A 340-GHz thin film polarisation converter

This paper introduces a 340-GHz polarisation converter. The polarisation converter consists of a bilayer metamaterial, which is processed by the flexible thin film lithography method. Two-dimensional metamaterial offer such advantages over traditional quarter-wave-plates as compactness, low profile and simple fabrication process, but the insertion loss is comparably high. In order to lower the insertion loss, the 10-μm ultrathin polyimide substrate and a bilayer structure separated by a quarter wavelength air impedance match layer are adopted. After the horn antenna’s far-field test by vector network analyser and the Gaussian beam test based on Terahertz time-domain spectroscopy (THz-TDS), the results show that in the band of 325–350 GHz, the transmission loss is less than 2 dB, and the axial ratio is less than 3 dB; besides, simulation and test insertion loss performance difference is less than 1 dB, which indicates that the method we adopt is effective. In addition, based on transmission line theory, we analyse its working principle and the influence from the fabrication error.