Terahertz reflection type linear-circular polarization converter based on biarc leaf-shaped structure

Abstract

The invention relates to the technical field of electromagnetic metasurfaces and polarization regulation and control, and provides a terahertz reflection type linear-circular polarization converter based on a biarc leaf-shaped structure. The polarization converter is composed of a bottom metal reflecting layer, a middle dielectric layer and a top metal resonance layer. The top layer is of a double-arc leaf-shaped structure, the structure can decompose incident linearly polarized terahertz waves into two orthogonal characteristic components, the two characteristic components generate a 90-degree phase difference in a specific frequency band and keep equal reflection amplitudes, and therefore the incident linearly polarized waves are efficiently converted into circularly polarized waves. When y polarized waves are vertically incident, right-handed circularly polarized waves are generated in the frequency range of 1.71-2.57 THz, and high-efficiency left-handed circularly polarized waves are generated in the frequency range of 3.45-5.41 THz. Linear-circular polarization conversion with the axial ratio larger than-3dB is achieved in the two frequency ranges. The invention has the advantages of compact structure, wide axial ratio bandwidth, high conversion efficiency and the like, and has important application value in terahertz communication and imaging systems.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic metasurfaces and polarization control technology, and proposes a terahertz reflective linear-circular polarization converter based on a double-circular-arc leaf-shaped structure. Background Technology

[0002] Terahertz waves typically refer to electromagnetic waves with frequencies ranging from 0.1 THz to 10 THz, corresponding to wavelengths of approximately 30 μm to 3000 μm, with a spectrum between microwaves and infrared radiation. Terahertz waves possess strong penetrating power, non-ionizing biocompatibility, high signal-to-noise ratio, and good spectral resolution, thus holding significant application potential in communication, sensing, spectral analysis, and imaging. Metamaterials are composite media composed of artificial periodic structures at subwavelength scales. Their structural unit sizes are much smaller than the operating wavelength, enabling them to exhibit specific electromagnetic response behaviors not found in natural materials, such as negative refraction, perfect absorption, super-resolution imaging, and anomalous reflection. The emergence of metamaterials has propelled the further development of terahertz technology, leading to the proposal of various functional devices based on metamaterials, including filters, modulators, polarization converters, wavefront modulators, and energy absorbers. In terahertz polarization manipulation, by rationally designing the structural configuration of metamaterials, the conversion from linear polarization to circular polarization or between linear polarizations can be achieved, thereby enabling precise control of the polarization state of terahertz waves, demonstrating significant research value and application potential. Traditional terahertz polarization devices mostly rely on natural anisotropic crystals or mechanically rotating elements for polarization modulation, which suffers from limitations such as large size, limited modulation rate, and complex fabrication processes. In contrast, metamaterial-based polarization converters utilize the electromagnetic coupling between a subwavelength metallic resonant structure and a dielectric substrate to excite highly efficient polarization conversion responses within a specific frequency band. By designing metallic resonant units of different shapes and arrangements, different modes of electromagnetic resonance can be excited at multiple operating frequencies, thereby achieving multi-band, high-efficiency, and even wideband polarization conversion functions. This type of structure provides a new technological path for realizing lightweight, integrated, and high-performance terahertz polarization control devices.

[0003] In monolayer metamaterial structures, the polarization conversion efficiency for linearly polarized waves is typically around 50%; however, in reflective metamaterials, this efficiency can be increased to approximately 90%. Furthermore, by introducing a metal substrate to form a bilayer or composite structure, a polarization conversion efficiency approaching 100% can theoretically be achieved. These results highlight the technological advantages of reflective metamaterials in achieving efficient polarization control. Based on this mechanism, subsequent research focuses on developing polarization conversion structures with multi-band and broadband response characteristics. Zhao et al. proposed a complementary L-type reflective polarization converter, which can achieve efficient linear-to-circular polarization conversion at multiple discrete frequency points, verifying the feasibility of multi-band operation through resonant unit morphology design. To further expand the operating bandwidth, Li et al. adopted a three-layer F-type resonant structure, effectively enhancing the frequency band coverage of polarization conversion by introducing an interlayer coupling mechanism. Based on this, further research realized the multi-polarization control function of a single structure. Huang et al. used a continuous U-shaped metal thin film design, which can simultaneously achieve cross-polarization conversion and linear-to-circular polarization conversion in different frequency bands. Hao et al. further proposed that by using an asymmetric H-type resonant unit structure, they achieved dynamic control of the polarization direction of the reflected wave, providing a new approach for the design of reconfigurable or tunable polarization devices. These developments demonstrate that, through rational structural design and electromagnetic coupling control, reflective metamaterials can achieve high-efficiency, multi-band, broadband, and dynamically tunable terahertz polarization conversion, laying the foundation for the practical application of such devices.

[0004] While existing metamaterial structures have made some progress in linear polarization conversion, their functionality remains primarily limited to converting linear polarization to orthogonal linear polarization. For linear-to-circular polarization conversion, when the incident polarization direction forms a specific angle with the structure's axis of symmetry, the conversion process is often accompanied by the evolution of an intermediate polarization state, resulting in a narrow effective conversion bandwidth. High conversion efficiency is only achieved near a few discrete frequency points, and overall performance remains significantly insufficient. Furthermore, current ultra-narrowband linear-to-circular polarization converters based on multi-frequency resonance principles, while exhibiting good performance at the designed frequency, generally face practical problems such as sensitivity to out-of-band response, high structural fabrication complexity, and poor compatibility with existing terahertz systems. Therefore, achieving a linear-to-circular polarization reflector converter with ultra-wideband, high efficiency, and stability for large-angle incident events, while maintaining a simple and reliable structural design, has become a key challenge in driving the practical application of terahertz metamaterial polarization devices. Summary of the Invention

[0005] Given the limitations of existing terahertz polarization converters, such as limited bandwidth, low conversion efficiency, and high structural complexity, this invention aims to provide a terahertz reflective linear-to-circular polarization converter based on a double-circular-leaf structure. This converter, through precise design and optimization of the geometric parameters of the double-circular-leaf structure, achieves near-theoretical reflectivity within a specific terahertz frequency band and efficiently converts incident linearly polarized waves into circularly polarized waves. The design employs a single-level resonant structure, characterized by its compact structure and high integration. Its advantages include high energy utilization, achieving extremely high reflection and polarization conversion efficiency within the operating frequency band. Furthermore, the unit structure is simple and easily fabricated using existing micro-nano fabrication processes, exhibiting good fabrication feasibility and reliability. This polarization converter can be widely applied in terahertz communication, high-resolution imaging, target stealth technology, and precision sensing, providing a high-performance, miniaturized key polarization control device for terahertz systems.

[0006] To achieve the above and other related objectives, this invention provides a terahertz reflective linear-circular polarization converter based on a double-circular-leaf structure. Its structural feature is that the converter consists of several periodically arranged square structural units. Each square structural unit comprises three layers from top to bottom: a top metal resonant layer, an intermediate dielectric layer, and a bottom metal reflective layer. The top metal resonant layer and the metal reflective layer are respectively attached to the upper and lower surfaces of the intermediate dielectric layer, and their geometric centers are aligned on a straight line. The periodic side length of the structural unit is P = 20 μm.

[0007] The top-layer metal resonant layer features a double-arc leaf-shaped structure consisting of the intersecting region of two circles. The radius of each circle is R = 20 μm, and the centers of the two circles are located at the two endpoints of a diagonal line of the square unit structure. The metal resonant layer is made of gold, with a conductivity of 4.56 × 10⁻⁶. 7 S / m, the thickness of the metal resonant layer is h3=0.2μm.

[0008] The intermediate dielectric layer is made of polyimide, which has a dielectric constant of 3.5, a loss tangent of 0.0027, and a thickness h2 of 10 μm.

[0009] The metal substrate is made of gold and has a thickness of h1 = 0.2 μm.

[0010] Full-wave electromagnetic simulation was performed using the finite element method (FEM) with CST Microwave Studio 2022 electromagnetic simulation software. A frequency domain solver was used, and the mesh type was tetrahedral adaptive mesh. Under perpendicular incidence, the electric field of the terahertz wave was along the y-axis, and the magnetic field was along the x-axis. Boundary conditions were set as follows: unit cell boundaries in the x and y directions, and open boundaries in the z direction. The S-parameters of the structure were obtained through simulation and used to calculate the modulus, phase, phase difference, normalized elliptic susceptibility, and angles formed by the major and minor axes of the ellipse with the x and y axes of the co-polarization transmission factor and the cross-polarization factor of the reflection of the converter. The approximation rate of an ellipse to a standard circle And the axial ratio AR.

[0011] As described above, a terahertz reflective linear-to-circular polarization converter based on a double-circular-leaf structure is proposed. Through precise design and optimization of the geometric parameters of the double-circular-leaf structure, this converter achieves near 100% reflection efficiency and highly efficient linear-to-circular polarization conversion in the terahertz band. When a y-polarized wave is incident perpendicularly, a right-handed circularly polarized wave is generated in the 1.71-2.57 THz frequency range, and a highly efficient left-handed circularly polarized wave is generated in the 3.45-5.41 THz frequency range. Linear-to-circular polarization conversion with an axial ratio greater than -3 dB is achieved in both frequency ranges. This invention has advantages such as compact structure, wide axial ratio bandwidth, and high conversion efficiency, and has significant application value in terahertz communication and imaging systems. Attached Figure Description

[0012] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0013] Figure 1 This is a three-dimensional diagram and a side view of a terahertz reflective linear-circular polarization converter structure unit based on a double-circular-leaf structure according to the present invention.

[0014] Figure 2 This invention relates to a top-layer metal resonant layer pattern of a terahertz reflective linear-circular polarization converter structure unit based on a double-circular-leaf structure.

[0015] Figure 3 This invention relates to the common reflection coefficient and cross reflection coefficient of a terahertz reflective linear-circular polarization converter based on a double-circular-arc leaf structure.

[0016] Figure 4 This is a phase difference curve diagram of a terahertz reflective linear-circular polarization converter based on a double circular arc leaf structure according to the present invention.

[0017] Figure 5This is a normalized elliptic polarizability curve of a terahertz reflective linear-circular polarization converter based on a double-circular-leaf structure according to the present invention.

[0018] Figure 6 The angle between the major and minor axes of the ellipse and the xy-axis of a terahertz reflective linear-circular polarization converter based on a double-circular-leaf structure according to the present invention. The approximation rate of an ellipse to a standard circle Line graph.

[0019] Figure 7 This is an AR curve diagram of the axial ratio of a terahertz reflective linear-circular polarization converter based on a double circular arc leaf structure according to the present invention. Detailed Implementation

[0020] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0021] Please see Figure 1 and Figure 2 As shown, this invention provides a terahertz reflective linear-to-circular polarization converter based on a double-circular-leaf structure. This converter consists of several periodically arranged square structural units. Each square structural unit comprises three layers from top to bottom: a top metal resonant layer, an intermediate dielectric layer, and a bottom metal reflective layer. The top metal resonant layer and the metal reflective layer are respectively attached to the upper and lower surfaces of the intermediate dielectric layer, and their geometric centers are aligned on a straight line. The periodic side length of the structural unit is P = 20 μm.

[0022] The top-layer metal resonant layer features a double-arc leaf-shaped structure consisting of the intersecting region of two circles. The radius of each circle is R = 20 μm, and the centers of the two circles are located at the two endpoints of a diagonal line of the square unit structure. The metal resonant layer is made of gold, with a conductivity of 4.56 × 10⁻⁶. 7 S / m, the thickness of the metal resonant layer is h3=0.2μm.

[0023] The intermediate dielectric layer is made of polyimide, which has a dielectric constant of 3.5, a loss tangent of 0.0027, and a thickness h2 of 10 μm.

[0024] The metal substrate is made of gold and has a thickness of h1 = 0.2 μm.

[0025] To further illustrate the performance characteristics of the polarization-insensitive terahertz multiband perfect absorber described in this invention, simulation analysis was performed using CST Microwave Studio 2022 software. A finite element-based frequency domain solver was used in the simulation, with a tetrahedral adaptive mesh. Unit cell boundary conditions were set in the x and y directions to simulate a periodic structure; an open boundary condition was used in the z direction to simulate free-space radiation. A linearly polarized plane wave, perpendicularly incident along the z-axis, was excited through a Floquet port, with the electric field direction along the y-axis. To accurately simulate far-field conditions, the distance between the port and the top layer of the structure was set to be greater than two wavelengths within the operating frequency band to avoid the influence of near-field coupling on the results.

[0026] Please see Figure 3 and Figure 4 As shown, in the frequency ranges of 1.71–2.57 THz and 3.45–5.41 THz, the moduli of the cross-polarization and co-polarization transfer factors of the reflected signals are... And their phase difference The polarization is stable around -90° and -270°. The characteristics of equal amplitude and phase difference close to ±90° and integer multiples thereof are the key conditions for converting incident ray polarization into high-quality circular polarization: when the amplitudes of the orthogonal reflection components are equal and the phase difference is -90°, the reflected wave exhibits right-hand circular polarization; when the phase difference is -270°, the reflected wave exhibits left-hand circular polarization, thus achieving approximately ideal circular polarization output within the above frequency bands.

[0027] To quantitatively evaluate the quality of linear-to-circular polarization conversion, normalized ellipticity is used. To characterize.

[0028]

[0029] Please see Figure 5 As shown, the conversion device achieves a normalized ellipticity of over 80% in the frequency ranges of 1.71-2.57THz and 3.45-5.41THz, demonstrating its efficient conversion of linear wave to circular polarization in the field of polarization conversion.

[0030] To analyze the characteristics of circularly polarized reflection fields in greater depth, polarization azimuth angle α and ellipticity angle β are introduced to quantitatively describe their polarization state.

[0031]

[0032]

[0033] Please see Figure 6As shown, an azimuth angle is introduced to analyze the polarization characteristics of the reflected wave. With elliptical angle As a key evaluation parameter. Used to describe the orientation direction of the principal axis of elliptically polarized light relative to a reference coordinate system, while Used to characterize the degree of deviation between elliptic polarization and standard circular polarization. When At ±45°, the polarization state corresponds to an ideal left-handed or right-handed circularly polarized wave. Simulation calculations show that at 2.23 THz and 4.05 THz, The values ​​of -44.74° and 44.93° are extremely close to the ideal circular polarization state (±45°). This indicates that at the above two frequencies, although the reflected wave does not reach a completely circular polarization state, its polarization characteristics are very close to those of ideal right-handed and left-handed circularly polarized waves, exhibiting strong circular polarization features. This result demonstrates that the structure described in this invention can achieve high-quality linear-to-circular polarization conversion over a wide frequency range. Its superior performance is mainly attributed to the stable electromagnetic coupling effect between metamaterial units and the highly symmetric structural design, thereby maintaining a stable amplitude ratio and phase difference along the reflection path, achieving efficient circular polarization reflection.

[0034] Axial ratio (AR) is used to quantitatively characterize the roundness of a polarization ellipse, and it is defined as follows:

[0035]

[0036] Please see Figure 7 As shown, when AR is greater than -3dB, the polarized wave can be considered approximately circularly polarized. Simulation results show that AR is consistently greater than -3dB within the frequency ranges of 1.71-2.57THz and 3.45-5.41THz. This result demonstrates that the polarization conversion structure of this invention maintains stable and efficient circularly polarized reflection characteristics over a wide frequency range. Within the frequency ranges of 1.71-2.57THz and 3.45-5.41THz, for y-polarized waves incident along the +z direction, the polarization ellipse shape of the reflected wave is essentially consistent with that of a standard circularly polarized wave, with a major-to-minor axis ratio close to 1 and a uniform polarization direction distribution. That is, all polarization ellipses with AR greater than -3dB can be approximated as circular, further verifying the feasibility and effectiveness of this invention's structure in achieving high-quality circularly polarized reflection over a wide frequency range.

[0037] In summary, this invention provides a terahertz reflective linear-to-circular polarization converter based on a double-circular-leaf structure. This converter, based on a double-circular-leaf structure design, achieves efficient reflective polarization conversion of incident linearly polarized terahertz waves by adjusting the geometric parameters of the diagonal double-arrow structure. When a linearly polarized wave in the y-direction is incident perpendicularly, the reflected wave achieves right-hand circular polarization and left-hand circular polarization outputs in the frequency ranges of 1.71-2.57 THz and 3.45-5.41 THz, respectively, achieving linear-to-circular polarization conversion with an axial ratio greater than -3 dB in both frequency ranges. Analysis of relevant parameters verifies the polarization conversion mechanism of the device. The single-structure design of this invention has advantages such as compact structure, high energy utilization, high polarization conversion efficiency, and strong fabrication feasibility, making it suitable for terahertz communication, imaging, stealth, and sensing fields.

[0038] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Those skilled in the art will recognize that the present invention can be modified and varied in many ways. Several improvements and modifications can be made without departing from the original inventive technology, and all such modifications should be included within the scope of protection of the present invention.

Claims

1. A terahertz reflective linear-circular polarization converter based on a double-circular-leaf structure, comprising several periodically arranged square structural units. Each square structural unit consists of three layers from top to bottom: a top metal resonant layer, an intermediate dielectric layer, and a bottom metal reflective layer. The top metal resonant layer and the metal reflective layer are respectively bonded to the upper and lower surfaces of the intermediate dielectric layer, and their geometric centers are aligned on a straight line. The periodic side length of the structural unit is P = 20 μm.

2. A terahertz reflective linear-circular polarization converter based on a double-arc leaf structure as described in claim 1, characterized in that: the aforementioned double-arc leaf structure of the top metal resonant layer is an intersecting region of two circles. The radius of each of the two circles is R = 20 μm, and the centers of the two circles are located at the two endpoints of a diagonal of the square unit structure. The metal resonant layer is made of gold, with a conductivity of 4.56 × 10⁻⁶. 7 S / m, the thickness of the metal resonant layer is h3=0.2μm.

3. The terahertz reflective linear-circular polarization converter based on a double circular arc leaf structure as described in claim 2 is characterized in that: the material of the aforementioned intermediate dielectric layer is polyimide, which has a dielectric constant of 3.5, a loss tangent of 0.0027, and a thickness h2 of 10 μm.

4. The terahertz reflective linear-circular polarization converter based on a double-circular-leaf structure as described in claim 3, characterized in that: the aforementioned metal substrate layer is made of gold, and the thickness of the metal substrate layer is h1 = 0.2 μm.

5. A terahertz reflective linear-circular polarization converter based on a double-circular-arc leaf structure as described in any of the preceding claims, characterized in that: Through precise design and optimization of the geometric parameters of the double-circular-leaf structure, the device achieves a reflection efficiency of up to 100% and highly efficient conversion between linearly polarized and circularly polarized waves in the terahertz frequency band. When a y-polarized wave is incident perpendicularly, a right-handed circularly polarized wave is generated in the frequency range of 1.71-2.57 THz, and a highly efficient left-handed circularly polarized wave is generated in the frequency range of 3.45-5.41 THz. Linear-to-circular polarization conversion with an axial ratio greater than -3 dB is achieved in both frequency ranges. This invention has advantages such as compact structure, wide axial ratio bandwidth, and high conversion efficiency, and has significant application value in terahertz communication and imaging systems.

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