Terahertz polarization conversion and sensing with double-layer chiral metasurface**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0701000), the National Natural Science Foundation of China (Grant Nos. 61971242, 61831012, and 61671491), the Natural Science Foundation of Tianjin City, China (Grant No. 19JCYBJC16600), and the Young Elite Scientists Sponsorship Program by Tianjin, China (Grant No. TJSQNTJ-2017-12).

Abstract

The terahertz (THz) resonance, chirality, and polarization conversion properties of a double-layer chiral metasurface have been experimentally investigated by THz time domain spectroscopy system and polarization detection method. The special symmetric geometry of each unit cell with its adjacent cells makes a strong chiral electromagnetic response in this metasurface, which leads to a strong polarization conversion effect. Moreover, compared with the traditional THz transmission resonance sensing for film thickness, the polarization sensing characterized by polarization elliptical angle (PEA) and polarization rotation angle (PRA) shows a better Q factor and figure of merit (FoM). The results show that the Q factors of the PEA and PRA reach 43.8 and 49.1 when the interval film is 20 μm, while the Q factor of THz resonance sensing is only 10.6. And these PEA and PRA can play a complementary role to obtain a double-parameter sensing method with a higher FoM, over 4 times than that of resonance sensing. This chiral metasurface and its polarization sensing method provide new ideas for the development of high-efficiency THz polarization manipulation, and open a window to the high sensitive sensing by using THz polarization spectroscopy.