Spin-multiplexed full-space trifunctional terahertz metasurface [Invited]

The widespread use of multifunctional metasurfaces has started to revolutionize conventional electromagnetic devices due to their unprecedented capabilities and exceedingly low losses.Specifically, geometric metasurfaces that utilize spatially varied single-celled elements to impart arbitrary phase modulation under circularly polarized (CP) waves have attracted more attention.However, the geometric phase has intrinsically opposite signs for two spins, resulting in locked and mirrored functionalities for the right-handed and left-handed CP beams.Additionally, the demonstrated geometric metasurfaces so far have been limited to operating in either transmission or reflection modes at a single wavelength.Here, we propose a double-layered metasurface composed of complementary elliptical and reversal ring resonator structures to achieve simultaneous and independent control of the reflection and transmission of CP waves at two independent terahertz frequencies, which integrates three functions of reflected beam deflection, reflected Bessel beam generation, and transmitted beam focusing on the whole space.The high efficiency and simple design of our metasurface will open new avenues for integrated terahertz metadevices with advanced functionalities.