High-transmission linear-to-circular polarization converter based on a polymer relief grating structure.

The polarization manipulation of sub-terahertz waves is critical for sub-terahertz communication and sensitive measurement. This work demonstrates a simple, low-cost, and high-performance transmissive-type polarization converter based on polymer relief grating, which can realize linear-to-circular polarization conversion, i.e., it functions as a quarter-wave plate. The completely suspended grating design can significantly enhance the transmittance. A new synthetic criterion is proposed for a linear-to-circular polarization converter, and complete formulae are derived to uniquely obtain the polarization properties to measure the state of polarization of the wave quantitatively based on the proposed modified polarimetry measurements method using a polarimetric terahertz time-domain spectroscopy setup. The simulated and experimental results show good agreement, highlighting the converter featuring advantages of high transmittance (>96.99%), high polarization conversion ratio (>97.29%), and high ellipticity (0.97 ± 0.03) in a relatively wide bandwidth (28.57%) at the central frequency of D-band (140 GHz). Based on these results, we also demonstrate that by cascading multiple converters, our converter can easily function as an attenuator, or as a half-wave plate (which can rotate a linear polarization state into its orthogonal one) in manipulating sub-terahertz waves. Hence, our study here provides a promising solution and an invaluable complement to a sub-terahertz polarization converter, and can also be extended to other frequency regimes.