Terahertz waves are underdeveloped waves between microwaves and infrared waves that exhibit abundant spectral resources and have received wide attention in many fields, including medical detection, security imaging, and communication. Polarization is an important characteristic of electromagnetic waves, and a metasurface can flexibly control the propagation of light waves using its subwavelength structural unit, thus rendering it suitable for the design of lightweight and compact polarization converters. Research on terahertz polarization converters based on metasurfaces is expected to promote the development of terahertz wave bands. However, to realize practical applications of terahertz systems in the future, improving metasurface-based terahertz polarization converters is necessary. First, a simple structure can be designed to enable polarization conversion, thereby enhancing its advantages in the design process and practical applications. Second, the operating bandwidth of the polarization-conversion device can be expanded, and a wide bandwidth operating range can improve the spectrum utilization of terahertz systems. Furthermore, the polarization-conversion effect must be enhanced to ensure stable optical performance. In this study, the proposed terahertz-line polarization-conversion metasurface based on a double split-ring resonator features a simple design process, few structural parameters, high optimization efficiency, and a wide operating bandwidth. Moreover, through the structural optimization and design of notches on a double-split-ring resonator, the structure can achieve efficient polarization conversion over the entire operating frequency band. The metasurface structure designed in this study may promote the application of terahertz technology in fields such as wireless communication, liquid-crystal displays, and medical detection.

The structure proposed in this study comprises a long, metal, rectangular bar connected to a double split-ring resonator, with two symmetrical notches on the double split-ring resonator, which improves the overall performance of the polarization converter. Numerical simulations were performed to evaluate the effectiveness of the metasurface. The simulations were conducted using CST Microwave Studio, where a frequency-domain solver was used to calculate the metasurface reflection coefficients. Subsequently, these coefficients were processed to extract the evaluation parameters for the cross-polarization converter. The resonance modes at each resonant point were identified based on the distribution of surface currents, and the underlying physical mechanism facilitating cross-polarization conversion was analyzed based on the interactions of electric and magnetic dipoles corresponding to their respective dipole moments.

The metasurface structure proposed in this study can achieve excellent polarization conversion over a wide bandwidth operating range. In the range of 2.55?7.61 THz, Ryy is lower than 0.3 and Rxy exceeds 0.9. Based on these two parameters, the polarization conversion ratio (PCR), which reflects the polarization-conversion performance, exceeds 0.9, thus indicating the excellent polarization-conversion capability of the metasurface over the entire operating band. The polarization-converting metasurface has an operating bandwidth of 5.06 THz, with a relative bandwidth of 99.4%, thus demonstrating its excellent broadband performance (Fig. 2). The ability to realize polarization conversion in such a wide operating band is endowed by the two notches on the double-split-ring resonator. A comparison between notched and non-notched metasurface structures shows that the former exhibits a prominent resonance-enhancement effect at 6.31 THz and 7.49 THz, which is endowed by the notched structure on the entire polarization-converting metasurface, whose PCR exceeds 0.9 in the entire operating band (Fig. 3). The physical mechanisms facilitating polarization conversion were elucidated by analyzing the distribution of surface currents at various resonance points. The enhancement effect of the notched structure on cross-polarization conversion was analyzed via comparative analysis, where the dimensional parameter x₁ is shown to significantly affect polarization conversion. This effect was further clarified by decomposing the electric-field components (Figs. 4, 5, and 7). The effect of the metasurface structural parameters on polarization conversion was analyzed via parameter scanning. This paper clarifies the effects of structural parameters t₂, w₁, x₁, and y₁ on the PCR and reveals the geometric factors affecting the polarization-conversion capability of the metasurface (Fig. 6).

In this study, a terahertz polarization-converting metasurface with a simple structure and a wide bandwidth operating range is proposed based on a double split-ring resonator. Results of CST simulation show that the polarization converter encompasses the frequency band of 2.55?7.61 THz, has a PCR exceeding 0.9 in an entire frequency band with a bandwidth of 5.06 THz, and exhibits a relative bandwidth of 99.4%. The mechanism by which the notched structure affects the cross-polarization conversion effect was derived by decomposing the electric-field components. Additionally, the physical mechanism of the broadband cross-polarization conversion effect was derived by analyzing the surface current. The proposed structure is applicable to terahertz wireless communications, liquid-crystal displays, and security imaging.