Optically tunable extrinsic chirality of single-layer metal metasurface for terahertz wave

The extrinsic chirality of metasurfaces refers to the chirality produced by the oblique incidence of light, which is observed in achiral meta-structures. Here we propose a new scheme for tunable extrinsic chirality of terahertz metasurface controlled by near-infrared continuous wave. The metasurface is composed of silicon substrate and single-layer metal units, in which the silicon substrate is used as a photosensitive element. The simulated results show that the maximum circular dichroism (CD) in transmission reaches 0.58, when the terahertz wave is incident on the metasurface with an angle of 83 degree. Furthermore, the terahertz CD will decrease under an illumination of optical beam. Simultaneously changing the incidence angle and the power of the pump light, continuous and active control of CD from 0 to 0.58 can be observed. In experiment, the same trend can also be obtained. The metasurface is simple in structure, and it is the first report for the optical tunable extrinsic chirality in terahertz band as far as we know.