Diatomic terahertz metasurfaces for arbitrary-to-circular polarization conversion.

Polarization control is crucial for tailoring light-matter interactions. Direct manipulation of arbitrarily incident polarized waves could provide more degrees of freedom in the design of integrated and miniaturized terahertz (THz) devices. Metasurfaces with unprecedented wave manipulation capabilities could serve as candidates for fulfilling this requirement. Here, a kind of all-silicon metasurface is demonstrated to realize the conversion of arbitrary incident polarization states to circular polarization states in the THz band through the mutual interference of monolayer achiral meta-atoms. Also, we confirmed that the conversion intensities are controllable using the evolution behavior of arbitrary polarization states defined on the Poincaré sphere. Meta-platforms with circularly polarized incidence experience spin-selective destructive or constructive interference, exhibiting broadband circular dichroism (BCD) in the target frequency range. Based on the versatility of the proposed design, the feasibility of the theoretical derivation has been verified in the experiment process. By introducing the geometric phase principle, the proposed design is demonstrated to be an attractive alternative to achieve chiral wavefront manipulation. This work may provide a promising avenue to replace the cumbersome cascaded optical building blocks with an ultrathin meta-platform, which can be used in chiral spectroscopy, imaging, optical communication, and so on.