Polarization, as important information carried by electromagnetic waves, is crucial in imaging, encryption, image processing, and other fields. How to generate specific polarization states, how to identify the polarization state of electromagnetic waves, and how to utilize polarization are all research hotspots. For the recognition of polarization states, traditional approaches include measurement through rotating polarizers or phase retarders, as well as the divisions of amplitude and aperture. These traditional methods are bulky and not conducive to integration. Combining the wavefront control characteristics of metasurfaces, researchers have proposed some polarization detection devices based on methods such as polarization beam splitting, multi-focus metalens, and vortex pattern recognition. These polarization detection schemes need to obtain light fields with different polarization states. However, in the terahertz band, multiple measurements are mostly required to measure such light fields.

To solve this problem, a team led by Associate Professor Huaping Zang and Chenglong Zheng from Zhengzhou University, in collaboration with teams led by Professor Yan Zhang from Capital Normal University and Academician Jianquan Yao from Tianjin University, proposed a terahertz polarization detection scheme based on multi-dimensional polarization conversions. The relevant Research were published in the 3rd issue of Photonics Research in 2024 (Chenglong Zheng, Hui Li, Jingyu Liu, Mengguang Wang, Huaping Zang, Yan Zhang, and Jianquan Yao, "Full-Stokes metasurface polarimetry requiring only a single measurement," Photonics Research 12(3), 514-521 (2024)).

In this scheme, the proposed metasurface consists of six sub-arrays, as shown in Fig. 1. Each sub-array focuses on the x, y, 45°, 135°, LCP and RCP components of the incident wave respectively, and converts them into x-polarized waves. The polarization state of the incident wave can be reconstructed based on the intensities of these six foci.

Fig. 1 Schematic of the proposed multi-foci metalens for polarization detection.

The fabricated polarization detector sample was characterized by a laboratory-built terahertz digital holographic imaging system, as shown in Fig. 2. The incident waves were respectively set as linearly polarized states with different azimuth angles and circularly polarized states. The Stokes parameters reconstructed based on the measured intensities of the six foci were basically consistent with the theoretical values. The efficient polarization detection scheme has great application potentials in fields such as real-time terahertz detection and integrated optics.

Fig. 2 Experimental characterization of the proposed multi-foci metalens for polarization detection.