in the x-axis direction after polarization rotation, r yy It is the co-polarization reflection coefficient formed by the electromagnetic wave incident along the y-axis direction with almost no polarization rotation and directly reflected from the y-axis direction. The circularly polarized wave needs to meet the two requirements of equal polarization wave amplitude and ±90° phase difference.

[0059] In addition, the normalized ellipticity χ is introduced to further evaluate the polarization conversion state of the terahertz wave obtained by the reflection-type polarization converter. χ close to +1 indicates left-hand circular polarization (LCP), and close to -1 indicates right-hand circular polarization (RCP).

[0060] The normalized ellipticity calculation formula is:

[0061]

[0062] The linear polarization wave is incident on the polarization converter vertically, and the transmitted wave is a cross-polarization wave. 2When the width w4 ratio is 1:1, the transmission coefficient changes with frequency as shown in Figure 5 As shown, in the 1THz-2.85THz frequency band, the cross-polarization coefficient amplitude t xy Greater than the co-polarization coefficient amplitude t yy , which shows that the device converts the incident wave polarized in the y direction into a transmitted electromagnetic wave polarized in the x direction, thus realizing cross-polarization conversion.

[0063] like Figure 6 As shown, the terahertz polarization converter based on the "I"-shaped superposition structure has a polarization conversion rate greater than 0.9 in the 0.78THz-3.13THz frequency band, and the linear polarization conversion rate is greater than 0.98 in the 1THz-2.85THz band, indicating that the polarization converter has a high conversion rate in the working frequency band and realizes perfect linear polarization conversion, which can convert most of the incident y-polarized waves into outgoing x-polarized waves.

[0064] The bottom metal grating width w3 and VO 2 The width w4 ratio has no effect on the reflection mode. 2 The conductivity is 2×10 5 S / m, the simulated reflection coefficient and phase difference are as follows Figure 7 and Figure 8 As shown, the co-polarization reflection coefficient r yy and the cross-polarization reflection coefficient r xy The amplitudes are almost equal in the frequency bands of 0.8THz-1.83THz and 2.7THz-3.06THz, and the phase difference is maintained at around ±90°. The amplitude and phase conditions for the conversion from linear polarization to circular polarization in the reflection mode are met, which indicates that the designed structure can manipulate linearly polarized incident electromagnetic waves and realize circularly polarized reflected electromagnetic waves.

[0065] like Fig. 9 As shown in the figure, the axial ratio curves of the reflected wave in the 0.78THz-1.9THz and 2.37THz-3.18THz bands are kept below 3dB, and at 1.07THz and 1.69THz, the axial ratio AR of the reflected electromagnetic wave takes the minimum value of 0.1dB. Fig.10 It can be seen that in the frequency band of 0.8THz-1.83THz, χ is approximately +1, indicating that the y-polarized incident electromagnetic wave can be converted into a left-handed circularly polarized wave in the reflection mode. In the frequency band of 2.7THz-3.06THz, χ is almost -1, indicating that the reflected wave is converted into a right-handed circularly polarized wave.

[0066] In this preferred simulation example, the proposed polarization converter can be realized by changing VO 2The conductivity achieves the goal of dynamically switching between the function of linear polarization deflection and the function of circular polarization conversion, solving the problem of single function due to the non-tunability of the formed metasurface.

[0067] The principles, characteristics, and related advantages of the present invention are described in detail above, and the related structural parameters given are preferred implementation schemes. It should be pointed out that the present invention is not limited to the specific details in the above-mentioned implementation schemes, and within the scope of the technical concept of the present invention, any modifications made should also be regarded as the protection scope of the present invention.

Claims

1. A multifunctional polarization converter based on metasurface transflective switchable, characterized in that: It includes four layers, which are, from top to bottom, a TOPAS dielectric layer (1), a cross-I-shaped metal resonant layer (2), a SiO2 dielectric layer (3) and a metal-VO2 alternating grating layer (4); The metal-VO2 alternating grating layer (4) is formed by periodically arranging metal and VO2 along the x and y directions in an o-xyz spatial rectangular coordinate system to form a super surface microstructure, where o is the origin of the coordinates; The cross-I-shaped metal resonance layer (2) is symmetrical about the diagonal line in the xoy plane, and is composed of two I-shaped metal structures of different sizes rotated and superimposed, that is, the first I-shaped structure is rotated 45° counterclockwise on the xoy plane and the second I-shaped structure is rotated 45° clockwise before being superimposed, and the first I-shaped structure and the second I-shaped structure have the same center; wherein the length of the middle vertical axis of the first I-shaped metal structure is 17-23 μm, the width is 5-9 μm, the length of the horizontal axes on both sides is 5-9 μm, and the width is 29-35 μm; the length of the middle vertical axis of the second I-shaped metal structure is 30-45 μm, the width is 1-3 μm, the length of the horizontal axes on both sides is 0.5-3 μm, and the width is 1-10 μm; In the metal-VO2 alternating grating layer (4), the width of each metal grating w3=p / (4+4 / g), the width of VO2 w4=w3 / g, the value of g is 1-3, and p is the side length.

2. A multifunctional polarization converter based on switchable transflection and transmission of a metasurface as claimed in claim 1, characterized in that: The metal material used in the cross-I-shaped metal resonance layer (2) and the metal-VO2 alternating grating layer (4) is gold, and the electrical conductivity is 4.56×10 7 S / m, thickness d=0.5μm.

3. A multifunctional polarization converter based on switchable transflection and transmission of a metasurface as claimed in claim 1, characterized in that: The TOPAS dielectric layer (1) and the SiO2 dielectric layer (3) are square, have the same side length p=50 μm, but different thicknesses; the upper TOPAS dielectric layer (1) has a dielectric constant of 2.35, a loss tangent of 0.00007, and a thickness d2=15 μm; The lower SiO2 dielectric layer (3) has a dielectric constant of 3.90, a loss tangent of 0.001, and a thickness of d1 = 20 μm.

4. The multifunctional polarization converter based on switchable transflection and transmission of a metasurface as claimed in claim 1, characterized in that: The structural dimensions of the multifunctional polarization converter are 50 μm×50 μm×36 μm.

5. The multifunctional polarization converter based on switchable transflection and transmission of a metasurface as claimed in claim 1, characterized in that: The relative dielectric constant ε of the vanadium dioxide film is VO2 It is expressed as follows: Where ε represents the relative dielectric constant, VO2 represents vanadium dioxide, ε VO2 represents the relative dielectric constant of vanadium dioxide, δ is the volume fraction of vanadium dioxide in the metallic state, ε insulating represents the relative dielectric constant of the insulating state of vanadium dioxide, ε metallic Represents the relative dielectric constant of metallic vanadium dioxide.

6. The multifunctional polarization converter based on switchable transflection and transmission of a metasurface as claimed in claim 1, characterized in that: In the metal-VO2 alternating grating layer (4), changing the VO2 conductivity can realize the switching between transmission and reflection. The VO2 conductivity is 2×10 5 When the VO2 conductivity is 10S / m, it behaves in a metallic state and the device behaves in a reflective state. When the VO2 conductivity is 10S / m, it behaves in an insulating state and the device behaves in a transmissive state.

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