Spin-decoupled bifunctional metasurface for THz wavefront manipulation

Metasurfaces have been extensively studied in the field of THz wavefront manipulation as an effective means for light field manipulation. Among them, the metasurface with geometric phase shows its natural and robust phase control ability for spin wave beams. However, the phase responses of opposite spins are locked to be opposite to each other, resulting in symmetric transmission of light. The symmetric transmission is not conducive to the multifunctional integration of optical systems, resulting in decreased efficiency of the metasurface. As a phase component of the metasurface, the transmission phase has spin-independent properties. Moreover, the research discovered that combining transmission phase gradients with geometric phase can achieve spin-decoupled manipulation of light. Based on this, we designed three spin-decoupled bifunctional THz metasurface, and spin-selective focusing, asymmetric hologram and vortex beam generation are realized. The proposed metasurface consists of an arrangement of 16 all-dielectric elliptical silicon pillar meta-atoms with different electromagnetic responses. The co-simulation method of MATLAB and CST realizes the efficient design of these meta-atoms. The average cross-polarization transmissivity of these meta-atoms exceeds 87%, and the transmission phase covers 0~2π. Utilizing these meta-atoms allows free design of spin-decoupled bifunctional THz metadevice to independently manipulate opposite spins. This work is expected to promote the development of THz metasurface devices in practical applications.