Racemic dielectric metasurfaces for arbitrary terahertz polarization rotation and wavefront manipulation

Dielectric chiral metasurface is a new type of planar and efficient chiral optical device that shows strong circular dichroism or optical activity, which has important application potential in optical sensing and display. However, the two types of chiral optical responses in conventional chiral metasurfaces are often interdependent, as their modulation of the amplitudes and phases of orthogonal circularly polarized components is correlated, which limits the further progress of chiral meta-devices. Here we propose a new scheme for independently designing the circular dichroism and optical activity of chiral metasurfaces to further control the polarization and wavefront of transmitted waves. Inspired by mixtures of chiral molecular isomers, we use the dielectric isomer resonators to form "super-units" instead of single meta-atoms for chiral responses in terahertz band, which is called racemic metasurface. By introducing two levels of Pancharatnam-Berry phases between meta-atoms and "super-units", the polarization rotation angle and wavefront of the beam can be designed without the far-field circular dichroism. We demonstrate the strong control ability on terahertz waves of this scheme through simulation and experiments. In addition, this new type of device with near-field chirality but no far-field circular dichroism may also have important value in optical sensing and other technologies.