Fundamentals and applications of spin-decoupled Pancharatnam-Berry metasurfaces

Manipulating circularly polarized(CP)electromagnetic(EM)waves at will is significantly important for a wide range of applications ranging from chiral-molecule manipulations to optical communication.However,conventional EM devices based on natural materials suffer from limited functionalities,bulky configurations,and low efficiencies.Recently,Pancharatnam-Berry(PB)phase metasurfaces have shown excellent capabilities in controlling CP waves in different frequency domains,thereby allowing for multi-functional PB meta-devices that integrate distinct functionalities into single and flat devices.Nevertheless,the PB phase has intrinsically opposite signs for two spins,resulting in locked and mirrored functionalities for right CP and left CP beams.Here we review the fundamentals and applications of spin-decoupled metasurfaces that release the spin-locked limitation of PB metasurfaces by combining the orientation-dependent PB phase and the dimension-dependent propagation phase.This provides a general and practical guideline toward realizing spin-decoupled functionalities with a single metasurface for orthogonal circular polarizations.Finally,we conclude this review with a short conclusion and personal outlook on the future directions of this rapidly growing research area,hoping to stimulate new research outputs that can be useful in future applications.

Multi-feed multi-mode metasurface for independent orbital angular momentum communication in dual polarization

The wavefront control of spin or orbital angular momentum(OAM)is widely applied in the optical and radio fields.However,most passive metasurfaces provide limited manipulations,such as the spin-locked wavefront,a static OAM combination,or an uncontrollable OAM energy distribution.We propose a reflection-type multi-feed metasurface to independently generate multi-mode OAM beams with dynamically switchable OAM combinations and spin states,while simultaneously,the energy distribution of carrying OAM modes is controllable.Specifically,four elements are proposed to overcome the spin-locked phase limitation by combining propagation and geometric phases.The robustness of these elements is analyzed.By involving the amplitude term and multi-feed technology in the design process,the proposed metasurface can generate OAM beams with a controllable energy distribution over modes and switchable mode combinations.OAM-based radio communication with four independent channels is experimentally demonstrated at 14 GHz by employing a pair of the proposed metasurfaces.The powers of different channels are adjustable by the provided amplitude term,and the maximum crosstalk is−9 dB,proving the effectiveness and practicability of the proposed method.