High-performance quarter-wave plate based on the effective graded impedance matching configuration at 220 GHz

Broadband and low-loss quarter-wave plates (QWPs) operating in the sub-THz band are crucial for polarization control in the field of high-speed wireless communications and terahertz (THz) detection systems. However, their performances are typically constrained by narrow operation bandwidth and high losses. In this paper, we propose a high-performance QWP scheme based on the effective graded refractive index subwavelength grating structure to reduce the insertion loss. This QWP is fabricated from cyclic olefin polymer (COP), incorporating tapered grating structures with graded refractive index at both ends on the basis of rectangular subwavelength gratings to reduce the air/QWP interface as well as the absorption losses. Experimental measurements demonstrate that the designed QWP achieves a transmittance of 98.17% and a polarization conversion ratio (PCR) of 99.995% at the operating frequency of 220 GHz, with a relative bandwidth of 25% (covering 205-260 GHz) and PCR maintaining higher than 98.93% within this bandwidth. Meanwhile, this QWP also exhibits outstanding manufacturing robustness, with excellent performance under a ± 20% dimensional deviation and a surface roughness of about 20 µm. Combining the high transmittance feature, near-unit polarization conversion efficiency, and the broadband operation characteristics, the proposed QWP can effectively meet the high-performance manipulation device requirements of H-band (170-260 GHz) wireless THz communications.