In addition, the octagonal tube exhibits the lowest loss, whereas the circular and square tubes show slightly higher losses. This can be attributed to the fact that the octagonal tube combines the advantages of both circular and square tubes. First, the geometric shape matching between the input terahertz beam profile and the waveguide tube cross section is crucial. Considering that the terahertz Gaussian beam enters the waveguide tube and diverges from the center point before being reflected by the tube walls, an ideal cross section would be circular. In this case, all rays would reach the tube wall simultaneously and return to the origin, maintaining synchronization throughout the propagation along the waveguide. By contrast, for polygonal tubes with non-circular cross sections, the geometric shape mismatch leads to imperfect synchronization of the rays, with fewer sides resulting in poorer synchronization. Second, the core-antiresonance effect is generated by the oscillation of transverse electric (TE) polarized light between the waveguide’s tube walls. For a circular tube, the opposing tube walls are theoretically only two infinitesimal points within its cross section. However, in a polygonal tube, the opposing walls are much broader. Accordingly, polygonal tubes have an advantage over circular ones. The octagonal tube benefits from both aspects, which is the primary reason for its lower transmission loss. Moreover, the loss is reduced when the tube walls are made of magnetic paper than of ordinary paper. This is mainly because magnetic paper has a higher reflectivity for terahertz waves.

Regarding the uncertainties associated with the octagonal tube and U-shaped tube devices, the former primarily derives from material fatigue induced by repeated deformations of the octagonal origami structure under magnetic force. Over time, the tube may struggle to return to its initial regular octagonal shape, leading to a gradual decline in the control accuracy and stability of the terahertz polarization state. By contrast, the U-shaped tube offers better stability, as the magnetic paper only needs to lift or lower under the influence of the magnetic field. However, environmental humidity can alter the elasticity and toughness of the paper, potentially reducing the device’s precision in controlling the terahertz polarization state. A future work will address these issues. Specifically, we will experiment with plastic magnetic materials to enhance the control accuracy and stability of the devices.Future research should focus on further optimizing the structural parameters and performance of these terahertz polarization conversion waveguides. In addition, exploring alternative control methods beyond magnetic fields, such as thermal and temperature control, is essential to meet the diverse requirements for terahertz polarization modulation under various scenarios.

In recent years, terahertz technology has garnered widespread attention and experienced rapid development. Among the key research areas in this field, terahertz polarization control is one of the most significant, as it holds broad application prospects in terahertz imaging, sensing, communication, and radar. Consequently, the efficient manipulation of terahertz polarization states has become a major focus of current research. Although many terahertz polarization conversion devices offer diverse control capabilities, they face challenges in terms of device fabrication complexity, terahertz wave losses, and the ability to continuously modulate polarization states. For example, terahertz polarization converters based on metasurfaces are difficult to fabricate and involve complex manufacturing processes. In addition, these devices often experience significant terahertz losses. In reflective polarization converters, the terahertz wave transmission efficiency can vary from 50% to 80%. More importantly, because most terahertz polarization control devices have a fixed wavefront modulation phase once they are fabricated, they are typically limited to switching between two specific polarization states, such as from linear to circular polarization. Consequently, they often lack the ability to achieve continuous polarization state control.

To address the aforementioned challenges, this study innovatively proposes a magnetically controlled terahertz polarization conversion waveguide based on the antiresonance mechanism of the waveguide core. The study first utilized magnetic paper and 3D printing technology to fabricate octagonal paper tube waveguides and U-shaped tube waveguides, which transmit terahertz waves via a core-antiresonance mechanism. Then, when a magnetic field was applied near the output port of the waveguide, the orthogonal inner diameters of the waveguide underwent relative changes. Finally, these changes in the orthogonal inner diameters led to variations in the relative time delay of the orthogonal terahertz polarization components as they propagated through the tube.

The results of this study show that the hollow-core waveguide device is capable of controlling terahertz polarization, thus offering several advantages: 1) the waveguide structure and fabrication process are simple; 2) the terahertz wave transmission loss is low; 3) continuous control of the terahertz polarization state can be achieved; 4) the proposed polarization conversion method is flexible. In addition to magnetic control, replacing the magnetic paper with ordinary paper or 4D printed materials allows polarization adjustment through force or thermal control.

To address the challenges of complex fabrication processes, low transmission efficiency, and difficulty in achieving continuous adjustment in terahertz polarization converters, this study innovatively proposes two types of magnetically controlled terahertz polarization conversion waveguides based on the core-antiresonance transmission mechanism. These devices can be fabricated using simple origami techniques or 3D printing, where transmission losses are as low as 0.04 cm^-1 at peak frequencies. They also enable continuous adjustment and chirality switching among the three polarization states of linear, elliptical, and circular polarization under an applied magnetic field.