A multifunctional polarization converter based on switchable transflective and reflective metasurfaces

Abstract

The present invention relates to the technical field of electromagnetic wave polarization conversion, and specifically proposes a multifunctional polarization converter based on metasurface transflective switchable polarization. The polarization converter comprises four layers, which are, from top to bottom, a cyclic olefin copolymer (TOPAS) dielectric layer, a crossed I-shaped metal resonant layer, a silicon dioxide dielectric layer, and a metal-VO2 alternating grating layer. The design can achieve the conversion of transmission and reflection functions by changing the electrical conductivity of vanadium dioxide: 1. In transmission mode, the incident polarized wave can be converted into a cross-polarized wave with broadband characteristics; 2. In reflection mode, the incident linearly polarized wave can be converted into a left-handed circularly polarized wave in the 0.8THz-1.83THz frequency band, and into a right-handed circularly polarized wave in the 2.7THz-3.06THz frequency band. The present invention has more conversion functions, can realize cross-polarization conversion and linear-circular polarization conversion, and has stronger engineering applicability.

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 a fundamental property of electromagnetic waves, referring to the direction of the electric field's vibration during propagation. Effectively controlling the polarization of electromagnetic waves is crucial for their utilization. Electromagnetic wave polarization can be categorized as linear, circular, and elliptical. Different systems often have different polarization requirements. For example, circularly polarized waves offer resistance to multipath fading and polarization matching, so satellite communications typically use circularly polarized electromagnetic waves. Radar detection systems employ two orthogonal linearly polarized or circularly polarized waves for detection and scanning.

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

[0006] Based on how they interact with the incident wave, polarization converters can be categorized as reflective or transmissive. Reflective polarization converters offer significant advantages in energy efficiency and wide-angle adaptability. Transmissive polarization converters offer excellent broadband performance, low insertion loss, and greater flexibility for polarization control in complex electromagnetic environments. Therefore, the present invention designs a polarization converter that switches between reflective and transmissive modes to achieve multifunctional performance and meet diverse practical application requirements. Summary of the Invention

[0007] The present invention aims to solve the problem that existing polarization converters are non-tunable and have a single function. It provides a multifunctional polarization converter based on a metasurface with switchable transmission and reflection. By changing the conductivity of VO2, the morphology and function of VO2 can be changed so that the polarization converter can achieve the goal of freely switching between transmission and reflection, 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 structure, characterized by comprising four layers, from top to bottom, namely, a TOPAS dielectric layer, a cross-I-shaped metal resonant layer, a SiO2 dielectric layer, and a metal-VO2 alternating grating layer;

[0009] The metal-VO2 alternating grating layer is formed by periodically arranging metal and VO2 along the x and y directions in the o-xyz spatial rectangular coordinate system to form a metasurface microstructure, where o is the coordinate origin;

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

[0011] The TOPAS dielectric layer and the SiO2 dielectric layer are square, have 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 d2 = 15 μm; the lower SiO2 dielectric layer has a dielectric constant of 3.90, a loss tangent of 0.001, and a 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 structure is rotated 45° counterclockwise in the xoy plane and the second I-shaped structure is rotated 45° clockwise before being superimposed, and the centers of the first I-shaped and the second I-shaped structures are the same; 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] In the metal-VO2 alternating grating layer, 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.

[0015] The metal material used in the cross-I-shaped metal resonance layer and the metal-VO2 alternating grating layer is gold, and the electrical conductivity is 4.56×10 7 S / m, thickness d=0.5 μm.

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

[0017]

[0018] Where ε represents the relative dielectric constant, VO2 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-VO2 alternating grating layer can switch between transmission and reflection by changing the VO2 conductivity. 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.

[0020] In the above scheme, the thickness of the bottom metal-VO2 alternating grating layer and the middle metal resonant layer must be greater than the skin depth of the electromagnetic wave, and the materials used are all with a conductivity of 4.56×10 7The bottom metal layer is used to reflect the terahertz waves that pass through the dielectric layer, allowing the device to form a multi-interference cavity model and achieve a high polarization conversion rate. The metal resonant layer resonates with the incident wave and forms an impedance match with the incident wave, allowing the terahertz wave to 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 this invention does not require additional active devices and can achieve dynamic switching between transmission mode and reflection mode in the terahertz band simply by controlling the VO2 conductivity.

[0023] 2. The transmission-type polarization converter has a relatively wide operating bandwidth. When the ratio of the bottom metal grating width to the VO2 width 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-hand rotation in the 0.8THz-1.83THz band and right-hand 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 in 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 designed metasurface-based transflective multifunctional polarization converter structure.

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

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

[0032] Figure 5 The co-polarization and cross-polarization transmission coefficient diagrams are obtained 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 The linear polarization conversion rate change curve of the polarization converter when VO2 is in an insulating state and the device behaves in transmission mode.

[0034] Figure 7 The co-polarization and cross-polarization reflection coefficient diagrams are obtained 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 Phase difference curve of the polarization converter when VO2 is in the metallic state and the device behaves in reflection mode.

[0036] Figure 9 The AR curve of the polarization converter when VO2 is in the metallic state and the device behaves in reflection mode.

[0037] Figure 10 Normalized ellipticity curve of the polarization converter when VO2 is in metallic state and the device behaves in reflection mode.

[0038] In the figure: 1 TOPAS dielectric layer; 2 cross-I-shaped metal resonant layer; 3 SiO2 dielectric layer; 4 metal-VO2 alternating grating layer. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below with reference to the accompanying drawings and examples. 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] Combine Figures 1 to 4As shown, the present invention provides a metasurface-based transflective multifunctional polarization converter. The structure consists of four layers from top to bottom: two dielectric layers and two metal layers. The upper dielectric layer is TOPAS, the lower dielectric layer is SiO2, and the middle cross-shaped metal resonant layer 2 is formed by rotating and overlapping the first and second I-shaped structures. At the bottom, VO2 is attached to the metal grating to achieve switching between transmission and reflection. The device is periodically arranged in the x and y directions to form a metasurface microstructure.

[0041] The model was simulated and analyzed using the frequency-domain solver in CST electromagnetic simulation software. The device structure was set to periodic boundary conditions along the x- and y-axes and an open boundary condition along the z-axis. The incident electromagnetic wave was a plane electromagnetic wave incident along the -z axis.

[0042] Working principle: When electromagnetic waves irradiate anisotropic structures, they exhibit different equivalent electromagnetic parameters in two orthogonal directions. By adjusting the physical dimensions of the polarization converter, the amplitude and phase of the outgoing electromagnetic wave can be changed, making the states of the incident wave and the reflected wave or transmitted wave 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 multifunctional polarization converter based on VO2 that is both transmissive and reflective. When VO2 is in an insulating state, the device functions as a transmissive polarization converter, achieving perfect broadband cross-polarization conversion. When VO2 is in a metallic state, the device functions as a reflective polarization converter, producing left-handed circularly polarized waves in the 0.8-1.83 THz range and right-handed circularly polarized waves in the 2.7-3.06 THz range. This device features 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, it is similar to a "sandwich" structure, which includes, 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.

[0045] like Figure 2 The bottom view of the polarization converter is shown. The bottom metal-VO2 alternating grating layer 4 is formed by attaching VO2 to the metal grating. The width of the metal grating is w3 = 5.55 μm, the width of VO2 is w4 = 5.55 μm, the length p = 50 μm, and the thickness d = 0.5 μm.

[0046] like Figure 3As shown, the dielectric constant of the upper TOPAS dielectric layer 1 of the present invention is 2.35, the loss tangent is 0.00007, and the thickness d2 is 15 μm. The dielectric constant of the lower SiO2 dielectric layer 3 is 3.90, the loss tangent is 0.001, and the thickness d1 is 20 μm. The length and width of the dielectric substrate p 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 cross I-shaped metal resonance layer, which is formed by rotating the first I-shaped metal structure 45° counterclockwise and the second I-shaped metal structure 45° clockwise 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 = 20 μm, the width w1 = 7.5 μm, the length of the horizontal axes on both sides is L2 = 8.5 μm, the width w2 = 32 μm, the length of the middle vertical axis of the second I-shaped metal structure is b = 40 μm, the width a = 2 μm, the length of the horizontal axes on both sides is e = 1 μm, and the width c = 5 μm.

[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. Both layers 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-VO2 alternating grating layer and the middle I-shaped metal resonant layer in the polarization converter form a Fabry-Perot interference structure, which further enhances the cross-polarization transmission and reflection efficiency, polarization conversion ratio and working bandwidth, and can convert the linearly polarized incident wave in the y-direction into reflected electromagnetic waves and transmitted electromagnetic waves of the x-direction polarized wave or circularly polarized wave.

[0050] The transmission-type polarization converter proposed in this 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 based on 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 and can be defined as:

[0051]

[0052] Where, t xy It 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, t yyIt 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 along the y-axis direction. The two methods of obtaining 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 45° counterclockwise to obtain the u and v coordinate axes respectively. is the phase difference, m represents any operating frequency point within the operating bandwidth, It represents the phase difference of the co-polarized reflection along the u and v axes at 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, 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. A χ close to +1 indicates left-hand circular polarization (LCP), and a χ close to -1 indicates right-hand circular polarization (RCP).

[0060] The calculation formula of normalized ellipticity is:

[0061]

[0062] The linearly polarized wave is incident on the polarization converter in a vertical manner, and the transmitted wave is a cross-polarized wave. When the ratio of the bottom metal grating width w3 to the VO2 width w4 is 1:1, the transmission coefficient changes with frequency as shown in the following curve: Figure 5 As shown, in the 1THz-2.85THz frequency band, the cross-polarization coefficient amplitude txy 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, realizing cross-polarization conversion.

[0063] like Figure 6 As shown, the terahertz polarization converter based on the "I"-shaped stacking 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 within the working frequency band and achieves perfect linear polarization conversion, which can convert most of the incident y-polarized waves into outgoing x-polarized waves.

[0064] The ratio of the bottom metal grating width w3 to the VO2 width w4 has no effect on the reflection mode. When the VO2 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 0.8THz-1.83THz and 2.7THz-3.06THz frequency bands, 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 Figure 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. Figure 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 achieve the goal of dynamically switching between the function of linear polarization deflection and the function of circular polarization conversion by changing the VO2 conductivity, solving the problem of single function caused by the non-tunability of the formed metasurface.

[0067] The principles, characteristics, and related advantages of the present invention are described in detail above. The relevant 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. Within the scope of the technical concept of the present invention, any modifications made should also be regarded as the scope of protection 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 coordinate origin; 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. The multifunctional polarization converter based on metasurface transflective switchable polarization according to 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=0.5 μm.

3. The multifunctional polarization converter based on metasurface transflective switchable polarization according to 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 d1 = 20 μm.

4. The multifunctional polarization converter based on metasurface transflective switchable polarization according to claim 1, characterized in that: The multifunctional polarization converter has a structural size of 50 μm×50 μm×36 μm.

5. The multifunctional polarization converter based on metasurface transflective switchable polarization according to claim 1, characterized in that: The relative dielectric constant ε of the vanadium dioxide 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 metasurface transflective switchable polarization according to 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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