Design of polarization converter based on dynamical cross-polarized amplitude modulation of superconducting NbN metasurface

In this paper, we innovatively employ superconducting material niobium nitride (NbN) into the polarization converter, designing a C-shape split resonant ring (SRR) polarization converter that works in the frequency range from 0.21 THz to 0.39 THz with a relative bandwidth of 60 % and the polarization conversion ratio (PCR) is more than 90 % in the superconducting state. The employment of NbN also brings dynamic modulation characteristic of cross-polarized amplitude. The cross-polarized amplitude reaches the maximum of-1 dB at 4.5 K, and de-creases to the minimum of-10 dB at 16 K. Meanwhile, focusing on the output purity of cross-polarized amplitude at different temperatures, PCRs are more than 80 % in the frequency range of 0.25 to 0.33 THz. The characteristic is validated in the form of composite beam splitters. The simulation results show that the beam intensity controlled by NbN module decreases with the increase of temperature, achieving a switch in the number of beams. Variation trend is in a good agreement with the decreasing trend of cross-polarized amplitude of the polarization converter. This structure probably provides a new idea for the design of dynamically tuned polarization converter and expands the application of NbN which is beneficial for integration in low-temperature systems.