⚡ electrical engineering

UWB THz Reflective-type Linear-to-Circular Polarization Converter using Anisotropic Metallic Grating Embedded in TOPAS Material

This study presents a compact, ultra-wideband reflective linear-to-circular polarization converter utilizing an anisotropic gold grating embedded in TOPAS material, which achieves a 116.13% fractional axial-ratio bandwidth and robust performance under oblique incidence for advanced Terahertz communication and defense applications.

Original authors: Samaneh Aghayari, Mahmood Rafaei-Booket

Published 2026-09-11

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Samaneh Aghayari, Mahmood Rafaei-Booket

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the invisible world of wireless communication, the way a signal spins as it travels is just as important as the signal itself. This "spin," known as polarization, determines how a wave interacts with antennas and materials. For decades, engineers have struggled to control this spin, especially at the very high frequencies used for future ultra-fast networks. Traditional methods to change a wave's spin often require bulky equipment or long paths for the signal to travel, making them impractical for the compact, high-speed devices of tomorrow. A newer approach uses metasurfaces, which are artificially engineered sheets made of tiny patterns that can bend and twist light and radio waves in ways nature does not. These flat surfaces offer a path to compact devices that can manipulate waves with extreme precision, opening the door to better radar, clearer imaging, and faster data transmission.

A team of researchers at the University of Zanjan in Iran has taken a significant step forward in this field by designing a new type of flat surface capable of transforming a straight-line signal into a spinning one across a vast range of frequencies. Their work focuses on the terahertz band, a region of the electromagnetic spectrum that sits between microwaves and infrared light. This band is highly sought after for next-generation communication systems because it can carry enormous amounts of data, but it is also difficult to work with because few natural materials respond well to it. The researchers created a reflective device that acts like a specialized mirror. Instead of simply bouncing a signal back, this mirror catches a wave vibrating in a single direction and reflects it back as a wave that spins in a circle. This conversion is crucial because many advanced systems, such as satellite links and radar, rely on circularly polarized waves to function reliably without losing signal strength.

The device itself is a layered structure built from materials chosen for their ability to handle these high frequencies. At its base is a solid sheet of gold, which acts as a perfect reflector. Above this sits a layer of a clear plastic material called TOPAS, which is known for being transparent to terahertz waves and having very low signal loss. Embedded within this plastic layer is a patterned grid of gold lines. This grid is not a simple mesh; it is an anisotropic grating, meaning its properties change depending on the direction you look at it. The researchers designed the grid with a specific, repeating pattern of tiny metallic shapes that are rotated at an angle. When a signal hits this surface, the grid interacts differently with the parts of the wave aligned in different directions. This interaction creates a delay between the two parts of the wave, forcing them to combine into a spinning motion as they reflect off the surface.

To test their design, the researchers ran detailed computer simulations to see how the device would behave across a wide spectrum of frequencies. They found that the device works exceptionally well over an ultra-wideband range, meaning it can convert signals effectively across a massive span of frequencies rather than just a single narrow channel. In their initial tests, the device successfully converted linear signals to circular ones across a range from 5.3 to 19.65 terahertz. This represents a fractional bandwidth of 115 percent, a measure of how wide the operating range is relative to the center frequency. To put this in perspective, the device can handle a range of frequencies that is more than twice as wide as the entire range it sits in the middle of. The researchers then refined the design, adjusting the size of the grid and the thickness of the plastic layers to optimize performance for frequencies below 10 terahertz. In this optimized state, the device maintained its high performance from 2.85 to 10.35 terahertz, achieving a fractional bandwidth of 115 percent in this lower range as well.

A critical feature of this design is its ability to work even when the signal does not hit the surface straight on. In real-world applications, signals often arrive from different angles, and many devices fail to work correctly when the angle changes. The researchers tested their device by simulating signals hitting the surface at various angles, up to 60 degrees off-center. The results showed that the device continued to produce a clean, spinning signal even at these steep angles. This stability is vital for practical use, as it means the device can be used in systems where the direction of the signal might shift, such as in moving vehicles or satellites. The entire structure is remarkably compact, with the repeating unit of the pattern being only a tiny fraction of the wavelength of the lowest frequency it handles. This small size allows many of these units to be packed together to form larger surfaces for complex tasks like steering beams of energy without moving parts.

The study compares this new design to other recent attempts to create similar devices for the terahertz range. While other designs have achieved wide bandwidths or good angles, they often rely on complex, multi-layered structures that are difficult to manufacture. The design presented here achieves a wider operating range and better angle stability while using a simpler, single-layer patterned grid. The researchers note that this simplicity makes the device easier to build and potentially cheaper to produce. The simulations confirm that the device can suppress unwanted signal modes, ensuring that only the desired reflected wave propagates. This level of control suggests that the device could be a key component in future technologies, including advanced radar systems that need to hide their presence, beam-steering antennas for 6G networks, and sophisticated communication systems for aerospace and defense. The work demonstrates that by carefully engineering the geometry of a simple metallic grid, it is possible to master the complex behavior of terahertz waves, turning a challenging physical phenomenon into a reliable tool for the future of connectivity.

Technical Summary: UWB THz Reflective-type Linear-to-Circular Polarization Converter using Anisotropic Metallic Grating Embedded in TOPAS Material

Problem Statement
Precise control of electromagnetic (EM) polarization is critical for modern wireless communication, radar, and remote sensing. While traditional polarization conversion methods rely on mechanisms like the Faraday Effect or material birefringence, they often suffer from large physical dimensions, restricted operating bandwidths, and fabrication complexity. Although metasurfaces offer a compact alternative, many existing Terahertz (THz) designs utilize complex multilayer structures or intricate resonators that increase fabrication difficulty and limit practical implementation. Furthermore, naturally occurring materials in the THz regime often lack the strong, tunable responses required for effective wavefront engineering. Consequently, there is a need for a simple, compact, and broadband metallic metasurface capable of efficient Linear-to-Circular Polarization (LTCP) conversion within the THz spectrum.

Methodology
The authors propose a reflective LTCP converter based on an anisotropic metallic grating embedded within a dielectric substrate. The structure consists of a patterned gold resonator layer, a TOPAS dielectric superstrate, and a continuous gold ground plane.

Design: The unit cell features an orthotropic configuration with two perpendicular principal symmetry axes ( $u$ and $v$ ) rotated $\pm45^\circ$ relative to the $y$ -axis. The design employs a square lattice with periodicity $P$ .
Materials: The dielectric layers are made of TOPAS (relative permittivity $\varepsilon_r = 2.35$ , loss tangent $\tan \delta = 0.0005$ ), while the metallic components are gold films.
Mechanism: The device operates by exploiting the anisotropic response of the metasurface. When a linearly polarized wave is incident, the structure generates co-polarized and cross-polarized reflection components. For ideal LTCP conversion, the amplitudes of these components must be equal ( $|R_{xx}| = |R_{yx}|$ ), and their phase difference must be an odd integer multiple of $\pm\pi/2$ .
Simulation and Optimization: Full-wave electromagnetic simulations were conducted using CST Microwave Studio. A parametric analysis was performed to optimize the lattice constant ( $P$ ), substrate thickness ( $h_1$ ), superstrate thickness ( $h_2$ ), quarter-circle radius ( $r$ ), and metallic strip width ( $w$ ). The goal was to maximize the 3-dB Fractional Axial-Ratio Bandwidth (FARBW) specifically within the sub-10 THz range.

Key Results

Initial Design Performance: The initial configuration (periodicity $P = 8.0 \, \mu\text{m}$ ) demonstrated Ultra-Wideband (UWB) polarization conversion from 5.3 to 19.65 THz, achieving a 3-dB FARBW of 115%. The unit cell periodicity was $0.14\lambda_0$ (where $\lambda_0$ is the free-space wavelength at the lowest operating frequency).
Optimized Design Performance: Through parametric optimization, the design was scaled to target the sub-10 THz regime. The final optimized dimensions are $P = 16 \, \mu\text{m}$ , $r = 7.76 \, \mu\text{m}$ , $w = 1.44 \, \mu\text{m}$ , $h_1 = 5.0 \, \mu\text{m}$ , and $h_2 = 8.6 \, \mu\text{m}$ . This configuration achieves a maximum FARBW of approximately 115% (specifically 116.13% in the analysis), covering the frequency range of 2.85 to 10.35 THz.
Angular Stability: The converter maintains stable performance under oblique illumination. Simulations confirm that the Axial Ratio (AR) remains below the 3-dB threshold for incidence angles up to 60° for both TE and TM polarizations.
Compactness: The optimized unit cell dimensions are approximately $0.15\lambda_0 \times 0.15\lambda_0 \times 0.12\lambda_0$ , demonstrating a compact profile.
Comparative Performance: When compared to recent THz metallic grating-based converters, the proposed design offers a wider operating bandwidth (2.85–10.35 THz) and comparable or superior angular stability (60°) while utilizing a simpler, single-layer resonator architecture without complex multilayer configurations.

Significance and Claims
The paper claims that the proposed structure addresses the trade-off between bandwidth, compactness, and fabrication complexity in THz polarization control. By utilizing a simple anisotropic metallic grating embedded in TOPAS, the device achieves:

Ultra-Wideband Operation: A fractional bandwidth of 115% in the THz regime, which is significantly broader than many previously reported designs.
Robust Angular Tolerance: Stable circular polarization conversion up to 60° incidence, making it suitable for applications where the angle of wave incidence varies.
Simplicity: A planar, single-resonator-layer design that avoids the fabrication challenges associated with 3D chiral metastructures or complex multilayer stacks.

The authors conclude that these characteristics make the proposed LTCP converter a suitable platform for THz beam-steering technologies, Radar Cross-section (RCS) suppression, and next-generation aerospace and defense communication systems. The work is presented as a solution to the challenge of designing simple, compact, broadband metallic metasurfaces for efficient THz polarization conversion.

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