Terahertz reflection type linear-circular polarization conversion device based on diagonal double-arrow 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 conversion device based on a diagonal double-arrow structure. The conversion device is composed of a metal resonance layer, a dielectric layer and a metal reflection layer. By regulating and controlling the reflection phase difference of incident ray polarization terahertz waves in two orthogonal directions, high-efficiency conversion of linearly polarized waves and circularly polarized waves is realized. When the incident wave is y polarization, the ultra-wideband and high-efficiency right-hand circularly polarized wave is generated in the frequency range of 2.91-4.78 THz, and the left-hand circularly polarized wave also exists in the frequency range of 5.15-5.52 THz. Linear-circular polarization conversion with the axial ratio larger than-3dB is achieved in the two frequency ranges. Compared with a traditional transmission type structure, the reflection type design has the advantages of being compact in structure, high in energy utilization rate, high in polarization conversion efficiency, high in process realizability and the like, and is suitable for the fields of terahertz communication, imaging, stealth, sensing and the like.

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 conversion device based on a diagonal double-arrow structure. Background Technology

[0002] Terahertz waves are generally defined as electromagnetic waves with frequencies between 0.1 THz and 10 THz, corresponding to wavelengths ranging from approximately 30 μm to 3000 μm, with their spectrum lying between microwave and infrared radiation. Terahertz waves possess strong penetrating power, non-ionizing biocompatibility, high signal-to-noise ratio, and excellent spectral resolution, thus holding significant application value in communication, sensing, spectral analysis, and imaging technologies. Metamaterials are a class of composite media composed of subwavelength-scale artificial periodic structures. Their structural unit sizes are significantly 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 promoted the further development of terahertz technology, leading to the proposal of various metamaterial-based functional devices, such as filters, modulators, polarization converters, wavefront modulation, and energy absorption devices. In terahertz polarization control, by rationally designing the structural configuration of metamaterials, the conversion between linear polarization and circular polarization, or between linear polarizations, can be achieved, thereby precisely controlling the polarization state of terahertz waves, which has significant research value and application potential.

[0003] Traditional terahertz polarization devices typically rely on natural anisotropic crystals or mechanically rotating elements for polarization modulation, which suffers from limitations such as large structural 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 specific frequency bands. 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.

[0004] Based on the research of M et al. on the linear-circular polarization conversion mechanism of monolayer metamaterials, the polarization conversion efficiency can reach up to 50% when linearly polarized waves are incident on a monolayer structure; in a reflective structure, this efficiency can be increased to about 90%; furthermore, by introducing a planar metal substrate to construct a bilayer or composite structure system, the conversion efficiency can theoretically reach 100%. This result reveals the significant potential of reflective metamaterials in polarization control. Building on the above research, Zhao et al. proposed a complementary L-shaped multi-band reflective polarization converter, which can achieve efficient linear-circular polarization conversion in multiple frequency bands. This design verifies the feasibility of achieving multi-band polarization conversion through resonant structure morphology control. To further expand the operating bandwidth, Li et al. designed a three-layer F-shaped reflective structure, which effectively improved the frequency band coverage of polarization conversion by introducing a multi-layer coupling mechanism, promoting the development of broadband polarization converters. Subsequently, Huang et al. reported a multi-band reflective polarization converter based on a continuous U-shaped metal thin film. This structure can simultaneously achieve cross-polarization and multi-band linear-circular polarization conversion functions, demonstrating the ability of a single structure to achieve multiple polarization controls. Building on this, Hao et al. further proposed the H-type structure, which achieves active control of the polarization direction of the reflected wave through the design of asymmetric resonant units, providing a new path for dynamic polarization control.

[0005] Although existing structures have improved in linear-to-circular polarization conversion performance, most designs still primarily achieve the conversion from linear polarization to cross-linear polarization, i.e., the incident and reflected polarization directions remain orthogonal. When the polarization azimuth angle is 45°, the linear-to-circular polarization conversion requires an intermediate state evolution, resulting in limited conversion bandwidth. High conversion efficiency can only be achieved near a few discrete frequency points, and there is still considerable room for improvement in overall performance. Furthermore, while existing ultra-narrowband linear-to-circular polarization converters based on multi-frequency resonance exhibit good performance at specific frequencies, they generally suffer from problems such as sensitivity to out-of-band response, high fabrication difficulty, and insufficient compatibility with existing terahertz systems. Therefore, developing a linear-to-circular polarization reflector converter with ultra-wideband, high efficiency, and support for large-angle incident polarization, while maintaining a simple structural design, has become a key technical problem urgently needing to be solved in the field of terahertz metamaterial polarization devices. Summary of the Invention

[0006] 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 conversion device based on a diagonal double-arrow structure. This conversion device is designed based on a diagonal double-arrow structure unit. Through precise design and optimization of the geometric parameters of the diagonal double-arrow structure (including the length a, width b of the arrow body, the period L of the diagonal double-arrow structure unit, and the dielectric layer thickness h2), the device achieves a reflection efficiency of up to 100% and efficient conversion of linearly polarized waves to circularly polarized waves in the terahertz frequency band. The single-structure design offers 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 applications.

[0007] To achieve the above and other related objectives, this invention provides a terahertz reflective linear-to-circular polarization conversion device based on a diagonal double-arrow structure. The device is characterized by comprising several periodically arranged square structural units. Each square structural unit consists of three layers from top to bottom: a metal resonant layer, a dielectric layer, and a metal substrate layer. The metal resonant layer and the metal substrate layer are respectively bonded to the upper and lower surfaces of the dielectric layer, and their geometric centers are aligned on a straight line. The periodic side length of the structural unit is P = 30 μm.

[0008] The metal resonant layer is composed of two anti-symmetrical arrows connected by an inclined metal strip, arranged in a mirror-symmetrical manner along the diagonal direction. The periodic side length of the diagonal double-arrow structural unit is L = 25 μm. The length of the diagonal double-arrow body is a = 13.5 μm, and the width is b = 9.5 μm. 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=0.1μm.

[0009] The 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.

[0010] The metal substrate is made of gold and has a thickness of h1 = 0.5 μm.

[0011] 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.

[0012] As described above, a terahertz reflective linear-to-circular polarization conversion device based on a diagonal double-arrow structure is proposed. Based on the precise design and optimization of the geometric parameters of the diagonal double-arrow structure, including the arrow length *a*, width *b*, structural unit period *L*, and dielectric layer thickness *h2*, this conversion device achieves near 100% reflection efficiency and highly efficient linear-to-circular polarization conversion in the terahertz band. Under perpendicular incidence of y-polarized waves, this structure generates right-handed circularly polarized waves in the 2.91-4.78 THz frequency range and left-handed circularly polarized waves in the 5.15-5.52 THz frequency range, with an axial ratio greater than -3 dB in both frequency bands, exhibiting excellent linear-to-circular polarization conversion characteristics. This design is compact, possesses advantages such as high energy utilization, excellent polarization conversion performance, and strong fabrication feasibility, and is suitable for terahertz communication, imaging, stealth technology, and sensing systems. Attached Figure Description

[0013] 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.

[0014] Figure 1 This is a three-dimensional diagram and a side view of a structural unit of a terahertz reflection-type linear-circular polarization conversion device based on a diagonal double-arrow structure according to the present invention.

[0015] Figure 2 This invention relates to a resonant pattern of a metal resonant layer in a terahertz reflection-line-circular polarization conversion device structure unit based on a diagonal double-arrow structure.

[0016] Figure 3 This invention relates to the common reflection coefficient and cross reflection coefficient of a terahertz reflection-type linear-circular polarization conversion device based on a diagonal double-arrow structure.

[0017] Figure 4This is a phase difference curve diagram of a terahertz reflection-line to circular polarization conversion device based on a diagonal double-arrow structure according to the present invention.

[0018] Figure 5 This is a normalized elliptic polarizability curve of a terahertz reflection-type linear-circular polarization conversion device based on a diagonal double-arrow structure according to the present invention.

[0019] Figure 6 The present invention relates to a terahertz reflection profile linear-circular polarization conversion device based on a diagonal double-arrow structure, wherein the angle formed by the major and minor axes of the ellipse and the xy-axis is... The approximation rate of an ellipse to a standard circle Line graph.

[0020] Figure 7 This is an AR curve diagram of the axial ratio of a terahertz reflection profile linear-circular polarization conversion device based on a diagonal double-arrow structure according to the present invention. Detailed Implementation

[0021] 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.

[0022] Please see Figure 1 and Figure 2 As shown, this invention provides a terahertz reflective linear-to-circular polarization conversion device based on a diagonal double-arrow structure. The designed conversion device consists of several periodically arranged square structural units. Each square structural unit consists of three layers from top to bottom: a metal resonant layer, a dielectric layer, and a metal substrate layer. The metal resonant layer and the metal substrate layer are respectively bonded to the upper and lower surfaces of the dielectric layer, and their geometric centers are on a straight line. The periodic side length of the structural unit is P = 30 μm.

[0023] The aforementioned metallic resonant layer is composed of two anti-symmetrical arrows connected by an inclined metal strip, arranged in a mirror-symmetrical manner along the diagonal direction. The periodic side length of the diagonal double-arrow structural unit is L = 25 μm. The length of the diagonal double-arrow body is a = 13.5 μm, and the width is b = 9.5 μm. The metallic 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=0.1μm.

[0024] The 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.

[0025] The metal substrate is made of gold and has a thickness of h1 = 0.5 μm.

[0026] 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.

[0027] Please see Figure 3 and Figure 4 As shown, in the frequency ranges of 2.91-4.78 THz and 5.15-5.52 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 90°. The characteristics of equal amplitude and phase difference close to ±90° 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 +90°, the reflected wave exhibits left-hand circular polarization, thus achieving approximately ideal circular polarization output within the above frequency bands.

[0028] To quantitatively evaluate the quality of linear-to-circular (LTC) conversion, normalized ellipticity is used. To characterize.

[0029]

[0030] Please see Figure 5 As shown, the conversion device achieves a normalized ellipticity of over 80% in the frequency ranges of 2.91-4.78 THz and 5.15-5.52 THz, demonstrating its efficient conversion of linear wave to circular polarization in the field of polarization conversion.

[0031] 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.

[0032]

[0033]

[0034] Please see Figure 6 As 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 4.47 THz and 5.30 THz, The values ​​of -43.76° and -44.92° 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 shows that the structure described in this invention can achieve high-quality linear-to-circular polarization conversion over a wide frequency range. Its excellent 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.

[0035] Axial ratio (AR) is used to quantitatively characterize the roundness of a polarization ellipse; it is defined as follows: .

[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 2.91-4.78THz and 5.15-5.52THz. This result demonstrates that the polarization conversion structure of this invention maintains stable and efficient circularly polarized reflection characteristics across an ultra-wideband range. Within the frequency ranges of 2.91-4.78THz and 5.15-5.52THz, 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 conversion device based on a diagonal double-arrow structure. This device, based on the diagonal double-arrow structure design, achieves efficient reflective polarization conversion of incident linearly polarized terahertz waves by manipulating 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 2.91-4.78 THz and 5.15-5.52 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 this invention. 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 conversion device based on a diagonal double-arrow structure, comprising several periodically arranged square structural units. Each square structural unit consists of three layers from top to bottom: a metal resonant layer, a dielectric layer, and a metal substrate layer. The metal resonant layer and the metal substrate layer are respectively bonded to the upper and lower surfaces of the dielectric layer, and their geometric centers are aligned on a straight line. The periodic side length of the structural unit is P = 30 μm.

2. The terahertz reflective linear-to-circular polarization conversion device based on a diagonal double-arrow structure as described in claim 1, characterized in that: the aforementioned metal resonant layer is composed of two anti-symmetrical arrows connected by an inclined metal strip to form a whole, and the whole is arranged in a mirror-symmetrical manner along the diagonal direction. The periodic side length of the diagonal double-arrow structure unit is L = 25 μm. The length of the diagonal double-arrow body is a = 13.5 μm, and the width is b = 9.5 μm. The material of the metal resonant layer is gold, and its conductivity is 4.56 × 10⁻⁶. 7 S / m, the thickness of the metal resonant layer is h3=0.1μm.

3. The terahertz reflective linear-circular polarization conversion device based on a diagonal double-arrow structure as described in claim 2, characterized in that: the material of the aforementioned dielectric layer is polyimide, with 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 conversion device based on a diagonal double-arrow 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 = 0.5 μm.

5. A terahertz reflection-type linear-circular polarization conversion device based on a diagonal double-arrow structure as described in any of the preceding claims, characterized in that: Through precise design and optimization of the geometric parameters of the diagonal double-arrow structure (including the length *a* and width *b* of the arrow body, the periodic side length of the diagonal double-arrow structure unit, and the thickness *h2* of the dielectric layer), the device achieves a reflection efficiency of up to 100% and efficient conversion between linearly polarized and circularly polarized waves in the terahertz band. When a y-polarized wave is incident, an ultra-wideband and highly efficient right-hand circularly polarized wave is generated in the frequency range of 2.91–4.78 THz, and a left-hand circularly polarized wave is also generated in the frequency range of 5.15–5.52 THz. Linear-to-circular polarization conversion with an axial ratio greater than -3 dB is achieved in both frequency ranges. The single-structure design offers 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 applications.

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