Dual-band giant circular dichroism in terahertz spiral metasurface

Abstract The terahertz metamaterials and metasurfaces with strong optical chirality are vital in a wide range of applications, such as chirality detection, bimolecular sensing and terahertz communication. Here, we proposed a metal spiral metasurface (MSM) that can achieve strong circular dichroism with high circular polarization extinction ratio in terahertz region. The designed metasurface can support two kinds of electric resonance modes, which can be used to control the polarization and phase of light at dual frequency bands. The chiroptical responses of structure are enhanced by lowering symmetry of MSM and the thin-film interference effects. The simulation results demonstrate that the left-hand and right-hand circularly polarized light waves can be selectively converted into linearly polarized light waves with transmission efficiency up to 80% at dual frequency bands. The amplitude of the circular dichroism is up to 0.8 and the circular polarization extinction ratio is larger than 90 at each frequency band. Moreover, the sign of CD in each frequency band can be controlled and flipped by simply varying the rotation angle of spiral metasurface. The studied results have potential applications in polarization manipulation, chiral metasurface sensor, and biomedical diagnosis.